Fig.3
Structure of natural sialic acids．N-and O-Substituents together with their position（in parentheses）in the neuraminic acid molecule are indicated．Several combinations of these substituents are known and up to three O-acetyl residues have been found in one sialic acid molecule. N-Acetylneuraminic acid 9-Phosphate is an intermediate in sialic acid biosynthesis and has been found so far only as free sugar．