Near-IR irradiation of the S(sub)2 state of the water oxidizing complex of photosystem II at liquid helium temperatures produces the metalloradical intermediate attributed to S(sub)1Y(sub)z

The near-infra red excitation of the S(sub)2 state at liquid helium temperatures produces metalloradical intermediates assigned to S(sub)1Y(sub)z, suggesting a proton exchange between tyrosine Z and the manganese cluster and alteration of the magnetic properties of the manganese cluster.