Macrocyclic Polyamines as a Probe for Equilibrium Study of the Acid Functions of Zinc(II) Ion in Hydrolysis Enzymes

Abstract

Abstract Macrocyclic polyamine ligands have been used in studying intrinsic acid properties of Zn(II) ion to help elucidate the role of Zn(II) in Zn-enzymes such as carbonic anhydrase (CA), carboxypeptidase, etc. Among macrocyclic tri- and tetraamines, [12]aneN3 is the most appropriate ligand that mimics the ligand field surrounding the Zn(II) in CA. In its 1:1 [ZnIIL]2+ complex, the H2O bound at the fourth coordination site deprotonates with a pK a value of 7.30 at 25°C and I = 0.1, almost the same value reported for CA. Anion binding affinity to the [ZnIIL]2+ is determined by pH-metric titration and inhibition kinetics of 4-nitrophenyl acetate hydrolysis. The order and magnitude, OH– ≫ HCO– 3 > CH3COO– > I– > Br– > Cl– > F– are almost comparable with the anion inhibition for CA. The pH-metric determination of the interaction of Zn(II) and Cd(II) with dissociable (acidic) hydrogen-containing macrocyclic polyamines has served to distinguish acid and coordination properties of these two metal ions. Thanks ...