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  • 1.
    Andersson, David C.
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Martinez, N.
    Zeller, D.
    Rondahl, S. H.
    Koza, M. M.
    Frick, B.
    Ekstrom, F.
    Peters, J.
    Linusson, Anna
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Changes in dynamics of alpha-chymotrypsin due to covalent inhibitors investigated by elastic incoherent neutron scattering2017In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 19, no 37, p. 25369-25379Article in journal (Refereed)
    Abstract [en]

    An essential role of enzymes is to catalyze various chemical reactions in the human body and inhibition of the enzymatic activity by small molecules is the mechanism of action of many drugs or tool compounds used to study biological processes. Here, we investigate the effect on the dynamics of the serine protease alpha-chymotrypsin when in complex with two different covalently bound inhibitors using elastic incoherent neutron scattering. The results show that the inhibited enzyme displays enhanced dynamics compared to the free form. The difference was prominent at higher temperatures (240-310 K) and the type of motions that differ include both small amplitude motions, such as hydrogen atom rotations around a methyl group, and large amplitude motions, such as amino acid side chain movements. The measurements were analyzed with multivariate methods in addition to the standard univariate methods, allowing for a more in-depth analysis of the types of motions that differ between the two forms. The binding strength of an inhibitor is linked to the changes in dynamics occurring during the inhibitor-enzyme binding event and thus these results may aid in the deconvolution of this fundamental event and in the design of new inhibitors.

  • 2. Arafa, Wael A. A.
    et al.
    Kärkäs, Markus D.
    Lee, Bao-Lin
    Åkermark, Torbjörn
    Liao, Rong-Zhen
    Berends, Hans-Martin
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Messinger, Johannes
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Siegbahn, Per E. M.
    Åkermark, Björn
    Dinuclear manganese complexes for water oxidation: evaluation of electronic effects and catalytic activity2014In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 16, no 24, p. 11950-11964Article in journal (Refereed)
    Abstract [en]

    During recent years significant progress has been made towards the realization of a sustainable and carbon-neutral energy economy. One promising approach is photochemical splitting of H2O into O-2 and solar fuels, such as H-2. However, the bottleneck in such artificial photosynthetic schemes is the H2O oxidation half reaction where more efficient catalysts are required that lower the kinetic barrier for this process. In particular catalysts based on earth-abundant metals are highly attractive compared to catalysts comprised of noble metals. We have now synthesized a library of dinuclear Mn-2 (II,III) catalysts for H2O oxidation and studied how the incorporation of different substituents affected the electronics and catalytic efficiency. It was found that the incorporation of a distal carboxyl group into the ligand scaffold resulted in a catalyst with increased catalytic activity, most likely because of the fact that the distal group is able to promote proton-coupled electron transfer (PCET) from the high-valent Mn species, thus facilitating O-O bond formation.

  • 3. Beranová, Lenka
    et al.
    Humpolíčková, Jana
    Sýkora, Jan
    Benda, Aleš
    Cwiklik, Lukasz
    Jurkiewicz, Piotr
    Gröbner, Gerhard
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Hof, Martin
    Effect of heavy water on phospholipid membranes: experimental confirmation of molecular dynamics simulations2012In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 14, no 42, p. 14516-14522Article in journal (Refereed)
    Abstract [en]

    Although there were experimental indications that phospholipid bilayers hydrated with D(2)O express different biophysical properties compared with hydration by ordinary H(2)O, a molecular concept for this behavior difference was only recently proposed by a molecular dynamics simulations study [T. Róg et al., J. Phys. Chem. B, 2009, 113, 2378-2387]. Here we attempt to verify those theoretical predictions by fluorescence measurements on 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) membranes. Specifically, we determine the water isotope effect on headgroup hydration and mobility, lateral lipid diffusion and lipid backbone packing. Time-dependent fluorescence shift experiments show significantly slower dynamics and lower hydration of the headgroup region for a bilayer hydrated with D(2)O, an observation in good agreement with the calculated predicted differences in duration of lipid-lipid and lipid-water bridges and extent of water penetration into the bilayer, respectively. The water isotope effect on the lipid order parameter of the bilayer core (measured by fluorescence anisotropy) and lateral diffusion of lipid molecules (determined by two-focus fluorescence correlation spectroscopy) is close to the experimental errors of the experiments, however also refers to slightly more rigid organization of phospholipid bilayers in heavy water. This study confirms the view that the water isotope effect can be particularly found in time-resolved physicochemical properties of the membrane. Together with the simulations our experiments provide a comprehensive, molecular view on the effect of D(2)O on phospholipid bilayers.

  • 4.
    Boily, Jean-François
    Umeå University, Faculty of Science and Technology, Chemistry.
    Elucidation of oxyanion coordination geometries at solid surfaces of varied electric field strengths2009In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 11, no 37, p. 8133-5Article in journal (Refereed)
    Abstract [en]

    Density functional theory calculations of oxyanions exposed to external electric fields revealed systematic variations in molecular geometries and vibration stretching frequencies. These variations can be used in laboratory studies to determine coordination geometries of oxyanions adsorbed on solid surfaces using infrared spectroscopy.

  • 5.
    Boily, Jean-François
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Rosso, Kevin M
    Crystallographic controls on uranyl binding at the quartz/water interface2011In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 13, no 17, p. 7845-7851Article in journal (Refereed)
    Abstract [en]

    Molecular dynamics methods were used to simulate UO(2)(OH)(2)(0) binding to pairs of oxo sites (O(S)) on three low-index planes of α-SiO(2) in contact with water. Differences in binding site distributions on the (001), (010) and (101) planes produced distinct sets of stable U inner-sphere species. Steric constraints prevented bidentate coordination to the (001) surface, resulting in a mononuclear monodentate complex, [UO(2)(OH)(2)(H(2)O)(n)O(S)] (90% for n = 1 and 10% for n = 2 over 5 ns production runs). Binuclear bidentate coordination, [UO(2)(OH)(2)(H(2)O)(n)(O(S))(2)], was however favored on the (010) (99% for n = 0 and 1% for n = 1) and the (101) (72% for n = 0 and 28% for n = 1) planes. These results underscore a predominant four-coordinated equatorial shell for U when complexed to the quartz/water interface. Potential of mean force calculations uncovered a diversity of metastable outer- and inner-sphere complexes at local energy minima up to ∼0.4 nm from the surface. These calculations point to important differences in both energetic requirements and mechanisms for the approach of UO(2)(OH)(2)(0) to different quartz surfaces. Binding strengths are affected by binding site distribution, steric freedom, U hydration and OH orientation, and increase in the order (001) (3.7 kJ mol(-1)) < (101) (5.6 kJ mol(-1)) < (010) (6.5 kJ mol(-1)). A general binding mechanism involves (1) formation of monodentate outer-sphere complexes, (2) removal of oxo-bound waters, (3) formation of one (monodentate), then two (bidentate) direct U-O(S) bonds (inner-sphere), and (4) expulsion of excessive waters from the equatorial shell of U.

  • 6. Bonev, Boyan
    et al.
    Watts, Anthony
    Bokvist, Marcus
    Umeå University, Faculty of Science and Technology, Chemistry.
    Gröbner, Gerhard
    Umeå University, Faculty of Science and Technology, Chemistry.
    Electrostatic peptide-lipid interactions of amyloid-β peptide and pentalysine with membrane surfaces monitored by 31P MAS NMR2001In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 3, no 14, p. 2904-2910Article in journal (Refereed)
    Abstract [en]

    High-resolution 31P magic angle spinning (MAS) NMR spectroscopy is presented as a direct and non-perturbing method for measuring changes in surface charge density occurring in mixed phospholipid membranes upon binding of charged surface-active peptides. 31P MAS NMR was used to investigate mixed lipid membranes of neutral phosphatidylcholine and negatively charged phosphatidylglycerol where the molar fraction of the charged lipid was varied from 0 to 1. The chemical shifts of the  individual membrane lipids showed a simple variation in response to changes in the fraction of the negatively charged component phosphatidylglycerol. Addition of the positively charged amyloid-β1-40 peptide, a key substance in Alzheimer's disease, resulted in changes in the isotropic chemical shifts of the membrane lipid phosphates in a way consistent with reduction in the negative surface charge of the mixed lipid bilayers. Binding of different amounts of the positively charged peptide pentalysine to L-α-dioleoylphosphatidylcholine/L-α-dioleoylphosphatidylglycerol(DOPC/DOPG) vesicles (2 : 1 molar ratio) also showed a systematic variation of both chemical shift values. These changes were described by a simple two-site model and indicate purely electrostatic binding of pentalysine.

