pH-metric study of reaction centers from photosynthetic bacteria in micellular solutions: protonatable groups equilibrate with the aqueous bulk phase.

Hydrogen ion equilibria of the reaction center protein from photosynthetic purple bacteria Rhodobacter sphaeroides and Rhodobacter capsulatus dissolved in micellular solution were studied by acid-base titration to estimate the water accessibility of protonatable residues of the protein determined from structural data. The ionizable amino acids of the reaction center underwent protonation-deprotonation with protons from the interfacial layer, which, however, exchanged protons from the aqueous bulk phase. The equilibrium was described in terms of the buffering capacity of the multiphase system. The detergents decreased the proton activity coefficient (increased the buffering capacity) of the aqueous solution by a factor of 0.33 (in 0.03% Triton X-100 and LDAO) and 0.12 (0.04% dodecyl beta-D-maltoside). The observed buffering capacities of the reaction center protein were large and detergent-dependent. However, corrections for proton activities made the pH dependence of buffering capacities in different detergents uniform and similar to that expected from the number and pK values of protonatable groups of the protein. The vast majority of protonatable amino acids of the reaction center are in protonation equilibria with the aqueous bulk phase on an extended time scale.