Biomolecular electron transfer under high hydrostatic pressure

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_BT 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.