  • 7.
    Boulanger, Nicolas
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Yu, Victor
    Hilke, Michael
    Toney, Michael F.
    Barbero, David R.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Graphene induced electrical percolation enables more efficient charge transport at a hybrid organic semiconductor/graphene interface2018In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 20, no 6, p. 4422-4428Article in journal (Refereed)
    Abstract [en]

    Self-assembly of semiconducting polymer chains during crystallization from a liquid or melt dictates to a large degree the electronic properties of the resulting solid film. However, it is still unclear how charge transport pathways are created during crystallization. Here, we performed complementary in situ electrical measurements and synchrotron grazing incidence X-ray diffraction (GIXD), during slow cooling from the melt of highly regio-regular poly(3-hexylthiophene) (P3HT) films deposited on both graphene and on silicon. Two different charge transport mechanisms were identified, and were correlated to the difference in crystallites' orientations and overall amount of crystallites in the films on each surface as molecular self-assembly proceeded. On silicon, a weak charge transport was enabled as soon as the first edge-on lamellae formed, and further increased with the higher amount of crystallites (predominantly edge-on and randomly oriented lamellae) during cooling. On graphene however, the current remained low until a minimum amount of crystallites was reached, at which point interconnection of conducting units (face-on, randomly oriented lamellae and tie-chains) formed percolated conducting pathways across the film. This lead to a sudden rapid increase in current by approximate to 10 fold, and strongly enhanced charge transport, despite a much lower amount of crystallites than on silicon.

  • 8.
    Boulanger, Nicolas
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry. Nano-Engineered Materials and Organic Electronics Laboratory.
    Yu, Victor
    Hilke, Michael
    Toney, Michael F.
    Barbero, David R.
    Umeå University, Faculty of Science and Technology, Department of Chemistry. Nano-Engineered Materials and Organic Electronics Laboratory.
    In situ probing of the crystallization kinetics of rr-P3HT on single layer graphene as a function of temperature2017In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 19, no 12, p. 8496-8503Article in journal (Refereed)
    Abstract [en]

    We studied the molecular packing and crystallization of a highly regio-regular semiconducting polymer poly(3-hexylthiophene) (P3HT) on both single layer graphene and silicon as a function of temperature, during cooling from the melt. The onset of crystallization, crystallites' size, orientation, and kinetics of formation were measured in situ by synchrotron grazing incidence X-ray diffraction (GIXD) during cooling and revealed a very different crystallization process on each surface. A favored crystalline orientation with out of plane pi-pi stacking formed at a temperature of 200 degrees C on graphene, whereas the first crystallites formed with an edge-on orientation at 185 degrees C on silicon. The crystallization of face-on lamellae revealed two surprising effects during cooling: (a) a constant low value of the pi-pi spacing below 60 degrees C; and (b) a reduction by half in the coherence length of face-on lamellae from 100 to 30 degrees C, which corresponded with the weakening of the 2nd or 3rd order of the in-plane (k00) diffraction peak. The final ratio of face-on to edge-on orientations was 40% on graphene, and 2% on silicon, revealing the very different crystallization mechanisms. These results provide a better understanding of how surfaces with different chemistries and intermolecular interactions with the polythiophene polymer chains lead to different crystallization processes and crystallites orientations for specific electronic applications.

  • 9.
    Driver, Gordon W.
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Huang, Yang
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Laaksonen, Aatto
    Sparrman, Tobias
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Wang, Yonglei
    Westlund, Per-Olof
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Correlated/non-correlated ion dynamics of charge-neutral ion couples: the origin of ionicity in ionic liquids2017In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 19, no 7, p. 4975-4988Article in journal (Refereed)
    Abstract [en]

    Proton/Fluoride spin-lattice ($T_1$) nuclear magnetic relaxation dispersion (NMRD) measurements of 1-butyl-3-methyl-$1H$-imidazolium hexa-fluorophosphate, [$C_4mim][PF_6]$, have been carried out using high field spectrometers and fast-field-cycling instrument at proton Larmor frequencies ranging from 10kHz to 40 MHz, at different temperatures. The NMRD profiles are interpreted by means of a simple relaxation model based on the inter- and intra-ionic dipole-dipole relaxation mechanism. Using an atomic molecular-ion dynamic simulation at 323 K the relevant spin dipole-dipole(DD) correlation functions are calculated. The results indicate the NMRD profiles can be rationalized using intra- and inter-ionic spin DD interactions, however, both are mainly modulated by ionic reorientation because of temporary correlations with cations, where modulation by translational diffusion plays a minor role. Reorientational dynamics of charge-neutral ion couples (i.e. $[C_4mim]^{...}[PF_6]$) and $[C_4mim]^{+}$ ions are in the nano-second (ns) time range whereas the reorientation of $[PF_6]{^-}$ is characterized by a reorientational correlation time in the pico-second (ps) regime. Based on the NMRD profiles we conclude the main relaxation mechanism for $[PF_6]{^-}$ is, due to fast internal reorientational motion, a partially averaged F-F intra and a F-H inter-ionic DD coupling as the anion resides in close proximity to its temporary oppositely charged cation partner. The F-$T_1$- NMRD data display a ns dispersions which is interpreted as being due to correlated reorientational modulations resultant from H-containing charge-neutral ion couple $[C_4mim]^{...}[PF_6]$. The analysis of ionicity is based on the free anion fraction, $f$ and it increase with temperature with $f$ $\rightarrow$ 1 at the highest temperatures investigated. The fraction is obtained from the H-F NMRD profiles as correlated-non-correlated dynamics of the ions. The analysis of $T_1$ relaxation rates of C, H, F and P at high fields cannot generally give the fraction of ion but are consistent with the interpretation based on the NMRD profiles with relaxation contributions due to DD-intra and -inter, CSA-intra (and -inter for C), including spin rotation for P. The investigation has led to a description of the mechanics governing ion transport in the title ionic liquid via identification of transient correlated/non-correlated ion dynamics.

  • 10.
    Han, Guangye
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry. Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, USA .
    Huang, Yang
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Koua, Faisal Hammad Mekky
    Graduate School of Natural Science and Technology, Department of Biology, Faculty of Science, Okayama University, Tsushima, Japan .
    Shen, Jian-Ren
    Graduate School of Natural Science and Technology, Department of Biology, Faculty of Science, Okayama University, Tsushima, Japan.
    Westlund, Per-Olof
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Messinger, Johannes
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Hydration of the oxygen-evolving complex of photosystem II probed in the dark-stable S1 state using proton NMR dispersion profiles2014In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, no 16, p. 11924-11935Article in journal (Refereed)
    Abstract [en]

    The hydration of the oxygen-evolving complex (OEC) was characterized in the dark stable S1 state of photosystem II using water R1(ω) NMR dispersion (NMRD) profiles. The R1(ω) NMRD profiles were recorded over a frequency range from 0.01 MHz to 40 MHz for both intact and Mn-depleted photosystem II core complexes from Thermosynechococcus vulcanus (T. vulcanus). The intact-minus-(Mn)-depleted difference NMRD profiles show a characteristic dispersion from approximately 0.03 MHz to 1 MHz, which is interpreted on the basis of the Solomon-Bloembergen-Morgan (SBM) and the slow motion theories as being due to a paramagnetic enhanced relaxation (PRE) of water protons. Both theories are qualitatively consistent with the ST = 1, g = 4.9 paramagnetic state previously described for the S1 state of the OEC; however, an alternative explanation involving the loss of a separate class of long-lived internal waters due to the Mn-depletion procedure can presently not be ruled out. Using a point-dipole approximation the PRE-NMRD effect can be described as being caused by 1-2 water molecules that are located about 10 Å away from the spin center of the Mn4CaO5 cluster in the OEC. The application of the SBM theory to the dispersion observed for PSII in the S1 state is questionable, because the parameters extracted do not fulfil the presupposed perturbation criterion. In contrast, the slow motion theory gives a consistent picture indicating that the water molecules are in fast chemical exchange with the bulk (τw < 1 μs). The modulation of the zero-field splitting (ZFS) interaction suggests a (restricted) reorientation/structural equilibrium of the Mn4CaO5 cluster with a characteristic time constant of τZFS = 0.6-0.9 μs.

  • 11.
    Huang, Yang
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Nam, Kwangho
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Westlund, Per-Olof
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    The water R1(ω) NMRD profiles of a hydrated protein from molecular dynamics simulation2013In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 15, no 33, p. 14089-14097Article in journal (Refereed)
    Abstract [en]

     The hydration of a protein, peroxiredoxin 5, is obtained from a molecular dynamics simulation and compared with the picture of hydration which is obtained by analysing the water proton R1 NMRD profiles using a generally accepted relaxation model [K. Venu, V.P. Denisov and B. Halle, J. Am. Chem. Soc. 119,3122(1997)]. The discrepancy between the hydration pictures derived from the water R1 0)-NMRD profiles and MD is relevant in a discussion of the factors behind the stretched NMRD profile, the distribution of orientationalorder parameters and residence times of buried water used in the NMRD model.

