Secondary stabilization reactions and proton-coupled electron transport inphotosystem II investigated by electroluminescence and fluorescence spectroscopy

The oxidized primary electron donor in photosystem II, P-680(+), is reduced in several phases, extending over 4 orders of magnitude in time. Especiall y the slower phases may reflect the back-pressure exerted by water oxidatio n and provide information on the reactions involved. The kinetics of second ary electron-transfer reactions in the microseconds time range after charge separation were investigated in oxygen-evolving thylakoids suspended in H2 O or D2O. Flash-induced changes of chlorophyll fluorescence yield and elect ric field-induced recombination luminescence were decomposed into contribut ions from oxidation states S-0, S-1, S-2, and S-3 of the oxygen-evolving co mplex and interpreted in terms of stabilization kinetics of the initial cha rge-separated state S(j)Y(z)P(680)(+)Q(A)(-)Q(B). In approximately 10% of t he centers, only charge recombination took place. Otherwise, no static hete rogeneity was involved in the microsecond reduction of P-680(+) by Y-Z (sta bilization) or Q(A)(-) (recombination). The recombination component in acti ve centers occurs mainly upon charge separation in S3, and, in the presence of D2O, in S-2 as well and is tentatively attributed to the presence of (Y ZSj-1)-S-ox in equilibrium with YZSj. A 20-30 mus stabilization occurs in a ll S-states, but to different extents. Possible mechanisms for this compone nt are discussed. D2O was found to decrease: (i) the rate of the reaction ( YZS1)-S-ox to YZS2, (ii) the equilibrium constant between P680(+)Y(Z)S(2) a nd (P680YZS2)-S-ox, (iii) the rate of the slow phase of P-680(+) reduction for the S-3 --> S-0 transition, and (iv) the rate of electron transfer from Q(A)(-) to Q(B)/Q(B)(-). The increased 'miss probability' in D2O is due to (iii).