Molecular recognition of ferrioxamine B by host-guest complex formation with lasalocid A in chloroform

The polyether carboxylic acid antibiotic lasalocid A was employed as an ionophore in the extraction of ferrioxamine B (FeHDFB+) from H2O to CHCl3 at aqueous pH 3 and pH 9, Lasalocid can selectively recognize and extract FeHDFB+ in the presence of the cations Mg2+, Na+, and Li+ through formation of a neutral second-sphere complex composed of the anionic form of lasalocid and the cationic FeHDFB+. The extraction constant for the following reaction was determined to be log K-ex = -3.9 +/- 0.1: FeHDFBaq+ + HLas(org) reversible arrow (FeHDFB,Las)(org) + H-aq(+) (K-ex). While lasalocid is highly effective as an anionic ionophore, it does not act as a neutral ionophore even at low pH, despite its ability to assume pseudo-crown conformations. The extraction constant for the iron-free ligand H-4-DFB+ was determined to be log K-ex = -4.4 +/- 0.2, similar in magnitude to the constant for the complex, suggesting that the salicylate group of lasalocid does not enter the inner coordination sphere of the iron center. Lasalocid is selective for FeHDFB+ by 1-3 orders of magnitude over alkali metal ions. Lasalocid is 300 times as effective as a lariat ether (benzo-18-crown-6 with a 12-atom carboxylic acid side chain) in the extraction of FeHDFB+ from H2O to CHCl3 at pH 9. Since both the lariat ether and the lasalocid have easily ionizable protons to provide charge neutralization, the superiority of lasalocid is attributed to its open chain structure, which permits a stronger interaction between the backbone oxygens and the hydrogen atoms of the siderophore's pendant ammonium group.