His190 might directly bridge Yz and Mn-cluster: Mechanistic consequences for water oxidation deducedfrom DFT calculation

Abstract

Alcohols (e.g. MeOH, EtOH) are often used to probe the structure and function of the oxygen-evolving complex in PSII. It was earlier found that alcohols affect the EPR signals arising from the magnetic coupling inside the Mn-Cluster, e.g. S0 and S2 states EPR signals and the parallel mode signals from S1 and S3 states. The magnetic interaction between the Yz· radical and the Mn-cluster in the S2 state gives rise to a split radical EPR signal known as the S2Yz· signal. Here we report that small alcohols have interesting effects on two families of split S2Yz· signals. One signal, which was found by our group recently, is induced from the S3 state by increasing the pH, while another split signal is formed from Ca-depleted PSII after illumination at 0° C. In both cases, alcohols modify the split signal, but in different ways. For the high pH induced signal, the presence of alcohol (MeOH or EtOH) narrowed the width of the split signal from ~125G to ~85G, accompanied by an intensity increment. The saturation concentration of both alcohols was ~1.0%(vol/vol). For the split signal induced by Ca-depletion, two components could be distinguished. One was symmetric and narrow, the other was asymmetric and broad. The presence of alcohol resulted in the decrement of the symmetric signal, while no obvious change was observed for the broad asymmetric signal. In this condition, the saturation concentration of both alcohols was ~0.4%(vol/vol). These alcohol effects indicate that small alcohols interact closely with the Mn4Yz· system by changing the magnetic coupling.