Bulk liquid membrane transport of ferrioxamine B by neutral and ionizable carriers
Bulk liquid membrane (BLM) transport of ferrioxamine B (FeHDFB+) across an artificial hydrophobia membrane facilitated by neutral and ionizable carriers was studied. Under controlled conditions, ferrioxamine B transport by neutral ionophores follows a simple theoretical model for a diffusion controlled process which predicts a linear dependence of flux upon aqueous-to-chloroform extraction constants. Unsubstituted 18C6 crown ether exhibits the best transport properties among the neutral carriers investigated with a flux of 1.4 × 10-10 mol s-1 cm-2. When transport of FeHDFB+, metal-free desferrioxamine B, H4DFB+, and AlHDFB+ mediated by DC18C6 crown ether was followed, a selectivity was observed with a ratio of measured relative fluxes of 5:3:1, respectively. An ionizable carrier lasalocid {6-[7-[5-ethyl-5-(5-ethyltetrahydro-5-hydroxy-6-methyl-2H-pyran-2-yl)tetrahydro- 3-methyl-2-furanyl]-4-hydroxy-3,5-dimethyl-6-oxonoyl]-2-hydroxy-3-methylbenzoic acid}, negatively charged at neutral pH, outperformed the crown ethers as a ferrioxamine B transporting agent. However, transport by ionizable carriers follows a different mechanism and occurs only when an electrolyte is present in the receiving phase. This requirement was exploited by designing a BLM experiment in which H4DFB+ acts as a counter cation at the receiving interface, whereby FeHDFB+ and H4DFB+ were transported in opposite directions in a synergistic fashion. When the ionizable salicylic acid moiety of lasalocid is transformed into an ester, lasalocid almost entirely loses its transporting properties. This suggests a co-operative mode of binding in which electrostatic recognition of the FeHDFB+ cation by lasalocid anion is a prerequisite for a subsequent encapsulation of the amine site in a host-guest fashion.
