Solution equilibria and structural characterisation of the transition metal complexes of glycyl-L-cysteine disulfide.

Stoichiometry, stability constants and structure of the complexes formed in the reaction of copper(II), nickel(II), zinc(II), cobalt(II) and cadmium(II) with (GlyCys)2 containing disulfide bond were determined by potentiometric, UV-Vis, NMR and EPR spectroscopic methods. The complex [Ni(GlyCys)2·H2O] was prepared in the solid state and its structure determined by single crystal X-ray diffraction method. Disulfide sulfur atoms of (GlyCys)2 were not metal-binding sites in any of the systems studied in solution or in the solid state. It was found that copper(II) is able to induce deprotonation and coordination of the peptide amide nitrogen donor atoms. This resulted in the formation of [CuL], [Cu2H-2L] and [CuH-1L]- as the major species in solution. In the case of nickel(II), cobalt(II), zinc(II) and cadmium(II) the complex [ML] predominates, and the outstanding stability of this species was explained by the formation of a macrochelate between the (NH2, CO)-coordinated five-membered chelates in solution. Crystal structure of [Ni(GlyCys)2·H2O] revealed the octahedral geometry of nickel(II) in a polymeric structure.