Probing the Phosphoesterase Activity of [Mo7O24]6- Cluster

The half-life of phosphoester bonds, linking all nucleotide monomers in the DNA chain, is estimated at 1.3E5 by neutral pH and 298K, meaning that almost one billion years are required for complete hydrolysis of the DNA backbone. This kinetic stability is an excellent advantage important for the preservation of the genome. However, it certainly is a disadvantage when it comes to the repair of damaged DNA or to the destruction of foreign DNA.
Polyoxometalates (POMs) are extensively investigated as an important family of metal-oxygen clusters. Since such metal complexes accelerate the hydrolysis of phosphoester bonds up to four orders of magnitude, they are widely used in catalysis, medicine and material science. The mechanism of this hydrolysis is generally described as an interplay of several factors. Among other criteria, the complex has to have an overall positive charge. However, the cleavage of phosphoesters by highly negative POM clusters was recently likewise reported.
In this study the hydrolysis of a DNA model system, p-nitrophenyl phosphate (NPP), promoted by the polyoxomolybdate cluster [Mo7O24]6- was investigated. During the reaction, there is absolutely no free NPP in solution and two transient complexes are generated, giving finally rise to free phosphate and p-nitrophenol. 1D 1H and 31P NMR allowed to characterize one of the complexes as [(NPP)2Mo5O21]4- but did enable us to assign the second complex to either (NPP)Mo7O25(H2O) or [(NPP)Mo6O18(H2O)3]4-.
Diffusion Ordered NMR Spectroscopy (DOSY NMR) made it possible to identify effectively these catalytically active transient clusters through their diffusive behaviour. Available size and shape information of the complexes allowed to estimate the diffusion coefficient of the species in solution. Comparing estimated and experimental diffusion coefficients confirmed the characterization of the first complex as [(NPP)2Mo5O21]4- and to identify the second complex as [(NPP)2Mo12O36(H2O)6]4-, a dimer of (NPP)Mo6O18(H2O)3.
In conclusion, the combination of kinetic and diffusion based NMR results made it possible to propose a reaction mechanism for the hydrolysis of NPP catalyzed by a heptamolybdate.