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

Abstract

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 position and a pyrene fluorescent tag on the hydrophobic tail. When incorporated into bilayers of distearylphosphatidylcholine, the receptor-lipid 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- and nanoscales. 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.