Engineering A23187/EDTA-containing metal-sorbing vesicles for selective extraction of divalent heavy metal ions

Abstract

Phosphatidylcholine vesicles harboring the lipophilic metal ion carrier A23187 in the bilayer and encapsulating ethylenediamine tetraacetic acid (EDTA) exhibit broadly varied, pH-dependent affinity for Cu 2+, Pb 2+, Cd 2+, and Ca 2+ at micromolar levels. Initial rates of uptake by these metal-sorbing vesicles (MSVs) is strongly dependent on the metal ion affinity of A23187 and the dissociation rate of A23187 complexation with turnover numbers ranging over five orders of magnitude from 27.51 s −1 for Cu 2+ at pH 7.0 to <0.001 s −1 for Ca 2+ at pH 5.5. A modified Briggs–Haldane model fits initial MSV kinetics well and was used to estimate metal ion dissociation rate constants of A23187, which varied from 270 s −1 for Ca 2+ to 18.3 s −1 for Cd 2+. The experimental data support a “pore” type mechanism rather than a membrane-transport-limited model for metal ion uptake in qualitative agreement with the microsecond time-scale expected for carrier–metal ion complex translocation across the lipid bilayer wall of MSVs. The concentration factors achieved by these MSVs, which are controlled by the amount of encapsulated EDTA, varied from 674 for Cu 2+ to 166 for Ca 2+ at pH 7.0 to an immeasurable level for Ca 2+ at pH 5.5. Although several physicochemical properties impact metal ion uptake, MSV selectivity in initial uptake rate or in equilibrium capacity can be described at least semi-quantitatively in terms of the metal ion affinity of the carrier and chelator, as well as the dissociation rate of the metal–carrier complex.