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.

Photoelectron microscopy: a new approach to mapping organic and biological surfaces.

A general method of imaging organic and biological surfaces based on the photoelectric effect is reported. For the experiments, a photoelectron emission microscope was constructed. It is an ultrahigh vacuum instrument using electrostatic electron lenses, microchannel plate image intensifier, cold stage, hydrogen excitation source, and magnesium fluoride optics. The organic surfaces examined were grid patterns of acridine orange, fluorescein, and benzo(a)pyrene on a Butvar surface. A biological sample, sectioned rat epididymis, was also imaged by the new photoelectron microscope. Good contrast was obtained in these initial low magnification experiments. These data demonstrate the feasibility of mapping biological surfaces according to differences in ionization potentials of exposed molecules. A number of technical difficulties, such as the intensity of the excitation source, must be solved before high resolution experiments are practical. However, it is probable that this approach can be useful, even at low magnifications, in determination of the properties of organic and biological surfaces.