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

The decay of the excited state of the primary donor (1P∗) 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 ∼15 ns delayed emission (amplitude ∼5·10−5) is due to recombination of P+HA− (HA denoting bacteriopheophytin) when electron transport to the secondary quinone acceptor (QA) is blocked, the nature of (iii) intermediate fluorescence components with time constants ranging from ∼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 QA. Reconstitution of QA to QA-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− (BA 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 ∼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 ∼3% at 85 K. In QA-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 1P∗ and P+HA− changing from ΔG0≈−0.21 eV at about 70 ps to ΔG0≈−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.