An Asymmetric Dizinc Phosphodiesterase Model with Phenolate and Carboxylate Bridges

A phosphodiesterase model with two zinc centers has been synthesized and characterized. The compound,
[Zn2(L-2H)(AcO)(H2O)](PF6)·2H2O (Zn2L‘), was formed using an “end-off” type compartmental ligand, 2,6-bis{[(2-pyridylmethyl)(2-hydroxyethyl)amino]methyl}-4-methylphenol (L), and zinc acetate dihydrate. The X-ray crystallographic
analysis shows that Zn2L‘ contains a μ-acetato-μ-cresolato-dizinc(II) core comprised of a quasi-trigonal bipyramidal
Zn and a distorted octahedral Zn, and the distance between them is 3.421 Å which is close to the dizinc distance
in related natural metalloenzymes. Phosphodiesterase activity of Zn2L‘ was investigated using bis(4-nitrophenyl)
phosphate (BNPP) as the substrate. The pH dependence of the BNPP cleavage in aqueous buffer media shows
a sigmoid-shaped pH−kobs profile with an inflection point around pH 7.13 which is close to the first pKa value of
7.20 for Zn2L‘ obtained from the potentiometric titration. The catalytic rate constant (kcat) is 4.60 × 10-6 s-1 at pH
7.20 and 50 °C which is ca. 105-fold higher than that of the uncatalyzed reaction. The deprotonated alcoholic
group appended on Zn2L‘ is responsible for the cleavage reaction. The possible mechanism for the BNPP cleavage
promoted by Zn2L‘ is proposed on the basis of kinetic and spectral analysis. The dizinc complex formed in situ in
anhydrous DMSO exhibits a similar ability to cleave BNPP. This study provides a less common example for the
phosphodiesterase model in which the metal-bound alkoxide is the nucleophile.