Probing Excited-State Electron Transfer by Resonance Stark Spectroscopy. 2. Theory and Application

Abstract

A theory of the resonance Stark effect data reported in part 1 (preceding paper in this issue) is developed. The model used involves a weak charge resonance interaction between the excited state of an electron donor *D and a vibronically broad charge-separated state D+A-, where A is an electron acceptor. The theory predicts a series of unusual higher order Stark line shapes depending primarily on the driving force for electron transfer. These line shapes closely resemble the series of line shapes reported in part 1 for the BL absorption band of several reaction center variants. Analysis of the trends leads to the conclusion that the novel Stark line shapes are due to coupling between the 1BL and BL+HL- states. Analysis of the higher order Stark data gives information on the driving force and rates for the 1BL → BL+HL- electron-transfer reaction in this series of variants. The rate of this reaction in wild-type reaction centers is about 1 order of magnitude slower than energy transfer from 1BL to the special pair as the driving force is quite small; however, it becomes much faster when the driving force is larger, as in the (M)Y210F mutant. Quantitative information is extracted on the rates, relative driving force, and electronic coupling for this process. These results have implications for the mechanism of the primary charge-separation reaction (one-step vs two-step electron transfer) and the origins of unidirectional electron transfer. The experimental method and method of analysis should be generally applicable to any excited-state electron-transfer reaction.