Crown Ether Functionalized Lipid Membranes: Lead Ion Recognition and Molecular Reorganization

A fluorescent lipid bilayer, functionalized with 18-crown-6, was developed to examine the mechanism of chemical recognition-induced molecular reorganization events in a membrane system. The synthetic receptor-lipid, PS18C6, was prepared with the crown ether at the headgroup positionanda pyrene fluorescent tag on the hydrophobic tail. When incorporated into bilayers of distearylphosphatidylcholine, the receptorlipid aggregated into domains, evidenced by the relatively large pyrene excimer emission from the bilayer. Langmuir pressure-area (π-A) isotherm measurements and atomic force microscopy (AFM) further aided in characterizing the receptor aggregation at the macro- and nanoscale, respectively. The functionalized bilayer exhibited selective affinity for mercuric and lead ions in aqueous buffered solution (pH 7.4), with a fluorescence response that was linear over the concentration range 10^-7 to 10^-4 M metal ions. 1H NMR studies established that the binding stoichiometry of PS18C6 with lead was 1:1, with Ka = 10⁵ M^-1 in methanol. Recognition and binding of lead ions at the membrane surface resulted in a rapid and prominent reorganization of the receptor-lipids in the membrane that was measurable at both the macro-andnanoscales. Removal of the lead ions, through the addition of EDTA, resulted in recovery of the original fluorescence and the reaggregation of structures in the membrane.