Kõu Timpmann

Directed picosecond excitation transport in purple photosynthetic bacteria

Abstract Picosecond spectrally resolved fluorescent measurements together with the coupled kinetic rate equation model simulations of the data have been employed to determine the rates and pathways of heterogeneous excitation transport in the membranes of photosynthetic bacteria Rhodobacter sphaeroides and Chromatium minutissimum . Excitation transport from bacteriochlorophyll molecules B800 to B850, belonging to the same pigment-pigment-protein complex B800–850, proceeds in 1–2 ps at room temperature and at 77 K. The intercomplex excitation transport from B850 to B875 takes about 10 ps for most excitations. In the minor part of B800–850 complexes, the excitation transport to B875 complex takes much longer, about 50 ps. The macroscopic rate constant of excitation trapping by open reaction centres is shown to be the same for all the bacteria studied, although the number of antenna molecules per reaction centre differs significantly. This seems to be an indication of the intrinsic homogeneity of the long-wavelength bacteriochlorophyll band, which facilitates an additional localization of excitations in the vicinity of the reaction centre and, due to the shortening of the trapping time, increases the overall quantum yield of photosynthesis.

Exciton Self Trapping in Photosynthetic Pigment–Protein Complexes Studied by Single-Molecule Spectroscopy

Evidence for the formation of self-trapped exciton states in photosynthetic antenna complexes is provided by comparing single-molecule fluorescence-excitation and emission spectra that have been recorded from the same individual LH2 complex from Rhodopseudomonas acidophila . While the excitation spectra showed the signatures for the B800 and B850 bands as observed previously, two distinctively different types of emission spectra were found. One group of antenna complexes shows spectra with a relatively narrow spectral profile with a clear signature of a zero-phonon line, whereas the other group of complexes displays spectra that consist only of a broad featureless band. Analysis of these data reveals clear correlations between the spectral position of the emission, the width of the spectral profile, and the associated electron-phonon coupling strength.

Exciton Relaxation and Transfer in the LH2 Antenna Network of Photosynthetic Bacteria

There are still significant questions about the nature of interactions in the core LH1 or peripheral LH2 antenna rings and between the rings of photosynthetic purple bacteria that govern very efficient light energy collection, transfer and concentration into the photochemical reaction center. Recent measurements [1] of excitation wavelength dependence of early absorbance changes of Rhodobacter (R) sphaeroides mutant membranes that include only LH2 antenna proteins and are devoid of the core LH1 antenna and reaction center proteins once again [2] exposed that spectral heterogeneity plays an important role in determining the electronic structure of the antenna system.

A comparative spectroscopic and kinetic study of photoexcitations in detergent-isolated and membrane-embedded LH2 light-harvesting complexes ☆

Integral membrane proteins constitute more than third of the total number of proteins present in organisms. Solubilization with mild detergents is a common technique to study the structure, dynamics, and catalytic activity of these proteins in purified form. However beneficial the use of detergents may be for protein extraction, the membrane proteins are often denatured by detergent solubilization as a result of native lipid membrane interactions having been modified. Versatile investigations of the properties of membrane-embedded and detergent-isolated proteins are, therefore, required to evaluate the consequences of the solubilization procedure. Herein, the spectroscopic and kinetic fingerprints have been established that distinguish excitons in individual detergent-solubilized LH2 light-harvesting pigment-protein complexes from them in the membrane-embedded complexes of purple photosynthetic bacteria Rhodobacter sphaeroides. A wide arsenal of spectroscopic techniques in visible optical range that include conventional broadband absorption-fluorescence, fluorescence anisotropy excitation, spectrally selective hole burning and fluorescence line-narrowing, and transient absorption-fluorescence have been applied over broad temperature range between physiological and liquid He temperatures. Significant changes in energetics and dynamics of the antenna excitons upon self-assembly of the proteins into intracytoplasmic membranes are observed, analyzed, and discussed. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

Self-trapped excitons in circular cacteriochlorophyll antenna complexes

Fluorescence line narrowing and hole-burning spectroscopic studies of excitons in the LH2 pigment-protein complex, which is a part of the light harvesting system of purple bacteria, are combined with straightforward numerical simulations of the emission spectra based on exciton polaron model. The analysis provides evidence for self-trapping of all the excitons, except the lowest one.

Single-Molecule Spectroscopy Unmasks the Lowest Exciton State of the B850 Assembly in LH2 from Rps. acidophila

We have recorded fluorescence-excitation and emission spectra from single LH2 complexes from Rhodopseudomonas (Rps.) acidophila. Both types of spectra show strong temporal spectral fluctuations that can be visualized as spectral diffusion plots. Comparison of the excitation and emission spectra reveals that for most of the complexes the lowest exciton transition is not observable in the excitation spectra due to the cutoff of the detection filter characteristics. However, from the spectral diffusion plots we have the full spectral and temporal information at hand and can select those complexes for which the excitation spectra are complete. Correlating the red most spectral feature of the excitation spectrum with the blue most spectral feature of the emission spectrum allows an unambiguous assignment of the lowest exciton state. Hence, application of fluorescence-excitation and emission spectroscopy on the same individual LH2 complex allows us to decipher spectral subtleties that are usually hidden in traditional ensemble spectroscopy.

