Influence of the coordination environment of zinc(II) complexes of designed mannich ligands on phosphatase activity: A combined experimental and theoretical study

A mononucleating (HL1) and a dinucleating (HL2) \u201cendoff\u201d compartmental ligand have been designed and synthesized by controlled Mannich reaction using p-cresol and bis(2-methoxyethyl)amine, and their formation has been rationalized. Six complexes have been prepared on treating HL1 and HL2 with ZnIIX2 (X = Cl 12, Br 12, I 12) with the aim to investigate their hydrolytic activity on phosphoester bond cleavage. Interestingly, the mononucleating ligand was observed to yield dinuclear complexes, [Zn2(L1)2X2] (1 123), while the potential dinucleating ligand generated mononuclear complexes, [Zn(HL2)X2] (4 126). Four (1 124), out of six complexes studied, were characterized by single-crystal X-ray diffraction (XRD): the Zn ion exhibits trigonal bipyramidal and tetrahedral coordination spheres in the di- and mononuclear complex, respectively. The hydrolytic kinetics, followed spectrophotometrically with 4-nitrophenylphosphate (4-NPP) in buffered dimethylformamide (DMF) (97.5% DMF, v/v) because of solubility reasons, under excess substrate conditions (substrate:complex = 20:1), indicated that the complexes enormously accelerate the rate of phosphomonoester hydrolysis with first order rate constants (kcat) in the range 2 1210 s 121 at 25 \ub0C. In each case kinetic data analyses have been run by Michaelis 12Menten treatment. The efficacy in the order of conversion of substrate to product (pnitrophenolate ion) follows the trend 1 > 2 > 3 > 4 > 5 > 6, and the ratio of kcat of an analogous dinuclear to mononuclear complex is 432. An electrospray ionization-mass spectrometry (ESI-MS) study has revealed the dissociation of the centrosymmetric dinuclear complex to two mononuclear species instead of a syn-cooperative catalysis. Density functional theory (DFT) calculations have been performed to rationalize our proposed mechanistic pathway for phosphatase activity. The comparative analysis concludes the following facts under experimental conditions: (1) the halide bound to the active site affects the overall rate in the order: Cl 12 > Br 12 > I 12 regardless of nuclearity; (2) dinuclear complexes prevail over the mononuclear ones