Fig. 31.7 The crystallographically determined binding mode of sialyl–LewisX 31.13, exposed binding epitope of the PSGL-1 protein to the selectin surface domain (a). The four carbohydrate moieties: N-acetylglucosamine (violet), fucose (green), galactose (blue), and sialic acid (red) form numerous hydrogen bonds with their oxygen atoms to the protein in a shallow, bowl-shaped binding pocket (b). A calcium ion (violet sphere) is involved in the binding and interacts with multiple protein residues as well as with the ligand’s fucose moiety. Rational design allowed the development of the glycomimetic 31.19 (yellow), which binds to the protein with 19 µM.