Reversible O -Acetyl Migration within the Sialic Acid Side Chain and Its Influence on Protein Recognition.

O -Acetylation is a common naturally occurring modification of carbohydrates and is especially widespread in sialic acids, a family of nine-carbon acidic monosaccharides. O -Acetyl migration within the exocyclic glycerol-like side chain of mono- O -acetylated sialic acid reported previously was from the C7- to C9-hydroxyl group with or without an 8- O -acetyl intermediate, which resulted in an equilibrium that favors the formation of the 9- O -acetyl sialic acid. Herein, we provide direct experimental evidence demonstrating that O -acetyl migration is bidirectional, and the rate of equilibration is influenced predominantly by the pH of the sample. While the O -acetyl group on sialic acids and sialoglycans is stable under mildly acidic conditions (pH < 5, the rate of O -acetyl migration is extremely low), reversible O -acetyl migration is observed readily at neutral pH and becomes more significant when the pH increases to slightly basic. Sialoglycan microarray studies showed that esterase-inactivated porcine torovirus hemagglutinin-esterase bound strongly to sialoglycans containing a more stable 9- N -acetylated sialic acid analog, but these compounds were less resistant to periodate oxidation treatment compared to their 9- O -acetyl counterparts. Together with prior studies, the results support the possible influence of sialic acid O -acetylation and O -acetyl migration to host-microbe interactions and potential application of the more stable synthetic N -acetyl mimics.