Carrier-facilitated bulk liquid membrane transport of iron(III)-siderophore complexes utilizing first coordination sphere recognition.

Abstract

Carrier-facilitated bulk liquid membrane (BLM) transport from an aqueous source phase through a chloroform membrane phase to an aqueous receiving phase was studied for various hydrophilic synthetic and naturally occurring Fe(III)-siderophore complexes using first coordination sphere recognition. Iron transport systems were designed such that two cis coordination sites on a hydrophilic Fe(III) complex are occupied by labile aquo ligands, while the other four coordination sites are blocked by strong tetradentate ligands (siderophores). The labile aquo coordination sites can be "recognized" by a liquid membrane-bound hydrophobic bidentate ligand, which carries the hydrophilic Fe(III)-siderophore complex across the hydrophobic membrane to an aqueous receiving phase. The system is further designed for uphill transport of Fe(III) against a concentration gradient, driven by anti-port H(+) transport. Three tetradentate siderophore and siderophore mimic ligands were investigated: rhodotorulic acid (H(2)L(RA)), alcaligin (H(2)L(AG)), and N,N'-dihydroxy-N,N'-dimethyldecanediamide (H(2)L(8)). Flux values for the transport of Fe(L(x))(OH(2))(2)(+) (x = RA, AG, 8) facilitated by the hydrophobic lauroyl hydroxamic acid (HLHA) membrane carrier were the highest when x = 8, which is attributed to substrate lipophilicity. Ferrioxamine B (FeHDFB(+)) was also selectively transported through a BLM by HLHA. The process involves partial dechelation of ferrioxamine B to produce the tetradentate form of the complex (Fe(H(2)DFB)(OH(2))(2)(2+)), followed by ternary complex formation with HLHA (Fe(H(2)DFB)(LHA)(+)) and transport across the membrane into the receiving phase. Uphill transport of ferrioxamine B was confirmed by increased flux as [H(+)](source phase) < [H(+)](receiving phase). The membrane flux of ferrioxamine B occurs near neutral pH, as evidence that ternary complex formation and ligand exchange are viable processes at the membrane/receptor surface of microbial cells.