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Redox Reactions of the Non-Heme Iron of Photosystem II: An EPR Spectroscopic Study

  • James P. McEvoy
  • Gary W. Brudvig
Conference paper

Abstract

We have used EPR spectroscopy to investigate the redox chemistry of the non-heme iron of photosystem II, focusing on the cryogenic electron-transfer reactions of iron, QA and a variety of secondary electron donors, namely chlorophylls, carotenoids and tyrosine D. Glycolate coordination to the iron was used in combination with potassium ferricyanide to preoxidize the non-heme iron completely and prime it for photoreduction. The oxidized Fe3+ and QA centers operated as a single, one-electron acceptor site: EPR quantitation of both the oxidized donor yield and the Fe2+ QA − yield found no evidence for the formation of more than one oxidized secondary electron donor per PSII. The photooxidized chlorophyll and carotenoid secondary electron donors were shown, for the first time, to be capable of charge recombination with photoreduced Fe2+, oxidizing the iron in a temperature-dependent fashion below 300 K. Two redox populations of Fe3+ were revealed at low temperatures. One population was photoreduced at the lowest attainable temperatures, while the other was fully photoreduced only at temperatures above ca. 140 K. It is hypothesized that the redox activity of the non-heme iron depends upon the existence of a facile proton-transfer pathway linking the site to the stromal surface of the protein, and that the redox activity of the non-heme iron may probe redox-coupled proton-transfer reactions around the QB site.

Keywords

Photosystem II electron paramagnetic resonance EPR non-heme iron redox 

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Copyright information

© Springer Science + Business Media, B.V. 2008

Authors and Affiliations

  • James P. McEvoy
    • 1
  • Gary W. Brudvig
    • 2
  1. 1.Department of ChemistryRegis UniversityDenverUSA
  2. 2.Department of ChemistryYale UniversityNew HavenUSA

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