Sialic acids (Sias) and other nonulosonic acids (NulOs). As shown in the figure, Sias are a subset of NulOs. All NulOs carry a negatively charged carboxylate (C-1) and a 3-carbon exocyclic glycerol-like side-chain (C-7 to C-9). The anomeric center (C-2) is drawn with the carboxyl group in the axial orientation (α-anomer for Sias), the configuration that typically occurs in glycosidically bound states of Sias, but the β-anomer is more abundant in the unbound state in solution and is the norm in CMP-Sias. There are examples of most of the prokaryotic NulOs in α- and β- configurations. The inset box shows the structures of the core Sias. These molecules vary widely in the substituent attached to C-5. For Sias, the N-acetyl (Neu5Ac, N-acetylneuraminic acid) is the most common form, and the N-glycolyl (Neu5Gc, N-glycolylneuraminic acid) form is common in many nonhuman vertebrates and “higher” invertebrates. Depending on the species, 3-deoxy-D-glycero-D-galacto-non-2-ulosonic acid (also known as 2-keto-3-deoxy-nononic acid or Kdn, with a hydroxyl at the C-5), appears less abundant and is not found in the bound form in mammals, whereas the free amine form (Neu, neuraminic acid) is rare in nature and found only in the glycosidically linked form. Non-Sia NulOs reported to date differ from Neu5Ac in having no hydroxyl group at C-9, an additional often substituted amino group (e.g., acetyl, formyl, acetimidoyl) at C-7, and various different stereochemistries at C-5 and C-7 as well as other carbon positions (C-4 or C-8). Consensus among the experts on nomenclature has not been reached (e.g., both Leg and Leg5,7Ac₂ have been used to represent di-N-acetyllegionaminic acid). So far, 9-deoxy NulOs with free amino groups at C-5 and C-7 have not been reported in nature. Here, structures representing six types of prokaryotic NulOs—Leg5,7Ac₂ (di-N-acetyllegionaminic acid), 8eLeg5,7Ac₂ (8-epi-di-N-acetyllegionaminic acid), 4eLeg5,7Ac₂ (4-epi-di-N-acetyllegionaminic acid), Pse5,7Ac₂ (di-N-acetylpseudaminic acid), Aci5,7Ac₂ (di-N-acetylacinetaminic acid), and 8eAci5,7Ac₂ (8-epi-di-N-acetylacinetaminic acid)—are shown.