The Collapse of the Tyrosine Z^•−Mn Spin−Spin Interaction above 100 K Reveals the Spectrum of Tyrosine Z^•. An Application of Rapid-Scan EPR to the Study of Intermediates of the Water Splitting Mechanism of Photosystem II^†

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Tyr Z of photosystem II mediates electron transfer from the water splitting site, a Mn₄Ca cluster, to the specialized chlorophyll assembly P₆₈₀. Due to its proton-limited redox properties and the proximity to the Mn cluster, it is thought to play a critical role in the proton-coupled electron transfer reactions that constitute the four-step oxidation mechanism (so-called S-state transitions) of water to molecular oxygen. Spectroscopic evidence for the Tyr Z radical has been scarce in intact preparations (it is difficult to probe it optically, and too short-lived for EPR characterization) until recently. Advances in recent years have allowed the trapping at liquid helium temperatures and EPR characterization of metalloradical intermediates, attributed to tyrosyl Z^• magnetically interacting with the Mn cluster. We have extended these studies and examined the evolution of the spectra of five intermediates:  S₀Y_Z^•, S₀Y_Z^• (with 5% MeOH), S₁Y_Z^•, S₂Y_Z^•, and S₂Y_Z^• (with 5% MeOH) in the temperature range of 11−230 K. A rapid-scan EPR method has been applied at elevated temperatures. The tyrosyl radical decouples progressively from Mn, as the Mn relaxation rate increases with an increase in temperature. Above 100 K, the spectra collapse to the unperturbed spectrum of Tyr Z^•, which is found to be somewhat broader than that of the stable Tyr D^• radical. This study provides a simple means for recording the spectrum of Tyr Z^• and extends earlier observations that link the photochemistry at liquid helium temperatures to the photochemistry at temperatures that support S-state transitions.