Unified analysis of ensemble and single-complex optical spectral data from light-harvesting complex-2 chromoproteins for gaining deeper insight into bacterial photosynthesis.

Bacterial light-harvesting pigment-protein complexes are very efficient at converting photons into excitons and transferring them to reaction centers, where the energy is stored in a chemical form. Optical properties of the complexes are known to change significantly in time and also vary from one complex to another; therefore, a detailed understanding of the variations on the level of single complexes and how they accumulate into effects that can be seen on the macroscopic scale is required. While experimental and theoretical methods exist to study the spectral properties of light-harvesting complexes on both individual complex and bulk ensemble levels, they have been developed largely independently of each other. To fill this gap, we simultaneously analyze experimental low-temperature single-complex and bulk ensemble optical spectra of the light-harvesting complex-2 (LH2) chromoproteins from the photosynthetic bacterium Rhodopseudomonas acidophila in order to find a unique theoretical model consistent with both experimental situations. The model, which satisfies most of the observations, combines strong exciton-phonon coupling with significant disorder, characteristic of the proteins. We establish a detailed disorder model that, in addition to containing a C_{2}-symmetrical modulation of the site energies, distinguishes between static intercomplex and slow conformational intracomplex disorders. The model evaluations also verify that, despite best efforts, the single-LH2-complex measurements performed so far may be biased toward complexes with higher Huang-Rhys factors.

Fluorescence-Excitation and Emission Spectra from LH2 Antenna Complexes of Rhodopseudomonas acidophila as a Function of the Sample Preparation Conditions

The high sensitivity of optical spectra of pigment-protein complexes to temperature and pressure is well known. In the present study, we have demonstrated the significant influence of the environments commonly used in bulk and single-molecule spectroscopic studies at low temperatures on the LH2 photosynthetic antenna complex from Rhodopseudomonas acidophila. A transfer of this LH2 complex from a bulk-buffer solution into a spin-coated polymer film results in a 189 cm(-1) blue shift of the B850 excitonic absorption band at 5 K. Within the molecular exciton model, the origin of this shift could be disentangled into three parts, namely to an increase of the local site energies, a contraction of the exciton band, and a decrease of the displacement energy.

Fluctuations in the Electron–Phonon Coupling of a Single Chromoprotein

Sequences of high resolution low-temperature emission spectra from individual light-harvesting 2 (LH2) complexes from Rhodopseudomonas acidophila reveal a much larger variety of the emission profiles than previously observed. The results provide direct evidence for substantial variations in electron–phonon coupling and concomitantly of exciton (de)localization within single pigment-protein complexes.

Electron transfer and electronic energy relaxation under high hydrostatic pressure

The following question has been addressed in the present work. How external high (up to 8 kbar) hydrostatic pressure acts on photoinduced intramolecular electron transfer and on exciton relaxation processes? Unlike phenomena, as they are, have been studied in different systems: electron transfer in an artificial Zn-porphyrin-pyromellitimide (ZnP-PM) supramolecular electron donor-acceptor complex dissolved in toluene measured at room temperature; exciton relaxation in a natural photosynthetic antenna protein called FMO protein measured at low temperatures, between 4 and 100 K. Spectrally selective picosecond time-resolved emission technique has been used to detect pressure-induced changes in the systems. The following conclusions have been drawn from the electron transfer study: (i) External pressure may serve as a potential and sensitive tool not only to study, but also to control and tune elementary chemical reactions in solvents; (ii) Depending on the system parameters, pressure can both accelerate and inhibit electron transfer reactions; (iii) If competing pathways of the reaction are available, pressure can probably change the branching ratio between the pathways; (iv) The classical nonadiabatic electron transfer theory describes well the phenomena in the ZnP-PM complex, assuming that the driving force or/and reorganisation energy depend linearly on pressure; (v) A decrease in the ZnP-PM donor-acceptor distance under pressure exerts a minor effect on the electron transfer rate. The effect of pressure on the FMO protein exciton relaxation dynamics at low temperatures has been found marginal. This may probably be explained by a unique structure of the protein [D.E. Trondrud, M.F. Schmid, B.W. Matthews, J. Mol. Biol. 188 (1986) p. 443; Y.-F. Li, W. Zhou, E. Blankenship, J.P. Allen, J. Mol. Biol., submitted]. A barrel made of low compressibility beta-sheets may, like a diving bell, effectively screen internal bacteriochlorophyll a molecules from external influence of high pressure. The origin of the observed slow pico = and subnanosecond dynamics of the excitons at the exciton band bottom remains open. The phenomenon may be due to weak coupling of phonons to the exciton states or/and to low density of the relevant low-frequency ( approximately 50 cm(-1)) phonons. Exciton solvation in the surrounding protein and water-glycerol matrix may also contribute to this effect. Drastic changes of spectral, kinetic and dynamic properties have been observed due to protein denaturation, if the protein was compressed at room temperature and then cooled down, as compared to the samples, first cooled and then pressurised.