title: Pulse electron paramagnetic resonance studies of the interaction of methanol with the S2 state of the Mn4O5Ca cluster of photosystem II.
abstract: The binding of the substrate analogue methanol to the catalytic Mn4CaO5 cluster of the water-oxidizing enzyme photosystem II is known to alter the electronic structure properties of the oxygen-evolving complex without retarding O2-evolution under steady-state illumination conditions. We report the binding mode of (13)C-labeled methanol determined using 9.4 GHz (X-band) hyperfine sublevel-correlation (HYSCORE) and 34 GHz (Q-band) electron spin-echo electron nuclear double resonance (ESE-ENDOR) spectroscopies. These results are compared to analogous experiments on a mixed-valence Mn(III)Mn(IV) complex (2-OH-3,5-Cl2-salpn)2Mn(III)Mn(IV) (salpn = N,N'-bis(3,5-dichlorosalicylidene)-1,3-diamino-2-hydroxypropane) in which methanol ligates to the Mn(III) ion ( Larson et al. (1992) J. Am. Chem. Soc. , 114 , 6263 ). In the mixed-valence Mn(III,IV) complex, the hyperfine coupling to the (13)C of the bound methanol (Aiso = 0.65 MHz, T = 1.25 MHz) is appreciably larger than that observed for (13)C methanol associated with the Mn4CaO5 cluster poised in the S2 state, where only a weak dipolar hyperfine interaction (Aiso = 0.05 MHz, T = 0.27 MHz) is observed. An evaluation of the (13)C hyperfine interaction using the X-ray structure coordinates of the Mn4CaO5 cluster indicates that methanol does not bind as a terminal ligand to any of the manganese ions in the oxygen-evolving complex. We favor methanol binding in place of a water ligand to the Ca(2+) in the Mn4CaO5 cluster or in place of one of the waters that form hydrogen bonds with the oxygen bridges of the cluster.
authors.affiliation: Department of Chemistry, University of California-Davis , One Shields Avenue, Davis, California 95616, United States.
authors.forename: Paul H
authors.initials: PH
authors.lastname: Oyala
authors.forename: Troy A
authors.initials: TA
authors.lastname: Stich
authors.forename: Jamie A
authors.initials: JA
authors.lastname: Stull
authors.forename: Fangting
authors.initials: F
authors.lastname: Yu
authors.forename: Vincent L
authors.initials: VL
authors.lastname: Pecoraro
authors.forename: R David
authors.initials: RD
authors.lastname: Britt
mesh_terms: D002118:Calcium / Q000737:chemistry*; D018360:Crystallography, X-Ray; D004578:Electron Spin Resonance Spectroscopy; D006860:Hydrogen Bonding; D008274:Magnesium / Q000737:chemistry*; D000432:Methanol / Q000737:chemistry*; D045332:Photosystem II Protein Complex / Q000737:chemistry*; D018724:Spinacia oleracea / Q000201:enzymology*
chemical_list: D045332:Photosystem II Protein Complex; D008274:Magnesium; D002118:Calcium; D000432:Methanol