FORMATION OF A LONG-LIVED P(-)STATE IN PLANT PHEOPHYTIN-EXCHANGED REACTION CENTERS OF RHODOBACTER-SPHAEROIDES R26 AT LOW-TEMPERATURE()B(A)()

Femtosecond transient absorption spectroscopy in the range of 500-1040 nm was used to study electron transfer at 5 K in reaction centers of Rhodobacter sphaeroides R26 in which the bacteriopheophytins (BPhe) we re replaced by plant pheophytin a (Phe). Primary charge separation too k place with a time constant of 1.6 ps, similar to that found in nativ e RCs. Spectral changes around 1020 nm indicated the formation of redu ced bacteriochlorophyll (BChl) with the same time constant, and its su bsequent decay in 620 ps. This observation identifies the accessory BC hl as the primary electron acceptor. No evidence was found for electro n transfer to Phe, indicating that electron transfer from B-A(-) occur s directly to the quinone (Q(A)) through superexchange. The results ar e explained by a model in which the free energy level of P(+)Phe(-) li es above that of P+BA-, which itself is below P. Assuming that the pi gment exchange does not affect the energy levels of P and P+BA-, our results strongly support a two-step model for primary electron transfe r in the native bacterial RC, with no, or very little, admixture of su perexchange.