Electron transfer in pheophytin a-modified reaction centers from Rhodobacter sphaeroides (R-26).

The major part (> 90%) of bacteriopheophytin a in reaction centers (RCs) of Rhodobacter sphaeroides was substituted by plant pheophytin a. In modified RCs the photochemical formation of P+Qa- occurs with with a quantum efficiency of 79%. The intermediary state P+I- displayed a recombination time constant of 1.5 ns, and the electron transfer from I- to Qa was characterized by a time constant of 540 ps. On the basis of spectral properties of P+I- for native and modified RCs, it was suggested that bacteriopheophytin, as well as bacteriochlorophyll monomers located in L protein branch, have a transition at 545 nm with approx. equal extinction coefficients. Accordingly, the state P+I- in modified RCs is proposed to consist of a thermodynamic mixture of P+BL- (approximately 80%) and P+Phe- (approximately 20%).