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 +Q a − 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 Q a 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 − (∼ 80%) and P +Phe − (∼ 20%).