  • 12.
    Håkansson, Pär
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Westlund, Per-Olof
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Nuclear magnetic relaxation study of the microstructure of a bicontinuous cubic phase2004In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, no 6, p. 4321-4329Article in journal (Refereed)
    Abstract [en]

    A NMR relaxation and NMR translational diffusion study is performed on a bicontinuous cubic phase of the dodecyltrimethylammonium chloride/2H2O system with low water content. It is demonstrated how NMR relaxation measurements are combined with structural investigations of X-ray and NMR molecular diffusion measurements to create a consistent picture of the mesoscopic structure of the cubic phase. The 14N, spin-lattice (R1) and spin-spin relaxation rates (R2), were measured for the surfactant at two different field strengths and at four temperatures. The water and the surfactant translational diffusion coefficients were measured by NMR field gradient technique. The experimental spin relaxation rates, and translation diffusion coefficients were interpreted in a model based on a Brownian dynamics (BD) simulation of translational diffusion along curved interfaces (P. Håkansson, L. Persson and P.-O. Westlund, J. Chem. Phys., 2002,117(19), 8634-8643, ). The model surfaces used in our BD relaxation model are also frequently used in interpretations of X-ray crystallography and in free energy calculations on bicontinuous phases. One of our BD-models was able to quantify the influence of the water/lipid interface on the NMR relaxation and diffusion data. Together with X-ray data, a consistent picture of the microstructure for this system was obtained.

  • 13.
    Håkansson, Pär
    et al.
    Umeå University, Faculty of Medicine, Department of Medical Biochemistry and Biophysics.
    Westlund, Per-Olof
    Umeå University, Faculty of Medicine, Department of Medical Biochemistry and Biophysics.
    Lindahl, Erik
    Edholm, Olle
    A direct simulation of EPR slow-motion spectra of spin labelled phospholipids in liquid crystalline bilayers based on a molecular dynamics simulation of the lipid dynamics2001In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 3, no 23, p. 5311-5319Article in journal (Refereed)
    Abstract [en]

    EPR line shapes can be calculated from the stochastic Liouville equation assuming a stochastic model for the reorientation of the spin probe. Here we use instead and for the first time a detailed molecular dynamics (MD) simulation to generate the stochastic input to the Langevin form of the Liouville equation. A 0.1 μs MD simulation at T = 50°C of a small lipid bilayer formed by 64 dipalmitoylphosphatidylcholine (DPPC) molecules at the water content of 23 water molecules per lipid was used. In addition, a 10 ns simulation of a 16 times larger system consisting of 32 DPPC molecules with a nitroxide spin moiety attached at the sixth position of the sn2 chain and 992 ordinary DPPC molecules, was used to investigate the extent of the perturbation caused by the spin probe. Order parameters, reorientational dynamics and the EPR FID curve were calculated for spin probe molecules and ordinary DPPC molecules. The timescale of the electron spin relaxation for a spin-moiety attached at the sixth carbon position of a DPPC lipid molecule is 11.9 × 107 rad s−1 and for an unperturbed DPPC molecule it is 3.5 × 107 rad s−1.

  • 14. Ingman, Petri
    et al.
    Driver, Gordon W
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    A quantitative ionicity scale for liquid chloride salts2012In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 14, no 37, p. 13053-13057Article in journal (Refereed)
    Abstract [en]

    Knowledge of ionicity is requisite for successful identification of those salt qualities required to design and couple the most appropriate fluid for performance of an intended chemical function. We report on utilisation of 35Cl quadrupolar coupling constants (CQ) to quantitatively assess the ionicities of given chloride salts, by exploiting the electronic response of the quadrupolar chlorine atom as a function of its immediate chemical environment. We find that protic salts in particular, like their aprotic analogues, are highly ionised, while at the same time being highly associated, in stark contrast to literature reports claiming in general that they are of sub-ionic origin.

  • 15.
    Isaksson, Mikael
    et al.
    Umeå University, Faculty of Science and Technology, Chemistry.
    Hägglöf, Peter
    Håkansson, Pär
    Umeå University, Faculty of Science and Technology, Chemistry.
    Ny, Tor
    Umeå University, Faculty of Medicine, Medical Biochemistry and Biophsyics.
    Johansson, Lennart B-Å
    Umeå University, Faculty of Science and Technology, Chemistry.
    Extended Förster theory for determining intraprotein distances: 2. an accurate analysis of fluorescence depolarisation experiments2007In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 9, p. 3914-3922Article in journal (Refereed)
    Abstract [en]

    The extended Förster theory (EFT) is for the first time applied to the quantitative determination of the intramolecular distances in proteins. It is shown how the EFT (J. Chem. Phys., 1996, 105, 10896) can be adapted to the analyses of fluorescence depolarisation experiments based on the time-correlated single photon counting technique (TCSPC). The protein system studied was the latent form of plasminogen activator inhibitor type I (PAI-1), which was mutated and labelled by the thiol reactive BODIPY® derivative {N-(4,4-difluoro-5,7-dimethyl-4-bora-3a,4a-diaza-s-indacene-3-yl)methyl iodoacetamide}. The energy migration occurs within pairs of photophysically identical donor groups that undergo reorientational motions on the timescales of energy migration and fluorescence relaxation. Unlike all models currently used for analysing fluorescence TCSPC data, the EFT explicitly accounts for the time-dependent reorientations that influence the rate of electronic energy transfer/migration in a complex manner. The complexity is related to the 2 problem, which has been discussed for years. The EFT brings the analyses of DDEM data to the same level of molecular description as in ESR and NMR spectroscopy, i.e. it yields microscopic information about the reorientation correlation times, the order parameters, as well as inter-chromophoric distances.

  • 16.
    Isaksson, Mikael
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Kalinin, Stanislav
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Lobov, Sergei
    Umeå University, Faculty of Medicine, Department of Medical Biochemistry and Biophysics.
    Wang, Shouye
    Umeå University, Faculty of Medicine, Department of Medical Biochemistry and Biophysics.
    Ny, Tor
    Umeå University, Faculty of Medicine, Department of Medical Biochemistry and Biophysics.
    Johansson, Lennart B-Å
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Partial donor-donor energy migration (PDDEM): a novel fluorescence method for internal protein distance measurements2004In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 6, no 11, p. 3001-3008Article in journal (Refereed)
    Abstract [en]

    We show that the photophysics of chemically identical but photophysically non-identical fluorescent pairs can be used for measuring distances within proteins. For this purpose, the theory of partial donor-donor energy migration (PDDEM, S. Kalinin, J. G. Molotkovsky and L. B.-Angstrom. Johansson, Spectrochim. Acta, Part A, 2002, 58, 1057-1097) was applied for distance measurements between BODIPY groups covalently linked to cystein residues in plasminogen activator inhibitor of type 2 (PAI-2). Two sulfhydryl specific derivatives of BODIPY were used namely: N-(4,4-difluoro-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene-2-yl) iodoacetamide and N-(4.4-difluoro-5.7-ditriethyl-4-bona-3a,4a-diaza-s-indacene-3-yl) methyl iodoacetamide. To determine distances, the time-resolved fluorescence relaxation for two singly labelled forms of PAI-2, as well as the corresponding doubly labelled protein were combined and analysed in a global manner. Fluorescence depolarisation experiments on the labelled mutants were also analysed. The distances determined by PDDEM were in good agreement to those obtained from donor-donor energy migration (DDEM) experiments and structural data on PAI-2. The PDDEM approach allows for the use of very different fluorescent probes, which enables wide range of distances to be measured. The PDDEM model also provides a rational explanation to why previous observations of polyfluorophore-labelled proteins exhibit a shorter average fluorescence lifetime compared to the arithmetic average of lifetimes obtained for the corresponding single labelled proteins.

  • 17.
    Isaksson, Mikael
    et al.
    Umeå University, Faculty of Science and Technology, Chemistry.
    Norlin, Nils
    Umeå University, Faculty of Science and Technology, Chemistry.
    Westlund, Per-Olof
    Umeå University, Faculty of Science and Technology, Chemistry.
    Johansson, Lennart B-Å
    Umeå University, Faculty of Science and Technology, Chemistry.
    On the quantitative molecular analysis of electronic energy transfer within donor–acceptor pairs2007In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 9, p. 1941-51Article in journal (Refereed)
    Abstract [en]

    An extended Förster theory (EFT) on electronic energy transfer is presented for the quantitative analysis of time-resolved fluorescence lifetime and depolarisation experiments. The EFT, which was derived from the stochastic Liouville equation, yields microscopic information concerning the reorientation correlation times, the order parameters, as well as inter chromophoric distances. Weakly interacting donor and acceptor groups, which reorient and interact in a pair wise fashion, are considered, under isotropic and anisotropic conditions. For the analysis of experiments it is shown that not only do we need to consider the orientational distributions of the transition dipoles, but the internal reorienting molecular dynamics within the pair which is of even greater importance. The latter determines the shape as well as the rate of the observed donor fluorescence and depolarisation decays, which are most often not mono-exponential functions. It is shown that the commonly used Förster theory is a special case of the EFT. Strategies are presented for applying the EFT, which makes use of Brownian dynamics simulation.

