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Westlund, Per-Olof
Publications (10 of 41) Show all publications
Han, G., Huang, Y., Koua, F. H., Shen, J.-R., Westlund, P.-O. & Messinger, J. (2014). Hydration of the oxygen-evolving complex of photosystem II probed in the dark-stable S1 state using proton NMR dispersion profiles. Physical Chemistry, Chemical Physics - PCCP (16), 11924-11935
Open this publication in new window or tab >>Hydration of the oxygen-evolving complex of photosystem II probed in the dark-stable S1 state using proton NMR dispersion profiles
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2014 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, no 16, p. 11924-11935Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
Royal Society of Chemistry, 2014
National Category
Chemical Sciences
Identifiers
urn:nbn:se:umu:diva-87820 (URN)10.1039/c3cp55232b (DOI)000337122000012 ()24695863 (PubMedID)
Available from: 2014-04-10 Created: 2014-04-10 Last updated: 2018-02-12Bibliographically approved
Wennerström, H. & Westlund, P.-O. (2013). On Stern‐Gerlach coincidence measurements and their application to Bell's theorem. Physics essays, 26(2), 174-180
Open this publication in new window or tab >>On Stern‐Gerlach coincidence measurements and their application to Bell's theorem
2013 (English)In: Physics essays, ISSN 0836-1398, Vol. 26, no 2, p. 174-180Article in journal (Refereed) Published
Abstract [en]

We analyze a coincidence Stern-Gerlach measurement often discussed in connection with the derivation and illustration of Bell's theorem. The treatment is based on our recent analysis of the original Stern-Gerlach experiment (PCCP, 14, 1677‐1684 (2012)), where it is concluded that it is necessary to include a spin relaxation process to account for the experimental observations. We consider two limiting cases of a coincidence measurement using both an analytical and a numerical description. In on limit relaxation effects are neglected. In this case the correlation between the two spins present in the initial state is conserved during the passage through the magnets. However, at exit the z coordinate along the magnetic field gradient is randomly distributed between the two extreme values. In the other limit T2 relaxation is assumed to be fast relative to the time of flight through the magnet. In this case the z coordinate takes one of two possible values as observed in the original Stern‐Gerlach experiment. Due to the presence of a relaxation process involving transfer of angular momentum between particle and magnet the initially entangled spin state changes character leading to a loss of correlation between the two spins. In the original derivations of Bell's theorem based on a coincidence Stern‐Gerlach setup one assumes both a perfect correlation between the spins and only two possible values for the z‐coordinate on exit. According to the present calculations one can satisfy either of these conditions but not both simultaneously.

Place, publisher, year, edition, pages
Ottawa: Physics Essays Publication, 2013
Keyword
Stern‐Gerlach device, coincident measurement, entangled state, spin relaxation, Bell's theorem
National Category
Chemical Sciences
Identifiers
urn:nbn:se:umu:diva-64735 (URN)10.4006/0836-1398-26.2.174 (DOI)000320076300002 ()
Available from: 2013-02-06 Created: 2013-02-01 Last updated: 2018-01-22Bibliographically approved
Huang, Y., Nam, K. & Westlund, P.-O. (2013). The water R1(ω) NMRD profiles of a hydrated protein from molecular dynamics simulation. Physical Chemistry, Chemical Physics - PCCP, 15(33), 14089-14097
Open this publication in new window or tab >>The water R1(ω) NMRD profiles of a hydrated protein from molecular dynamics simulation
2013 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 15, no 33, p. 14089-14097Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
RSC Publishing, 2013
National Category
Chemical Sciences
Identifiers
urn:nbn:se:umu:diva-73724 (URN)10.1039/C3CP51147B (DOI)000322517800041 ()
Available from: 2013-06-27 Created: 2013-06-27 Last updated: 2017-12-06Bibliographically approved
Bergenstråhle-Wohlert, M., Berglund, L. A., Brady, J. W., Larsson, P. T., Westlund, P.-O. & Wohlert, J. (2012). Concentration enrichment of urea at cellulose surfaces: results from molecular dynamics simulations and NMR spectroscopy. Cellulose (London), 19(1), 1-12
Open this publication in new window or tab >>Concentration enrichment of urea at cellulose surfaces: results from molecular dynamics simulations and NMR spectroscopy
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2012 (English)In: Cellulose (London), ISSN 0969-0239, E-ISSN 1572-882X, Vol. 19, no 1, p. 1-12Article in journal (Refereed) Published
Abstract [en]

