CHLORIDE-DEPLETION OF PHOTOSYNTHETIC WATER OXIDASE - NO PROTON RELEASE DURING THE 2ND OXIDATION STEP, S2-ASTERISK-]S3-ASTERISK, AND A TRANSMEMBRANE RADICAL PAIR RECOMBINATION FROM THE 3RD ON

Chloride depletion blocks the normal four-step progress of photosynthe tic water oxidation. We studied proton release in chloride-depleted th ylakoids which were dark-adapted and excited by flashing light. Proton release was blocked from the second flash on, possibly leaving an unc ompensated positive charge in the catalytic centre. The reduction of P 680+ by Tyr(Z) was still very rapid (much less than 10 mus). From the third flash on, P680+ was reduced more slowly (70 mus/200 mus), and by an electrogenic back-reaction. The uncompensated positive charge may be the reason why the rapid reduction of P680+ by Tyr(Z) is prevented and the transmembrane charge-pair recombination is facilitated.