Proton Binding Is Part of Protein Relaxation of Flash-Excited Reaction Center from Photosynthetic Bacteria Rhodobacter sphaeroides

In the first part of this work, a brief review is presented of the recent knowledge of charge stabilization processes in flash-excited reaction center protein from photosynthetic bacteria Rhodobacter sphaeroides. The adaptation of the protein to charge separation is comprised of different manifestations of protein relaxation including proton binding at the late phase. In the second part, a unique method, comparative measurement of prompt and millisecond-delayed fluorescence of the bacteriochlorophyll dimer of the reaction center protein is used to determine the free energy levels of the charge-separated states with respect to that of the excited singlet state of the dimer: −910 ± 20 meV and −970 ± 20 meV were measured at pH 8.0 for reaction centers in the absence and presence of secondary quinone, respectively. The pH-dependence of the energetics of charge stabilization due to light-induced proton binding is described in reaction centers with and without secondary quinone activity. The range of free-energy change between pH 11 and pH 5 was −65 meV with a single (alkaline) inflection point (no active secondary quinone) and −175 meV with an additional (acidic) inflection point (active secondary quinone). The conclusions from fluorescence data agreed well with pH-dependence of integrated proton uptake (a model-independent method) and with calculations based on interaction of quinones with four key amino acid residues. The enthalpy and entropy parts of the free-energy changes were determined from van't Hoff analysis of the delayed fluorescence and compared with data of other methods. It is concluded that the charge stabilization including proton binding is a highly enthalpy-driven process.