Reaction centers of Rhodobacter sphaeroides R-26 with selective replacement of bacteriopheophytin a by pheophytin a .2. Temperature dependence of the quantum yield of P(+)Q(A)(-) and P-3 formation

The quantum yield of the formation of the charge-separated state P(+)Q(A)(-) in reaction centers (RCs) of Rhodobacter sphaeroides R-26, in which the bacteriopheophytins in both the active (A) and the inactive (B) branch are replaced by pheophytin (Pheo) a (Phi(A,B)-exchanged RCs), shows a positive temperature dependence: it is 38 +/- 5% between 10 and 60 K, increases with temperature to 72 +/- 5% at 200 K and shows a minor additional increase above this temperature. The temperature dependence of the quantum yield of P(+)Q(A)(-) formation in Phi(A,B)-exchanged RCs is modelled in the framework of a reaction scheme with the energy level of P(+)Pheo(A)(-) placed above P+BA- (Shkuropatov, A. Ya. and Shuvalov, V.A. (1993) FEES Lett. 322, 168-172), by the introduction of direct electron transfer from B-A(-) to Q(A), assisted by a superexchange-mechanism via P(+)Pheo(A)(-). The observed triples formation Phi(A,B)-exchanged RCs with pre-reduced Q(A) at cryogenic temperatures (quantum yield less than or equal to 12%) is attributed to a residual fraction of RCs in which only Phi(B) was exchanged for Pheo a. The lack of triplet formation in pre-reduced Phi(A,B)-exchanged RCs is consistent with our kinetic model, since this predicts that at low temperatures the state P(+)Pheo(A)(-) is not populated. (C) 1997 Elsevier Science B.V.