Coupled transport of protons and anions through lipid bilayer membranes containing a long-chain secondary amine

Summary

Transport of protons and halide ions through planar lipid bilayers made from egg lecithin and a long-chain secondary amine (n-lauryl [trialkylmethyl] amine) inn-decane was studied. Net proton fluxes were measured with a pH electrode, and halide fluxes were measured with⁸²Br⁻ and³⁶Cl⁻. In membranes containing the secondary amine, a large net proton flux was produced either by a Br⁻ gradient with symmetrical pH or by a pH gradient with symmetrical Br⁻, but not by a pH gradient in Br⁻-free solutions. This H⁺ flux was electrically silent (nonconductive), and the H⁺ permeability coefficient was >10⁻³ cm sec⁻¹ in 0.1m NaBr. In Br⁻-free solutions, H⁺ selectivity was observed electrically by measuring conductances and zero-current potentials generated by H⁺ activity gradients. The permeability coefficient for this ionic (conductive) H⁺ flux was about 10⁻⁵ cm sec⁻¹, several orders of magnitude smaller than the H⁺ permeability of the electroneutral pathway. Large electroneutral Br⁻ exchange fluxes occurred under symmetrical conditions, and the permeability coefficient for Br⁻ exchange was about 10⁻³ cm sec⁻¹ at pH 5. The one-way Br⁻ flux was inhibited by substituting SO ⁼₄ for Br⁻ on the “trans” side of the membrane. These results support a “titratable carrier” model in which the secondary amine exists in three forms (C, CH⁺ and CHBr). Protons can cross the membrane either as CHBr (nonconductive) or as CH⁺ (conductive), whereas Br⁻ crosses the membrane primarily as CHBr (nonconductive). In addition to these three types of transport, there is also a pH-dependent conductive flux of Br⁻ which has a permeability coefficient of about 10⁻⁷ cm sec⁻¹ at pH 5. Experiments with lipid monolayers suggest that the pH dependence of this conductive flux is caused by a change in surface potential of about +100 mV between pH 9.5 and 5.0.