Ion permeability through bilayer lipid membranes for biosensor development: control by chemical modification of interfacial regions between phase domains

Abstract

Based on the results of studies on cystic fibrosis, which implicated hydroxystearic acid (HSA) as a contributing factor in altered biomembrane function, solvent-free bilayer lipid membranes (BLMs) and monolayer films were prepared from a lipid mixture containing (by mass) 34% phosphatidylcholine, 19% dipalmitoylphosphatidyl serine, 47% cholesterol and variable amounts of 10- and 12-HSA (0–50%). Ion currents, resulting from K⁺ permeation through BLMs that were supported in 0.1 mol dm^–3 KCl solutions buffered to pH 7.4, were monitored with use of a d.c. circuit. The structures of monolayer films at the air–water interface of a Langmuir–Blodgett trough were studied by pressure–area correlations and by further correlation with microscopic phase separation as revealed by fluorescence microscopy. In order to elucidate the role of the hydroxyl moieties in ion permeability, the transmembrane ion current was corrected for the effect of the negative surface charge of the carboxylic acid by replacement of the HSA component with stearic acid. The ion current was found to increase with the molar proportion of the HSAs. Two models for ion conduction through BLMs were considered: ‘hopping’via hydrophilic sites within the hydrophobic zone of the BLMs, introduced by the hydroxyl moiety of 10- or 12-HSA; and transport through interfacial regions between phase domains that represent areas of low steric density and low structural order within monolayers. Although the two mechanisms are not distinct, the ion permeability results indicate a change in the response of ion current to HSA concentration at 35 mol-%, suggesting a change in the relative proportion of the mechanisms. The results implied that the magnitude and physical location of ion permeation through BLMs could be controlled by using specific concentrations of HSA such that permeation occurs predominantly through interfacial zones at the edges between domains, and that artificial ‘ion channels’ had been generated by virtue of the edge activity of HSA.