  • 18. Kazemi, Mehdi Mohammad
    et al.
    Namboodiri, Mahesh
    Umeå University, Faculty of Medicine, Department of Integrative Medical Biology (IMB).
    Donfack, Patrice
    Materny, Arnulf
    Kerle, Daniela
    Rathke, Bernd
    Kiefer, Johannes
    Influence of the alkyl side-chain length on the ultrafast vibrational dynamics of 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl) amide (C(n)mimNTf(2)) ionic liquids2017In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 19, no 24, p. 15988-15995Article in journal (Refereed)
    Abstract [en]

    Probing the vibrational dynamics of 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (CnmimNTf2) ionic liquids (ILs) using femtosecond time-resolved coherent anti-Stokes Raman scattering (fs-CARS) has indicated the ultrafast vibrational energy transfer between counter ions which is governed by interionic interactions and facilitated by hydrogen bonds. In this study, fs-CARS is used to investigate the ultrafast dynamics of the vibrational modes of the CnmimNTf2 ILs with n = 6, 8, 10, and 12 in a spectral region, which involves the imidazolium ring and the alkyl side-chain vibrations. The vibrational Raman modes with wavenumbers around 1418 cm−1 are excited through the CARS process and the ultrafast time evolution of the consequently excited vibrational modes is monitored. The investigation of the life times of the fs-CARS transient signals indicates that the time scale of the dynamics becomes much faster when the alkyl side-chain length of the CnmimNTf2 is longer than n = 8. This observation suggests an increase in the hydrogen bonding interactions due to the nano-structuring of the ionic liquids, which became evident with an increasing length of the alkyl side-chain. This behavior is also found in molecular dynamics simulations. There, an increase of the oxygen density around the C(2)–H moiety of the imidazolium ring, which is the predominant site for hydrogen bond formation, is observed. In other words, the longer the alkyl side-chain, the more reorganization of the ionic liquid into polar and non-polar domains occurs and the higher the probability of finding interionic hydrogen bonds at the C(2)–H position becomes.

  • 19. Langhals, Heinz
    et al.
    Walter, Andreas
    Rosenbaum, Erik
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Johansson, Lennart B-Å
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    A versatile standard for bathochromic fluorescence based on intramolecular FRET2011In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 13, no 23, p. 11055-11059Article in journal (Refereed)
    Abstract [en]

    A perylene and a terrylene tetracarboxylic bisimide dyad was prepared in which an efficient energy transfer from the former to the latter is observed. The absorption spectrum of this compound covers a broad range. Bathochromic fluorescence with a high quantum yield was obtained independent of excitation wavelengths (λ < 655 nm). The dyad can be recommended for the use of calibrating fluorescence spectrometers, as well as a fluorescence standard in the bathochromic region.

  • 20.
    Liang, Yongqi
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Messinger, Johannes
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Improving BiVO4 photoanodes for solar water splitting through surface passivation2014In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 16, no 24, p. 12014-12020Article in journal (Refereed)
    Abstract [en]

    BiVO4 has shown great potential as a semiconductor photoanode for solar water splitting. Significant improvements made during recent years allowed researchers to obtain a photocurrent density of up to 4.0 mA cm(-2) (AM1.5 sunlight illumination, 1.23 V-RHE bias). For further improvements of the BiVO4 photoelectrodes, a deep understanding of the processes occurring at the BiVO4-H2O interface is crucial. Employing an electrochemical loading and removal process of NiOx, we show here that carrier recombination at this interface strongly affects the photocurrents. The removal of NiOx species by electrochemical treatment in a phosphate electrolyte leads to significantly increased photocurrents for BiVO4 photoelectrodes. At a bias of 1.23 V-RHE, the Incident Photon-to-Current Efficiency (IPCE) at 450 nm reaches 43% for the passivated BiVO4 electrode under back side illumination. A model incorporating heterogeneity of NiOx centers on the BiVO4 surface (OER catalytic centers, recombination centers, and passivation centers) is proposed to explain this improved performance.

  • 21.
    Lindgren, Matteus
    et al.
    Umeå University, Faculty of Science and Technology, Chemistry.
    Laaksonen, Aatto
    Stockholms universitet, Avdelningen för fysikalisk kemi.
    Westlund, Per-Olof
    Umeå University, Faculty of Science and Technology, Chemistry.
    A theoretical spin relaxation and molecular dynamics simulation study of the Gd(H2O)93+ complex2009In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 11, p. 10368-76Article in journal (Refereed)
    Abstract [en]

    A theoretical analysis of the paramagnetically enhanced water proton spin–lattice relaxation of a hydrated Gd3+ ion is combined with Molecular Dynamics (MD) simulations. The electron–proton dipole–dipole correlation function, CDDp(), as well as the pseudo-rotation (PR) model of the transient zero-field splitting (ZFS) are evaluated with the help of the data from MD simulations. The fast local water motion in the first hydration shell, i.e. the wagging and rocking motions, is found not to change the mono exponential character of the dipole correlation function CDDp(), but is important in the time dependence of the transient ZFS interaction.The dynamics of the transient ZFS interaction is modeled as the water-induced electric field gradient tensor at the site of the metal ion. This approach follows the ideas of the pseudo-rotation model, describing the fluctuating zero-field interaction as a constant amplitude in the principal frame but reorienting according to a rotational diffusion equation of motion. The MD results indicate that the pseudo-rotation model gives a multi-exponential correlation function which oscillates at short times and is described by three exponential terms. The time scale is shorter than previously assumed but contain an intermediate time constant (1–2 ps). The electron spin resonance (ESR) spectral width at half height at frequencies of X-band, Q-band, 75 MHz, 150 MHz and 225 MHz can be reproduced at 320 K without any contributions from 4th or 6th rank ZFS interactions. Consequently, there are two mutually inconsistent dynamic models of the ZFS interaction which can describe the water proton T1-NMRD (nuclear magnetic resonance dispersion) profile and the field dependent ESR spectra of the hydrated Gd(III) complex equally well.

     

  • 22.
    Lindgren, Matteus
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Westlund, Per-Olof
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    On the stability of chymotrypsin inhibitor 2 in a 10 M urea solution: the role of interaction energies for urea-induced protein denaturation2010In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 12, no 32, p. 9358-9366Article in journal (Refereed)
    Abstract [en]

    Molecular dynamics simulations of chymotrypsin inhibitor 2 in both water and in 10 M urea have been compared with respect to the energies of interaction between protein and solvent. The analysis yield clear and detailed information regarding the enthalpic driving force of urea-induced protein denaturation. The protein is kept in the folded structure by applying positional restraints on the alpha-carbons, thereby creating an equilibrium system from which appropriate driving forces for denaturation can be obtained. All protein atoms are classified as belonging to the backbone, the polar side chains or to the hydrophobic side chains. The interaction energies are extracted for each class separately. The commonly proposed mechanisms of urea denaturation, i.e. that urea interacts mainly with the backbone or with the hydrophobic side chains, can then be tested. The results show that urea decreases the Lennard-Jones interaction energies between protein and solvent by a large amount. The electrostatic energies are almost unaffected by the switch of solvent. The energetically favorable interaction between CI2 and the urea solvent will function as a driving force for the protein to increase its solvent accessible surface area as compared to the folded protein in water. The magnitude of the decrease in the Lennard-Jones energies for the hydrophobic and the hydrophilic side chains and for the backbone were similar. We therefore conclude that urea interacts favorably with the whole protein surface and that all parts of the protein are important in urea-induced denaturation.

