PROTON AND METAL-ION INTERACTIONS WITH GLYCYLGLYCYLHISTAMINE, A SERUM-ALBUMIN MIMICKING PSEUDOPEPTIDE

The macro- and microprotonations of glycylglycylhistamine (GGHA) have been determined by combined potentiometric and H-1-NMR methods. The co mplexation of GGHA with Co(II), Ni(II), and Cu(II) has been studied by potentiometric, EPR, and H-1-NMR methods. In the pH range 3-11.2, mor e or less deprotonated monomeric complexes (MLH, ML, MLH(-1), MLH(-2), MLH(-3)) formed in all systems. In the case of Ni(II) and Cu(II) at p hysiological pH, the MLH(-2) species is predominant with four nitrogen coordination sites (one amino, two peptide, and one imidazole-N-3 nit rogens) in square planar arrangement. In Co(II) containing systems how ever, CoL is the predominant complex near pH 7 with a macrochelate coo rdination of terminal amino and imidazole nitrogens, while CoLH(-2) sp ecies forms at much higher pH. In accordance with NMR measurements, th e formation of MLH(-3) species can be assigned to the further deproton ation of the N-1-pyrrolic nitrogen in the imidazole ring without metal coordination. The formation constants determined were compared with t hose of the analogous histidine derivatives. Single-crystal X-ray anal ysis of CuLH(-2). 3H(2)O verified the expected four nitrogen coordinat ion in the equatorial plane of Cu(II).