A Mechanistic Study on the Non‐enzymatic Hydrolysis of Kdn Glycosides

Sialic acids are biologically important carbohydrates that are prevalent throughout nature. We are interested in their intrinsic reactivity in aqueous solution and how such reactivity affects the design of substrates for investigation of enzymes that process these sugars. To probe the reactivity differences between two sialic acid family members N‐acetylneuraminic acid and Kdn we measured the rate constants for hydrolysis of 4‐nitrophenyl 3‐deoxy‐d‐glycero‐α‐d‐galacto‐non‐2‐ulosonide in aqueous solution. The kinetic data is consistent with glycosidic C−O bond cleavage occurring via four mechanistic pathways, and these are: (i) hydronium ion‐catalyzed hydrolysis of the neutral sugar; (ii) hydronium ion‐catalyzed hydrolysis of the glycosidic carboxylate; (iii) water‐catalyzed hydrolysis of the anionic glycoside; and (iv) base‐promoted reaction of the anionic glycoside. To study the effects of C‐5 substitution on the Kdn glycoside we made 4‐nitrophenyl 5‐O‐methyl‐α‐Kdn glycoside and determined its rate constants for hydrolysis. All hydrolytic rate constants for both Kdn glycosides were larger than those reported for the parent N‐acetyl‐α‐neuraminide. The water‐catalyzed reaction (pathway iii) exhibited a βlg value of −1.3±0.1. We conclude that the larger rate constants associated with C5‐oxygen containing sialosides results from less steric congestion at the hydrolytic transition states than for the parent C‐5 acetamido glycoside.