Effective cleavage of phosphodiester promoted by the zinc(II) and copper(II) inclusion complexes of β-cyclodextrin.

To construct the model of metallohydrolase, two inclusion complexes [MLCl(β-CD)] (1, M=Zn(II); 2, M=Cu(II); L=N,N'-bis(2-pyridylmethyl)amantadine; β-CD=β-cyclodextrin) were synthesized by mixing β-CDs with the pre-synthesized complexes G1, [ZnLCl] and G2, [CuLCl]. Structures of G1, G2, 1 and 2 were characterized by X-ray crystallography, respectively. In solution, two chloride anions of G1 and G2 underwent ligand exchange with solvent molecules according to ESI-MS analysis. The chemical equilibrium constants were determined by potentiometric pH titration. The kinetics of bis(4-nitrophenyl) phosphate (BNPP) hydrolysis catalyzed by G1, G2, 1 and 2 were examined at pHs ranging from 7.50 to 10.50 at 308±0.1K. The pH profile of rate constant of BNPP hydrolysis catalyzed by 1 exhibited an exponential increase with the second-order rate constant of 2.68×10Ms assigned to the di-hydroxo species, which was approximately an order of magnitude higher than those of reported mono-Zn(II)-hydroxo species. The high reactivity was presumably hydroxyl-rich microenvironment provided by β-CDs, which might effect in stabilizing either the labile zinc-hydroxo species or the catalytic transition state.