Chloride-depletion of photosynthetic water oxidase. No proton release during the second oxidation step, S2*==>S3*, and a transmembrane radical pair recombination from the third on.

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