Interaction of YZ with its environment in acetate treated photosystem II membranes and reaction center cores

The photosynthetic oxidation of water to oxygen occurs in photosystem II (PSII) at an active site composed of a tetranuclear cluster of manganese ions, a redox active tyrosine, YZ, and two essential cofactors, calcium and chloride. Recently, several experimental observations have led to the proposal of a metalloradical catalytic cycle in which water oxidation occurs via hydrogen-atom abstraction by the tyrosyl radical from water bound to the manganese cluster. This model predicts a close proximity between the radical tyrosine, Yz•, and the Mn cluster and the involvement of the radical in a bifurcated hydrogen bond. Magnetic resonance techniques have been used in this work to probe the interaction of the tyrosyl radical with its environment in PSII samples in which the catalytic cycle is blocked by acetate treatment and the enzyme is trapped in a paramagnetic S2Yz• state. Radical interaction with the metal cluster has been studied via simulations of the EPR spectra obtained for this state. The simulations were based on a radical-pair model and included terms for both electron−electron dipolar and exchange interactions. The results show a dominant exchange interaction between the radical and the manganese cluster in these preparations and led to an estimate of 8−9 Å for the spin−spin distance. ESEEM spectroscopy and 1H2O/2H2O exchange were used to study interactions of the S2Yz• state with exchangeable hydrogen nuclei in the site. Two-pulse ESEEM data show features expected for a radical-pair species, 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 four deuterons that exhibit an electron−nuclear dipole−dipole coupling of 0.42 MHz. The validity of this analysis and its implications for the oxygen-evolving apparatus are discussed.