Structure-based analysis of the initial electron transfer step in bacterial photosynthesis: Electric field Induced fluorescence anisotropy

Abstract

The fluorescence intensity from randomly oriented, immobilized Rb. sphaeroides reaction centers at 77 K increases in the presence of an externally applied electric field. We have proposed that this increase is due to a net decrease in the rate of the forward electron transfer reaction which competes with the prompt fluorescence. This decrease results from the change in the free energy difference between the reactant and very dipolar product state in the presence of the electric field. Because the free energy change and thus the electron transfer rate for a given reaction center depends on its orientation relative to the field, the intensity of the competing fluorescence likewise becomes orientation dependent. An expression is derived relating the degree of this electric field induced fluorescence anisotropy to the angle ζet between the fluorescence transition moment and the effective dipole moment whose interaction with the field results in the change in the rate of the electron transfer reaction which competes with fluorescence. ζet is determined to be about 69°. This angle can be estimated from the x-ray crystal structure coordinates for possible identities of the initial electron acceptor. The results are inconsistent with a two-step hopping mechanism in which the bacteriochlorophyll on the L side is the initial electron acceptor whose formation competes with fluorescence. Effects of an electric field on the electronic coupling for a superexchange mechanism are discussed. The theoretical and experimental approach should be generally applicable for studying long-range electron transfer reactions. © 1988 American Institute of Physics.