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  • 51. Ryderfors, Linus
    et al.
    Mukhtar, Emad
    Johansson, Lennart B-Å
    Umeå University, Faculty of Science and Technology, Chemistry.
    The Symmetry of Two-Photon Excited States as Determined by Time-Resolved Fluorescence Depolarization Experiments2007In: The Journal of Physical Chemistry, Vol. 111, no 45, p. 11531-9Article in journal (Refereed)
    Abstract [en]

    A new experimental and theoretical approach is presented for the quantitative determination and assignment of the two-photon absorption tensor of fluorophores dissolved in liquid solutions. Two linearly independent time-resolved fluorescence anisotropies and the two-photon polarization ratio were determined from experiments based on using the time-correlated single photon counting technique. The data were analyzed in a global manner under the assumption of prevailing diffusive molecular reorientations and when accounting for the influence of rapid unresolved reorientations. The method has been applied in fluorescence studies of perylene, two-photon excited at 800 nm. The analysis suggests that the two-photon transition is mediated via vibronic coupling including at least two vibrations of different symmetry, and also that the first singlet excited electronic state acts as a dominating intermediate state.

  • 52. Ryderfors, Linus
    et al.
    Mukhtar, Emad
    Johansson, Lennart B-Å
    Umeå University, Faculty of Science and Technology, Chemistry.
    Two-Photon Excited Fluorescence and Molecular Reorientations in Liquid Solutions2007In: Journal of Fluorescence, ISSN 1053-0509 (Print) 1573-4994 (Online), Vol. 17, no 5, p. 466-80Article in journal (Refereed)
    Abstract [en]

    Theoretical expressions are derived that relate the two-photon excited fluorescence depolarisation experiments to the molecular symmetry and the rotational motions of fluorescent molecules. Diffusive rotational motions in liquid solvents are considered, as well as the influence of fast unresolved motions (e.g. librations). The results obtained are compared with one-photon excited fluorescence depolarisation experiments. The derived theoretical expressions can be applied for detailed analyses of the molecular rotation in solvent. Several of the results are useful for determining and assigning the components of two-photon absorption tensors.

  • 53. Ryderfors, Linus
    et al.
    Mukhtar, Emad
    Johansson, Lennart B-Å
    Umeå University, Faculty of Science and Technology, Chemistry.
    Two-photon excited fluorescence depolarisation experiments: II. The proper response function for analysing TCSPC data2005In: CHEMICAL PHYSICS LETTERS, ISSN 0009-2614, Vol. 411, no 1-3, p. 51-60Article in journal (Refereed)
    Abstract [en]

    The instrumental response function (IRF) needed for the analyses of fluorescence time-correlated single-photon counting (TCSPC) experiments is usually determined by measuring the Rayleigh scattering (RS) of colloidal particles. Since RS is a one-photon process the response function obtained is referred to as a one-photon IRF (OP-IRF). Hence, in a two-photon excited (TPE) fluorescence experiment the hyper-Rayleigh scattering (HRS) might suit the determination of the two-photon IRF better [A. Habenicht, J. Hjelm, E. Mukhtar, F. Bergstrom, L.B.-angstrom. Johansson, Chem. Phys. Lett. 354 (2002) 3671.

    In the present study, we explore the importance of OP- and TP-IRF in fluorescence depolarisation studies. POPOP in ethanol and n-octanol is analysed when using OP- and TP-IRF. It is demonstrated that the TP-IRF yields significantly better statistics.

  • 54.
    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.

  • 55.
    Sachl, Radek
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Humpolickova, Jana
    J. Heyrovský Institute of Physical Chemistry, v.v.i., Academy of Sciences of the Czech Republic, Prague, Czech Republic.
    Stefl, Martin
    J. Heyrovský Institute of Physical Chemistry, v.v.i., Academy of Sciences of the Czech Republic, Prague, Czech Republic.
    Johansson, Lennart B-Å
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Hof, Martin
    J. Heyrovsky´ Institute of Physical Chemistry, v.v.i., Academy of Sciences of the Czech Republic, Prague, Czech Republic.
    Limitations of electronic energy transfer in the determination of lipid nanodomain sizes2011In: Biophysical Journal, ISSN 0006-3495, E-ISSN 1542-0086, Vol. 101, no 11, p. L60-L62Article in journal (Refereed)
    Abstract [en]

    Even though superresolution microscopy indicates that size of plasma membrane rafts is <20 nm, those structures have never been observed. Forster resonance energy transfer (FRET) is therefore still the most powerful optical method for characterization of such domains. In this letter we investigate relation between nanodomain affinity of a donor-acceptor (D/A) pair and the detectable nanodomain size/area. We show that probes with high affinity to the liquid-ordered (L(o)) phase are required for detecting domain sizes of a few nanometers, and/or domains that occupy a few percent of the bilayer area. A combination of donors and acceptors that prefer different phases is the more favorable approach. For instance, a D/A pair with the distribution constant of donors K(D) = 5 and acceptors K(A) = 0.01 can resolve a broad spectrum of nanodomain sizes. On the other hand, currently available donors and acceptors that prefer the same phase, either the liquid-disordered (L(d)) or L(o) phase, are not so convenient for determining domain sizes <20 nm. Here the detection limits of FRET experiments employing several commonly used D/A pairs have been investigated.