  • 23.
    Lindström, Fredrick
    et al.
    Umeå University, Faculty of Science and Technology, Chemistry.
    Bokvist, Marcus
    Umeå University, Faculty of Science and Technology, Chemistry.
    Sparrman, Tobias
    Umeå University, Faculty of Science and Technology, Chemistry.
    Gröbner, Gerhard
    Umeå University, Faculty of Science and Technology, Chemistry.
    Association of amyloid-β peptide with membrane surfaces monitored by solid state NMR2002In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 4, no 22, p. 5524-5530Article in journal (Refereed)
    Abstract [en]

    Amyloid-β peptide (Aβ), a key substance in Alzheimers disease (AD), is characterized by its abnormal folding into neurotoxic aggregates. Since Aβ comprises an extracellular and transmembrane domain, some of its neurotoxic actions might be exerted via interactions with neuronal membranes. Wideline and magic angle spinning 14N and 31P NMR have been used in combination with differential scanning calorimetry and circular dichroism spectroscopy to investigate the association between Aβ1–40 peptide and membranes with different electrostatic surface potentials. Calorimetric measurements showed that all membrane systems were in the liquid crystalline state at 308 K. Binding of Aβ1–40 at a 30 1 lipid/peptide ratio to membranes composed of neutral dimyristoyl-phosphatidylcholine (DMPC) and negatively charged dimyristoylphosphatidylglycerol (DMPG) at a 4 : 1 molar ratio is mainly driven electrostatically, reflected in characteristic changes of the isotropic 31P chemical shift values for both lipids. In addition, the average orientation of the choline headgroup of DMPC, with its electric P–N+(CH3)3 dipole, changed directly in response to the reduced negative membrane surface potential. The deviation in tilt angle of the PN vector relative to the membrane surface is manifested in the observed 14N NMR quadrupole splitting and can therefore be described semiquantitatively. Adding Aβ1–40 to membranes with nominal neutral surface charge, but composed of a ternary mixture of DMPC with DMPG and the cationic amphiphile didodecyldimethyl–ammonium bromide (DDAB) at a 3 : 1 : 1 molar ratio revealed surprisingly electrostatic interactions visible in the NMR spectra. Since Aβ1–40 does not bind to neutral DMPC bilayers a model is proposed, in which on a molecular level the charged residues of Aβ1–40 peptide can interact independently with lipid headgroups of various charges in these microscopically heterogeneous systems.

  • 24.
    Messinger, Johannes
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Lubitz, Wolfgang
    Shen, Jian-Ren
    Photosynthesis: from natural to artificial2014In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 16, no 24, p. 11810-11811Article in journal (Other academic)
  • 25.
    Mitev, Pavlin D.
    et al.
    Department of Chemistry - Ångström Laboratory, Uppsala University, Box 538, S-751 21 Uppsala, Sweden .
    Eriksson, Anders
    Department of Chemistry - Ångström Laboratory, Uppsala University, Box 538, S-751 21 Uppsala, Sweden .
    Boily, Jean-Francois
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Hermansson, Kersti
    Department of Chemistry - Ångström Laboratory, Uppsala University, Box 538, S-751 21 Uppsala, Sweden .
    Vibrational models for a crystal with 36 water molecules in the unit cell: IR spectra from experiment and calculation2015In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 17, no 16, p. 10520-10531Article in journal (Refereed)
    Abstract [en]

    We present experimental and calculated IR spectra of the water molecules in crystalline aluminium nitrate nonahydrate and a method to generate a realistic and well resolved isotope-isolated spectrum from periodic DFT calculations. Our sample crystal contains 18 structurally different OH groups and is a perfect benchmark compound to validate vibrational models and the structure-property relationship of bound water molecules. FTIR spectra (ATR technique) were recorded for the Al(NO3)(3)center dot 9H(2)O crystal at 138 and 298 K, and due to a multitude of OH contributions and couplings, they are naturally poorly resolved and yield a broad OH band in the range 3500 to 2700 cm(-1) at both temperatures. Isotope-isolated IR spectra have the clear advantage over non-deuterated spectra that they are better resolved and easier to interpret - here we have extended the experimental study by simulating the isotope-isolated IR spectrum, using PBE-D2 and auxiliary B3LYP calculations and an anharmonic OH vibrational model. We find excellent agreement between the shapes and frequency ranges of the experimental and calculated OH spectral bands. We make use of four different vibrational models: (i) a harmonic lattice-dynamical model for the isotope-isolated crystal with 1 H among 71 D, (ii) a harmonic lattice-dynamical model for the normal undeuterated crystal involving all the vibrational couplings, (iii) a harmonic 1-dimensional uncoupled OH vibrational model, and (iv) the anharmonic variant of the previous model, which yields the final spectrum. We also use the individual frequencies, resolved by the calculations, to quantify new or extended relationships involving OH frequencies versus local electric fields and H-bond distances. We explore the correlation between OH frequency and molecular dipole moment for bound water molecules.

  • 26.
    Opanasyuk, Oleg
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Johansson, Lennart B-Å
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Extended Förster theory: a quantitative approach to the determination of inter-chromophore distances in biomacromolecules2010In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 12, no 28, p. 7758-7767Article in journal (Refereed)
    Abstract [en]

    This review highlights recent theoretical and experimental advances in the study of biomacromolecular structure by using electronic transfer. The considered electronic transport in the extended Förster theory occurs within donor–acceptor pairs, donor–donor pairs, as well as within regular arrangements of many donors which may undergo reorienting and translational dynamics. The classical and the extended FoЁ rster theory are compared. Applications concern the determination of structural properties of proteins and non-covalent protein polymers. Studies of energy migration by means of two-photon excited fluorescence spectroscopy, as well as the relevant extension of the Förster theory are presented.

  • 27.
    Opanasyuk, Oleg
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Johansson, Lennart B-Å
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    On the analyses of fluorescence depolarisation data in the presence of electronic energy migration: Part I. Theory and general description2012In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 14, p. 1907-1916Article in journal (Refereed)
    Abstract [en]

    A new and general procedure is described for a detailed analysis of time-resolved fluorescence depolarisation data in the presence of electronic energy migration. An isotropic ensemble of bifluorophoric molecules (D1-R-D2) has been studied to demonstrate its utility. Intramolecular donor-donor energy migration occurs between the two donor groups (D), which are covalently connected to a rigid linker group (R). These groups undergo restricted reorientational motions with respect to the R group. The analysis of depolarisation data basically involves the search for best-fit parameters which describe the local reorienting motions, the intermolecular D1-D2 distance, as well as the mutual orientations of the donors. For this, the analysis is partly performed in the Fourier domain and the best-fit parameters are determined by using an approach based on a Genetic Algorithm. The energy migration process has been described by using Monte Carlo simulations and an extended Förster theory (EFT). It is found that the EFT provides the least time-consuming computational method. Since one-photon and two-photon excited fluorescence experiments can be applied for energy migration studies, a general and unified theoretical formulation is given.

  • 28.
    Opanasyuk, Oleg
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Mikaelsson, Therese
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Ryderfors, Linus
    Department of Photochemistry and Molecular Science, Uppsala University, P. O. Box 523, S-751 20 Uppsala, Sweden.
    Mukhtar, Emad
    Department of Photochemistry and Molecular Science, Uppsala University, P. O. Box 523, S-751 20 Uppsala, Sweden.
    Johansson, Lennart B-Å
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    On the analyses of fluorescence depolarisation data in the presence of electronic energy migration.: II. Applying & Evaluating Two-Photon Excited Fluorescence2012In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 14, p. 1917-1922Article in journal (Refereed)
    Abstract [en]

    Electronic energy migration within a bifluorophoric molecule has been studied by time-resolved two-photon excited (TPE) fluorescence depolarisation experiments. Data were analysed by using a recently developed quantitative approach [Opanasyuk, O. & Johansson, L. B.-Å., On the Analyses of Fluorescence Depolarisation Data in the Presence of Electronic Energy Migration. I. Theory & General Description. Phys. Chem. Chem. Phys., Submitted.]. The energy migration occurs between the 9-anthrylmethyl groups of the bifluorophoric molecule, bis-(9-anthrylmethylphosphonate) bisteroid. These groups undergo local reorientations, while overall tumbling of the bisteroid is strongly hampered in the used viscous solvent, 1,2-propanediol. To solely obtain information about local reorientations of the 9-anthrylmethyl group, also the mono-(9-anthrylmethylphosphonate) bisteroid was studied, which enabled modelling of the ordering potential shape. The analysis of data is partly performed in the Fourier domain and the best-fit parameters are determined by using an approach based on a Genetic Algorithm. The energy migration process was described by an extended Förster theory (EFT). A reasonable value of the distance between the 9-anthrylmethyl groups is found, as well as for the mutual orientation of the ordering potentials. Furthermore, values of the two-photon tensor components were obtained.

