Trapping of the S2 to S3 state intermediate of the oxygen-evolving complex of photosystem II

Photosystem II preparations poised in the S-2 center dot center dot center dot Q(A) state produce no detectable intermediate during straightforward illumination at liquid helium temperatures. However, upon flash illumination in the range of 77-190 K, they produce a transient state which at -10 degrees C advances to S-3 or after rapid cooling to 10 K gives rise to a 116 G wide metalloradical EPR signal. The latter decays with half-times on the order of a few minutes, presumably by charge recombination, and can be regenerated repeatedly by illumination at 10 K. The constraints for Tyr Z oxidation are attributed to the presence of excess positive charge in S2. Elevated temperatures are required presumably to overcome a thermal barrier in the deprotonation of Tyr Z(+) or most likely to allow secondary proton transfer away from the base partner of Tyr Z. Treatment with 5% (v/v) MeOH appears to remove the constraints for Tyr Z oxidation, and a 160 G wide metalloradical EPR signal is produced by illumination at 10 K, which decays with a half-time of ca. 80 s. Formation of the metalloradical signals is accompanied by reversible changes in the Mn multiline signal. The intermediates are assigned to Tyr Z(center dot) magnetically interacting with the Mn cluster in S-2, S2YZ center dot. A molecular model which extends an earlier suggestion and provides a plausible explanation of a number of observations, including the binding of small molecules to the Mn cluster, is presented.