RAPID PROTON-TRANSFER UNDER FLASHING LIGHT AT BOTH FUNCTIONAL SIDES OF DARK-ADAPTED PHOTOSYSTEM-II PARTICLES

By exposing dark-adapted Photosystem II core particles to a series of light flashes, we aimed at kinetic resolution of proton release during the four steps of water oxidation. The signal-to-noise ratio was impr oved by averaging under repetitive dark adaptation. The previously obs erved kinetic damping of pH-transients by particle aggregation was pre vented by detergent. The complicating superimposition of protolytic ev ents at the donor side (water oxidation) and at the acceptor side (qui none oxide-reduction) was unravelled by characterizing the rate consta nts of electron and proton transfer at the acceptor side (Q(A)(-) . nH (+) + DCBQ --> Q(A) + DCBQ(-) + nH(+): k = 1.7 . 10(6) M(-1) s(-1)//2 DCBQ(-) + 2H(+) --> DCBQ + DCBQH(2): k = 4 . 10(8) M(-1) s(-1)). Contr asting with the pronounced period of four oscillations of the oxygen-e volving centre, the extent of proton release was practically constant. The apparent half-rise time of the stepped acidification was shortene d upon lowering of the pH (250 mu s at pH 7.5, 70 mu s at pH 6.0 and 1 2 mu s at pH 5.2). This kinetic behaviour was independent of the natur e and the concentration of the added pH-indicator. We conclude that th is reflects the protolysis of several electrostatically interacting ac ids at the surface of the protein in response to a new positive charge on Y-Z(+), and persisting upon electron transfer from the manganese c luster to Y-Z(+).