title: The S1YZ* metalloradical EPR signal of photosystem II contains two distinct components that advance respectively to the multiline and g = 4.1 conformations of S2.
abstract: The S2 state of the oxygen-evolving complex (OEC) of photosystem II is heterogeneous, exhibiting two main EPR spectral forms, the multiline and the g = 4.1 signal. It is not clearly established whether this heterogeneity develops during the S1 to S2 transition or is already present in the precursor states. We have compared the spectra of the S1YZ* intermediate, obtained by visible light excitation (induction of charge separation) of the S1 state at liquid He temperatures, (S1YZ*)vis, or by near-infrared (NIR) light excitation of the S2 state (utilization of the unusual property of the Mn cluster to act as an oxidant of Yz when excited by NIR), (S1YZ*)NIR. The decay kinetics of the (S1YZ*)vis spectrum at 11 K was also studied by the application of rapid-scan EPR. The two spectra share in common a signal with a characteristic feature at g = 2.035, but the (S1YZ*)vis spectrum contains in addition a fast decaying component 26 G wide. The analysis of the surface of the rapid-scan spectra yielded 270 +/- 35 and 90 +/- 15 s for the respective half-times of the two components of the (S1YZ*)vis spectrum at 11 K. (S1YZ*)vis advances efficiently to S2 when annealed at 200 K; notably the g = 2.035 signal advances to the multiline while the 26 G component advances to the g = 4.1 conformation. The "26 G" component is absent or very small, respectively, in thermophilic cyanobacteria or glycerol-containing spinach samples, in correlation to vanishing or very small amounts of the g = 4.1 component in the S2 spectrum. The results validate the assignment of S1YZ* to a true S1 to S2 intermediate and imply that the heterogeneity observed in S2 is already present in S1. Tentative valences are assigned to the individual Mn ions of the OEC in the two heterogeneous conformations of S1.
authors.affiliation: Institute of Materials Science, NCSR Demokritos, 153 10 Aghia Paraskevi Attikis, Greece.
authors.forename: George
authors.initials: G
authors.lastname: Sioros
authors.forename: Dionysios
authors.initials: D
authors.lastname: Koulougliotis
authors.forename: George
authors.initials: G
authors.lastname: Karapanagos
authors.forename: Vasili
authors.initials: V
authors.lastname: Petrouleas
mesh_terms: D004578:Electron Spin Resonance Spectroscopy; D007700:Kinetics; D008345:Manganese / Q000737:chemistry; D008958:Models, Molecular; D008968:Molecular Conformation; D010084:Oxidation-Reduction; D045332:Photosystem II Protein Complex / Q000737:chemistry*; D019265:Spectroscopy, Near-Infrared; D014443:Tyrosine / Q000031:analogs & derivatives* / Q000737:chemistry
chemical_list: D045332:Photosystem II Protein Complex; C114979:tyrosyl radical YZ; D014443:Tyrosine; D008345:Manganese