Hydrolysis of Dipeptides Catalyzed by a Zirconium(IV)-Substituted Lindqvist Type Polyoxometalate

The hydrolysis of a series of unactivated dipeptides in the presence of a zirconium(IV)-substituted Lindqvist type polyoxometalate, (Me4N)(2)[W5O18Zr(H2O)(3)] (designated as ZrW5), was studied by kinetic experiments and NMR spectroscopy. Among the dipeptides examined, those with the X-Ser amino acid sequence were most effectively hydrolyzed. The kinetics of the hydrolysis of histidylserine (His-Ser) was studied in detail; a rate constant of 95.3 (+/- 0.1)x10(-7) s(-1) (pD 7.4 and 60 degrees C) in the presence of an equimolar amount of ZrW5 was calculated. The binding of His-Ser to ZrW5 was examined by UV/Vis, H-1, C-13, and W-183 NMR spectroscopy, and the data indicate that at physiological pD His-Ser chelates the Zr-IV through its imidazole nitrogen, amine nitrogen, and amide carbonyl oxygen. In the presence of ZrW5, the pD profile of k(obs) is bell-shaped, with a maximum reaction rate at pD 7.5. At high pD values an inactive complex is formed as a result of the deprotonation of the amide nitrogen, resulting in inhibition of His-Ser hydrolysis. The effects of pH, temperature, inhibitors, and ionic strength on the hydrolysis rate constant were also investigated, and a full account of the mechanism of this novel reaction is given.