title: Thin film voltammetry of spinach photosystem II. Proton-gated electron transfer involving the Mn4 cluster.
abstract: Thin film voltammetry was used to obtain direct, reversible, electron-transfer peaks between electrodes and the spinach photosystem II (PS II) reaction center in lipid films for the first time. Three well-defined pairs of reduction-oxidation peaks were found using cyclic and square wave voltammetry at 4 degrees C at pH 7.5, reflecting direct, reversible electron transfer involving cofactors of PS II. These peaks were assigned to the oxygen-evolving complex (OEC) tetramanganese cluster (Em = 0.2 V vs NHE), quinones (Em = -0.29 V), and pheophytin (Em = -0.72 V). PS II that was depleted of the OEC did not give the peak at 0.2 V. Observed Em values, especially for the OEC, may be influenced by protein-lipid interactions and electrode double-layer effects. Voltammetry at pH 6 and at pH 7.5 with a time window of >100 ms revealed that the manganese cluster oxidation is gated by slow deprotonation of a reduced form. Additional rapid protonation/deprotonation steps are also involved in the electrochemical reduction-oxidation pathways.
authors.affiliation: Department of Chemistry, University of Connecticut, Storrs, CT 06269-3060, USA.
authors.forename: Khrisna
authors.initials: K
authors.lastname: Alcantara
authors.forename: Bernard
authors.initials: B
authors.lastname: Munge
authors.forename: Zeus
authors.initials: Z
authors.lastname: Pendon
authors.forename: Harry A
authors.initials: HA
authors.lastname: Frank
authors.forename: James F
authors.initials: JF
authors.lastname: Rusling
mesh_terms: D004579:Electron Transport; D008345:Manganese / Q000737:chemistry*; D010084:Oxidation-Reduction; D045332:Photosystem II Protein Complex / Q000737:chemistry*; D011522:Protons; D013056:Spectrophotometry, Ultraviolet; D018724:Spinacia oleracea / Q000737:chemistry*
chemical_list: D045332:Photosystem II Protein Complex; D011522:Protons; D008345:Manganese