title: Spin-lattice relaxation of the phyllosemiquinone radical of photosystem I.
abstract: The spin-lattice relaxation time (T1) of the phyllosemiquinone anion radical, A1-, of the photosystem I (PSI) reaction center, were measured between 4.5 and 85 K by electron spin-echo spectroscopy. The selective removal of the iron-sulfur centers, FA, FB, and FX, from PSI allowed the measurement of the intrinsic T1 of the A1- radical. The temperature dependence of the intrinsic (T1)-1 for A1- was found to be approximately T1.3 +/- 0.1. The spin-lattice relaxation of the reduced form of iron-sulfur center FX was also measured at low temperatures, in FA/FB-depleted PSI membranes. It was found that the fast-relaxing FX center enhances the spin-lattice relaxation of the phyllosemiquinone due to dipolar coupling. The effect of the reduced forms of FA/FB on the T1 of the phyllosemiquinone was minor compared to the effect of FX. By analyzing the data with a dipolar model in the light of limitations imposed by other information present in the literature, the distance between the phyllosemiquinone and FX in PSI is estimated to be 14.8 +/- 4 A.
authors.affiliation: Section de Bioénergétique (URA CNRS 2096), Département de Biologie Cellulaire et Moléculaire, CEA Saclay, Gif-sur-Yvette, France.
authors.forename: Y
authors.initials: Y
authors.lastname: Deligiannakis
authors.forename: J
authors.initials: J
authors.lastname: Hanley
authors.forename: A W
authors.initials: AW
authors.lastname: Rutherford
mesh_terms: D016227:Benzoquinones / Q000737:chemistry; D000458:Cyanobacteria / Q000737:chemistry; D004578:Electron Spin Resonance Spectroscopy; D004579:Electron Transport; D005609:Free Radicals / Q000737:chemistry; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry*
chemical_list: D016227:Benzoquinones; D005609:Free Radicals; D045322:Photosynthetic Reaction Center Complex Proteins; C025232:semiquinone radicals