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

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Abstract
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.