Hydrolysis of Tetraglycine by a Zr(IV)-Substituted Wells–Dawson Polyoxotungstate Studied by Diffusion Ordered NMR Spectroscopy

Abstract

The use of diffusion ordered NMR spectroscopy (DOSY) for the analysis of complex reaction mixtures involving polyoxometalates (POMs) was demonstrated for the hydrolysis of the peptide tetraglycine by the K15H[Zr(α2-P2W17O61)2]·25H2O Wells–Dawson type cluster. 1H DOSY NMR studies have shown that severe signal overlap observed in the one-dimensional 1H NMR spectrum of reaction mixtures containing a POM and peptides could be overcome by the two-dimensional character of a DOSY NMR measurement. A clear distinction between the 1H NMR signals of the products formed during the hydrolysis of 5.0 mM of tetraglycine catalyzed by 1.0 mM of K15H[Zr(α2-P2W17O61)2]·25H2O was observed based on the extra dimension containing information about diffusion coefficients that distinguishes a typical DOSY measurement from conventionally used 1D 1H NMR. The spectrum clearly shows the presence of 5 species with diffusion coefficients of 3.71 × 10−10 m2/s (3.91; 3.84; 3.82 and 3.62 ppm), 4.39 × 10−10 m2/s (3.87; 3.76 and 3.61 ppm), 5.26 × 10−10 m2/s (3.67 and 3.63 ppm), and 7.46 × 10−10 m2/s (3.37 ppm) that are assigned to the non-hydrolyzed tetraglycine, the hydrolysis intermediate products triglycine and glycylglycine, and the end product of hydrolysis glycine, respectively. In addition, a signal assigned to cyclic glycylglycine, with a diffusion coefficient practically identical to the diffusion coefficient of glycylglycine was observed at 3.86 ppm. In addition, 1H and 31P NMR spectroscopy were further used to study the binding of tetraglycine to K15H[Zr(α2-P2W17O61)2]·25H2O and the solution speciation of K15H[Zr(α2-P2W17O61)2]·25H2O.