![Sample our Physical Sciences journals, sign in here to start your FREE access for 14 days]({"type":"image" , "src" : "/sda/110819/TOC-Subject-Sample-2021-Physical-Sciences.jpg"})
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 to macrocyclic stabilities the metal-promoted amide proton dissociations were observed for the first time, particular with the more acidic Zn(II). To yield the same complexes with less acidic Cd(II), higher pH was required. Thus, monooxocyclam is proven to be the first Zn(II)-selective chelating agent. Taking all of these equilibrium data (from our model complexes) into consideration, we find that the pK a value for Zn-OH2 ⇄ Zn-OH, 1:1 anion affinity constants, blood pH, and the pK a values for H2CO3 ⇄ HCO– 3 ⇄ CO2 – 3 are all ideally consistent for CA activities.