ELECTROGENICITY AT THE SECONDARY QUINONE ACCEPTOR SITE OF CYANOBACTERIAL PHOTOSYSTEM-II

Flash-induced generation of the electric potential difference (Delta p si) by a direct electrometrical method was studied in Anacystis nidula ns photosystem II-containing proteoliposomes associated with a phospho lipid-impregnated collodion film. Besides a rapid phase of Delta psi g eneration corresponding to charge separation between P680 and Q(A), an additional electrogenic phase with a characteristic time of 0.27 ms a t pH 7.0 was observed after the second laser flash. The maximal amplit ude of this phase was approx. 4% of that related to the P680(+)Q(A)(-) formation. The sensitivity of this phase to DCMU, the flash-number de pendence of its amplitude as well as the amplitude and the rate consta nt pH-dependences, indicate that it is due to the dismutation of Q(A)( -) and Q(B)(-) and to subsequent protonation of a doubly reduced plast oquinone Q(B)(2-).