| Abstract
| - Molecular recognition of ferrioxamine B by lasalocid occursvia formation of a second-sphere complex. A host−guest assemblyis produced by the interaction of the lasalocid anion and its backboneoxygens with the protonated amine side chain of the siderophorecomplex. Lasalocid is more effective in the extraction offerrioxamine B from water to chloroform than lariat or crown ethers, byvirtue of its open chain structure, which allows formation of a morestable second-sphere complex.
- The polyether carboxylic acid antibiotic lasalocid A was employedas an ionophore in the extraction of ferrioxamineB (FeHDFB+) from H2O to CHCl3 ataqueous pH 3 and pH 9. Lasalocid can selectively recognize andextractFeHDFB+ in the presence of the cationsMg2+, Na+, and Li+ throughformation of a neutral second-sphere complexcomposed of the anionic form of lasalocid and the cationicFeHDFB+. The extraction constant for thefollowingreaction was determined to be log Kex = −3.9± 0.1: FeHDFB+aq + HLasorg⇌ (FeHDFB,Las)org + H+aq(Kex).While lasalocid is highly effective as an anionic ionophore, itdoes not act as a neutral ionophore even at low pH,despite its ability to assume pseudo-crown conformations. Theextraction constant for the iron-free ligandH4DFB+ was determined to be log Kex= −4.4 ± 0.2, similar in magnitude to the constant for thecomplex, suggestingthat the salicylate group of lasalocid does not enter the innercoordination sphere of the iron center. Lasalocidis selective for FeHDFB+ by 1−3 orders of magnitudeover alkali metal ions. Lasalocid is 300 times aseffectiveas a lariat ether (benzo-18-crown-6 with a 12-atom carboxylic acid sidechain) in the extraction of FeHDFB+from H2O to CHCl3 at pH 9. Since both thelariat ether and the lasalocid have easily ionizable protons toprovidecharge neutralization, the superiority of lasalocid is attributed toits open chain structure, which permits a strongerinteraction between the backbone oxygens and the hydrogen atoms of thesiderophore's pendant ammonium group.
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