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, C-13 and N-14 NMR methods. M acroscopic 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.