Electrostatic potentials in Rhodopseudomonas viridis reaction centers: Implications for the driving force and directionality of electron transfer

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 (B-L) toward the bacteriopheophytin electron acceptor (HL) The effect of the field is to favor electron transfer to B-L by approximately 0.4 eV and to H-L 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 water with and without a membrane. While the calculated site potentials are sensitive to the assumptions about the dielectric response of the protein and surrounding medium, the conclusion that there is a large static field favoring charge separation along the L branch is independent of the detailed model used to describe the system.