  • 29.
    Opanasyuk, Oleg
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Ryderfors, Linus
    Department of Photochemistry and Molecular Science, Uppsala University, P. O. Box 523, S-751 20 Uppsala, Sweden.
    Mukhtar, Emad
    Department of Photochemistry and Molecular Science, Uppsala University, P. O. Box 523, S-751 20 Uppsala, Sweden.
    Johansson, Lennart B-Å
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Two-photon excited fluorescence depolarisation and electronic energy migration within donor–donor pairs2009In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, no 11, p. 7152-7160Article in journal (Refereed)
    Abstract [en]

    A unified theoretical description is presented for one- and two-photon excited fluorescence depolarisation and electronic energy migration within pairs of chromophores. Two weakly coupled donor groups are linked via a rigid macromolecule with the ability to undergo restricted reorienting motions. Describing these reorienting motions as well as their influence on the coupling is rather complex, but can be accounted for by using the extended Förster theory. Here explicit expressions have been derived for chromophores belonging to the point groups D2h,D2 andC2v when residing in uniaxial potentials (i.e. C∞v symmetry). From the given basic equations, it is possible however, to derive the relevant equations for molecules of arbitrary symmetry in any uniaxial orienting potential. The expected time-resolved fluorescence anisotropy for different two-photon absorption tensors are compared for reorienting fluorophores in liquids, as well as in anisotropic systems. Simulated fluorescence depolarisation data are also displayed that mimic energy migration within pairs of two-photon excited donor molecules, which simultaneously undergo reorienting motions within effectively isotropic and uniaxially anisotropic environments. The obtained results demonstrate that the time-resolved fluorescence anisotropy strongly depends on the properties of the two-photon absorption tensor, as well as on using a linear or a circular polarisation of the excitation field.

  • 30. Pantazis, Dimitrios A
    et al.
    Orio, Maylis
    Petrenko, Taras
    Zein, Samir
    Lubitz, Wolfgang
    Messinger, Johannes
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Neese, Frank
    Structure of the oxygen-evolving complex of photosystem II: information on the S(2) state through quantum chemical calculation of its magnetic properties.2009In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 11, no 31, p. 6788-6798Article in journal (Refereed)
    Abstract [en]

    Twelve structural models for the S(2) state of the oxygen-evolving complex (OEC) of photosystem II are evaluated in terms of their magnetic properties. The set includes ten models based on the 'fused twist' core topology derived by polarized EXAFS spectra and two related models proposed in recent mechanistic investigations. Optimized geometries and spin population analyses suggest that Mn(iii), which is most often identified with the manganese ion at site D, is always associated with a penta-coordinate environment, unless a chloride is directly ligated to the metal. Exchange coupling constants were determined by broken-symmetry density functional theory calculations and the complete spectrum of magnetic sublevels was obtained by direct diagonalization of the Heisenberg Hamiltonian. Seven models display a doublet ground state and are considered spectroscopic models for the ground state corresponding to the multiline signal (MLS) of the S(2) state of the OEC, whereas the remaining five models display a sextet ground state and could be related to the g = 4.1 signal of the S(2) state. It is found that the sign of the exchange coupling constant between the Mn centres at positions A and B of the cluster is directly related to the ground state multiplicity, implying that interconversion between the doublet and sextet can be induced by only small structural perturbations. The recently proposed quantum chemical method for the calculation of (55)Mn hyperfine coupling constants is subsequently applied to the S(2) MLS state models and the quantities that enter into the individual steps of the procedure (site-spin expectation values, intrinsic site isotropic hyperfine parameters and projected (55)Mn isotropic hyperfine constants) are analyzed and discussed in detail with respect to the structural and electronic features of each model. The current approach performs promisingly. It reacts sensitively to structural distortions and hence may be able to distinguish between different structural proposals. Thus it emerges as a useful contributor to the ongoing efforts that aim at establishing correlations between the body of spectroscopic data available for the various S(i) states of the OEC and their actual geometric features.

  • 31. Perera, Ishanie Rangeeka
    et al.
    Gupta, Akhil
    Xiang, Wanchun
    Daeneke, Torben
    Bach, Udo
    Evans, Richard A.
    Ohlin, C. Andre
    Spiccia, Leone
    Introducing manganese complexes as redox mediators for dye-sensitized solar cells2014In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 16, no 24, p. 12021-12028Article in journal (Refereed)
    Abstract [en]

    The abundance and low toxicity of manganese have led us to explore the application of manganese complexes as redox mediators for dye sensitized solar cells (DSCs), a promising solar energy conversion technology which mimics some of the key processes in photosynthesis during its operation. In this paper, we report the development of a DSC electrolyte based on the tris(acetylacetonato) manganese(III)/(IV), [Mn(acac)(3)](0/1+), redox couple. PEDOT-coated FTO glass was used as a counter electrode instead of the conventionally used platinum. The influence of a number of device parameters on the DSC performance was studied, including the concentration of the reduced and oxidized mediator species, the concentration of specific additives (4-tert-butylpyridine, lithium tetrafluoroborate, and chenodeoxycholic acid) and the thickness of the TiO2 working electrode. These studies were carried out with a new donor-pi-acceptor sensitizer K4. Maximum energy conversion efficiencies of 3.8% at simulated one Sun irradiation (AM 1.5 G; 1000 W m(-2)) with an open circuit voltage (V-OC) of 765 mV, a short-circuit current (J(SC)) of 7.8 mA cm(-2) and a fill factor (FF) of 0.72 were obtained. Application of the commercially available MK2 and N719 sensitizers resulted in an energy conversion efficiency of 4.4% with a V-OC of 733 mV and a J(SC) of 8.6 mA cm(-2) for MK2 and a V-OC of 771 mV and a J(SC) of 7.9 mA cm(-2) for N719. Both dyes exhibit higher incident photon to current conversion efficiencies (IPCEs) than K4.

  • 32.
    Revoju, Srikanth
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Biswas, Subhayan
    Eliasson, Bertil
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Sharma, Ganesh D.
    Asymmetric triphenylamine–phenothiazine based small molecules with varying terminal acceptors for solution processed bulk-heterojunction organic solar cells2018In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 20, no 9, p. 6390-6400Article in journal (Refereed)
    Abstract [en]

    Three compounds consisting of the electron-donating triphenylamine–phenothiazine conjugate backbone and each of the electron-withdrawing groups 3-ethylrhodanine, malononitrile and 1,3-indandione have been synthesized and used as donors in blends with [6,6]-phenyl-C70-butyric acid methyl ester (PC71BM) for organic solar cell devices. After improvements of the active layer structure by a selected donor-to-acceptor weight ratio and a two-step solvent and thermal annealing, the organic solar cells showed power conversion efficiency (PCE) values in the range of 4.79–7.25%. The highest PCE was obtained for the bulk heterojunction device with the indandione compound, which can be attributed to its better absorption profile, higher crystallinity, more balanced electron and hole transport, higher charge collection efficiency and reduced recombination, in comparison with the photovoltaic cells from the other two compounds. DFT-calculated characteristics, absorption spectra and cyclic voltammetry of the compounds, along with X-ray diffraction patterns of the blend films, are used to validate the photovoltaic results.

  • 33.
    Sachl, Radek
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Boldyrev, Ivan
    Shemyakin & Ovchinnikov Institute, Institute of Bioorganic Chemistry, Russian Academy of Sciences, Moscow, Russia.
    Johansson, Lennart B-Å
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Localisation of BODIPY-labelled phosphatidylcholines in lipid bilayers2010In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 12, p. 6027-6034Article in journal (Refereed)
    Abstract [en]

    A series of sn-2 acyl-labelled phosphatidyl-cholines (PC), bearing 4,4-difluoro-1-3-5-7-tetra-methyl-4-bora-3a,4a-diaza-s-indacene-8-yl (Me4-BODIPY) at the end of the Cn-acyl chains were solubilised in unilamellar vesicles and studied with respect to the order and location of the Me4-BODIPY (denoted: B) group. The obtained results are based on time-resolved electronic energy transfer from donors (2-(9-anthroyloxy)-stearic acid) localised in the lipid–water interface to acceptors BnPC (n = 3, 5, 7, 9, 11, 13, 15), as well as the energy migration among the Me4-BODIPY groups of BnPC:s. The donor–acceptor and the donor–donor experiments strongly suggest that the Me4-BODIPY group in BnPC tends to loop back close to the lipid–water interface. The Me4-BODIPY groups, residing in the two bilayer leaflets, are located at approximately the same depth, and transversally separated by ca. 27 Å for all n-values. Close to the interface, the optimal transversal distribution widens somewhat with increasing length of the sn-2 acyl chain. The obtained order parameter profile of the BnPC:s is also compatible with such a location.

