Contribution of Protonation to the Dielectric Relaxation Arising from Bacteriopheophytin Reductions in the Photosynthetic Reaction Centers of Rhodobacter sphaeroides

The pH dependence of the free energy level of the flash-induced primary charge pair P⁺I_A^- was determined by a combination of the results from the indirect charge recombination of P⁺Q_A^- and from the delayed fluorescence of the excited dimer (P*) in the reaction center of the photosynthetic bacterium <i>Rhodobacter sphaeroides</i>, where the native ubiquinone at the primary quinone binding site Q_A was replaced by low-potential anthraquinone (AQ) derivatives. The following observations were made: (1) The free energy state of P⁺I_A^- was pH independent below pH 10 (-370 ± 10 meV relative to that of the excited dimer P*) and showed a remarkable decrease (about 20 meV/pH unit) above pH 10. A part of the dielectric relaxation of the P⁺I_A^- charge pair that is not insignificant (about 120 meV) should come from protonation-related changes. (2) The single exponential decay character of the kinetics proves that the protonated/unprotonated P⁺I_A^- and P⁺Q_A^- states are in equilibria and the rate constants of protonation <i>k</i>_on^H +<i>k</i>_off^H are much larger than those of the charge back reaction <i>k</i>_back ~10³ s^-1. (3) Highly similar pH profiles were measured to determine the free energy states of P⁺Q_A^- and P⁺I_A^-, indicating that the same acidic cluster at around Q_B should respond to both anionic species. This was supported by model calculations based on anticooperative proton distribution in the cluster with key residues of GluL212, AspL213, AspM17, and GluH173, and the effect of the polarization of the aqueous phase on electrostatic interactions. The larger distance of I_A^- from the cluster (25.2 Å) compared to that of Q_A^- (14.5 Å) is compensated by a smaller effective dielectric constant (6.5 ± 0.5 and 10.0 ± 0.5, respectively). (4) The P* → P⁺Q_A^- and I_A^-Q_A → I_AQ_A^- electron transfers are enthalpy-driven reactions with the exemption of very large (>60%) or negligible entropic contributions in cases of substitution by 2,3-dimethyl-AQ or 1-chloro-AQ, respectively. The possible structural consequences are discussed.