The S2 to S3 transition for water oxidation in PSII (photosystem II), revisited

The formation of O-2 from water requires four transitions, each one after the absorption of one light flash. The mechanism for one of these transitions, the one from S-2 to S-3, has been under considerable debate for the past five years. There has since long been a consensus that one water molecule becomes inserted into the OEC (oxygen evolving complex) in this transition, but the mechanism for this insertion is not clear. A major conclusion obtained in the recent studies has been that a, so called, closed cubane structure of the OEC is a key factor in this transition. Leading suggestions for water insertion have been the so called pivot and carousel mechanisms. In the present study, inspired by recent experiments, it is found that the closed cubane structure is never involved in the mechanism for this transition, nor are the different suggestions mentioned above for water insertion. In the experiments, a high spin S-2 (HS-S-2) intermediate has been detected. Here, it is suggested that this state has an open structure of the OEC.