Selective transport of Pb2+ and Cd2+ across a phospholipid bilayer by a cyclohexanemonocarboxylic acid-capped 15-crown-5 ether

Abstract A cyclohexanemonocarboxylic acid-capped 15-crown-5 ether was synthesized and found to be effective as an ionophore for Pb 2+ and Cd 2+ , transporting them across a phospholipid bilayer membrane. Transport studies were carried out using 1-palmitoyl-2-oleoyl- sn -glycerophosphatidylcholine (POPC) vesicles containing the chelating indicator 2-([2-bis(carboxymethyl)amino-5-methylphenoxy]methyl)-6-methoxy-8-bis(carboxymethyl)aminoquinoline (Quin-2). Data obtained at pH 7.0 using this system, show that the synthetic ionophore transports divalent cations with the selectivity sequence Pb 2+  > Cd 2+  ≫ Zn 2+  > Mn 2+  > Co 2+  > Ni 2+  > Ca 2+  > Sr 2+ . Selectivity factors, based on the ratio of individual initial cation transport rates, are 280 (Pb 2+ /Ca 2+ ), 62 (Pb 2+ /Zn 2+ ), 68 (Cd 2+ /Ca 2+ ), and 16 (Cd 2+ /Zn 2+ ). Plots of log initial rate versus log M n + or log ionophore concentration suggest that Pb 2+ and Cd 2+ are transported primarily as a 1:1 cation–ionophore complex, but that complexes with other stoichiometries may also be present. The ionophore transports Pb 2+ and Cd 2+ by a predominantly electrogenic mechanism, based upon an enhanced rate of transport that is produced by agents which dissipate transmembrane potentials. The rate of Pb 2+ transport shows a biphasic pH dependence with the maximum occurring at pH ∼ 6.5. The high selectivity for Pb 2+ and Cd 2+ displayed by the cyclohexanecarboxylic acid-capped 15-crown-5 ether suggests potential applications of this ionophore for the treatment of Pb and Cd intoxication, and removal of these heavy metals from wastewater.