Detailed Mechanism of Phosphoanhydride Bond Hydrolysis Promoted by a Binuclear Zr^IV-Substituted Keggin Polyoxometalate Elucidated by a Combination of ³¹P, ³¹P DOSY, and ³¹P EXSY NMR Spectroscopy

A detailed reaction mechanism is proposed for the hydrolysis of the phosphoanhydride bonds in adenosine triphosphate (ATP) in the presence of the binuclear Zr^IV-substituted Keggin type polyoxometalate (Et₂NH₂)₈[{α-PW₁₁O₃₉Zr­(μ-OH)­(H₂O)}₂]·7H₂O (ZrK 2:2). The full reaction mechanism of ATP hydrolysis in the presence of ZrK 2:2 at pD 6.4 was elucidated by a combination of ³¹P, ³¹P DOSY, and ³¹P EXSY NMR spectroscopy, demonstrating the potential of these techniques for the analysis of complex reaction mixtures involving polyoxometalates (POMs). Two possible parallel reaction pathways were proposed on the basis of the observed reaction intermediates and final products. The 1D ³¹P and ³¹P DOSY spectra of a mixture of 20.0 mM ATP and 3.0 mM ZrK 2:2 at pD 6.4, measured immediately after sample preparation, evidenced the formation of two types of complexes, I1A and I1B, representing different binding modes between ATP and the Zr^IV-substituted Keggin type polyoxometalate (ZrK). Analysis of the NMR data shows that at pD 6.4 and 50 °C ATP hydrolysis in the presence of ZrK proceeds in a stepwise fashion. During the course of the hydrolytic reaction various products, including adenosine diphosphate (ADP), adenosine monophosphate (AMP), pyrophosphate (PP), and phosphate (P), were detected. In addition, intermediate species representing the complexes ADP/ZrK (I2) and PP/ZrK (I5) were identified and the potential formation of two other intermediates, AMP/ZrK (I3) and P/ZrK (I4), was demonstrated. ³¹P EXSY NMR spectra evidenced slow exchange between ATP and I1A, ADP and I2, and PP and I5, thus confirming the proposed reaction pathways

Phosphate Ester Bond Hydrolysis Promoted by Lanthanide-Substituted Keggin-type Polyoxometalates Studied by a Combined Experimental and Density Functional Theory Approach

Hydrolytic cleavage of 4-nitrophenyl phosphate (NPP), a commonly used DNA model substrate, was examined in the presence of series of lanthanide-substituted Keggin-type polyoxometalates (POMs) [Me₂NH₂]₁₁­[Ce^III(PW₁₁O₃₉)₂], [Me₂NH₂]₁₀­[Ce^IV(PW₁₁O₃₉)₂] (abbreviated as (Ce^IV(PW₁₁)₂), and K₄[EuPW₁₁O₃₉] by means of NMR and luminescence spectroscopies and density functional theory (DFT) calculations. Among the examined complexes, the Ce­(IV)-substituted Keggin POM (Ce^IV(PW₁₁)₂) showed the highest reactivity, and its aqueous speciation was fully determined under different conditions of pD, temperature, concentration, and ionic strength by means of ³¹P and ³¹P diffusion-ordered NMR spectroscopy. The cleavage of the phosphoester bond of NPP in the presence of (Ce^IV(PW₁₁)₂) proceeded with an observed rate constant <i>k</i>_obs = (5.31 ± 0.06) × 10^–6 s^–1 at pD 6.4 and 50 °C. The pD dependence of NPP hydrolysis exhibits a bell-shaped profile, with the fastest rate observed at pD 6.4. The formation constant (<i>K</i>_f = 127 M^–1) and catalytic rate constant (<i>k</i>_c = 19.41 × 10^–5 s^–1) for the NPP-Ce­(IV)-Keggin POM complex were calculated, and binding between Ce^IV(PW₁₁)₂ and the phosphate group of NPP was also evidenced by the change of the chemical shift of the ³¹P nucleus in NPP upon addition of the POM complex. DFT calculations revealed that binding of NPP to the parent catalyst Ce^IV(PW₁₁)₂ is thermodynamically unlikely. On the contrary, formation of complexes with the monomeric 1:1 species, Ce^IVPW₁₁, is considered to be more favorable, and the most stable complex, [Ce^IVPW₁₁(H₂O)₂(NPP-κO)₂]^7–, was found to involve two NPP ligands coordinated to the Ce^IVcenter of Ce^IVPW₁₁ in the monodentate fashion. The formation of such species is considered to be responsible for the hydrolytic activity of Ce^IV(PW₁₁)₂ toward phosphomonoesters. On the basis of these findings a principle mechanism for the hydrolysis of NPP by the POM is proposed