title: Asymmetric binding of the 1- and 4-C=O groups of QA in Rhodobacter sphaeroides R26 reaction centres monitored by Fourier transform infra-red spectroscopy using site-specific isotopically labelled ubiquinone-10.
abstract: Using 1-, 2-, 3- and 4-13C site-specifically labelled ubiquinone-10, reconstituted at the QA site of Rhodobacter sphaeroides R26 reaction centres, the infra-red bands dominated by the 1- and 4-C = O vibration of QA are assigned in the QA(-)-QA difference spectra. The mode dominated by the 4-C = O vibration is drastically downshifted in the reaction centres as compared with its absorption frequency in free ubiquinone-10. In contrast, the mode dominated by the 1-C = O vibration absorbs at similar frequencies in the free and the bound forms. The frequency shift of the 4-C = O vibration is due to a large decrease in bond order and indicates a strong interaction with the protein microenvironment in the ground state. In the charge-separated state the mode dominated by the semiquinone 4-C = O vibration is characteristic of strong hydrogen bonding to the microenvironment, whereas the mode dominated by the 1-C = O vibration indicates a weaker interaction. The asymmetric binding of the 1- and 4-C = O groups to the protein might contribute to the factors governing different redox reactions of ubiquinone-10 at the QA site as compared with its reactions at the QB site.
authors.affiliation: Institut für Biophysik, Ruhr-Universität Bochum.
authors.forename: R
authors.initials: R
authors.lastname: Brudler
authors.forename: H J
authors.initials: HJ
authors.lastname: de Groot
authors.forename: W B
authors.initials: WB
authors.lastname: van Liemt
authors.forename: W F
authors.initials: WF
authors.lastname: Steggerda
authors.forename: R
authors.initials: R
authors.lastname: Esmeijer
authors.forename: P
authors.initials: P
authors.lastname: Gast
authors.forename: A J
authors.initials: AJ
authors.lastname: Hoff
authors.forename: J
authors.initials: J
authors.lastname: Lugtenburg
authors.forename: K
authors.initials: K
authors.lastname: Gerwert
mesh_terms: D001665:Binding Sites; D002247:Carbon Isotopes; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry*; D012242:Rhodobacter sphaeroides / Q000737:chemistry*; D017550:Spectroscopy, Fourier Transform Infrared; D014451:Ubiquinone / Q000737:chemistry*
chemical_list: D002247:Carbon Isotopes; D045322:Photosynthetic Reaction Center Complex Proteins; D014451:Ubiquinone; C025204:Ubiquinone Q2