Electrostatic Potentials inRhodopseudomonas viridisReaction Centers: Implications for the Driving Force and Directionality of Electron Transfer

Abstract

Finite difference solutions to the Poisson equation are used to characterize electrostatic interactions in reaction centers from Rhodopseudomonas viridis. A sizable potential gradient resulting primarily from charged amino acid side chains is found in the protein. This static field favors the observed electron transfers from the primary donor (P) along the L branch of the protein via the bacteriochlorophyll monomer (BL) toward the bacteriopheophytin electron acceptor (HL). The effect of the field is to favor electron transfer to BL by approximately 0.4 eV and to HL by approximately 0.8 eV. The electric field along the M branch is significantly smaller, thus providing a straightforward explanation for the directionality of electron transfer. The large static field in the protein appears to be necessary to overcome the intrinsic cost of charge separation in a low dielectric medium. Electrostatic potentials were calculated for the protein in uniform low dielectric medium and for the protein surrounded by wat...