title: Recent pulsed EPR studies of the photosystem II oxygen-evolving complex: implications as to water oxidation mechanisms.
abstract: The pulsed electron paramagnetic resonance (EPR) methods of electron spin echo envelope modulation (ESEEM) and electron spin echo-electron nuclear double resonance (ESE-ENDOR) are used to investigate the structure of the Photosystem II oxygen-evolving complex (OEC), including the paramagnetic manganese cluster and its immediate surroundings. Recent unpublished results from the pulsed EPR laboratory at UC-Davis are discussed, along with aspects of recent publications, with a focus on substrate and cofactor interactions. New data on the proximity of exchangeable deuterons around the Mn cluster poised in the S(0)-state are presented and interpreted. These pulsed EPR results are used in an evaluation of several recently proposed mechanisms for PSII water oxidation. We strongly favor mechanistic models where the substrate waters bind within the OEC early in the S-state cycle. Models in which the O-O bond is formed by a nucleophilic attack by a Ca(2+)-bound water on a strong S(4)-state electrophile provide a good match to the pulsed EPR data.
authors.affiliation: Department of Chemistry, University of California, Davis, 1 Shields Avenue, Davis, CA 95616, USA. rdbritt@ucdavis.edu
authors.forename: R David
authors.initials: RD
authors.lastname: Britt
authors.forename: Kristy A
authors.initials: KA
authors.lastname: Campbell
authors.forename: Jeffrey M
authors.initials: JM
authors.lastname: Peloquin
authors.forename: M Lane
authors.initials: ML
authors.lastname: Gilchrist
authors.forename: Constantino P
authors.initials: CP
authors.lastname: Aznar
authors.forename: Michelle M
authors.initials: MM
authors.lastname: Dicus
authors.forename: John
authors.initials: J
authors.lastname: Robblee
authors.forename: Johannes
authors.initials: J
authors.lastname: Messinger
mesh_terms: D004578:Electron Spin Resonance Spectroscopy; D006859:Hydrogen / Q000737:chemistry; D008345:Manganese / Q000737:chemistry; D008958:Models, Molecular; D010084:Oxidation-Reduction; D045332:Photosystem II Protein Complex / Q000737:chemistry* / Q000378:metabolism*; D014867:Water / Q000737:chemistry / Q000378:metabolism
chemical_list: D045332:Photosystem II Protein Complex; D014867:Water; D008345:Manganese; D006859:Hydrogen