Structure, equilibrium and ribonuclease activity of copper(II) and zinc(II) complexes formed with a dinucleating bis-imidazole ligand

The syntheses. crystal structures, solution equilibria and ribonuclease act ivity are reported for copper(II) and zinc(II) complexes of a new potential ly dinucleating, bis-imidazole ligand N,N'-bis(5-methylimidazol-4-ylmethyl) -1,3-diamino-propan-2-ol (bimido). The zinc(II) ion in [Zn(bimido)Cl]NO3 is coordinated in a slightly distorted square pyramidal environment, with the four N atoms of bimido in the basal positions and the Cl- ion in the axial one. The two copper(II) ions in [Cu-2(bimido(-1H))(DPP)(ClO4)(CH3OH)]ClO4. 1/2 H2O (DPP = diphenyl phosphate) are bridged by the deprotonated alkoxo group of bimido and by the phosphate group of DPP in a 1,3-bridging mode. D epending on the [M]/[L] ratio, the ML and M2L-2H species are present in sol ution in the neutral pH range, having analogous structures as described abo ve for the crystalline complexes. The zinc(II)-bimido (2/1) system, in a 65 % EtOH-H2O mixed solvent, shows an important increase of hydrolytic activit y, parallel with the formation of the Zn2L-2H species, with a sigmoidal pH- rate profile model Ling both steps of RNA hydrolysis. The k(cat) value for the transesterification of 2-hydroxypropyl-p-nitrophenyl phosphate by Zn2L- 2H,determined from saturation kinetic measurements (T = 298 K), corresponds to a maximum rate acceleration (k(cat)/k(uncat)) of ca. 10(4). The obsenre d pseudo-first order rate constant for the hydrolysis of uridine 2',3'-cycl ic monophosphate, under nearly physiological conditions (T = 310 K, pH = 8, [Zn2L-2H] = 3.6 mM, k(obs) = 2.10(-5) s(-1)) reflects a higher hydrolytic activity of Zn2L-2H towards this biologically relevant substrate (k(obs)/k( uncat) indicates ca. 10(4) fold rate acceleration). The proposed mechanisms include bifunctional Lewis-acid and general base catalysis.