Alpha-secondary tritium isotope effects in the aqueous hydrolysis of glycopyranosides of N-acetyl-beta-D-glucosamine

Secondary tritium isotope effects were used to study the aqueous hydrolysis of a series of α- and β-glycopyranosides of N-acetyl-d-glucosamine. The magnitude of the secondary tritium isotope effects, and their dependence on the structure of the aglycone, are compatible with the carbonium ion mechanism suggested for the specific acid catalysis of these compounds by Piszkiewicz and Bruice (1–3). The secondary tritium isotope effect determined for the spontaneous hydrolysis of the p-nitrophenyl-2-acetamido-2-deoxy-β-d-glucopyranoside is not consistent with an intramolecular, nucleophilic displacement mechanism. A mechanism involving the equilibrium formation of a carbonium ion-anion pair is proposed. The relevance of these model studies to hydrolysis of oligosaccharides of N-acetyl-d-glucosamine by lysozyme is discussed.