PROBING REACTION-CENTER PROTONATION BY ELECTROCHROMIC ABSORPTION CHANGES OF COFACTORS IN RHODOBACTER-SPHAEROIDES

Electrochromic changes of the Q(Y) absorption bands of chromophores (d imeric and monomeric bacteriochlorophylls and bacteriopheophytins) wer e measured in reaction centres from purple photosynthetic bacteria Rho dobacter sphaeroides. The observed electrochromic effects could be dec omposed to changes due to negative charge on the acceptor quinone comp lex and positive charge on the dimer. The confirmed additivity (superp osition) of the electrochromic effects indicated that the chromophores had fixed geometry in redox transitions. The Q(Y) absorption band of the bacteriopheophytin proved to be a very sensitive indicator of the relaxational processes of electric charges (protons and/or cations) in the vicinity of the quinones: (1) the bandwidth was affected by pH ar ound 10 which was close to the pK of protonation of the reduced primar y quinone, (2) absorption changes at 748 nm were observed after flash excitation with time constant characteristic to that of light-induced proton binding in reaction centres.