Hydrolysis of DNA model substrates catalyzed by metal-substituted Wells–Dawson polyoxometalates

Abstract

In this study we report the first example of phosphoester bond hydrolysis in 4-nitrophenyl phosphate (NPP) and bis-4-nitrophenyl phosphate (BNPP), two commonly used DNA model substrates, promoted by metal-substituted polyoxometalates (POMs). Different transition metal and lanthanide ions were incorporated into the Wells–Dawson polyoxometalate framework and subsequently screened for their hydrolytic activity towards the cleavage of the phosphoester bonds in NPP and BNPP. From these complexes, the Zr(IV)-substituted POM showed the highest reactivity. At pD 7.2 and 50 °C a NPP hydrolysis rate constant of 7.71 × 10⁻⁴ min⁻¹ (t_1/2 = 15 h) was calculated, representing a rate enhancement of nearly two orders of magnitude in comparison with the spontaneous hydrolysis of NPP. The catalytic (k_c = 1.73 × 10⁻³ min⁻¹) and formation constant (K_f = 520.02 M⁻¹) for the NPP–Zr(IV)–POM complex were determined from kinetic experiments. The reaction proceeded faster in acidic conditions and ³¹P NMR experiments showed that faster hydrolysis is proportional to the presence of the 1 : 1 monosubstituted Zr(IV)–POM at acidic pD values. The strong interaction of the 1 : 1 monosubstituted Zr(IV)–POM with the P–O bond of NPP was evidenced by the large chemical shift and the line broadening of the ³¹P nucleus in NPP observed upon addition of the metal complex. Significantly, a ten-fold excess of NPP was fully hydrolyzed in the presence of the Zr(IV)–POM, proving the principles of catalysis. The NMR spectra did not show sign of any paramagnetic species, excluding an oxidative cleavage mechanism and suggesting purely hydrolytic cleavage.