title: Sequential and Coupled Proton and Electron Transfer Events in the S2→ S3Transition of Photosynthetic Water Oxidation Revealed by Time-Resolved X-ray Absorption Spectroscopy.
abstract: The choreography of electron transfer (ET) and proton transfer (PT) in the S-state cycle at the manganese-calcium (Mn4Ca) complex of photosystem II (PSII) is pivotal for the mechanism of photosynthetic water oxidation. Time-resolved room-temperature X-ray absorption spectroscopy (XAS) at the Mn K-edge was employed to determine the kinetic isotope effect (KIE = τD2O/τH2O) of the four S transitions in a PSII membrane particle preparation in H2O and D2O buffers. We found a small KIE (1.2-1.4) for manganese oxidation by ET from Mn4Ca to the tyrosine radical (YZ•+) in the S0n→ S1+and S1n→ S2+transitions and for manganese reduction by ET from substrate water to manganese ions in the O2-evolving S3n→ S0nstep, but a larger KIE (∼1.8) for manganese oxidation in the S2n→ S3+step (subscript, number of accumulated oxidizing equivalents; superscript, charge of Mn4Ca). Kinetic lag phases detected in the XAS transients prior to the respective ET steps were assigned to S3+→ S3n(∼150 μs, H2O; ∼380 μs, D2O) and S2+→ S2n(∼25 μs, H2O; ∼120 μs, D2O) steps and attributed to PT events according to their comparatively large KIE (∼2.4, ∼4.5). Our results suggest that proton movements and molecular rearrangements within the hydrogen-bonded network involving Mn4Ca and its bound (substrate) water ligands and the surrounding amino acid/water matrix govern to different extents the rates of all ET steps but affect particularly strongly the S2n→ S3+transition, assigned as proton-coupled electron transfer. Observation of a lag phase in the classical S2→ S3transition verifies that the associated PT is a prerequisite for subsequent ET, which completes Mn4Ca oxidation to the all-Mn(IV) level.
authors.affiliation: Freie Universität Berlin , Department of Physics, 14195 Berlin, Germany.
authors.forename: Ivelina
authors.initials: I
authors.lastname: Zaharieva
authors.affiliation: Freie Universität Berlin , Department of Physics, 14195 Berlin, Germany.
authors.forename: Holger
authors.initials: H
authors.lastname: Dau
authors.affiliation: Freie Universität Berlin , Department of Physics, 14195 Berlin, Germany.
authors.forename: Michael
authors.identifier: http://orcid.org/0000-0001-7008-1764
authors.initials: M
authors.lastname: Haumann
mesh_terms: D004578:Electron Spin Resonance Spectroscopy; D004579:Electron Transport; D004583:Electrons; D007700:Kinetics; D008345:Manganese / Q000737:chemistry*; D010084:Oxidation-Reduction; D045332:Photosystem II Protein Complex / Q000737:chemistry*; D011522:Protons; D018724:Spinacia oleracea / Q000378:metabolism*; D014867:Water / Q000737:chemistry*; D056928:X-Ray Absorption Spectroscopy / Q000379:methods*
chemical_list: D045332:Photosystem II Protein Complex; D011522:Protons; D014867:Water; D008345:Manganese