The cationic plastoquinone radical of the chloroplast water splitting complex: Hyperfine splitting from a single methyl group determines the EPR spectral shape of Signal II |
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Authors: | Pádraig J. O Malley,Gerald T. Babcock,Roger C. Prince |
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Affiliation: | 1. Department of Chemistry, Michigan State University, East Lansing, MI 48824-1322 U.S.A.;2. Department of Biochemistry and Biophysics, University of Pennsylvania, Philadelphia, PA 19104 U.S.A. |
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Abstract: | The proposal that EPR Signal II in spinach chloroplasts is due to a plastoquinone cation radical (O'Malley, P.J. and Babcock, G.T. (1983) Biophys. J. 41, 315a) has been investigated in further detail. The similarity in spectral shape between Signal II and the 2-methyl-5-isopropylhydroquinone cation radical is shown to arise from hyperfine coupling to one methyl group for both radicals. A well-resolved four line EPR spectrum of approximate relative intensity 1:3:3:1 for membrane orientation parallel and perpendicular to the applied magnetic field direction also indicates that the partially resolved structure of Signal II is due to hyperfine interaction with one methyl group, i.e., the 2-CH3 group of the plastoquinone cation radical. The ENDOR band observed for this coupling is similar to that observed for methyl group bands of model quinone radicals. The principal hyperfine tensor values obtained for the methyl group interactions are A⊥ = 27.2 MHz and . The large isotropic coupling value (28.6 MHz) of the plastoquinone cation radical's 2-methyl group in vivo indicates that the antisymmetric orbital is the sole contributor to the spin-density distribution of Signal II. The orientation data also suggest that the plastoquinone cation radical is oriented such that the C-CH3 bond direction, and hence the aromatic ring plane, lies perpendicular to the membrane plane. |
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Keywords: | Plastoquinone cation radical Signal II Photosynthesis ESR ENDOR ENDOR electron nuclear double resonance P680 primary donor reaction center chlorophyll of Photosystem II Q McConnell proportionality factor between carbon spin density and proton-electron hyperfine coupling values Hepes 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid |
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