Zinc Complexes of Amino Acids and Peptides, 6. Zinc Complexes of Oligopeptides Containing Histidine at Both Termini

Abstract

AbstractOne tripeptide, five tetrapeptides, and one pentapeptide, all containing His–Xn–His sequences and being blocked at the N and C termini with acyl and amide functions, respectively, were synthesized by solid‐phase methods. With one exception their reaction with various zinc salts led to the precipitation of 1:1 (zinc/peptide) complexes. Analytically pure compounds were obtained from zinc tetrafluoroborate and His–Gly–His (1), from zinc chloride (resp. bromide) and His–Gly –Gly–His (2), His–Ala–Gly–His (3), His–Leu–Gly–His (4), His–Pro–Gly–His (5), and His–Pro–Asn–His (6) as well as from zinc sulfate and His–Leu–Gly–His (4) and His–Ala–Pro–Gly–His (7). 1H‐NMR data, when available, indicate the coordination of both histidine units to zinc in all cases. The low solubility of the complexes points to their polymeric nature. The only 1:2 (zinc/peptide) complex in this series was obtained from zinc perchlorate and His–Gly–Gly–His (2). An EXAFS study revealed that it contains zinc symmetrically coordinated by four histidine imidazole ligands. Based on the available information it is proposed that all complexes are one‐dimensional polymers containg [–Zn–His–Xn–His–Zn–]x backbones.