Protein Relaxation and Kinetics of P ⁺₆₈₀ Reduction in Photosystem II

Abstract

Electron transfer reactions in proteins are often described in the framework of the non-adiabatic transition-state approximation assuming fast solvent relaxation compared to the rate of electron transfer. This theory predicts mono-exponential kinetics of electron transfer. It occurs at the crossing point of the two potential wells characterizing the initial and final state of the system [1]. Unlike polar liquids for which this approach has been developed, proteins have a wider frequency spectrum of the non-optical dielectric polarizability. It extends into the range of nano- and microseconds. The “slow” relaxation of a protein may even control the dynamics of charge transfer. If the frequency of relaxation mode is distinctly lower than the rate of electron transfer, then the electron equilibration between donor and acceptor would look like a bi-exponential process (see the paper of Cherepanov and Mulkidjanian in these proceedings). In this work we consider effects of slow relaxation on the kinetics of electron transfer between photo-oxidized primary electron donor P ⁺₆₈₀ and its immediate donor tyrosine Y _Z in core particles of photosystem II.