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(-) AND P-3 FORMATION()Q(A)()

The quantum yield of the formation of the charge-separated state P(+)Q (A)(-) in reaction centers (RCs) of Rhodobacter sphaeroides R-26, in w hich 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 dep endence of the quantum yield of P(+)Q(A)(-) formation in Phi(A,B)-exch anged RCs is modelled in the framework of a reaction scheme with the e nergy level of P(+)Pheo(A)(-) placed above P+BA- (Shkuropatov, A. Ya. and Shuvalov, V.A. (1993) FEES Lett. 322, 168-172), by the introductio n of direct electron transfer from B-A(-) to Q(A), assisted by a super exchange-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 la ck of triplet formation in pre-reduced Phi(A,B)-exchanged RCs is consi stent with our kinetic model, since this predicts that at low temperat ures the state P(+)Pheo(A)(-) is not populated. (C) 1997 Elsevier Scie nce B.V.