A combined solid-state NMR and Molecular Dynamics simulation study of cellulose in urea aqueous solution and in pure water was conducted. It was found that the local concentration of urea is significantly enhanced at the cellulose/solution interface. There, urea molecules interact directly with the cellulose through both hydrogen bonds and favorable dispersion interactions, which seem to be the driving force behind the aggregation. The CP/MAS 13C spectra was affected by the presence of urea at high concentrations, most notably the signal at 83.4 ppm, which has previously been assigned to C4 atoms in cellulose chains located at surfaces parallel to the (110) crystallographic plane of the cellulose Iβ crystal. Also dynamic properties of the cellulose surfaces, probed by spin-lattice relaxation time 13CT 1 measurements of C4 atoms, are affected by the addition of urea. Molecular Dynamics simulations reproduce the trends of the T 1measurements and lends new support to the assignment of signals from individual surfaces. That urea in solution is interacting directly with cellulose may have implications on our understanding of the mechanisms behind cellulose dissolution in alkali/urea aqueous solutions.

Place, publisher, year, edition, pages
Springer Science+Business Media B.V., 2012
Keyword
Cellulose, Urea, Solid state NMR, Molecular dynamics simulation
National Category
Chemical Sciences
Identifiers
urn:nbn:se:umu:diva-50768 (URN)10.1007/s10570-011-9616-x (DOI)
Note
Published online 15 November 2011Available from: 2011-12-21 Created: 2011-12-21 Last updated: 2017-12-08Bibliographically approved
Jonsson, S., Skyllberg, U., Nilsson, M. B., Westlund, P.-O., Shchukarev, A., Lundberg, E. & Björn, E. (2012). Mercury methylation rates for geochemically relevant HgII species in sediments. Environmental Science and Technology, 46(21), 11653-11659
Open this publication in new window or tab >>Mercury methylation rates for geochemically relevant HgII species in sediments
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2012 (English)In: Environmental Science and Technology, ISSN 0013-936X, E-ISSN 1520-5851, Vol. 46, no 21, p. 11653-11659Article in journal (Refereed) Published
Abstract [en]

Monomethylmercury (MeHg) in fish from freshwater, estuarine and marine environments are a major global environmental issue. Mercury levels in biota are mainly controlled by the methylation of inorganic mercuric mercury (HgII) to MeHg in water, sediments and soils. There is, however, a knowledge gap concerning the mechanisms and rates of methylation of specific geochemical HgII species. Such information is crucial for a better understanding of variations in MeHg concentrations among ecosystems and, in particular, for predicting the outcome of currently proposed measures to mitigate mercury emissions and reduce MeHg concentrations in fish. To fill this knowledge gap we propose an experimental approach using HgII isotope tracers, with defined and geochemically important adsorbed and solid HgII forms in sediments, to study MeHg formation. We report HgII methylation rate constants, km, in estuarine sediments which span over two orders of magnitude depending on chemical form of added tracer: metacinnabar (β-201HgS(s)) < cinnabar (α-199HgS(s)) < HgII reacted with mackinawite (≡FeS-202HgII) < HgII bonded to natural organic matter (NOM-196HgII) < a typical aqueous tracer (198Hg(NO3)2(aq)). We conclude that a combination of thermodynamic and kinetic effects of HgII solid-phase dissolution and surface desorption control the HgII methylation rate in sediments and causes the large observed differences in km-values. The selection of relevant solid-phase and surface adsorbed HgII tracers will therefore be crucial to achieving biogeochemically accurate estimates of ambient HgII methylation rates.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2012
National Category
Chemical Sciences
Identifiers
urn:nbn:se:umu:diva-60708 (URN)10.1021/es3015327 (DOI)23017152 (PubMedID)
Available from: 2012-10-29 Created: 2012-10-23 Last updated: 2017-10-24Bibliographically approved
Lindgren, M. & Westlund, P.-O. (2012). The molecular mechanism of urea denaturation. In: James C. Taylor (Ed.), Advances in Chemistry Research: Volume 11. Nova Science Publishers, Inc.
Open this publication in new window or tab >>The molecular mechanism of urea denaturation
2012 (English)In: Advances in Chemistry Research: Volume 11 / [ed] James C. Taylor, Nova Science Publishers, Inc., 2012Chapter in book (Refereed)
Abstract [en]

