Electrostatic control of the efficiency of light-induced electron transfer across membranes

The energetics and efficiency of light-induced electron transfer across membranes is examined on a molecular level. It is found that the activation energies that control the efficiency are determined by the solvation energies of the charge-transfer states, the redox potentials of the donors and acceptors, and the dielectric relaxation of the system. The formalism developed allows one to assess the efficiency of any artificial photosynthetic system in terms of its molecular components and their local environment. It is pointed out that the key problem in designing an efficient photosynthetic system is the transfer of a charge through a low dielectric environment and that this problem cannot be overcome by choosing the position of the primary donor and acceptor in the membrane. It is predicted that artificial photosynthetic systems can be optimized by placing the acceptors in polar sites that provide a large effective dielectric constant and low dielectric relaxation and by arranging the acceptors in order of increasing redox potentials. The implication regarding bacterial photosynthesis is discussed.