MODELS FOR METAL-SULFUR COORDINATION IN COPPER PROTEINS

The oxidation state of Cu in several model compounds containing Cu coordinated to S, some of which are suggested model compounds for naturally occurring Cu-S chromophores, was determined employing X-ray photoelectron spectrometry. Cu was present in a 3d10 state, which is characteristic for Cu(I). An initial photoreduction is excluded, as Cu(ethylenediamine)2 (SCN)2, in which a Cu-S bond is also present, contained Cu(II) (3d9). As was the case with native parsley plastocyanin, the model compounds reacted effectively with superoxide anion radicals. The electrons of the metal-sulfur chromophore in blue Cu proteins are probably delocalized and an equilibrium: RS-Cu(II) .dblarw. RS .cntdot. Cu(I) exists.