EPR Spectroscopy and the Electronic Structure of the Oxygen-Evolving Complex of Photosystem II

This article provides an overview of the application of electron paramagnetic resonance (EPR) and related hyperfine spectroscopies for the investigation of the electronic and geometric structure of the tetramanganese cluster that constitutes the oxygen-evolving complex (OEC) of photosystem II. Starting from the spin physics of Mn ions, a quantum-mechanical description of exchange-coupled oligonuclear Mn systems is given. Then, the focus shifts to the characterization of the OEC with emphasis on the two half-integer spin states of its catalytic cycle, namely the S₀ and S₂. The development of electronic models based on EPR and related spectroscopies, such as electron nuclear double resonance (ENDOR), is described in detail. The role of broken-symmetry density functional theory (BS-DFT) is also outlined. The characterization of Mn ligands and bound substrate waters via electron nuclear magnetic interactions is illustrated. Implications for the mechanism of water oxidation catalysis are discussed.