title: Trapping of the S2 to S3 state intermediate of the oxygen-evolving complex of photosystem II.
abstract: Photosystem II preparations poised in the S(2)...Q(A) state produce no detectable intermediate during straightforward illumination at liquid helium temperatures. However, upon flash illumination in the range of 77-190 K, they produce a transient state which at -10 degrees C advances to S(3) or after rapid cooling to 10 K gives rise to a 116 G wide metalloradical EPR signal. The latter decays with half-times on the order of a few minutes, presumably by charge recombination, and can be regenerated repeatedly by illumination at 10 K. The constraints for Tyr Z oxidation are attributed to the presence of excess positive charge in S(2). Elevated temperatures are required presumably to overcome a thermal barrier in the deprotonation of Tyr Z(+) or most likely to allow secondary proton transfer away from the base partner of Tyr Z. Treatment with 5% (v/v) MeOH appears to remove the constraints for Tyr Z oxidation, and a 160 G wide metalloradical EPR signal is produced by illumination at 10 K, which decays with a half-time of ca. 80 s. Formation of the metalloradical signals is accompanied by reversible changes in the Mn multiline signal. The intermediates are assigned to Tyr Z(*) magnetically interacting with the Mn cluster in S(2), S(2)Y(Z)(*). A molecular model which extends an earlier suggestion and provides a plausible explanation of a number of observations, including the binding of small molecules to the Mn cluster, is presented.
authors.affiliation: Institute of Materials Science, NCSR Demokritos, 153 10 Aghia Paraskevi Attikis, Greece.
authors.forename: Nikolaos
authors.initials: N
authors.lastname: Ioannidis
authors.forename: Georgia
authors.initials: G
authors.lastname: Zahariou
authors.forename: Vasili
authors.initials: V
authors.lastname: Petrouleas
mesh_terms: D004578:Electron Spin Resonance Spectroscopy; D005609:Free Radicals / Q000737:chemistry / Q000378:metabolism; D018698:Glutamic Acid / Q000235:genetics / Q000378:metabolism; D008345:Manganese / Q000378:metabolism; D000432:Methanol / Q000378:metabolism / Q000494:pharmacology; D009154:Mutation; D010084:Oxidation-Reduction; D010100:Oxygen / Q000378:metabolism*; D045332:Photosystem II Protein Complex / Q000737:chemistry* / Q000378:metabolism*; D019265:Spectroscopy, Near-Infrared; D018724:Spinacia oleracea / Q000737:chemistry / Q000166:cytology; D013696:Temperature; D014443:Tyrosine / Q000031:analogs & derivatives / Q000378:metabolism
chemical_list: D005609:Free Radicals; D045332:Photosystem II Protein Complex; C114979:tyrosyl radical YZ; D018698:Glutamic Acid; D014443:Tyrosine; D008345:Manganese; D010100:Oxygen; D000432:Methanol