Peptide Bond Hydrolysis Catalyzed by the Wells–Dawson Zr(α2-P2W17O61)2 Polyoxometalate

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Abstract
In this paper we report the first example of peptide hydrolysis catalyzed by a polyoxometalate complex. A series of metal-substituted Wells–Dawson polyoxometalates were synthesized, and their hydrolytic activity toward the peptide bond in glycylglycine (GG) was examined. Among these, the Zr(IV)- and Hf(IV)-substituted ones were the most reactive. Detailed kinetic studies were performed with the Zr(IV)-substituted Wells–Dawson type polyoxometalate K15H[Zr(α2-P2W17O61)2]·25H2O which was shown to act as a catalyst for the hydrolysis of the peptide bond in GG. The speciation of K15H[Zr(α2-P2W17O61)2]·25H2O which is highly dependent on the pD, concentration, and temperature of the solution, was fully determined with the help of 31P NMR spectroscopy and its influence on the GG hydrolysis rate was examined. The highest reaction rate (kobs = 9.2 (±0.2) × 10–5 min–1) was observed at pD 5.0 and 60 °C. A 10-fold excess of GG was hydrolyzed in the presence of K15H[Zr(α2-P2W17O61)2]·25H2O proving the principles of catalysis. 13C NMR data suggested the coordination of GG to the Zr(IV) center in K15H[Zr(α2-P2W17O61)2]·25H2O via its N-terminal amine group and amide carbonyl oxygen. These findings were confirmed by the inactivity of K15H[Zr(α2-P2W17O61)2]·25H2O toward the N-blocked analogue acetamidoglycylglycinate and the inhibitory effect of oxalic, malic, and citric acid. Triglycine, tetraglycine, and pentaglycine were also fully hydrolyzed in the presence of K15H[Zr(α2-P2W17O61)2]·25H2O yielding glycine as the final product of hydrolysis. K15H[Zr(α2-P2W17O61)2]·25H2O also exhibited hydrolytic activity toward a series of other dipeptides.

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