Internal Electrostatic Control of the Primary Charge Separation and Recombination in Reaction Centers from Rhodobacter sphaeroides Revealed by Femtosecond Transient Absorption

We report the observation of two conformational states of closed RCs from Rhodobacter sphaeroides characterized by different P<sup>+</sup>H<sub>A</sub><sup>−</sup> → PH<sub>A</sub> charge recombination lifetimes, one of which is of subnanosecond value (700 ± 200 ps). These states are also characterized by different primary charge separation lifetimes. It is proposed that the distinct conformations are related to two protonation states either of reduced secondary electron acceptor, Q<sub>A</sub><sup>−</sup>, or of a titratable amino acid residue localized near Q<sub>A</sub>. The reaction centers in the protonated state are characterized by faster charge separation and slower charge recombination when compared to those in the unprotonated state. Both effects are explained in terms of the model assuming modulation of the free energy level of the state P<sup>+</sup>H<sub>A</sub><sup>−</sup> by the charges on or near Q<sub>A</sub> and decay of the P<sup>+</sup>H<sub>A</sub><sup>−</sup> state via the thermally activated P<sup>+</sup>B<sub>A</sub><sup>−</sup> state.