Early Binding of Substrate Oxygen Is Responsible for a Spectroscopically Distinct S2 State in Photosystem II

The biological generation of oxygen by the oxygen-evolving complex (OEC) in photosystem II (PS II) is one of nature's most important reactions. The OEC is a Mn4Ca cluster that has multiple Mn-O-Mn and Mn-O-Ca bridges and binds four water molecules. Previously, binding of an additional oxygen was detected in the S2 to S3 transition. Here we demonstrate that early binding of the substrate oxygen to the five-coordinate Mn1 center in the S2 state is likely responsible for the S2 high-spin EPR signal. Substrate binding in the Mn1-OH form explains the prevalence of the high-spin S2 state at higher pH and its low-temperature conversion into the S3 state. The given interpretation was confirmed by X-ray absorption spectroscopic measurements, DFT, and broken symmetry DFT calculations of structures and magnetic properties. Structural, electronic, and spectroscopic properties of the high-spin S2 state model are provided and compared with the available S3 state models. New interpretation of the high-spin S2 state opens opportunity for analysis of factors controlling the oxygen substrate binding in PS II.