Abstract
Oxygen evolution in plant photosynthesis takes place at the donor side of the oxygen-evolving photosystem II (PS II) of higher plants and algae [1]. The oxidation of H2O and the concomitant release of oxygen occurs in the Oxygen Evolving Complex (OEC). Near the OEC the electron donor D is localized. Recently D was identified as a tyrosine 160 on the D2 protein subunit of the PS II reaction center [2]. The oxidation of D to a stable tyrosine radical D gives rise to a well known EPR signal, the so-called Signal II (S II). Tyrosine forms two possible oxidation products: one is a protonated cation, characterized by a g-value of 2.0033 and hyperfine (hf) splittings that agree well with the spin density distribution by simple Huckel theory in ref [3] for free tyrosine. The other is a neutral, possibly hydrogen-bonded tyrosine radical with a g-value of 2.0049 and a spin density distribution given in refs [3,4].
Published Version
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