Proteins are known to denature in high concentrations of compounds such as urea or guanidinium chloride. However, the mechanism by which urea and guanidinium chloride destabilizes proteins is not yet known, despite many decades of reasearch. Attempts have been made to understand protein denaturation on a thermodynamic level as well as on a molecular level. The long term goal in the field is to merge the results of these two types of studies into one mechanism that covers both the microscopic and the macroscopic level. In this text we firstly review thermodynamic studies as well as spectroscopic and computer simulation studies of chemical denaturation. The results of the different types of studies is then merged together in order to find a consistent view on chemical denaturation. In contrast to common belief in the field, a high degree of consensus is found between the different studies and a molecular mechanism of urea-induced protein denaturation can therefore be proposed.

Place, publisher, year, edition, pages
Nova Science Publishers, Inc., 2012
Series
Advances in Chemistry Research
National Category
Chemical Sciences
Identifiers
urn:nbn:se:umu:diva-54624 (URN)978-1-61324-815-7 (ISBN)
Available from: 2012-05-02 Created: 2012-05-02 Last updated: 2018-03-15Bibliographically approved
Wennerström, H. & Westlund, P.-O. (2012). The Stern-Gerlach experiment and the effects of spin relaxation. Physical Chemistry, Chemical Physics - PCCP, 14, 1677-1684
Open this publication in new window or tab >>The Stern-Gerlach experiment and the effects of spin relaxation
2012 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 14, p. 1677-1684Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
RSC Publishing, 2012
National Category
Chemical Sciences
Identifiers
urn:nbn:se:umu:diva-50825 (URN)10.1039/c2cp22173j (DOI)22193591 (PubMedID)
Note
First published on the web 21 Dec 2011 Available from: 2011-12-27 Created: 2011-12-27 Last updated: 2017-12-08Bibliographically approved
Westlund, P.-O. (2012). Theoretical reason for the lack of influence of 1H–14N cross-relaxation on the water proton T 1 NMRD profile in slow tumbling proteins. Molecular Physics, 110(18), 2251-2255
Open this publication in new window or tab >>Theoretical reason for the lack of influence of 1H–14N cross-relaxation on the water proton T 1 NMRD profile in slow tumbling proteins
2012 (English)In: Molecular Physics, ISSN 0026-8976, E-ISSN 1362-3028, Vol. 110, no 18, p. 2251-2255Article in journal (Refereed) Published
Abstract [en]

For immobilized protein the water proton T 1-NMRD profile displays three enhanced relaxation peaks (QP). For slow tumbling proteins these relaxation peaks are not experimentally observed. However, the theoretically determined QP effect on the amide proton T 1-NMRD profile displays a distorted Lorentzian dispersion profile. The question arises as to whether there is also a distortion of the water-proton T 1-NMRD profile due to QP. The model of Sunde and Halle [J. Magn. Reson. 203, 257 (2010)] predicts a decreasing QP relaxation contribution and, with the aid of a model for tumbling proteins [P.-O. Westlund, Phys. Chem. Chem. Phys, 12, 3136 (2010)], it is shown that the QP effect is absent in water-proton T 1-NMRD profiles for slow tumbling proteins with τR < 1 µs, τI.

Keyword
H–N cross-relaxation, quadrupole peak effect, stochastic Liouville equation, water T 1-NMRD profiles, slow tumbling proteins
National Category
Chemical Sciences
Identifiers
urn:nbn:se:umu:diva-54602 (URN)10.1080/00268976.2012.674566 (DOI)
Note

Available online: 20 Mar 2012

Available from: 2012-05-02 Created: 2012-05-02 Last updated: 2017-12-07Bibliographically approved
Gustafsson, H., Ahrén, M., Söderlind, F., Córdoba Gallego, J. M., Käll, P.-O., Nordblad, P., . . . Engström, M. (2011). Magnetic and Electron Spin Relaxation Properties of (GdxY1−x)2O3 (0 ≤ x ≤ 1) Nanoparticles Synthesized by the Combustion Method. Increased Electron Spin Relaxation Times with Increasing Yttrium Content. The Journal of Physical Chemistry C, 115(13), 5469-5477
Open this publication in new window or tab >>Magnetic and Electron Spin Relaxation Properties of (GdxY1−x)2O3 (0 ≤ x ≤ 1) Nanoparticles Synthesized by the Combustion Method. Increased Electron Spin Relaxation Times with Increasing Yttrium Content
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2011 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 115, no 13, p. 5469-5477Article in journal (Refereed) Published
Abstract [en]