  • 34.
    Sachl, Radek
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Mikhalyov, Ilya
    Shemyakin & Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, Moscow, Russia.
    Gretskaya, Natalia
    Shemyakin & Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, Moscow, Russia.
    Olżyńska, Agnieszka
    J. Heyrovsky´ Institute of Physical Chemistry, v.v.i., Academy of Sciences of the Czech Republic, Dolejsˇkova 3, 182 23 Prague 8, Czech Republic.
    Hof, Martin
    J. Heyrovsky´ Institute of Physical Chemistry, v.v.i., Academy of Sciences of the Czech Republic, Dolejsˇkova 3, 182 23 Prague 8, Czech Republic.
    Johansson, Lennart B-Å
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Distribution of BODIPY-labelled phosphatidylethanolamines in lipid bilayers exhibiting different curvatures2011In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 13, p. 11694-11701Article in journal (Refereed)
    Abstract [en]

    In this paper we have investigated the behaviour of newly synthesised mono-palmitoyl- and dipalmitoyl-phosphatidylethanolamine probes (abbreviated as mPE and dPE, respectively) labelled in the polar headgroup region by either the FL-BODIPY or the 564/570-BODIPY fluorophore and solubilised in lipid systems that exhibit different curvatures. Because of the bulky BODIPY-groups, the monoacyl-form derivatives have a conic-like shape, whereas that for the diacyl derivatives is rather cylindrical. A careful analysis of time-resolved resonance energy transfer experiments by means of analytical models as well as Monte Carlo simulations shows that the mPE derivatives have a comparable affinity to highly curved bilayer regions (torroidal pores formed by magainin-2 in lipid bilayers, or the rims of discoid bicelles) and to planar bilayer regions (i.e. the flat region of lipid bilayers and bicelles). Furthermore, the monoacyl-probes are as compared to the diacyl-probes effectively closer to each other in a lipid bilayer, while none of these probes seems to be randomly distributed. Self-aggregation is most efficiently induced by the larger aromatic 564/570-BODIPY chromophore, but it is suppressed when using the diacyl instead of the monoacyl-form, and/or by attaching BODIPY-groups to the acyl-chain.

  • 35.
    Sachl, Radek
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Mikhalyov, Ilya
    Shemyakin & Ovchinnikov Institute, Institute of Bioorganic Chemistry, Russian Academy of Sciences, Moscow, Russia.
    Hof, Martin
    J. Heyrovsky´ Institute of Physical Chemistry of the Academy of Sciences of the Czech Republic, Dolejsˇkova 3 182 23 Prague 8,Czech Republic.
    Johansson, Lennart B-Å
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    A comparative study on ganglioside micelles using electronic energy transfer, fluorescence correlation spectroscopy and light scattering techniques2009In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 11, no 21, p. 4335-4343Article in journal (Refereed)
    Abstract [en]

    Ganglioside (GM1) micelles have been studied by means of three different techniques: fluorescence correlation spectroscopy (FCS), electronic energy transfer, as monitored by time-resolved fluorescence spectroscopy, as well as static and dynamic light scattering. The aggregation numbers obtained, 168 ± 4, remain constant over a wide range of GM1 concentrations (0.764-156 M), are very consistent when using different donor-acceptor energy transfer pairs and have served as reference values in tests of the FCS method. It is recommended to calibrate the focal volume by using known dye concentrations. For this the rhodamine dye, 5-TAMRA, turns out to be most suitable. It is also shown that FCS provides correct values of the aggregation numbers, provided that the focal volume is calibrated by using updated values of the diffusion constant of Rhodamine 6G. These results also support recent methodological advances in FCS.

  • 36. Sharma, Rupali
    et al.
    Zhang, Jie
    Ohlin, C. André
    Predicting O-17 NMR chemical shifts of polyoxometalates using density functional theory2016In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 18, no 11, p. 8235-8241Article in journal (Refereed)
    Abstract [en]

    We have investigated the computation of O-17 NMR chemical shifts of a wide range of polyoxometalates using density functional theory. The effects of basis sets and exchange-correlation functionals are explored, and whereas pure DFT functionals generally predict the chemical shifts of terminal oxygen sites quite well, hybrid functionals are required for the prediction of accurate chemical shifts in conjunction with linear regression. By using PBE0/def2-tzvp//PBE0/cc-pvtz(H-Ar), lanl2dz(K-) we have computed the chemical shifts of 37 polyoxometalates, corresponding to 209 O-17 NMR signals. We also show that at this level of theory the protonation-induced pH dependence of the chemical shift of the triprotic hexaniobate Lindqvist anion, [HxNb6O19]((8-x)), can be reproduced, which suggests that hypotheses regarding loci of protonation can be confidently tested.

  • 37.
    Shimizu, Kenichi
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Nyström, J.
    Geladi, P.
    Lindholm-Sethson, Britta
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Boily, Jean-Francois
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Electrolyte ion adsorption and charge blocking effect at the hematite/aqueous solution interface: an electrochemical impedance study using multivariate data analysis2015In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 17, no 17, p. 11560-11568Article in journal (Refereed)
    Abstract [en]

    A model-free multivariate analysis using singular value decomposition is employed to refine an equivalent electrical circuit model in order to probe the electrochemical properties of the hematite/water interface in dilute NaCl and NH4Cl solutions using electrochemical impedance spectroscopy. The result shows that the surface protonation is directly related to the mobility and trapping of charge carriers at the mineral surface. Moreover, the point of zero charge can be found at pH where the charge transfer resistance is the highest, in addition to the minimum double layer capacitance. The innersphere interaction of the NH4+ ion with the surface is indicated by an increase of capacitance for charge carrier trapping from the protonated surface as well as lower double layer capacitance and open circuit potential. It is clear that the intrinsic electrochemical activity of hematite depends on the degree of surface (de) protonation and other inner-sphere adsorption, as these processes affect the charge carrier density in the surface state. This work also highlights an important synergistic effect of the two spectral analyses that enables EIS to be utilized in an in-depth investigation of mineral/water interfaces.

  • 38.
    Shukla, Shashi Kant
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Mikkola, Jyri-Pekka
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Intermolecular interactions upon carbon dioxide capture in Deep-Eutectic Solvents2018In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084Article in journal (Refereed)
    Abstract [en]

    Herein we report the CO2 uptake in potential Deep-Eutectic Solvents (DESs) formed between hydrogen bond acceptors (HBAs) such as monoethanolammonium chloride ([MEA.Cl]), 1-methylimidazolium chloride ([HMIM.Cl]) and tetra-n-butylammonium bromide ([TBAB]) and hydrogen bond donors (HBDs) like ethylenediamine ([EDA]), diethylenetriamine ([DETA]), tetraethylenepentamine ([TEPA]), pentaethylenehexamine ([PEHA]), 3-amino-1-propanol ([AP]) and aminomethoxypropanaol ([AMP]) and analyzed the outcome in terms of the specific polarity parameters. Among various combinations of HBAs and HBDs, [MEA.Cl][EDA]-, [MEA.Cl][AP]-, [HMIM.Cl][EDA]- and [HMIM.Cl][AP] showed excellent CO2 uptake which was further improved upon increasing the mole ratio of HBA:HBD from 1:1 to 1:4. The lowest CO2 uptake in [MEA.Cl][PEHA] (12.7 wt%) and [HMIM.Cl][PEHA] (8.4 wt%) despite the highest basicity of [PEHA] infers that the basicity is not the sole criteria for guiding the CO2 uptake but, in reality, CO2 capture in a DES relies on the interplay of H-bonding interactions between each HBA and HBD. The role of HBAs in guiding CO2 uptake was more prominent with weak HBDs such as [TEPA] and [PEHA]. The speciation of absorbed CO2 into carbamate, carbonate, and bicarbonate was favorable in DES characterized by comparable hydrogen bond donor acidity (α) and hydrogen bond acceptor basicity (β) values, whereas the conversion of carbamate to carbonate/bicarbonate was observed to depend on α. The addition of water in DES resulted in lower CO2 uptake due to the decreased basicity (β). ER

  • 39. Skvarla, Jiri
    et al.
    Kanuchova, Maria
    Shchukarev, Andrey
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Brezani, Ivan
    Skvarla, Juraj
    Accumulation of counterions and coions evaluated by cryogenic XPS as a new tool for describing the structure of electric double layer at the silica/water interface2017In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 19, no 43, p. 29047-29052Article in journal (Refereed)
    Abstract [en]

    We introduce a new method of evaluating the structure of electric double layer (EDL) at the native solid/liquid interface using cryogenic X-ray photoelectron spectroscopy technique. This method is based on evaluating the atomic concentration ratio of counterions and co-ions of supporting electrolyte at the close-to-in situ state surface of colloid particles by the cryo-XPS and comparing it with analogous ratio predicted by EDL models. For silica colloids in aqueous KCl solutions at pH 6 to 8 it has been found that the latter ratio is higher than unity, as expected for the negatively charged surface of silica, but does not correspond with the prediction of the basic Gouy-Chapman EDL model for the ideal interface. However, it agrees with that deduced from experiments on electrolytic coagulation kinetics of analogous silica colloids by applying a simple EDL model of swellable ion-permeable (Donnanian) polyelectrolyte gel layer. It turns out that the traditional Stern layer-based concept of EDL at solid/liquid interfaces is not justified for metal oxides at least in KCl solutions.

