Trapping of the S2to S3State Intermediate of the Oxygen-Evolving Complex of Photosystem II

Abstract

Photosystem II preparations poised in the S₂···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 °C advances to S₃ 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^• magnetically interacting with the Mn cluster in S₂, S₂Y_Z^•. 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.