Zinc complexation of cyclic dipeptides containing cysteine and/or histidine

The cyclic dipeptides c-HisHis, c-GlyCys, c-HisCys and c-CysCys were prepar ed in an analytically pure form. Potentiometric titrations in the presence of zinc nitrate showed that they all form a species of composition ZnL in s olution, which in case of c-CysCys is dimeric. Except for c-CysCys they als o form the complex ZnL2, which in case of c-GlyCys is polymeric. Contrary t o the expectation the preorganization of the peptide donors does not enhanc e the complex stabilities in comparison to those of the non-cyclic peptides . Preparative reactions have yielded the compounds Zn(c-HisHiS)Cl-2, Zn(c-G lyCys)(2), Zn(c-GlyCys)(2). neocuproin, Zn-2(c-HisCys)(3)I, and Zn(c-CysCys )(N-methylimidazole)(2). (C) 2001 Elsevier Science B.V. All rights reserved .