Excited states of photosynthetic reaction centers at low recox potentials

In preparations of photochemical reaction centers from Rhodopseudomonas spheroides R-26, lowering the recox potential so as to reduce the primary electron acceptor prevents the photochemical transfer of an electron from bacteriochlorophyll to the acceptor. Measuring absorbance changes under these conditions, we found that a 20-ns actinic flash converts the reaction center to a new state, P-F, which then decays with a half-time that is between 1 and 10 ns at 295 degrees K. At 25 degrees K, the decay half-time is approx. 20 ns. The quantum yield of state P-F appears to be near 1.0, both at 295 and at 15 degrees K. State P-F could be an intermediate in the photochemical electron-transfer reaction which occurs when the acceptor is in the oxidized form. Following the decay of state P-F, we detected another state, P-R, with a decay half-time of 6 mus at 295 degrees K and 120 mus at 15 degrees K. The quantum yield of state P-R is approx. 0.1 at 295 degrees K, but rises to a value nearer 1.0 at 15 degrees K. The kinetics and quantum yields are consistent with the view that state P-R forms from P-F. State P-R seems likely to be a side-product, rather than an intermediate in the electron-transfer process. The decay kinetics indicate that state P-F cannot be identical with the lowest excited singlet state of the reaction center. One of the two states, P-F or P-R, probably is the lowest excited triplet state of the reaction center, but it remains unclear which one.