Probing the Catalytically Active Species in POM-Catalysed DNA-Model Hydrolysis**

Phosphoester hydrolysis is an important chemical step in DNA repair. One archetypal molecular model of phosphoesters is para-nitrophenylphosphate (pNPP). It has been shown previously that the presence of molecular metal oxide Mo₇O₂₄]^6? may catalyse the hydrolysis of pNPP through the partial decomposition of polyoxomolybdate framework resulting in a (PO₄)₂Mo₅O₁₅]^6? product. Real-time monitoring of the catalytic system using electrospray ionisation mass spectrometry (ESI-MS) provided a glance into the species present in the reaction mixture and identification of potential catalytic candidates. Following up on the obtained spectrometric data, Density Functional Theory (DFT) calculations were carried out to characterise the hypothetical intermediate Mo₅O₁₅(pNPP)₂(H₂O)₆]^6? that would be required to form under the hypothesised transformation. Surprisingly, our results point to the dimeric Mo₂O₈]^4? anion resulting from the decomposition of Mo₇O₂₄]^6? as the active catalytic species involved in the hydrolysis of pNPP rather than the originally assumed {Mo₅O₁₅} species. A similar study was carried out involving the same species but substituting Mo by W. The mechanism involving W species showed a higher barrier and less stable products in agreement with the non-catalytic effect found in experimental results.