Metal complexes of imidazole ligands containing histamine-like donor sets: equilibrium, solution structure and hydrolytic activity‡

Abstract

The equilibrium and structural properties of copper(II) and zinc(II) complexes of N,N′-di-L-histidylethane-1,2-diamine (dhen) and those of the strongly related histamine have been characterized by pH-metric and spectroscopic (UV/VIS, CD, EPR and NMR) methods. In both dhen systems a dimeric M₂L₂ species is dominant near the physiological pH, having bis(histamine-like) 2N_im, 2NH₂ co-ordination. The MLH_–2 complex, also formed in both systems above pH 10, has different structures with the two metal ions. A hydroxo mixed-ligand complex is formed in the case of zinc(II), while the base-consuming processes are assigned to metal-promoted deprotonation of amide nitrogens in the copper(II) system. Between these two dominant species (pH 7–10) tetrameric complexes are formed in each case (as suggested by the CD, EPR and NMR results), with the participation of imidazole-N¹ (pyrrolic) nitrogen in the co-ordination. The catalytic activity of the zinc(II)-containing systems towards the hydrolysis of uridine 2′,3′-cyclic monophosphate as nuclease model has been examined. The zinc(II)–histamine complexes efficiently catalyse the hydrolysis. A kinetic study performed at different pH, concentrations and metal-to-ligand ratios, combined with the equilibrium data, revealed three reaction pathways involving Zn(OH), ZnL and ZnL(OH) complexes as active species, in order of activity ZnL  Zn(OH) < ZnL(OH).