Cobalt-(cysteinyl)4 tetrahedra in yeast cobalt(II)-thionein.

The conversion of yeast Cu(I)-thionein into the Co(II) derivative was successful. 2.6 Co atoms were incorporated per mole of protein yielding a Co : S ratio of 1 : 3. The electronic absorption of this highly air sensitive Co(II)-thionein is virtually identical to those of the Co(II) derivatives of other metallothioneins originating from vertebrates and N. crassa. Weaker Cotton extrema are noticed and the two doublet splittings of Cu-thionein disappeared. Throughout the molar ellipticities of the cobalt protein were markedly lower compared to those of the Cu-thionein. Owing to the characteristic charge transfer bands and d-d transitions a tetrahedral Co-thiolate coordination was deduced. The best fit proposal maintaining the above Co : S ratio of 1 : 3 was a six-membered ring with three bridging cysteine sulphurs.