Hydrolytic cleavage of DNA-model substrates promoted by polyoxovanadates

Hydrolysis of 4-nitrophenyl phosphate (NPP) and bis-4-nitrophenyl phosphate (BNPP), two commonly used DNA model substrates, was examined in vanadate solutions by means of ¹H, ³¹P and ⁵¹V NMR spectroscopy. The hydrolysis of the phosphoester bond in NPP at 50 °C and pH 5.0 proceeds with a rate constant of 1.74 × 10⁻⁵ s⁻¹. The cleavage of the phosphoester bond in BNPP at 70 °C and pH 5.0 proceeds with a rate constant of 3.32 × 10⁻⁶ s⁻¹, representing an acceleration of four orders of magnitude compared to the uncatalyzed cleavage. Inorganic phosphate and nitrophenol (NP) were the only products of hydrolysis. The NMR spectra did not show evidence of any paramagnetic species, excluding the possibility of V(V) reduction to V(IV), indicating that the cleavage of the phosphoester bond is purely hydrolytic. The pH dependence of k_obs revealed that the hydrolysis proceeds fastest in solutions of pH 5.5. Comparison of the rate profile with the concentration profile of polyoxovanadates shows a striking overlap of the k_obs profile with the concentration of decavanadate (V₁₀). Kinetic experiments at 37 °C using a fixed amount of NPP and increasing amounts of V₁₀ permitted the calculation of catalytic (k_c = 5.67 × 10⁻⁶ s⁻¹) and formation constants for the NPP-V₁₀ complex (K_f = 71.53 M⁻¹). Variable temperature ³¹P NMR spectra of a reaction mixture revealed broadening and shifting of the ³¹P resonance upon addition of increasing amounts of decavanadate and upon increasing temperature, implying the dynamic exchange process between free and bound NPP at higher temperatures. The origin of the hydrolytic activity of V₁₀ is most likely due its high lability and its dissociation into smaller fragments which may allow the attachment of NPP and BNPP into the polyoxovanadate framework.