title: ENDOR and special TRIPLE resonance spectroscopy of photoaccumulated semiquinone electron acceptors in the reaction centers of green sulfur bacteria and heliobacteria.
abstract: Photoaccumulation at 205 K in the presence of dithionite produces EPR signals in anaerobically prepared membranes from Chlorobium limicola and Heliobacterium chlorum that resemble the EPR spectrum of phyllosemiquinone (A1*-) photoaccumulated in photosystem I. We have used ENDOR and special TRIPLE resonance spectroscopy to demonstrate conclusively that these signals arise from menasemiquinone electron acceptors reduced by photoaccumulation. Hyperfine couplings to two protons H-bonded to the semiquinone oxygens have been identified by exchange of H. chlorum into D2O, and hyperfine couplings to the methyl group, and the methylene group of the phytyl side chain, of the semiquinone have also been assigned. The electronic structure of these menasemiquinones in these reaction centers is very similar to that of phyllosemiquinone in PSI, and shows a distorted electron spin density distribution relative to that of phyllosemiquinone in vitro. Special TRIPLE resonance spectrometry has been used to investigate the effect of detergents and oxygen on membranes of C. limicola. Triton X-100 and oxygen affect the menaquinone binding site, but n-dodecyl beta-D-maltoside preparations exhibit a relatively unaltered special TRIPLE spectrum for the photoaccumulated menasemiquinone.
authors.affiliation: School of Biological Sciences, Queen Mary and Westfield College, University of London, Mile End Road, London E1 4NS, UK.
authors.forename: I P
authors.initials: IP
authors.lastname: Muhiuddin
authors.forename: S E
authors.initials: SE
authors.lastname: Rigby
authors.forename: M C
authors.initials: MC
authors.lastname: Evans
authors.forename: J
authors.initials: J
authors.lastname: Amesz
authors.forename: P
authors.initials: P
authors.lastname: Heathcote
mesh_terms: D001419:Bacteria / Q000737:chemistry; D016227:Benzoquinones / Q000737:chemistry; D002462:Cell Membrane / Q000737:chemistry; D019414:Chlorobi / Q000737:chemistry*; D004227:Dithionite; D004578:Electron Spin Resonance Spectroscopy / Q000379:methods; D004583:Electrons; D006860:Hydrogen Bonding; D017830:Octoxynol; D010100:Oxygen / Q000633:toxicity; D010777:Photochemistry; D010788:Photosynthesis / Q000187:drug effects; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry*; D010971:Plastoquinone / Q000031:analogs & derivatives* / Q000737:chemistry; D011522:Protons
chemical_list: D016227:Benzoquinones; D045322:Photosynthetic Reaction Center Complex Proteins; D011522:Protons; C100461:menasemiquinone; D004227:Dithionite; C025232:semiquinone radicals; D017830:Octoxynol; D010971:Plastoquinone; D010100:Oxygen