Amide bond hydrolysis in peptides and cyclic peptides catalyzed by a dimeric Zr(IV)-substituted Keggin type polyoxometalate

Detailed kinetic studies on the hydrolysis of glycylserine (Gly-Ser) and glycylglycine (Gly-Gly) in the presence of the dimeric zirconium(IV)-substituted Keggin type polyoxometalate (Et2NH2)(8)[{alpha-PW11O39Zr(mu-OH)(H2O)}(2)]center dot 7H(2)O (1) were performed by a combination of H-1, C-13 and P-31 NMR spectroscopy. The observed rate constants for the hydrolysis of Gly-Ser and Gly-Gly at pD 5.4 and 60 degrees C were 63.3 x 10(-7) s(-1) and 4.44 x 10(-7) s(-1) respectively, representing a significant acceleration as compared to the uncatalyzed reactions. The pD dependence of the rate constant for both reactions exhibited a bell-shaped profile with the fastest hydrolysis observed in the pD range of 5.5-6.0. Interaction of 1 with Gly-Ser and Gly-Gly via their amine nitrogen and amide oxygen was proven by C-13 NMR spectroscopy. The effective hydrolysis of Gly-Ser in the presence of 1 is most likely a combination of the polarization of the amide oxygen due to its binding to the Zr(IV) ion in 1 and the intramolecular attack of the Ser hydroxyl group on the amide carbonyl carbon. The effect of temperature, inhibitors, and ionic strength on the hydrolysis rate constant was also examined. The solution structure of 1 was investigated by means of P-31 NMR spectroscopy, revealing that its stability is highly dependent on pH, concentration and temperature. A 2.0 mM solution of 1 was found to be fully stable under hydrolytic conditions (pD 5.4 and 60 degrees C) both in the presence and in the absence of the dipeptides.