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

Abstract

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.