The performance of a magnetic resonance imaging contrast agent (CA) depends on several factors, including the relaxation times of the unpaired electrons in the CA. The electron spin relaxation time may be a key factor for the performance of new CAs, such as nanosized Gd2O3 particles. The aim of this work is, therefore, to study changes in the magnetic susceptibility and the electron spin relaxation time of paramagnetic Gd2O3 nanoparticles diluted with increasing amounts of diamagnetic Y2O3. Nanoparticles of (GdxY1−x)2O3 (0 ≤ x ≤ 1) were prepared by the combustion method and thoroughly characterized (by X-ray diffraction, transmission electron microscopy, thermogravimetry coupled with mass spectroscopy, photoelectron spectroscopy, Fourier transform infrared spectroscopy, and magnetic susceptibility measurements). Changes in the electron spin relaxation time were estimated by observations of the signal line width in electron paramagnetic resonance spectroscopy, and it was found that the line width was dependent on the concentration of yttrium, indicating that diamagnetic Y2O3 may increase the electron spin relaxation time of Gd2O3 nanoparticles.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2011
National Category
Chemical Sciences
Identifiers
urn:nbn:se:umu:diva-50769 (URN)10.1021/jp111368t (DOI)
Available from: 2011-12-21 Created: 2011-12-21 Last updated: 2017-12-08Bibliographically approved
Lindgren, M., Sparrman, T. & Westlund, P.-O. (2010). A combined molecular dynamic simulation and Urea 14N NMR relaxation study of the Urea - lysozyme system. Spectrochimica Acta Part A - Molecular and Biomolecular Spectroscopy, 75(3), 953-9
Open this publication in new window or tab >>A combined molecular dynamic simulation and Urea 14N NMR relaxation study of the Urea - lysozyme system
2010 (English)In: Spectrochimica Acta Part A - Molecular and Biomolecular Spectroscopy, ISSN 1386-1425, E-ISSN 1873-3557, Vol. 75, no 3, p. 953-9Article in journal (Refereed) Published
Abstract [en]

Urea in the lysozyme solvation shell has been studied by utilizing a combination of urea , water NMR relaxation experiments and a molecular dynamics simulation of the urea–lysozyme system. Samples with lysozyme in the native fold in water as well as in 3 M urea have been studied, as well as denatured lysozyme in a 8.5 M urea solvent. The spin relaxation rates of the samples with folded protein show a clear field dependence, which is consistent with a few urea molecules having long residence times on the protein surface and preferentially located in pockets and grooves on the protein. By combining the 3 M urea NMR relaxation data and data from the MD simulation, a full parameter set of the relaxation model is found which can successfully predict the experimental relaxation rates of the 3 M urea sample. However, in the parameter fitting it is evident that the rotational dynamics of urea in the MD simulation is slightly too fast to be consistent with the NMR relaxation rates, perhaps a result of the fast dynamics of the TIP3P water model. The relaxation rates of urea in the proximity of the unfolded lysozyme lack field dependence, which can be interpreted as a loss of pockets and grooves on the denatured protein. The extracted model parameters from the 3 M sample are adjusted and tested on a simple model of the unfolded protein sample and are seen to be in agreement with the relaxation rates. It is shown that the combination of NMR relaxation and MD simulations can be used to create a microscopic picture of the solvent at the protein interface, which can be used for example in the study of chemical denaturation.

Place, publisher, year, edition, pages
Elsevier, 2010
Keyword
14N-urea NMR-relaxation, Molecular dynamics simulation, Urea–lysozyme system
Identifiers
urn:nbn:se:umu:diva-30184 (URN)10.1016/j.saa.2009.11.054 (DOI)000275584000002 ()20061179 (PubMedID)
Available from: 2009-12-10 Created: 2009-12-10 Last updated: 2017-12-12Bibliographically approved
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