EPR studies of the water oxidizing complex in the S 1 and the higher S states: the manganese cluster and Y Z radical

Abstract

The parallel polarization electron paramagnetic resonance (EPR) method has been applied to investigate manganese EPR signals of native S 1 and S 3 states of the water oxidizing complex (WOC) in photosystem (PS) II. The EPR signals in both states were assigned to thermally excited states with S=1, from which zero-field interaction parameters D and E were derived. Three kinds of signals, the doublet signal, the singlet-like signal and g=11–15 signal, were detected in Ca 2+-depleted PS II. The g=11–15 signal was observed by parallel and perpendicular modes and assigned to a higher oxidation state beyond S 2 in Ca 2+-depleted PS II. The singlet-like signal was associated with the g=11–15 signal but not with the Y Z (the tyrosine residue 161 of the D1 polypeptide in PS II) radical. The doublet signal was associated with the Y Z radical as proved by pulsed electron nuclear double resonance (ENDOR) and ENDOR-induced EPR. The electron transfer mechanism relevant to the role of Y Z radical was discussed.