Zinc complexation of cyclic dipeptides containing cysteine and/or histidine

The cyclic dipeptides c-HisHis, c-GlyCys, c-HisCys and c-CysCys were prepared in an analytically pure form. Potentiometric titrations in the presence of zinc nitrate showed that they all form a species of composition ZnL in solution, which in case of c-CysCys is dimeric. Except for c-CysCys they also form the complex ZnL2, which in case of c-GlyCys is polymeric. Contrary to the expectation the preorganization of the peptide donors does not enhance the complex stabilities in comparison to those of the non-cyclic peptides. Preparative reactions have yielded the compounds Zn(c-HisHis)Cl2, Zn(c-GlyCys)2, Zn(c-GlyCys)2·neocuproin, Zn2(c-HisCys)3I, and Zn(c-CysCys)(N-methylimidazole)2.