Significance of Asymmetric Sites in Choosing Siderophores as Deferration Agents

The syntheses of the microbial iron chelators l-fluviabactin, its unnatural enantiomer, d-fluviabactin, l-homofluviabactin, and l-agrobactin, are described. The key steps involve the selective bis-acylation of the terminal nitrogens of norspermidine, spermidine, or homospermidine with 2,3-bis(benzyloxy)benzoic acid in the presence of 1,1-carbonyldiimidazole, followed by coupling of the N-hydroxysuccinimide ester of CBZ-protected l- or d-threonine with the central nitrogen. The effectiveness of each of these ligands in supporting the growth of Paracoccus denitrificans in a low-iron environment and the ability of these compounds to promote iron uptake are evaluated. The stereochemical configuration of the oxazoline ring is shown to be the major structural factor controlling both microbial growth stimulation and iron uptake. l-Fluviabactin, l-homofluviabactin, and l-agrobactin all promoted growth and iron uptake; d-fluviabactin was only marginally active. As with the microorganism's native siderophore, l-parabactin, all three ligands in the l-configuration investigated exhibited biphasic, i.e., both high-affinity and low-affinity, kinetics. The high-affinity system (iron concentration < 1 μM) yielded Km values between 0.11 and 0.23 μM and Vmax values from 157 to 129 pg-atoms Fe min-1 (mg of protein)-1, whereas the low-affinity scheme (iron concentration > 1 μM) gave Km values from 0.53 to 3.5 μM and Vmax values between 96 and 413 pg-atoms Fe min-1 (mg of protein)-1. Both l- and d-fluviabactin are very effective at clearing iron from the bile duct-cannulated rodent; when given subcutaneously at a dose of 150 μmol/kg, both ligands had iron clearing efficiencies of >13%, which is much greater than that of desferrioxamine in this model. Thus, by altering the stereochemistry of certain microbial siderophores, it is possible to generate deferration agents that are still effective at clearing iron from animals, yet do not promote microbial growth.