Hydrolysis of Serine-Containing Peptides at Neutral pH Promoted by [MoO4]2- Oxyanion

Hydrolysis of the dipeptides glycylserine (GlySer), leucylserine (LeuSer), histidylserine (HisSer), glycylalanine (GlyAla), and serylglycine (SerGly) was examined in oxomolybdate solutions by means of H-1, C-13, and Mo-95 NMR spectroscopy. In the presence of a mixture of oxomolybdates, the hydrolysis of the peptide bond in GlySer proceeded under neutral pD conditions (pD = 7.0, 60 degrees C) with a rate constant of k(obs) = 5.9 X 10(-6) s(-1). NMR spectra did not show evidence of the formation of paramagnetic species, excluding the possibility of Mo(VI) reduction to Mo(V), indicating that the cleavage of the peptide bond is purely hydrolytic. The pD dependence of k(obs) exhibits a bell-shaped profile, with the fastest cleavage observed at pD 7.0. Comparison of the rate profile with the concentration profile of oxomolybdate species implicated monomolybdate MoO42- as the kinetically active complex. Kinetics experiments at pD 7.0 using a fixed amount of GlySer and increasing amounts of MoO42- allowed for calculation of the catalytic rate constant (k(2) = 9.25 x 10(-6) s(-1)) and the formation constant for the GlySer-MoO42- complex (K-f= 15.25 M-1). The origin of the hydrolytic activity of molybdate is most likely a combination of the polarization of amide oxygen in GlySer due to the binding to molybdate, followed by the intramolecular attack of the Ser hydroxyl group.