EPR/ENDOR characterization of the physical and electronic structure of the OEC Mn cluster

Electron paramagnetic resonance (EPR) spectroscopy has often played a crucial role in characterizing the various cofactors and processes of photosynthesis, and photosystem II and its oxygen evolving chemistry is no exception. Until recently, the application of EPR spectroscopy to the characterization of the oxygen evolving complex (OEC) has been limited to the S2-state of the Kok cycle. However, in the past few years, continuous wave-EPR signals have been obtained for both the S0- and S1-state as well as for the S2 (radical)(Z)-state of a number of inhibited systems. Furthermore, the pulsed EPR technique of electron spin echo electron nuclear double resonance spectroscopy has been used to directly probe the 55Mn nuclei of the manganese cluster. In this review, we discuss how the EPR data obtained from each of these states of the OEC Kok cycle are being used to provide insight into the physical and electronic structure of the manganese cluster and its interaction with the key tyrosine, Y(Z).