ON THE ENERGETICS OF THE PRIMARY ELECTRON-TRANSFER PROCESS IN BACTERIAL REACTION CENTERS

The energetics of the initial electron-transfer steps in photosyntheti c bacterial reaction centers are evaluated by free-energy perturbation calculations using several different treatments of the ionizable amin o acid residues and of solvent molecules in and around the protein. Th e calculations illustrate the problems associated with incomplete trea tments of dielectric effects. Calculations that do not include mobile solvent lead to large overestimates of the effects of ionized amino ac id side-chains, and can give errors of up to 20 kcal mol-1 in the free -energy difference between the ion-pair states P+B- and P+H-. When mob ile solvent molecules are included, these two states are calculated to be relatively close together in free energy.