*3.1. Monosaccharides*

Repeated disaccharide units of glycosamine and uronic acids with a non-uniform distribution of sulfated and acetylated groups along the chain constitute the main structural features of sulfated GAGs. Despite the high diversity of potential structures, only 28 unique monosaccharide structures occur in GAGs. Three of them correspond to 4,5 unsaturated uronic acids resulting from the eliminative cleavage of GAGs oligo- or polysaccharides containing (1->4)-linked d-glucuronate or l-iduronate residues and (1->4)-alpha-linked 2-sulfoamino-2-deoxy-6-sulfo-d-glucose residues to give oligosaccharides with terminal 4-deoxy-alpha-d-gluco-4-enuronosyl groups at their non-reducing ends.

The cartoon representation of monosaccharides was extended [42] in compliance with the SNFG representation of glycans [22] to link this description with the GlycoCT [58] and condensed IUPAC [56] codes of the monosaccharides.

While these nomenclatures have become widely popular in the field of glycoscience, they are not used to identify and describe monosaccharides in the PDB, which has its carbohydrate nomenclature in its ligand dictionary [69]. Therefore, we established the cross-references between some of these nomenclatures (Figure 5).

**Figure 5.** Cross-references between the common representations and nomenclature of monosaccharides: condensed IUAPC nomenclature, symbol nomenclature for glycan; PDB Chemical Component Nomenclature, 2-dimensional structure.

Except for l-idopyranosides, and the 4,5 unsaturated uronic acids, all monosaccharides exist as hexopyranosides. The predominant conformation being <sup>4</sup>C1. As for l-idopyranosides, the following <sup>1</sup>C4, <sup>4</sup>C1, and <sup>2</sup>S<sup>0</sup> conformations may be found. Figure 6 depicts the 3-dimensional representations of these unusual conformations, along with the corresponding SNFG extensions.

**Figure 6.** 3-D structures, descriptors, and schematic representations of L-Idopyranosides, the following <sup>1</sup>C<sup>4</sup> , <sup>4</sup>C<sup>1</sup> , and <sup>2</sup>S<sup>0</sup> ; and 4,5 unsaturated uronic acids (drawn with pyMol (Schrödinger)).
