title: Semiquinone-iron complex of photosystem II: EPR signals assigned to the low-field edge of the ground state doublet of QA•-Fe2+ and QB•-Fe2+.
abstract: The quinone-iron complex of the electron acceptor complex of Photosystem II was studied by EPR spectroscopy in Thermosynechococcus elongatus. New g ∼ 2 features belonging to the EPR signal of the semiquinone forms of the primary and secondary quinone, i.e., Q(A)(•-)Fe(2+) and Q(B)(•-)Fe(2+), respectively, are reported. In previous studies, these signals were missed because they were obscured by the EPR signal arising from the stable tyrosyl radical, TyrD(•). When the TyrD(•) signal was removed, either by chemical reduction or by the use of a mutant lacking TyrD, the new signals dominated the spectrum. For Q(A)(•-)Fe(2+), the signal was formed by illumination at 77 K or by sodium dithionite reduction in the dark. For Q(B)(•-)Fe(2+), the signal showed the characteristic period-of-two variations in its intensity when generated by a series of laser flashes. The new features showed relaxation characteristics comparable to those of the well-known features of the semiquinone-iron complexes and showed a temperature dependence consistent with an assignment to the low-field edge of the ground state doublet of the spin system. Spectral simulations are consistent with this assignment and with the current model of the spin system. The signal was also present in Q(B)(•-)Fe(2+) in plant Photosystem II, but in plants, the signal was not detected in the Q(A)(•-)Fe(2+) state.
authors.affiliation: iBiTec-S, CNRS URA 2096, CEA Saclay, 91191 Gif-sur-Yvette, France.
authors.forename: Arezki
authors.initials: A
authors.lastname: Sedoud
authors.forename: Nicholas
authors.initials: N
authors.lastname: Cox
authors.forename: Miwa
authors.initials: M
authors.lastname: Sugiura
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Lubitz
authors.forename: Alain
authors.initials: A
authors.lastname: Boussac
authors.forename: A William
authors.initials: AW
authors.lastname: Rutherford
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry; D016227:Benzoquinones / Q000737:chemistry* / Q000378:metabolism; D000458:Cyanobacteria / Q000201:enzymology*; D004578:Electron Spin Resonance Spectroscopy; D004579:Electron Transport; D005296:Ferrous Compounds / Q000737:chemistry* / Q000378:metabolism; D005609:Free Radicals / Q000737:chemistry; D007501:Iron / Q000737:chemistry*; D045342:Light-Harvesting Protein Complexes / Q000737:chemistry; D008872:Microwaves; D045332:Photosystem II Protein Complex / Q000737:chemistry*; D018724:Spinacia oleracea / Q000201:enzymology*; D014443:Tyrosine / Q000737:chemistry
chemical_list: D001426:Bacterial Proteins; D016227:Benzoquinones; D005296:Ferrous Compounds; D005609:Free Radicals; D045342:Light-Harvesting Protein Complexes; D045332:Photosystem II Protein Complex; C422369:photosystem II, psbA subunit; C408694:tyrosine radical; C076726:chlorophyll A binding protein CP43, Cyanobacteria; C025232:semiquinone radicals; C004532:quinone; D014443:Tyrosine; D007501:Iron