Multinuclear NMR and potentiometric study on tautomerism during protonation and zinc(II) complex formation of some imidazole-containing peptide derivatives

The acid–base properties and zinc(II) complexes of glycylhistamine, sarcosylhistamine, carcinine and carnosine have been studied by potentiometric, 13C and 14N NMR methods. Macroscopic species for the three states of protonation (LH22+, LH+, L) and the corresponding microspecies involving three protonation sites (terminal amino, N-1 and -3 imidazole nitrogens) are quantitatively estimated for the metal-free ligands. Zinc(II) complexation is shown to reverse the tautomeric preference between 1- and 3-H tautomeric forms of the imidazole ring (in LH+ and L), as compared to the free ligands where the 1-H tautomer is predominant.