High-resolution 1H-NMR spectroscopy of free and glycosidically linked O-acetylated sialic acids.

A number of naturally occurring and synthetic, partially O-acetylated derivatives of N-acetylneuraminic and N-glycoloylneuraminic acids have been investigated by 360-MHz 1H-NMR spectroscopy. O-Acetylation causes strong downfield shifts for the resonances of neighbouring sugar-skeleton protons. The chemical shifts of these resonances, together with their characteristic multiplet shapes, can be used for localisation of the position of O-acetyl substituents in the molecule. The number of such substituents in the molecule can be inferred from the number of acetyl-methyl singlets, which also have characteristic resonance positions. This method for determination of the number and position of O-acetyl substituents in sialic acid residues is very powerful for structural analysis of underivatized carbohydrate chains derived from glycoconjugates. This is demonstrated for the oligosaccharide N-acetyl-4-O-acetylneuraminosyl-(alpha 2 leads to 3)-lactose isolated from echidna milk.