Aleksandr Ellervee

Pressure effects on spectra of photosynthetic light-harvesting pigment-protein complexes

Abstract The influence of high (up to 9 kbar) hydrostatic pressure on the absorption and fluorescence emission spectra of photosynthetic light-harvesting pigment-protein complexes isolated from purple bacteria Rhodospirillum rubrum has been studied at room temperature and at 77 K. Under pressure at room temperature all spectral bands exhibit a red-shift at a rate of between 30 and 120 cm −1 /kbar for different bands. From these pressure shifts the compressibility of the protein matrix can be estimated. The compressibility is remarkably different for the protein surrounding bacteriochlorophyll a molecules (κ≥25±5 Mbar −1 ) than for the one surrounding spirilloxanthin molecules (κ=10±2 Mbar −1 ). This indicates that the elastic properties of a protein are locally specific.

Spectral hole burning at high hydrostatic pressure

Abstract The influence of hydrostatic pressure up to about 5 kbar at 4.2 K on the spectral holes burnt and measured at fixed values of pressure has been studied for chlorin molecules embedded in a polystyrene matrix. The narrowing of spectral holes by nearly a factor of two takes place at 5.1 kbar as compared with the results obtained at normal pressure. An incomplete restoration of the widths of the holes burnt at normal pressure after pressure cycling is observed. Possible mechanisms of these phenomena are discussed.

Hydrostatic pressure effects on spectral hole burning in a Shpol’skii system

Hydrostatic-pressure effects on persistent spectral holes in a polycrystalline solid solution of chlorin (Chl) in n-octane (C8:Chl, a Shpol’skii system) at 4.2 K were studied at high-pressure [up to 8.4 kbars (1 kbar = 750.06 kTorr)] isobaric (burning and recording at the same pressure) conditions as well as in low-pressure (21–39 bars) nonisobaric (burning and recording at different pressures) experiments. A significant decrease of the hole width was seen at high pressures, with the FWHM of holes in the 15 730-cm−1 line being 130 and 45 MHz at 1 bar and 5.5 kbars, respectively. The latter value approaches the normal-pressure lifetime limit of 40 MHz. A pressure-induced site-dependent instability of spectral holes was found at pressures above 5.5 kbars. High-pressure shifts of Shpol’skii lines and low-pressure hole shifts gave the same pressure coefficient, −4.8 cm−1/kbar, for the S1 ← S0 (0–0) transition energy. This coefficient yielded the value of 13.2 Mbars−1 for the local compressibility. Reversible non-Gaussian non-area-conserving hole broadening was observed in nonisobaric low-pressure experiments.

High pressure effects on low temperature relaxation in solids

Abstract Persistent spectral hole burning (SHB) was used to probe the influence of high hydrostatic pressure (up to 8.4 kbar) on optical relaxation in chlorin (Chl)-doped glassy (polystyrene, PS) and (poly)crystalline (n-octane, C 8 Shpolskii system) samples at 4.2 K. Pressure induced width reduction of isobarically burnt and measured holes was found, FWHM of holes being for PS:Chl 4.3 and 2.4 GHz at 1 atm and 5.1 kbar, respectively, and for C 8 :Chl (15730 cm −1 line) 130 and 45 MHz at 1 atm and 5.5 kbar, respectively. The latter value approaches the lifetime limit of 40 MHz. For the glassy matrix the effect of hole narrowing gives evidence of the dominant role of low frequency quasilocal vibrations in determining the hole width. Pressure induced instability of photoproduct was found in C 8 :Chl at pressures above 5.5 kbar.

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.

Pressure effects on absorption spectra of the isolated reaction center of Photosystem II

Absorption spectra of the D1-D2-cytochrom b559 complex at 4°C were investigated at pressures up to 300 MPa. Pressure effects were mostly reversible and independent of the detergent used (CHAPS or dodecyl-β-D-maltoside). Red-shifts were observed under pressure for the chlorophyll Qy- and the β-carotene S0 → S2 bands. The relatively small Qy-shift of approximately 0.15 cm-1/MPa is an indication for the absence of strongly coupled chlorophyll dimers within the reaction center and supports earlier reports from low-temperature measurements (Chang HC, Jankowiak R, Reddy NRS and Small GJ (1995) Chem Phys 197: 307–321). The carotene red-shift (seen in CHAPS) is much larger (0.5 – 0.6 cm-1/MPa) and within the range observed for excitonically coupled chlorophylls. However, since carotenes are more sensitive to changes of refractive index, we do not consider this evidence for excitonically coupled carotenes. Varying the pH and the detergent induced only small effects. Pigment exchange using high pressure instead of elevated temperature was not possible under the conditions tested.

Biomolecular electron transfer under high hydrostatic pressure

Abstract The dependence of the photoinduced electron transfer rate on hydrostatic pressure up to 8 kbar was studied at 295 K in a bridged Zn-porphyrin donor and pyromellitimide acceptor supermolecule dissolved in toluene. A picosecond fluorescence emission kinetics of the donor, limited by the electron transfer rate, was detected by using synchroscan streak camera. The experiment was complemented with model calculations based on modified classical and semi-classical nonadiabatic electron transfer theory. A peculiar asymmetric inverted parabola-like dependence of the electron transfer rate on pressure was observed. The dependence was successfully reproduced by nonadiabatic theory in the high-temperature limit assuming that the reorganisation free energy or both the reorganisation free energy and the reaction driving force (linearly) changed with pressure. The reaction driving force dependence on pressure alone failed to explain the asymmetry, suggesting that the electron transfer was accompanied with vibration frequency changes. It was inferred that the effective frequency in the product state should be larger than in the reactant state. The usage of the nonadiabatic theory is well justified due to the fulfilment of the inequalities V 《 k B T and V 《〈 τ L 〉 −1 ( V is the electronic coupling matrix element, 〈 τ L 〉 −1 is the solvent relaxation rate). The influence of the donor-acceptor distance reduction under compression on the electron transfer rate was found to be minor.

An immersion cryostat for mounting a high-pressure optical cell surrounded by nonboiling liquid nitrogen

A special bubble-free cryostat for precise optical measurements of samples compressed in a high-pressure cell has been built and put into operation. Liquid nitrogen around the high-pressure cell is prevented from boiling thanks to a slight overpressure in the closed cryovessel; instead, cryoliquid is boiling at atmospheric pressure inside an evaporator.