Probing Excited-State Electron Transfer by Resonance Stark Spectroscopy. 1. Experimental Results for Photosynthetic Reaction Centers

Higher order Stark spectroscopy has recently been introduced and applied to characterize the electrooptic properties of chromophores in bacterial photosynthetic reaction centers.1 In the course of these studies, an unusually large and broad higher order Stark effect with a novel line shape was discovered in the region of the monomeric bacteriochlorophyll absorption band. The origin of this new feature has been explored by comparing results from reaction centers in which the chromophores are modified or the environment around the chromophores has an altered amino acid residue composition. Taken together, these results demonstrate that this unusual higher order Stark effect is related to both the monomeric bacteriochlorophyll and bacteriopheophytin on the electron-transfer pathway of the reaction center. The effects of mutations and the oxidation of the special pair on this signal specifically suggest the involvement of charge-separated species between these monomeric chromophores. In part 2 (following paper in this issue) we develop a general treatment of this phenomenon based on a charge resonance interaction between a strongly allowed transition and a charge-separated state. This leads to a variety of predicted higher order Stark line shapes which span the range observed in part 1 and from which we can obtain information on these potentially important, but heretofore experimentally inaccessible, charge-separated states.