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

Abstract

Abstract One tripeptide, five tetrapeptides, and one pentapeptide, all containing His–X n –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 ). 1 H‐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–X n –His–Zn–] x backbones.