title: Theoretical study of the EPR spectrum of the S3TyrZ•metalloradical intermediate state of the O2-evolving complex of photosystem II.
abstract: The intermediates trapped during the transitions between the consecutive S-states of the oxygen-evolving complex (OEC) of photosystem II (PSII) contain the free radical TyrZ•interacting magnetically with the Mn-cluster (Mn4Ca). In this paper, we present a theoretical study of the EPR spectrum of the S3TyrZ•metalloradical intermediate state, which has been recently detected in MeOH-containing PSII preparations. For this analysis, we use two different approximations: the first, simpler one, is the point-dipole approach, where the two interacting spins are the S = 1/2 of TyrZ•and the ground spin state of S = 3 of the OEC being in the S3state. The second approximation is based on previous proposals indicating that the ground spin state (SG= 3) of the S3state arises from an antiferromagnetic exchange coupling between the S = 9/2 of the Mn(IV)3CaO4and the S = 3/2 of the external Mn(IV) of the OEC. Under the above assumption, the second approximation involves three interacting spins, denoted SA(Mn(IV)3Ca) = 9/2, SB(Mn(IV)) = 3/2 and SC(TyrZ•) = 1/2. Accordingly, the tyrosine radical is exposed to dipolar interactions with both fragments of the OEC, while an antiferromagnetic exchange coupling within the "3 + 1" structural motif of the OEC is also considered. By application of the first-point-dipole approach, the inter-spin distance that simulates the experimental spectrum is not consistent with the theoretical models that were recently reported for the OEC in the S3state. Instead, the recent models are consistent with the results of the analysis that is performed by using the second, more detailed, approach.
authors.affiliation: Institute of Nanoscience & Nanotechnology, NCSR "Demokritos", 15310, Athens, Greece. zahariougeorgia@gmail.com.
authors.forename: Georgia
authors.initials: G
authors.lastname: Zahariou
authors.affiliation: Institute of Nanoscience & Nanotechnology, NCSR "Demokritos", 15310, Athens, Greece.
authors.forename: Nikolaos
authors.initials: N
authors.lastname: Ioannidis
mesh_terms: D004578:Electron Spin Resonance Spectroscopy; D005609:Free Radicals / Q000378:metabolism; D010100:Oxygen / Q000378:metabolism; D045332:Photosystem II Protein Complex / Q000737:chemistry / Q000502:physiology*
chemical_list: D005609:Free Radicals; D045332:Photosystem II Protein Complex; D010100:Oxygen
keywords: Dipolar interactions; EPR; Exchange interactions; Photosystem II; S3 oxidation state of the OEC; S3TyrZ• metalloradical intermediate state