Effects of Applied Electric Fields on the Quantum Yields of the Initial Electron-Transfer Steps in Bacterial Photosynthesis. 1. Quantum Yield Failure

The effect of an applied electric field has been measured for the quantum yield of formation of the P H , P+QA-, and 3P states of nonoriented Rb. sphaeroides reaction centers a t 80 K in poly(viny1 alcohol) (PVA) films and in glycerol-buffer glasses. Both the shape and magnitude of the field dependence are very similar for all three observables in both media where the data overlap. This result suggests that the reduction of the quantum yield occurs a t a very early step in the charge separation process. More than 50% reduction in the quantum yield is observed at fields of 2.5 X lo6 V/cm in PVA films. Possible mechanisms include field-dependent nonradiative decay of lP, a field-dependent rate of formation or decay of P+Bif P+Bis important, and field-dependent formation or decay of an unrelaxed form of P H . Small electric field effects are observed on the recombination rate of the radical pair P+Hand the magnetic field dependence of the 3P quantum yield. This is likely because those orientational subpopulations of reaction centers which survive to become P+Hand 'Pin an electric field are those which did not experience quantum yield failure a t an early step. The field-induced effects provide a basis for a possible mechanism to regulate photosynthetic charge separation by transmembrane potentials.