title: Low-temperature electron transfer in photosystem II: a tyrosyl radical and semiquinone charge pair.
abstract: The states induced by illumination at 7 K in the oxygen-evolving enzyme (PSII) from Thermosynechococcus elongatus were studied by EPR. In the S(0) and S(1) redox states, two g approximately 2 EPR signals, a split signal and a g = 2.03 signal, respectively, were generated by illumination with visible light. These signals were comparable to those already reported in plant PSII in terms of their g value, shape, and stability at low temperatures. We report that the formation and decay of these signals correlate with EPR signals from the semiquinone of the first quinone electron acceptor, Q(A)(-). The light-induced EPR signals from oxidized side-path electron donors (Cyt b(559), Car, and Chl(Z)) were also measured, and from these and the signals from Q(A)(-), estimates were made of the proportion of centers involved in the formation of the g approximately 2 signals (approximately 50% in S(0) and 40% in S(1)). Comparisons with the signals generated in plant PSII indicated approximately similar yields for the S(0) split signal. A single laser flash at 7 K induced more than 75% of the maximum split and g = 2.03 EPR signal observed by continuous illumination, with no detectable oxidation of side-path donors. The matching electron acceptor side reactions, the high quantum yield, and the relatively large proportion of centers involved support earlier suggestions that the state being monitored is Tyr(Z)(*)Q(A)(-), with the g approximately 2 EPR signals arising from Tyr(Z)(*) interacting magnetically with the Mn complex. The current picture of the photochemical reactions occurring in PSII at low temperatures is reassessed.
authors.affiliation: Service de Bioénergétique, CNRS URA 2096, Département de Biologie Joliot-Curie, CEA Saclay, 91191 Gif-sur-Yvette, Cedex, France.
authors.forename: Chunxi
authors.initials: C
authors.lastname: Zhang
authors.forename: Alain
authors.initials: A
authors.lastname: Boussac
authors.forename: A William
authors.initials: AW
authors.lastname: Rutherford
mesh_terms: D016227:Benzoquinones / Q000737:chemistry*; D003080:Cold Temperature; D000458:Cyanobacteria / Q000737:chemistry; D004578:Electron Spin Resonance Spectroscopy; D004579:Electron Transport; D005609:Free Radicals / Q000737:chemistry*; D005615:Freezing; D007834:Lasers; D044367:Phase Transition; D010782:Photolysis; D045332:Photosystem II Protein Complex / Q000737:chemistry*; D018724:Spinacia oleracea / Q000737:chemistry; D055672:Static Electricity; D014443:Tyrosine / Q000031:analogs & derivatives* / Q000737:chemistry*
chemical_list: D016227:Benzoquinones; D005609:Free Radicals; D045332:Photosystem II Protein Complex; C114979:tyrosyl radical YZ; C025232:semiquinone radicals; D014443:Tyrosine