ZINC-COMPLEXES OF AMINO-ACIDS AND PEPTIDES .5. ZINC-COMPLEXES OF HISTIDINE-CONTAINING DIPEPTIDES AND TRIPEPTIDES

Nine dipeptides and one tripeptide containing histidine were converted into analytically pure zinc complexes. Four different compositions were observed. The peptides used as or converted into the monoacids LH (HisGly, GlyHis, HisPhe, HisHis) form the compounds ZnL(2) (11a, 12-14) and alternatively (HisGly, HisGlyGly) the compounds ZnL(BF4) (11b, 20). The peptides used as amides (HisGlyNH(2), HisMetNH(2)) act as neutral Ligands in the compounds ZnL(2)(ClO4)(2) (15, 16). The three remaining peptides (HisAsp, AlaHis, beta-AlaHis) behave Like diprotonic acids LH(2) forming the compounds ZnL (17-19). Spectra and solubilities indicate that complexes 11a, 13, 15, and 16 are mononuclear containing two chelating peptides bound by their amino and imidazole nitrogen atoms. All other complexes seem to be coordination polymers in some of which the amide N and O atoms are involved in the coordination. This was proven by a structure determination for Zn(GlyHis)(2) (12) in which the zinc ions are coordinated octahedrally by two histidine N, two amino N, and two amide O atoms of four peptide residues.