Model complexes suggest certain S‐state changes of the photosynthetic water‐oxidation enzyme may involve an Mn(II)–Mn(III) transition

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Abstract
The ultraviolet‐visible absorbance differences spectra of Mn(II,III) and Mn(III,III) oxo‐bridged carboxylate complexes are reported. The difference spectra are remarkably similar to those of the photosynthetic water‐oxidation enzyme complex reported by Dekker et al. [(1984) Biochim. Biophys. Acta 764, 301–309] which were interpreted as being due exclusively to Mn(II→IV) transitions. This result indicates that certain S‐state changes of the enzyme complex may instead involve Mn(II→III) transitions, and that difference spectra alone cannot be used with confidence to assign the Mn oxidation state changes during water oxidation.