Energetics and mechanism of primary charge separation in bacterial photosynthesis.: A comparative study on reaction centers of Rhodobacter sphaeroides and Chloroflexus aurantiacus

The high efficiency of charge separation in photosynthetic reaction centers arises from the interplay of energetics, electronic couplings, and reorganization energies relevant for the fast charge separation and slow recombination processes. All these parameters can be determined unambiguously only from magnetic-field-dependent measurements of the recombination dynamics of the intermediate radical pair P+HA- and the lifetime of the recombination product P-3*. Results Obtained on Q(A)-depleted reaction centers of Chloroflexus aurantiacus are compared with those for the well-characterized reaction centers of Rhodobacter sphaeroides. In contrast to Rb. sphaeroides, the magnetic field dependence of the triplet yield in Cf. aurantiacus has a pronounced resonance structure, allowing the direct determination of the exchange interaction of P+HA-, J = 21 G. The recombination rate k(T) is slightly larger for Cf. aurantiacus and shows a different temperature dependence. All these differences can be explained by the free energy of P+HA-, found to be larger by 0.04 eV in Cf. aurantiacus compared to Rb. sphaeroides. We propose that this different energy arises largely from the different amino acid at position L104, which is glutamic acid in the case of Rb. sphaeroides and glutamine in the case of Cf. aurantiacus. The electronic couplings and the reorganization energies, on the other hand, are very similar in both reaction centers. Implications for the mechanism of primary charge separation are discussed. The pronounced nonexponential kinetics of charge separation in Cf. aurantiacus is explained by the energetic inhomogeneity of the primary radical pair P+BA-.