New alcohol-pendant 1,5,9-triazacyclododecane ([12]aneN3) ligands, L2, L3, and L*, have been synthesized and characterized. A complexation study on the Zn11 complexes of these macrocyclic polyamines has revealed that the pendant alcohol of 9 (ZnnL2) deprotonates with an extremely low pXa value of 7.4 at 25 °C to become an alkoxide anion donor at the fourth coordination site. This is a novel chemical illustration that the serine residue located at the center of zinc enzymes can be deprotonated at physiological pH. The alkoxide-coordinating complex 10 was crystallized as the dimeric complex 17 from an aqueous solution of L2 and ( 04)2 at pH 9. The X-ray study of 17 shows each Zn" ion to be 5-coordinate with a short intramolecular Znn-O(alkoxide) bond (1.950(6) Á) and a relatively longer intermolecular Znn-O(alkoxide) bond (2.079(5) Á). Crystals of 17-(C104)2 (CnH24N305ClZn determined as the monomer) are monoclinic, space group P2\/n with a = 8.655(1) A,b = 19.874(1) A,c = 9.351(2) Á, ß = 95.90(1)°, V = 1600(4) Á3, and Z = 4. A full-matrix least-squares refinement yielded R = 0.071 and Rw = 0.099 for 1765 independent reflections. In CH3CN solution, the main species is the dimer 17, whereas in aqueous solution, it is the monomer 10, as found by NMR and potentiometric pH titration studies. The Znn-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 °C, I = 0.10 (NaN03), and pH 9.3 (20 mM CHES buffer), has established a second-order rate constant of 1.4 X 10~' M_1 s_l, which is almost 4 times greater than the corresponding value of 3.6 X l(h2 M™1 s™1 for the Zn11 [12]aneN3 complex 7. Thus, our present model study shows for the first time that the Znn-bound alkoxide is a better nucleophile than the Znn-bound hydroxide. Moreover, in the course of NA hydrolysis by 10, we have observed the occurrence of a transient acetyl group transfer from the substrate NA to the pendant alkoxide to yield the O-acetylated species 23, which was very rapidly hydrolyzed at alkaline pH. The intermediate 23 was independently isolated from the reaction in CH3CN solution and fully identified as the Znn-free ligand 24. The mechanism of catalysis by 10 is compared to the one already proposed for serine-containing enzymes (e.g., hydrolytic serine enzymes and alkaline phosphatase).