  • 56. Sachl, Radek
    et al.
    Johansson, Lennart B-Å
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Hof, Martin
    Förster Resonance Energy Transfer (FRET) between Heterogeneously Distributed Probes: Application to Lipid Nanodomains and Pores2012In: International Journal of Molecular Sciences, ISSN 1422-0067, E-ISSN 1422-0067, Vol. 13, no 12, p. 16141-16156Article in journal (Refereed)
    Abstract [en]

    The formation of membrane heterogeneities, e.g., lipid domains and pores, leads to a redistribution of donor (D) and acceptor (A) molecules according to their affinity to the structures formed and the remaining bilayer. If such changes sufficiently influence the F&#246;rster resonance energy transfer (FRET) efficiency, these changes can be further analyzed in terms of nanodomain/pore size. This paper is a continuation of previous work on this theme. In particular, it is demonstrated how FRET experiments should be planned and how data should be analyzed in order to achieve the best possible resolution. The limiting resolution of domains and pores are discussed simultaneously, in order to enable direct comparison. It appears that choice of suitable donor/acceptor pairs is the most crucial step in the design of experiments. For instance, it is recommended to use DA pairs, which exhibit an increased affinity to pores (i.e., partition coefficients K(D,A) &gt; 10) for the determination of pore sizes with radii comparable to the F&#246;rster radius R(0). On the other hand, donors and acceptors exhibiting a high affinity to different phases are better suited for the determination of domain sizes. The experimental setup where donors and acceptors are excluded from the domains/pores should be avoided.

  • 57.
    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.

  • 58.
    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.

  • 59.
    Sachl, Radek
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Rosenbaum, Erik
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Sellstedt, Magnus
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Almqvist, Fredrik
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Johansson, Lennart B-Å
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Locations and reorientations of multi-ring-fused 2-Pyridones in Ganglioside GM1 Micelles2011In: Langmuir, ISSN 0743-7463, E-ISSN 1520-5827, Vol. 27, no 5, p. 1662-1667Article in journal (Refereed)
    Abstract [en]

    Fluorescent multi-ring-fused 2-pyridones, with chemical resemblance to other biologically active 2-pyridone systems, were solubilized in spherical micelles formed by the gangloiside GM1 and studied with respect to their spatial localization and rotational mobility. For this, electronic energy transfer between the multi-ring-fused 2-pyridone (donor) and BODIPY-FL-labeled GM1 was determined, as well as their fluorescence depolarization. From the obtained efficiency of energy transfer to the acceptor group (BODIPY-FL), either localized in the polar or in the nonpolar part of the ganglioside, it has been possible to estimate the most likely localization of the multi-ring-fused 2-pyridones. The center of mass of the studied multi-ring-fused 2-pyridones are located at approximately 33 Å from the micellar center of mass, which corresponds to the internal hydrophobic-hydrophilic interfacial region. At this location, the reorienting rates of the multi-ring-fused 2-pyridones are surprisingly slow with typical correlation times of 35-55 ns. No evidence was found for the formation of ground and excited state dimers, even when two monomers were forced to be near each other via a short covalent linker.

  • 60.
    Sellstedt, Magnus
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Nyberg, Anders
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Rosenbaum, Erik
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Engström, Patrik
    Umeå University, Faculty of Medicine, Department of Molecular Biology (Faculty of Medicine). Umeå University, Faculty of Medicine, Molecular Infection Medicine Sweden (MIMS).
    Wickström, Malin
    Department of Medicinal Sciences, Division of Clinical Pharmacology, Uppsala University Hospital, 75185 Uppsala, Sweden.
    Gullbo, Joachim
    Department of Medicinal Sciences, Division of Clinical Pharmacology, Uppsala University Hospital, 75185 Uppsala, Sweden.
    Bergström, Sven
    Umeå University, Faculty of Medicine, Department of Molecular Biology (Faculty of Medicine). Umeå University, Faculty of Medicine, Molecular Infection Medicine Sweden (MIMS). Umeå University, Faculty of Medicine, Umeå Centre for Microbial Research (UCMR).
    Johansson, Lennart B-Å
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Almqvist, Fredrik
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Synthesis and characterization of a multi ring-fused 2-pyridone-based fluorescent scaffold2010In: European Journal of Organic Chemistry, ISSN 1434-193X, E-ISSN 1099-0690, no 32, p. 6171-6178Article in journal (Refereed)
    Abstract [en]

