title: Spin-lattice relaxation of the pheophytin, Pheo-, radical of photosystem II.
abstract: The spin-lattice relaxation times (T1) of the pheophytin anion radical, Pheo-, of the PSII reaction center, were measured between 5 and 80 K by electron spin-echo spectroscopy. The Pheo- was studied in Mn-depleted PSII reaction centers in which the primary quinone, QA, was doubly reduced. The selective conversion of the non-heme Fe2+ into its low-spin (S = O) state, in CN-treated PSII, allowed the measurement of the intrinsic T1 of the Pheo- radical. The temperature dependence of the intrinsic (T1)-1 was found to be approximately T1.3 +/- 0.1. In Mn-depleted PSII membranes the high-spin (S = 2) non-heme iron, enhances the spin-lattice relaxation of Pheo-. By analyzing the data with a dipolar model, the dipolar interaction (k1d) between the Pheo and the Fe2+ (S = 2) is estimated over the temperature range 5-80 K. Comparison with the dipolar coupling between the iron and the tyrosine, YD+, shows that the Pheo is much closer to the iron than the YD+ in the PSII reaction center. By scaling the reported Fe(2+)-YD+ distance by the ratio [k1dPheo-]/[k1dYD+], we estimate the Fe(2+)-Pheo- distance in PSII to be 20 +/- 4.2 A. This distance is close to the Fe(2+)-BPheo- distance in the bacterial reaction center, and this result provides further evidence that the acceptor sides of the reaction centers in PSII and bacteria are homologous.
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: A W
authors.initials: AW
authors.lastname: Rutherford
mesh_terms: D016227:Benzoquinones / Q000378:metabolism*; D003486:Cyanides / Q000494:pharmacology; D004578:Electron Spin Resonance Spectroscopy; D004579:Electron Transport; D007700:Kinetics; D008027:Light; D045342:Light-Harvesting Protein Complexes; D008345:Manganese / Q000378:metabolism; D008667:Metalloproteins / Q000737:chemistry / Q000378:metabolism; D019159:Nonheme Iron Proteins; D010084:Oxidation-Reduction; D010674:Pheophytins / Q000737:chemistry* / Q000378:metabolism; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry* / Q000378:metabolism; D045332:Photosystem II Protein Complex; D010940:Plant Proteins / Q000737:chemistry / Q000378:metabolism; D018724:Spinacia oleracea; D013696:Temperature
chemical_list: D016227:Benzoquinones; D003486:Cyanides; D045342:Light-Harvesting Protein Complexes; D008667:Metalloproteins; D019159:Nonheme Iron Proteins; D010674:Pheophytins; D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; D010940:Plant Proteins; C025232:semiquinone radicals; D008345:Manganese; C061694:pheophytin a