Cadmium(II) Complexes: Mimics of Organophosphate Pesticide Degrading Enzymes and Metallo-β-lactamases
作者:Lena J. Daumann、Lawrence. R. Gahan、Peter Comba、Gerhard Schenk
DOI:10.1021/ic300687y
日期:2012.7.16
Cadmium(II) complexes of ethyl 4-hydroxy-3,5-bis(((2-hydroxyethyl)(pyridin-2-ylmethyl)amino)methyl)-benzoate (CO(2)EtH(3)L1) and ethyl 4-hydroxy-3,5-bis(((2-methoxyethyl)(pyridin-2-ylmethyl)amino)methyl)benzoate (CO(2)EtHL2) are described. The two ligands possess an ethyl ester (CO2Et-) at the position para to the phenolic -OH; CO(2)EtHL2, with methyl ether donors in contrast to potentially nucleophilic alkoxide donors in CO(2)EtH(3)L1, offers a direct comparison of potential ligand-centered nucleophiles. The complex with CO(2)EtH(3)L1 was characterized using H-1 and C-13 NMR spectroscopy, mass spectrometry and microanalysis; X-ray crystallography defined a tetranuclear structure [Cd-4(CO(2)EtH(2)L1)(2)(CH3COO)(3.75)Cl-0.25(H2O)(2)](PF6)(2). Functional studies of the cadmium(II) complexes were undertaken with the substrates bis(2,4-dinitrophenyl)phosphate (BDNPP), and nitrocelin to assess their phosphatase and beta-lactamase activities, respectively. The complexes with CO(2)EtH(3)L1 and CO(2)EtHL2 are competent phosphoesterase mimics with K-M = 9.4 +/- 2.1 mM and 10.1 +/- 3.4 mM, k(cat) = 9.4 +/- 0.2 x 10(-3) s(-1) and 9.7 +/- 2.7 x 10(-3) s(-1), respectively. Use of a solvent mixture containing (H2O)-O-18/(H2O)-O-16 in the reaction with BDNPP showed that for the complex with CO(2)EtH(3)L1 the O-18 label was incorporated in the reaction product suggesting that the nucleophile involved is a Cd-OH moiety and not a metal bound alkoxide; for CO(2)EtHL2 the presence of the methyl-ether dictates that the active nucleophile must also be a hydroxide. The cadmium(II) complex with CO(2)EtH(3)L1 was furthermore found to be a competent beta-lactamase mimic with k(cat) = 1.39 x 10(-2) +/- 3 x 10(-3) s(-1), K-M = 0.11 +/- 0.03 mM, and pK(a) = 7.9 +/- 0.1. Mass spectral evidence suggested that the active nucleophile in this reaction is the alkoxide; lack of beta-lactamase activity of the complex with CO(2)EtHL2 supports this assignment. Similar to enzyme-catalyzed reactions, a blue reaction intermediate in the beta-lactamase reaction of the CO(2)EtH(3)L1 complex was also identified. It is proposed that the Cd(II) complexes of CO(2)EtH(3)L1 and CO(2)EtHL2 react identically as phosphatases, with a terminal hydroxide as the nucleophile; the former exhibits beta-lactamase activity with the alkoxide as a nucleophile, while the latter, without a potentially nucleophilic alkoxide, is inactive.