CARBOXYESTER HYDROLYSIS PROMOTED BY A NEW ZINC(II) MACROCYCLIC TRIAMINE COMPLEX WITH AN ALKOXIDE PENDANT - A MODEL STUDY FOR THE SERINE ALKOXIDE NUCLEOPHILE IN ZINC ENZYMES

New alcohol-pendant 1,5,9-triazacyclododecane ([12]aneN(3)) ligands, L (2), L(3), and L(4), have been synthesized and characterized. A comple xation study on the Zn-II complexes of these macrocyclic polyamines ha s revealed that the pendant alcohol of 9 (Zn(II)L(2)) deprotonates wit h an extremely low pK(a) value of 7.4 at 25 OC to become an alkoxide a nion donor at the fourth coordination site. This is a novel chemical i llustration that the serine residue located at the center of zinc enzy mes can be deprotonated at physiological pH. The alkoxide-coordinating complex 10 was crystallized as the dimeric complex 17 from an aqueous solution of L(2) and Zn(ClO4)1 at pH 9. The X-ray study of 17 shows e ach Zn-II ion to be 5-coordinate with a short intramolecular Zn-II-O-( alkoxide) bond (1.950(6) Angstrom) and a relatively longer intermolecu lar Zn-II-O-(alkoxide) bond (2.079(5) Angstrom). Crystals of 17.(ClO4) (2) (C11H24N3O5ClZn determined as the monomer) are monoclinic, space g roup P2(1)/n with a = 8.655(1) Angstrom, b = 19.874(1) Angstrom, c = 9 .351(2) Angstrom, beta = 95.90(1)degrees, V = 1600(4) Angstrom(3), and Z = 4. A full-matrix least-squares refinement yielded R = 0.071 and R (w) = 0.099 for 1765 independent reflections. In CH3CN solution, the m ain species is the dimer 17, whereas in aqueous solution, it is the mo nomer 10, as found by NMR and potentiometric pH titration studies. The Zn-II-bound alkoxide of 10 is shown to be a very reactive nucleophile and catalyzes 4-nitrophenyl acetate (NA) hydrolysis. A kinetic study of NA hydrolysis by 10 in 10% (v/v) CH3CN at 25 degrees C, 1 = 0.10 (N aNO3), and pH 9.3 (20 mM CHES buffer), has established a second-order rate constant of 1.4 x 10(-1) M(-1) s(-1), which is almost 4 times gre ater than the corresponding value of 3.6 x 10(-2) M(-1) s(-1) for the Zn-II [12]aneN(3) complex 7. Thus, our present model study shows for t he first time that the Zn-II-bound alkoxide is a better nucleophile th an the Zn-II-bound hydroxide. Moreover, in the course of NA hydrolysis by 10, we have observed the occurrence of a transient acetyl group tr ansfer from the substrate NA to the pendant alkoxide to yield the O-ac etylated species 23,which was very rapidly hydrolyzed at alkaline pH. The intermediate 23 was independently isolated from the reaction in CH 3CN solution and fully identified as the Zn-II-free ligand 24. The mec hanism of catalysis by 10 is compared to the one already proposed for serine-containing enzymes (e.g., hydrolytic serine enzymes and alkalin e phosphatase).