BIOMOLECULAR ELECTRON-TRANSFER UNDER HIGH HYDROSTATIC-PRESSURE

The dependence of the photoinduced electron transfer rate on hydrostat ic pressure up to 8 kbar was studied at 295 K in a bridged Zn-porphyri n donor and pyromellitimide acceptor supermolecule dissolved in toluen e. A picosecond fluorescence emission kinetics of the donor, limited b y 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 transf er theory. A peculiar asymmetric inverted parabola-like dependence of the electron transfer rate on pressure was observed. The dependence wa s successfully reproduced by nonadiabatic theory in the high-temperatu re limit assuming that the reorganisation free energy or both the reor ganisation free energy and the reaction driving force (linearly) chang ed with pressure. The reaction driving force dependence on pressure al one failed to explain the asymmetry, suggesting that the electron tran sfer was accompanied with vibration frequency changes. It was inferred that the effective frequency in the product state should be larger th an in the reactant state. The usage of the nonadiabatic theory is well justified due to the fulfilment of the inequalities V much less than k(B)T and V much less than[tau(L)](-1) (V is the electronic coupling m atrix element, [tau(L)](-1) is the solvent relaxation rate). The influ ence of the donor-acceptor distance reduction under compression on the electron transfer rate was found to be minor, (C) 1998 Elsevier Scien ce B.V. All rights reserved.