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

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 degrees C a NPP hydrolysis rate constant of 7.71 x 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 x 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 P-31 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 P-31 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.