EFFECT OF PH AND SURFACE-POTENTIAL ON THE RATE OF ELECTRIC-POTENTIAL GENERATION DUE TO PROTON UPTAKE BY SECONDARY QUINONE ACCEPTOR OF REACTION CENTERS IN RHODOBACTER-SPHAEROIDES CHROMATOPHORES

An electrometric method was used to investigate the effect of pH and i onic strength on the second flash-induced formation of the transmembra ne electric potential difference (DELTApsi) arising from proton uptake by the quinone complex of the photosynthetic reaction center (RC) in chromatophores of non-sulphur purple bacterium Rhodobacter sphaeroides (wild-type). The characteristic time of this DELTApsi phase is approx . 70 mus at pH 7.5 in the presence of 20 mM KCl. Increases in both pH and salt concentration were found to slow the time of generation of th e electric potential due to quinol formation (Q(B)H-2) after the secon d flash by more than 10-times. At higher pH, the effects of salts were more pronounced. The pH dependence of the Q(B)2- protonation rate is explained by the change in the surface proton concentration near the R C protein. The density of surface Charges calculated on the basis of G ouy-Chapman theory is approx. 0.002 e/angstrom (pH 7) or approx. 10 ne gative charges per RC protein from the cytoplasmic side of the membran e, in good agreement with previous estimates for chromatophores. The s ensitivity of the rate constant of the second flash-induced RC protona tion to the salt concentration probably resolves the contradiction amo ng results reported by different laboratories (Wraight, C.A. (1979) Bi ochim. Biophys. Acta 548, 309-327; Kleinfeld, D., Okamura, M.Y. and Fe her, G. (1985) Biochim. Biophys. Acta 809, 291-310) for the pH depende nce of the rate constant of the reaction Q(A)-Q(B)- --> Q(A)Q(B)H-2.