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) 2N1m, 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).
|Number of pages||8|
|Journal||Journal of the Chemical Society - Dalton Transactions|
|Publication status||Published - Apr 7 1998|
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