A ZINC(II) COMPLEX OF 1,5,9-TRIAZACYCLODODECANE ([12]ANEN3) AS A MODEL FOR CARBONIC-ANHYDRASE

Among macrocyclic tri- and tetraamines tested, a 12-membered triamine, [12]aneN3, is the most appropriate ligand that mimics the ligand field surrounding ZnII in carbonic anhydrases. In its 1:1 ZnII complex, the H2O bound at the fourth coordination site deprotonates with the pKa value of 7.30 at 25 °C, I= 0.1 (NaClO4), almost the same value being reported for the ZnII-enzymes. The resulting hydroxo complex is precipitated as a trimer from pH 8 aqueous solution, which with a formula of [ZnIIL(OH)]3(ClO4)3·HClO4 has been analyzed by X-ray crystal study. The crystals of (11)3·(ClO4)·HClO4, C27H67N9O19Cl4Zn3, are trigonal, space group R3c with six molecules of 11 in the unit cell of dimensions a = 22.103 (1) Å, c = 16.514 (2) Å. Anion binding affinity to the ZnIIL complex is determined by pH titration to have an order of OH- (log K = 6.4) ≫ CH3COO- (2.6) > SCN- (2.4) > I- (1.6) > Br- (1.5) > F- (0.8), which is almost comparable with the anion inhibition order and magnitude reported for carbonic anhydrase activities. Moreover, like the ZnII-enzymes, the [ZnIIL(OH)]+ species catalyzes methyl acetate hydrolysis and acetaldehyde hydration, where the ZnII-bound OH- commonly acts as a nucleophile to the carbonyl carbons. The plots of these rate constants vs pH in either case show the kinetic pKa values of ZnIIL(OH2) to be nearly the same as the thermodynamically obtained values of 7.3 at 25 °C and 7.9 at 0 °C. Various outstanding properties of ZnII in enzymes (over other metal ions such as CoIII), which contribute to its biological significance, have been well demonstrated by the present macrocyclic triamine complex behaviors. © 1990, American Chemical Society. All rights reserved.