S-state dependence of the miss probability in Photosystem II.

Abstract

The oxygen production of dark-adapted Photosystem II upon illumination by a series of single-turnover flashes shows a damped period four oscillation with flash number. The damping is attributed to 'misses' resulting from a statistical probability that a reaction center fails to produce a stable charge separation after a saturating flash. The origin of misses is of interest because its probable dependence on flash number, in principle, affects the quantitative interpretation of all measurements on phenomena associated with the period four oscillation. We show that the kinetics of chlorophyll fluorescence yield transients induced by a flash series can be used to estimate the relative amplitudes of the miss probability on each flash. It is concluded that a major part of the misses must be caused by failure of the reduction of the oxidized primary electron donor chlorophyll P680(+) by the secondary donor tyrosine Y(Z) before the charge separation is lost by recombination. The probability of this failure is found to increase with the oxidation state of the oxygen-evolving complex: more than half of it occurs upon charge separation in the S(3) state, which is attributed to the presence of Y(Z) (ox) S(2) in Boltzmann equilibrium with Y(Z)S(3).