    A series of compounds based on a novel fluorescent scaffold have been synthesized. Most of the compounds displayed high quantum yields of fluorescence and unusually long fluorescence lifetimes. HeLa cells were treated with one of the compounds and its use as a fluorescent dye was demonstrated with fluorescence confocal microscopy.

  • 61.
    Stefl, Martin
    et al.
    Acad Sci Czech Republic, J Heyrovsky Inst Phys Chem, Dept Biophys Chem, Prague, Czech Republic .
    Sachl, Radek
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Humpolickova, Jana
    Acad Sci Czech Republic, J Heyrovsky Inst Phys Chem, Dept Biophys Chem, Prague, Czech Republic .
    Cebecauer, Marek
    Acad Sci Czech Republic, J Heyrovsky Inst Phys Chem, Dept Biophys Chem, Prague, Czech Republic .
    Machan, Radek
    Acad Sci Czech Republic, J Heyrovsky Inst Phys Chem, Dept Biophys Chem, Prague, Czech Republic .
    Kolarova, Marie
    Acad Sci Czech Republic, J Heyrovsky Inst Phys Chem, Dept Biophys Chem, Prague, Czech Republic .
    Johansson, Lennart B-Å
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Hof, Martin
    Acad Sci Czech Republic, J Heyrovsky Inst Phys Chem, Dept Biophys Chem, Prague, Czech Republic .
    Dynamics and size of cross-linking-induced lipid Nanodomains in model Membranes2012In: Biophysical Journal, ISSN 0006-3495, E-ISSN 1542-0086, Vol. 102, no 9, p. 2104-2113Article in journal (Refereed)
    Abstract [en]

    Changes of membrane organization upon cross-linking of its components trigger cell signaling response to various exogenous factors. Cross-linking of raft gangliosides GM1 with cholera toxin ( CTxB) was shown to cause microscopic phase separation in model membranes, and the CTxB-GM1 complexes forming a minimal lipid raft unit are the subject of ongoing cell membrane research. Yet, those subdiffraction sized rafts have never been described in terms of size and dynamics. By means of two-color z-scan fluorescence correlation spectroscopy, we show that the nanosized domains are formed in model membranes at lower sphingomyelin (Sph) content than needed for the large-scale phase separation and that the CTxB-GM1 complexes are confined in the domains poorly stabilized with Sph. Forster resonance energy transfer together with Monte Carlo modeling of the donor decay response reveal the domain radius of similar to 8 nm, which increases at higher Sph content. We observed two types of domains behaving differently, which suggests a dual role of the cross-linker: first, local transient condensation of the GM1 molecules compensating for a lack of Sph and second, coalescence of existing nanodomains ending in large-scale phase separation.

  • 62. Viarbitskaya, Sviatlana
    et al.
    Ryderfors, Linus
    Mikaelsson, Therese
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Mukhtar, Emad
    Johansson, Lennart B-Å
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Luminescence enhancement from silica-coated gold nanoparticle agglomerates following multi-photon excitation2011In: Journal of Fluorescence, ISSN 1053-0509, E-ISSN 1573-4994, Vol. 21, no 1, p. 257-264Article in journal (Refereed)
    Abstract [en]

    Multi-photon absorption induced luminescence (MAIL) from bare gold nanoparticles, silica-coated particles, as well as silica-coated agglomerated gold nanoparticles suspended in aqueous solution was studied by using time-resolved and steady-state luminescence spectroscopy. The nanoparticles were excited by femtosecond pulses of wavelengths ranging from 630 nm to 900 nm. The luminescence from the particles exhibits a broad spectrum in the UV and VIS region. The time-resolved measurements indicate a luminescence lifetime of a few ps, limited by the response of the experimental system. The studied dependence of the MAIL efficiency on the excitation wavelength showed that the luminescence from silica-coated agglomerates was enhanced over the whole range of excitation wavelengths, when compared to the luminescence from individual gold nanoparticles. The agglomerates show an almost excitation wavelength independent efficiency of the MAIL, while for individual nanoparticles a rapid decrease of the MAIL efficiency was observed with increasing excitation wavelength. The observed enhancement of the MAIL from the agglomerated nanostructures can be attributed to the presence of localized surface plasmon resonances in the spectral region corresponding to the excitation wavelengths. The high MAIL efficiency from the agglomerated nanoparticle structures in the near-infrared could be an advantage in the expanding field of luminescence-based-imaging, as well as in biosensor technology.

