(Fig. 2) the oxygens linked to iron are derived from 2,3-dihydroxybenzoyl-L-serine and D chelates result. These correspond, respectively, to left- and right-hand coordination propellers. Geometrical isomers, cis and trans, are also possible, except in the two archetypal siderophores just cited steric restraints confine the isomers to the cis form.