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

The parallel polarization electron paramagnetic resonance (EPR) method has been applied to investigate manganese EPR signals of native S-1 and S-3 sta tes of the water oxidizing complex (WOC) in photosystem (PS) II. The EPR si gnals in both states were assigned to thermally excited states with S = 1, from which zero-field interaction parameters D and E were derived. Three ki nds of signals, the doublet signal, the singlet-like signal and g = 11-15 s ignal, were defected in Ca2+-depleted PS II. The g = 11-15 signal was obser ved by parallel and perpendicular modes and assigned to a higher oxidation state beyond S-2 in Ca2+-depleted PS II. The singlet-like signal was associ ated 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. (C) 2001 Elsevier Science B.V. All rights reserved.