THE CATALYTIC CARBOXYESTER HYDROLYSIS BY A NEW ZINC(II) COMPLEX WITH AN ALCOHOL-PENDANT CYCLEN (1-(2-HYDROXYETHYL)-1,4,7,10-TETRAAZACYCLODODECANE) - A NOVEL MODEL FOR INDIRECT ACTIVATION OF THE SERINE NUCLEOPHILE BY ZINC(II) IN ZINC ENZYMES

A new macrocyclic tetraamine (cyclen) having a strategically appended alcohol group, 1-(2-hydroxyethyl)1,4,7,10-tetraazacyclododecane (8), has been synthesized. The functionalized cyclen (8) forms a 1:1 ZnL complex (5) at pH ca. 6. The X-ray crystal study has disclosed a five-coordinate structure with the undeprotonated alcohol OH coordinating at an apical position. Crystals of 5.(ClO4)(2) (C10H24N4O9Cl2Zn) are monoclinic, space group P2(1/n) with a = 8.813 (2) Angstrom, b = 23.662 (2) Angstrom, c = 8.814 (2) Angstrom, beta = 90.21 (1)degrees, V = 1838.0 (5) Angstrom(3), Z = 4, R = 0.073, and R(w) = 0.125. The potentiometric pH titration of 5 showed dissociation of a proton with a pK(a) value of 7.60 +/- 0.02 at 25 degrees C and I = 0.10 (NaClO4). From the NMR and competitive anion-binding studies, the structure of the deprotonated species was assigned to be a OH--bound ZnL-OH- complex (11). During unsuccessful attempts to isolate the deprotonated species, we have obtained a trimeric phosphate complex, (ZnL-O-)(3)P=O, as its PF6- salts (15.(PF6)(3)(H2O)(1.5)) from pH 9.5 aqueous solution containing 5.(ClO4)(2), K2HPO4, and NH4PF6. Crystals of 15.(PF6)(3)(H2O)(1.5) (determined as the trimer of C10H25N4O17/6P4/3F6Zn) are trigonal, space group R ($) over bar 3 with a = 23.353 (3) Angstrom, c = 17.527 (5) Angstrom, V = 8278 (2) Angstrom(3), Z = 18, R = 0.070, and R(w) = 0.112. Among the known Zn-II complexes, the ZnL-OH- complex 11 is the most active catalyst for 4-nitrophenyl acetate (NA) hydrolysis. In the kinetic studies using 5 in 10% (v/v) CH3CN at 25 degrees C, I = 0.10 (NaNO3), and pH 6.4-9.5, the pH-rate profile gave a sigmoidal curve with an inflection point at pH 7.7, which corresponds to the pK(a) value for ZnL (5) reversible arrow ZnL-OH- (11) + H+. The second-order (first-order each in [11] and [NA]) rate constant of 0.46 +/- 0.01 M(-1) s(-1) is approximately ten times greater than the corresponding value of (4.7 +/- 0.1) x 10(-2) M(-1) s(-1) for N-methylcyclen-Zn-II-OH- complex 12b catalyst. Furthermore, we found the NA hydrolysis to occur through a double-displacement reaction of the acetyl group. In the first rate-determining reaction, the nucleophile is the pendant alcoholic OH ''activated'' by the adjacent Zn-II-OH-, which attacks NA to yield an ''acyl intermediate'' (16). This intermediate was independently synthesized by the reaction of 5 with acetic anhydride in CH3CN. In the second reaction, 16 is subject to extremely fast hydrolysis (e.g., t(1/2) = 6 s at pH 9.3), as monitored on spectral change of pH-indicators. A plot of the observed first-order rate constants against pH (=6.1-9.3) for the second process gave a sigmoidal curve with its inflection point at pH 7.7, which is a similar value for the pK(a)'s of the Zn-II-cyclen complexes 4a and 12a. It is concluded that the very fast second reaction occurs through the nucleophilic attack of the Zn-II-OH- (16a) at the intramolecular acetyl group. The overall NA hydrolysis by 11 is catalytic. Thus, the OH--bound ZnL plays dual roles: as a general base in the first acyl-transfer reaction to activate the remote alcoholic OH and as a nucleophile to attack the electrophilic center in the second hydrolysis step. Such a dyad of Zn-II-OH- and the adjacent alcoholic OH may account for the strong nucleophilicity of serine at the active center of zinc enzymes such as alkaline phosphatase.