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.     

Energy transfer in the inhomogeneously broadened core antenna of purple bacteria: a simultaneous fit of low-intensity picosecond absorption and fluorescence kinetics.

The excited state decay kinetics of chromatophores of the purple photosynthetic bacterium Rhodospirillum rubrum have been recorded at 77 K using picosecond absorption difference spectroscopy under strict annihilation free conditions. The kinetics are shown to be strongly detection wavelength dependent. A simultaneous kinetic modeling of these experiments together with earlier fluorescence kinetics by numerical integration of the appropriate master equation is performed. This model, which accounts for the spectral inhomogeneity of the core light-harvesting antenna of photosynthetic purple bacteria, reveals three qualitatively distinct stages of excitation transfer with different time scales. At first a fast transfer to a local energy minimum takes place (approximately 1 ps). This is followed by a much slower transfer between different energy minima (10-30 ps). The third component corresponds to the excitation transfer to the reaction center, which depends on its state (60 and 200 ps for open and closed, respectively) and seems also to be the bottleneck in the overall trapping time. An acceptable correspondence between theoretical and experimental decay kinetics is achieved at 77 K and at room temperature by assuming that the width of the inhomogeneous broadening is 10-15 nm and the mean residence time of the excitation in the antenna lattice site is 2-3 ps.     

High-pressure tuning of primary photochemistry in bacterial photosynthesis: membrane-bound versus detergent-isolated reaction centers

Photosynthesis Research - While photosynthesis thrives at close to normal pressures and temperatures, it is presently well known that life is similarly commonplace in the hostile environments of...     

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.     

Excitation energy transfer in chlorosomes of green bacteria: theoretical and experimental studies.

A theory of excitation energy transfer within the chlorosomal antennae of green bacteria has been developed for an exciton model of aggregation of bacteriochlorophyll (BChl) c (d or e). This model of six exciton-coupled BChl chains with low packing density, approximating that in vivo, and interchain distances of approximately 2 nm was generated to yield the key spectral features found in natural antennae, i.e., the exciton level structure revealed by spectral hole burning experiments and polarization of all the levels parallel to the long axis of the chlorosome. With picosecond fluorescence spectroscopy it was demonstrated that the theory explains the antenna-size-dependent kinetics of fluorescence decay in chlorosomal antenna, measured for intact cells of different cultures of the green bacterium C. aurantiacus, with different chlorosomal antenna size determined by electron microscopic examination of the ultrathin sections of the cells. The data suggest a possible mechanism of excitation energy transfer within the chlorosome that implies the formation of a cylindrical exciton, delocalized over a tubular aggregate of BChl c chains, and Forster-type transfer of such a cylindrical exciton between the nearest tubular BChl c aggregates as well as to BChl a of the baseplate.     

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.     

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.     

Investigation of spatial relationships and energy transfer between complexes B800-850 and B890-RC from Chromatium minutissimum reconstituted into liposomes.

Spatial relationships between different pigment-protein complexes in the membranes of the purple photosynthetic bacterium, Chromatium minutissimum, have been studied. The possibility of restoring the function of efficient excitation energy transfer from bacteriochlorophyll molecules to the reaction centers in the system of soybean liposomes, reconstituted with pigment-protein complexes B800-850 and B890-RC from C. minutissimum, has been explored. The chemical cross-linking method, together with stationary and picosecond spectrally resolved fluorescence measurements were employed. It has been shown that after the incorporation of the complexes into the liposome membranes conditions for directed excitation energy transfer from the light-harvesting pigments to the reaction centers are created, which are less optimal, however, than those in the native state. Possible reasons are considered.