ELECTRON-TRANSFER AND ELECTRONIC-ENERGY RELAXATION UNDER HIGH HYDROSTATIC-PRESSURE

The following question has been addressed in the present work. How ext ernal high (up to 8 kbar) hydrostatic pressure acts on photoinduced in tramolecular electron transfer and on exciton relaxation processes? Un like phenomena, as they are, have been studied in different systems: e lectron 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 photosyn thetic antenna protein called FMO protein measured at low temperatures , between 4 and 100 K. Spectrally selective picosecond time-resolved e mission technique has been used to detect pressure-induced changes in the systems. The following conclusions have been drawn from the electr on transfer study: (i) External pressure may serve as a potential and sensitive tool not only to study, but also to control and tune element ary chemical reactions in solvents; (ii) Depending on the system param eters, pressure can both accelerate and inhibit electron transfer reac tions; (iii) If competing pathways of the reaction are available, pres sure can probably change the branching ratio between the pathways; (iv ) The classical nonadiabatic electron transfer theory describes well t he 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 e ffect on the electron transfer rate. The effect of pressure on the FMO protein exciton relaxation dynamics at low temperatures has been foun d marginal. This may probably be explained by a unique structure of th e protein [D.E. Trondrud, M.F. Schmid, B.W. Matthews, J. Mol. Biol. 18 8 (1986) p. 443; Y.-F. Li, W. Zhou, E. Blankenship, J.P. Alien, J. Mel . Biol., submitted]. A barrel made of low compressibility beta-sheets may, like a diving bell, effectively screen internal bacteriochlorophy ll a molecules from external influence of high pressure. The origin of the observed slow pico = and subnanosecond dynamics of the excitons a t the exciton band bottom remains open. The phenomenon may be due to w eak coupling of phonons to the exciton states or/and to low density of the relevant low-frequency (approximate to 50 cm(-1)) phonons. Excito n solvation in the surrounding protein and water-glycerol matrix may a lso contribute to this effect. Drastic changes of spectral, kinetic an d dynamic properties have been observed due to protein denaturation, i f the protein was compressed at room temperature and then cooled down, as compared to the samples, first cooled and then pressurised. (C) 19 97 Elsevier Science B.V.