The first room-temperature X-ray absorption spectra of higher oxidation states of the tetra-manganese complex of photosystem II.

The manganese (Mn) complex of photosystem II catalyzes water oxidation. For the first time, its advancement through the reaction cycle was monitored by time-resolved X-ray absorption measurements at the Mn K-edge at room temperature. The complex was stepped through its four oxidation states by nano-second-laser flashes applied to samples exposed to the X-ray beam. Time courses of the X-ray fluorescence intensity were recorded during a flash sequence. Extended X-ray absorption fine-structure spectra were recorded with the S(1), S(2), and S(3) oxidation states highly populated. The room temperature data is compatible with the formation of a third di-mu-oxo bridge between two Mn atoms upon the S(2)-->S(3) transition.