  • 40.
    Song, Xiaowei
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Boily, Jean-François
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Structural controls on OH site availability and reactivity at iron oxyhydroxide particle surfaces2012In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 14, no 8, p. 2579-86Article in journal (Refereed)
    Abstract [en]

    Iron oxyhydroxides (FeOOH) are highly reactive minerals of widespread occurrence in natural and industrial settings. These minerals chiefly occur as nano- to submicron-sized particles and are covered by hydroxyl functional groups coordinated to one (-OH), two (μ-OH), or three (μ(3)-OH) underlying iron atoms. These groups are reaction centers for gases, solutes as well as solvents and thereby play important roles in the fate and transformation of natural and industrial compounds. In this work we provide tools to identify hydroxyl groups on distinct crystallographic planes of two important FeOOH polymorphs, namely lepidocrocite (γ-FeOOH) and goethite (α-FeOOH). Fourier transform infrared spectroscopy was used to monitor O-H stretching vibrations of groups on particles with well-defined and distinct morphologies. Spectral responses to proton loadings and thermal gradients were used to assign bands to hydroxyl groups. These efforts were facilitated by the extraction of pure spectral components obtained by multivariate curve resolution. Molecular dynamics simulations of dominant crystallographic planes of the particles guided band assignment procedures by identifying feasible hydrogen bond networks between surface groups. Our findings provide new possibilities for molecular-scale resolution of important gas-phase processes on the surfaces of these important minerals.

  • 41.
    Tonpheng, Bounphanh
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Yu, Junchun
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Andersson, Ove
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Effects of cross-links, pressure and temperature on the thermal properties and glass transition behaviour of polybutadiene2011In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 13, no 33, p. 15047-15054Article in journal (Refereed)
    Abstract [en]

    The thermal conductivity κ, heat capacity per unit volume ρcp and glass transition behaviour under pressure have been established for medium and high vinyl content polybutadiene PB with molecular weights 2600 and 100 000 and their highly cross-linked (ebonite) states obtained purely by high-pressure high-temperature treatments. Cross-linking eliminates the glass transitions and increases κ by as much as 50% at 295 K and 1 atm, and decreases ρcp to a limiting level close to that of the glassy state of PB, which is reached before the ultimate cross-link density is achieved. The pressure and temperature behaviours of κ are strongly changed by cross-links, which increases the effect of temperature but decreases the effect of pressure. We attribute these changes to a cross-linked induced permanent densification and consequential increase of phonon velocity simultaneously as conduction along polymer chains is disrupted. The glass transition temperatures for a time scale of 1 s are described to within 0.5 K by: Tg(p) = 202.5 (1 + 2.94 p)0.286 and Tg(p) = 272.3 (1 + 2.57 p)0.233 (p in GPa and T in K) up to 1 GPa, for PB2600 and PB100000, respectively, and can be estimated for medium and high vinyl content PBs with molecular weights in between by a constant, pressure independent, shift in temperature.

  • 42.
    Wennerström, Håkan
    et al.
    Division of Physical Chemistry, Chemical Center, P.O. Box 124, University of Lund, SE 22100 Lund, Sweden.
    Westlund, Per-Olof
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    The Stern-Gerlach experiment and the effects of spin relaxation2012In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 14, p. 1677-1684Article in journal (Refereed)
    Abstract [en]

    The classical Stern-Gerlach experiment is analyzed with an emphasis on the spin dynamics. The central question asked is whether there occurs a relaxation of the spin angular momentum during the time the particle passes through the Stern-Gerlach magnet. We examine in particular the transverse relaxation, involving angular momentum exchange between the spin of the particles and the spins of the magnet. A method is presented describing relaxation effects at an individual particle level. This leads to a stochastic equation of motion for the spins. This is coupled to a classical equation of motion for the particle translation. The experimental situation is then modeled through simulations of individual trajectories using two sets of parameter choices and three different sets of initial conditions. The two main conclusions are: (A) if the coupling between the magnet and the spin is solely described by the Zeeman interaction with the average magnetic field the simulations show a clear disagreement with the experimental observation of Stern and Gerlach. (B) If one, on the other hand, also allows for a T(2) relaxation time shorter than the passage time one can obtain a practically quantitative agreement with the experimental observations. These conclusions are at variance with the standard textbook explanation of the Stern-Gerlach experiment.

  • 43.
    Westlund, Per-Olof
    Umeå University, Faculty of Science and Technology, Chemistry.
    The quadrupole enhanced (1)H spin-lattice relaxation of the amide proton in slow tumbling proteins2010In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 12, no 13, p. 3136-40Article in journal (Refereed)
    Abstract [en]

    An analysis, based on the stochastic Liouville approach, is presented of the R(1)-NMRD or field dependent spin-lattice relaxation rate of amide protons. The proton relaxivity, displayed as R(1)-NMRD profiles, is calculated for a reorienting (1)H-(14)N spin group, where the inter spin coupling is due to spin dipole-dipole coupling or the scalar coupling. The quadrupole nucleus (14)N has an asymmetry parameter eta = 0.4 and a quadrupole interaction which is modulated by the overall reorientational motion of the protein. In the very slow reorientational regime, omega(Q)tau(R)>> 1 and tau(R)>/= 2.0 mus, both the dipole-dipole coupling and the scalar coupling yield a T(1)-NMRD profile with three marked peaks of proton spin relaxation enhancement. These peaks appear when the proton Larmor frequency, omega(I), matches the nuclear quadrupole spin transition frequencies: omega(1) = omega(Q)2eta/3, omega(2) = omega(Q)(1 -eta/3) and omega(3) = omega(Q)(1 + eta/3), and the quadrupole spin system thus acts as a relaxation sink. The relative relaxation enhancements of the peaks are different for the dipole-dipole and the scalar coupling. Considering the dipole-dipole coupling, the low frequency peak, omega(1), is small compared to the high field peaks whereas for the scalar coupling the situation is changed. For slow tumbling proteins with a correlation time of tau(R) = 400 ns, omega(2) and omega(3) are not resolved but become one relatively broad peak. At even faster reorientation, tau(R) < 60 ns, the marked peaks disappear. In this motional regime, the main effect of the cross relaxation phenomenon is a subtle perturbation of the main amide proton T(1) NMRD dispersion. The low field part of it can be approximately described by a Lorentzian function: R(DD,SC)(0.01)/(1 + (omega(I)tau(R)3/2)(2)) whereas the high field part coincides with R(DD,SC)(0.01)/(1 + (omega(I)tau(R))(2)).

  • 44.
    Westlund, Per-Olof
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Wennerström, Håkan
    Division of Physical Chemistry, Chemical Center, Lund University.
    Electron spin relaxation at low field2010In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 12, no 1, p. 201-206Article in journal (Refereed)
    Abstract [en]

    The low field ESR lineshape and the electron spin-lattice relaxation correlation function are calculated using the stochastic Liouville theory for an effective electron spin quantum number S = 1. When an axially symmetric permanent zero field splitting provides the dominant relaxation mechanism, and when it is much larger than the rotational diffusion constant, it is shown that both electron spin correlation functions S(0)S(t) (n = 0,1) are characterized by the same relaxation time tau(S) = (4D(R))(-1). This confirms the conjectures made by Schaefle and Sharp, J. Chem. Phys., 2004, 121, 5287 and by Fries and Belorizky, J. Chem. Phys., 2005, 123, 124510, based on numerical results using a different formalism. The stochastic Liouville approach also gives the paramagnetically enhanced nuclear spin relaxation time constants, T(1) and T(2), and the ESR lineshape function I(omega). In particular, the L-band (B(0) = 0.035 T) ESR spectrum of a low symmetry Ni(ii)-complex with a cylindrical ZFS tensor is shown to be detectable at sufficiently slowly reorientation of the complex. The analysis shows that the L-band spectrum becomes similar to the zero-field spectrum with a electron spin relaxation time tau(S) = (4D(R))(-1).

  • 45.
    Zietz, Burkhard
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Macpherson, Alisdair N
    Gillbro, Tomas
    Resolution of ultrafast excited state kinetics of bilirubin in chloroform and bound to human serum albumin2004In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 6, no 19, p. 4535-4537Article in journal (Refereed)
    Abstract [en]

    The Z–E-isomerisation of bilirubin upon excitation with visible light is a fundamental step in phototherapy of newborns with neonatal jaundice. Here we report results of an ultrafast optical spectroscopy study of bilirubin in CHCl3 as well as bound to human serum albumin. The data show that the initially excited singlet state has sub-ps decay times with major amplitude. Transient absorption measurements reveal that the ultrafast decay of the emission is accompanied by the formation of a transient intermediate which decays on the 15–20 ps timescale. The initial photoprocesses are thus considerably faster than the previously reported fastest lifetimes for bilirubin and this is, to our knowledge, the first time that the earliest processes in excited bilirubin have been resolved.

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