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.
Freiberg
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Institute of Physics, Riia 142, EE2400 Tartu, Estonia. freiberg@park.tartu.ee
Ellervee
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Tars
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Timpmann
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Laisaar
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