INTERACTION OF Y-Z(CENTER-DOT) WITH ITS ENVIRONMENT IN ACETATE-TREATED PHOTOSYSTEM-II MEMBRANES AND REACTION-CENTER CORES

The photosynthetic oxidation of water to oxygen occurs in photosystem II (RSII) at an active site composed of a tetranuclear cluster of mang anese ions, a redox active tyrosine, Y-z, and two essential cofactors, calcium and chloride. Recently, several experimental observations hav e led to the proposal of a metalloradical catalytic cycle in which wat er oxidation occurs via hydrogen-atom abstraction by the tyrosyl radic al from water bound to the manganese cluster. This model predicts a cl ose proximity between the radical tyrosine, Y-z(.), and the Mn cluster and the involvement of the radical in a bifurcated hydrogen bond. Mag netic resonance techniques have been used in this work to probe the in teraction of the tyrosyl radical with its environment in PSII samples in which the catalytic cycle is blocked by acetate treatment and the e nzyme is trapped in a paramagnetic S2Yz. state. Radical interaction wi th the metal cluster has been studied via simulations of the EPR spect ra obtained for this state. The simulations were based on a radical-pa ir model and included terms for both electron-electron dipolar and exc hange interactions. The results show a dominant exchange interaction b etween the radical and the manganese cluster in these preparations and led to an estimate of 8-9 Angstrom for the spin-spin distance. ESEEM spectroscopy and (H2O)-H-1/(H2O)-H-2 exchange were used to study inter actions of the S2Yz. state with exchangeable hydrogen nuclei in the si te. Two-pulse ESEEM data show features expected for a radical-pair spe cies, in support of our analysis of the continuous-wave EPR spectrum. An ESEEM analysis based on an electron spin 1/2, nuclear spin 1 model shows that both two- and three-pulse ESEEM data are consistent with fo ur deuterons that exhibit an electron-nuclear dipole-dipole coupling o f 0.42 MHz. The validity of this analysis and its implications for the oxygen-evolving apparatus are discussed.