  • 63.
    Wallgren, Marcus
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Ådén, Jörgen
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Pylypenko, Olena
    Department of Physical Biochemistry, Max-Planck-Institute for Molecular Physiology, 44202 Dortmund, Germany.
    Mikaelsson, Therese
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Johansson, Lennart B-Å
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Rak, Alexey
    Department of Physical Biochemistry, Max-Planck-Institute for Molecular Physiology, 44202 Dortmund, Germany.
    Wolf-Watz, Magnus
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Extreme temperature tolerance of a hyperthermophilic protein coupled to residual structure in the unfolded state2008In: Journal of Molecular Biology, ISSN 0022-2836, E-ISSN 1089-8638, Vol. 379, no 4, p. 845-858Article in journal (Refereed)
    Abstract [en]

    Understanding the mechanisms that dictate protein stability is of large relevance, for instance, to enable design of temperature-tolerant enzymes with high enzymatic activity over a broad temperature interval. In an effort to identify such mechanisms, we have performed a detailed comparative study of the folding thermodynamics and kinetics of the ribosomal protein S16 isolated from a mesophilic (S16meso) and hyperthermophilic (S16thermo) bacterium by using a variety of biophysical methods. As basis for the study, the 2.0 Å X-ray structure of S16thermo was solved using single wavelength anomalous dispersion phasing. Thermal unfolding experiments yielded midpoints of 59 and 111 °C with associated changes in heat capacity upon unfolding (ΔCp0) of 6.4 and 3.3 kJ mol− 1 K− 1, respectively. A strong linear correlation between ΔCp0 and melting temperature (Tm) was observed for the wild-type proteins and mutated variants, suggesting that these variables are intimately connected. Stopped-flow fluorescence spectroscopy shows that S16meso folds through an apparent two-state model, whereas S16thermo folds through a more complex mechanism with a marked curvature in the refolding limb indicating the presence of a folding intermediate. Time-resolved energy transfer between Trp and N-(4,4-difluoro-5,7-dimethyl-4-bora-3a,4a-diaza-s-indacene-3-yl)methyl iodoacetamide of proteins mutated at selected positions shows that the denatured state ensemble of S16thermo is more compact relative to S16meso. Taken together, our results suggest the presence of residual structure in the denatured state ensemble of S16thermo that appears to account for the large difference in quantified ΔCp0 values and, in turn, parts of the observed extreme thermal stability of S16thermo. These observations may be of general importance in the design of robust enzymes that are highly active over a wide temperature span.

  • 64. Wan, Chi-Wai
    et al.
    Burghart, Armin
    Chen, Jiong
    Bergström, Fredrik
    Umeå University, Faculty of Science and Technology, Chemistry.
    Johansson, Lennart B-Å
    Umeå University, Faculty of Science and Technology, Chemistry.
    Wolford Matthew, F
    Gyum, Kim Taeg
    Topp, Michael R
    Hochstrasser, Robin M
    Burgess, Kevin
    Anthracene-BODIPY Cassettes: Syntheses and Energy Transfer2003In: Chemistry - A European Journal, Vol. 9, no 18, p. 4430-41Article in journal (Refereed)
    Abstract [en]

    Compounds based on the 4,4-difluoro-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene (BODIPY) framework are excellent fluorescent markers. When BODIPY dyes of this type are conjugated to functionalities that absorb at relatively short wavelengths, those functionalities can, in some molecules, transmit the absorbed energy to the BODIPY which then fluoresces. In such cases the BODIPY fragment acts as an acceptor while the other group serves as a donor. Energy transfer efficiencies in such donor-acceptor cassette systems must vary with the relative orientation of the two components, and with the structure of the linkers that attach them. This study was designed to probe these issues for the special case in which the linkers between the donor and acceptor fragments are conjugated. To do this, the cassettes 3-10 were prepared. Electrochemical studies were performed to provide insight into the degree of donor-acceptor conjugation in these systems. X-ray Crystallographic studies on single crystals of compounds 7 and 9 revealed the favored conformations of the donor and acceptor fragments in the solid state. Absorption, fluorescence, and time-resolved fluorescence spectra of the compounds were recorded, and quantum yields for the cassettes excited at the donor max were measured. Fluorescence steady-state anisotropy data were determined for cassettes 3 and 9 to provide information about the mutual direction of the transition dipole moments.

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