Kinetic studies of phosphoester hydrolysis promoted by a dimeric tetrazirconium(iv) Wells–Dawson polyoxometalate

Abstract

The catalytic hydrolysis of a phosphoester bond in the DNA-model substrate 4-nitrophenyl phosphate (NPP) promoted by Zr(IV)-substituted Wells–Dawson Na₁₄[Zr₄(P₂W₁₆O₅₉)₂(μ₃-O)₂(OH)₂(H₂O)₄]·57H₂O polyoxometalate (ZrWD 4 : 2) was followed by means of ¹H and ³¹P NMR spectroscopy. The hydrolytic reaction proceeded with a rate constant of 8.44 (±0.36) × 10⁻⁵ s⁻¹ at pD 6.4 and 50 °C, representing a 300-fold rate enhancement in comparison with the spontaneous hydrolysis of NPP (k_obs = 2.81 (±0.25) × 10⁻⁷ s⁻¹) under the same reaction conditions. The ZrWD 4 : 2 was also active towards hydrolysis of bis(4-nitrophenyl) phosphate (BNPP) and the RNA model substrate 2-hydroxypropyl-4-nitrophenyl phosphate (HPNP). The pD dependence of k_obs shows that the rate constants for NPP hydrolysis decrease significantly when the pD values of the reaction mixtures increase. The formation constant (K_f = 190 M⁻¹) and catalytic rate constant (k_c = 6.40 × 10⁻⁴ s⁻¹) for the NPP-ZrWD 4 : 2 complex, activation energy (E_a) of 110.15 ± 7.06 kJ mol⁻¹, enthalpy of activation (ΔH^‡) of 109.03 ± 6.86 kJ mol⁻¹, entropy of activation (ΔS^‡) of 15.20 ± 2.49 J mol⁻¹ K⁻¹, and Gibbs activation energy (ΔG^‡) of 104.32 ± 6.09 kJ mol⁻¹ at 37 °C were calculated from kinetic studies. The recyclability of ZrWD 4 : 2 was examined by adding an extra amount (5.0 mM) of NPP twice to a fully hydrolyzed mixture of 5.0 mM NPP and 1.0 mM ZrWD 4 : 2. The interaction between ZrWD 4 : 2 and the P–O bond of NPP was evidenced by a change in the ³¹P chemical shift of the ³¹P atom in NPP upon addition of ZrWD 4 : 2. Based on ³¹P NMR experiments and the kinetic studies, a mechanism for NPP hydrolysis promoted by ZrWD 4 : 2 has been proposed.