Figure 1.

(a) Representation of the chemical structures of polymyxin B and colistin, with components B1, B2, E1, and E2, as well as colistimethate. The shaded boxes represent the fixed portions of the molecule, while the white boxes represent the structures that differ among polymyxin B, colistin, and/or colistimethate. The numbers in the blue circles correspond to the numbering of the amino acids from 1 to 10. The linear part of the molecule consists of a tripeptide (amino acids 1–3), while the cyclic part consists of a heptapeptide (amino acids 4–10). Dab = L-α,γ-diaminobutyric acid, Thr = Threonine; Leu = Leucine. (b) The groups A, B, and C in section (a) are described, and they represent the chemical moieties with variation between the polymyxins B1, B2, E1, E2 or between them and the prodrug colistimethate.