Dispersive charge separation and conformational cooling of P+HA- in reaction centers of Rb. sphaeroides R26:: a spontaneous emission study

The decay of the excited state of the primary donor (P-1*) of reaction centers (RC) of Rb. sphaeroides R26 has been monitored via its spontaneous emission. Although there is general agreement that (i) a prominent drop of prompt emission within a few picoseconds results from fast charge separation of the majority of RCs and that (ii) a similar to 15 ns delayed emission (amplitude similar to 5 . 10(-5)) is due to recombination of P+HA- (H-A denoting bacteriopheophytin) when electron transport to the secondary quinone acceptor (Q(A)) is blocked, the nature of (iii) intermediate fluorescence components with time constants ranging from similar to 100 ps to several nanoseconds has been unclear. In this paper these components are studied by manipulating the lifetime of P+HA- via the presence or absence of Q(A). Reconstitution of Q(A) to Q(A)-free RCs leads to a reduction of this lifetime from 15 ns to 100-200 ps, thus eliminating any delayed emission at later times. In such preparations small components from prompt emission extend up to a nanosecond exhibiting dispersive charge-separation kinetics, which we attribute to an energetic dispersion of the primary radical pair P+BA- (B-A is a bacteriochlorophyll). The high-energy wing of this energetic distribution is responsible for charge separation slower than 100 ps, which we find in a fraction of similar to 0.3% of the RCs at 280 K. Since at low temperatures correspondingly smaller activation barriers are sufficient for such a retardation, this fraction increases to similar to 3% at 85 K, In Q(A)-free RCs with a long P+HA- Lifetime prompt emission still dominates at early times. However, its contribution is smaller than 50% of the total emission at about 600 ps at 280 K and 1 ns at 85 K, respectively. The amplitudes of this delayed emission are time dependent and reveal a relaxation of the average free energy between P-1* and P+HA- changing from Delta G(0) approximate to -0.21 eV at about 70 ps to Delta G(0) approximate to -0.29 eV after 50 ns. This relaxation is attributed to the slow protein response to charge separation (conformational cooling) and can be described by a Kohlrausch relaxation function with a time constant of 4.21 ns and a stretching exponent of 0.456. (C) 1999 Elsevier Science B.V. All rights reserved.