Electron transfer in a genetically modified bacterial reaction center containing a heterodimer.

Time-resolved optical measurements encompassing the femtoseconds to seconds time scales have been used to investigate Rhodobacter capsulatus reaction centers (RCs) in which the histidine residue at position 200 on the M polypeptide has been changed to a leucine by site-directed mutagenesis. The HisM200----Leu RC, which has a heterodimer consisting of a bacteriochlorophyll and a bacteriopheophytin, is capable of the primary photochemistry observed in wild-type Rb. capsulatus RCs, but with an overall quantum yield reduced by about half. The lower yield resides in the initial electron transfer reaction and may be associated in part with substantial charge transfer character of the excited heterodimer. These and other comparisons between Rb. capsulatus wild-type and HisM200----Leu RCs have important implications for our understanding of the mechanism of electron transfer in the RC and the efficiency of the charge separation process.