Primary photochemical processes in isolated reaction centers of Rhodopseudomonas viridis

Picosecond and nanosecond spectroscopic techniques have been used to study the primary electron transfer processes in reaction centers isolated from the photosynthetic bacterium Rhodopseudomonas viridis. Following flash excitation, the first excited singlet state (P1) of the bacteriochlorophyll complex (P) transfers an electron to an intermediate acceptor (I) in less than 20 ps. The radical pair state (P⁺I^?) subsequently transfers an electron to another acceptor (X) in about 230 ps. There is an additional step of unknown significance exhibiting 35 ps kinetics. P⁺ subsequently extracts an electron from a cytochrome, with a time constant of about 270 ns. At low redox potential (X reduced before the flash), the state P⁺I^? (or P^F) lives approx. 15 ns. It decays, in part, into a longer lived state (P^R), which appears to be a triplet state. State P^R decays with an exponential time of approx. 55 μs. After continuous illumination at low redox potential (I and X both reduced), excitation with an 8-ps flash produces absorption changes reflecting the formation of the first excited singlet state, P1. Most of P1 then decays with a time constant of 20 ps. The spectra of the absorbance changes associated with the conversion of P to P1 or P⁺ support the view that P involves two or more interacting bacteriochlorophylls. The absorbance changes associated with the reduction of I to I^? suggest that I is a bacteriopheophytin interacting strongly with one or more bacteriochlorophylls in the reaction center.