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 charac terize electrostatic interactions in reaction centers from Rhodopseudo monas viridis. A sizable potential gradient resulting primarily from c harged amino acid side chains is found in the protein. This static fie ld favors the observed electron transfers from the primary donor (P) a long the L branch of the protein via the bacteriochlorophyll monomer ( B-L) toward the bacteriopheophytin electron acceptor (HL) The effect o f 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 expl anation 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 p otentials were calculated for the protein in uniform low dielectric me dium and for the protein surrounded by water with and without a membra ne. While the calculated site potentials are sensitive to the assumpti ons about the dielectric response of the protein and surrounding mediu m, the conclusion that there is a large static field favoring charge s eparation along the L branch is independent of the detailed model used to describe the system.