title: Endor characterization and D2O exchange in the [Formula: see text] radical in photosystem II.
abstract: The early suggestion by Lozier and Butler (Photochem. Photobiol. 17, 133-137 (1973)) that EPR Signal II arises from radicals associated with the water-splitting process in PSII has been confirmed and extended over the intervening years. Recent work has identified the Signal II radicals, [Formula: see text] and [Formula: see text], with plastosemiquinone cation species. In the experiments presented here we have used ENDOR spectroscopy and D2O/H2O exchange to characterize these paramagnets in more detail. The ENDOR matrix region, which arises from protons which interact weakly with the unpaired electron spin, is well-resolved at 4 K and at least seven resonances are apparent. A number of hyperfine couplings in the 3-8 MHz range are observed and are suggested to arise from methyl or hydroxyl protons which occur as substituents on the plastosemiquinone cation ring or from amino acid protons hydrogen-bonded to the 1,4-hydroxyl groups. Orientation selection experiments are consistent with these possibilities. D2O/H2O exchange shows that the D(+)/Z(+) site is accessible to solvent. However, the exchange occurs slowly and is not complete even after 72 hours which suggests that the free radicals are functionally isolated from solvent water.
authors.affiliation: Department of Chemistry, Michigan State University, 48824-1322, E. Lansing, MI, USA.
authors.forename: T K
authors.initials: TK
authors.lastname: Chandrashekar
authors.forename: P J
authors.initials: PJ
authors.lastname: O'malley
authors.forename: I
authors.initials: I
authors.lastname: Rodriguez
authors.forename: G T
authors.initials: GT
authors.lastname: Babcock