The role of hydrogen bonds for the multiphasic P680+• reduction by Yz in photosystem II with intact oxygen evolution capacity.: Analysis of kinetic H/D isotope exchange effects

The mechanism of multiphasic P680(+.) reduction by Yz has been analyzed by studying H/D isotope exchange effects on flash-induced changes of 830 nm absorption, Delta A(830)(t), and normalized fluorescence yield, F(t)/F-0, in dark-adapted thylakoids and PS II membrane fragments from spinach. It was found that (a) the characteristic period four oscillations of the normalized components of Delta A(830)(t) relaxation and of F(t)/F-0 rise in the nanosecond and microsecond time domain are significantly modified when exchangeable protons are replaced by deuterons; (b) in marked contrast to the normalized steady-state extent of the microsecond kinetics of 830 nm absorption changes which increases only slightly due to H/D exchange (about 10%) the S-i state-dependent pattern exhibits marked effects that are most pronounced after the first, fourth, fifth, and eighth flashes; (c) regardless of data evaluation by different fit procedures the results lead to a consistent conclusion, that is, the relative extent of the back reaction between P680(+.)Q(A)(-.) becomes enhanced in samples suspended in D2O; and (d) this enhancement is dependent on the S-i state of the WOC and attains maximum values in S-2 and S-3, most likely due to a retardation of the "35 mu s kinetics" of P680(+.) reduction. In an extension of our previous suggestion on the functional role of hydrogen bonding of Y-Z by a basic group X (Eckert, H.-J., and Renger, G. (1988) FEES Lett. 236, 425-431), a model is proposed for the origin of the multiphasic P680(+.) reduction by Y-Z. Two types of different processes are involved: (a) electron transfer in the nanosecond time domain is determined by strength and geometry of the hydrogen bond between the O-H group of Y-Z and acceptor X, and (b) the microsecond kinetics reflect relaxation processes of a hydrogen bond network giving rise to a shift of the equilibrium P680(+.)Y(Z) reversible arrow P680Y(Z)(OX) toward the right side. The implications of this model are discussed.