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

Abstract

A detailed reaction mechanism is proposed for the hydrolysis of the phosphoanhydride bonds in adenosine triphosphate (ATP) in the presence of the binuclear ZrIV-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 Zrᶦᵛ-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.