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• W2078561526 abstract "The effect of Zn2+ or Cu2+ ions on Mn-depleted photosystem II (PS II) has been investigated using EPR spectroscopy. In Zn2+-treated and Cu2+-treated PS II, chemical reduction with sodium dithionite gives rise to a signal attributed to the plastosemiquinone, QA•-, the usual interaction with the non-heme iron being lost. The signal was identified by Q-band EPR spectroscopy which partially resolves the typical g-anisotropy of the semiquinone anion radical. Illumination at 200 K of the unreduced samples gives rise to a single organic free radical in Cu2+-treated PS II, and this is assigned to a monomeric chlorophyll cation radical, Chl a•+, based on its 1H-ENDOR spectrum. The Zn2+-treated PS II under the same conditions gives rise to two radical signals present in equal amounts and attributed to the Chl a•+ and the QA•- formed by light-induced charge separation. When the Cu2+-treated PS II is reduced by sodium ascorbate, at ≥77 K electron donation eliminates the donor-side radical leaving the QA•- EPR signal. The data are explained as follows: (1) Cu2+ and Zn2+ have similar effects on PS II (although higher concentrations of Zn2+ are required) causing the displacement of the non-heme Fe2+. (2) In both cases chlorophyll is the electron donor at 200 K. It is proposed that the lack of a light-induced QA•- signal in the unreduced Cu2+-treated sample is due to Cu2+ acting as an electron acceptor from QA•- at low temperature, forming the Cu+ state and leaving the electron donor radical Chl a•+ detectable by EPR. (3) The Cu2+ in PS II is chemically reducible by ascorbate prior to illumination, and the metal can therefore no longer act as an electron acceptor; thus QA•- is generated by illumination in such samples. (4) With dithionite, both the Cu2+ and the quinone are reduced resulting in the presence of QA•- in the dark. The suggested high redox potential of Cu2+ when in the Fe2+ site in PS II is in contrast to the situation in the bacterial reaction center where it has been shown in earlier work that the Cu2+ is unreduced by dithionite. It cannot be ruled out however that QA−Cu2+ is formed and a magnetic interaction is responsible for the lack of the QA- signal when no exogenous reductant is present. With this alternative possibility, the effects of reductants would be explained as the loss of Cu2+ (due to formation of Cu+) leading to loss of the Cu2+ from the Fe2+ site due to the binding equilibrium. The quite different binding and redox behavior of the metal in the iron site in PS II compared to that of the bacterial reaction center is presumably a further reflection of the differences in the coordination of the iron in the two systems." @default.
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• W2078561526 date "1999-09-01" @default.
• W2078561526 modified "2023-10-15" @default.
• W2078561526 title "Effects of Copper and Zinc Ions on Photosystem II Studied by EPR Spectroscopy" @default.
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• W2078561526 doi "https://doi.org/10.1021/bi990236j" @default.
• W2078561526 hasPubMedId "https://pubmed.ncbi.nlm.nih.gov/10493813" @default.
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