Permeation of Halide Anions through Phospholipid Bilayers Occurs by the Solubility-Diffusion Mechanism

Abstract

Two alternative mechanisms are frequently used to describe ionic permeation of lipid bilayers. In the first, ions partition into the hydrophobic phase and then diffuse across (the solubility-diffusion mechanism). The second mechanism assumes that ions traverse the bilayer through transient hydrophilic defects caused by thermal fluctuations (the pore mechanism). The theoretical predictions made by both models were tested for halide anions by measuring the permeability coefficients for chloride, bromide, and iodide as a function of bilayer thickness, ionic radius, and sign of charge. To vary the bilayer thickness systematically, liposomes were prepared from monounsaturated phosphatidylcholines (PC) with chain lengths between 16 and 24 carbon atoms. The fluorescent dye MQAE (N-(ethoxycarbonylmethyl)-6-methoxyquinolinium bromide) served as an indicator for halide concentration inside the liposomes and was used to follow the kinetics of halide flux across the bilayer membranes. The observed permeability coefficients ranged from 10(-9) to 10(-7) cm/s and increased as the bilayer thickness was reduced. Bromide was found to permeate approximately six times faster than chloride through bilayers of identical thickness, and iodide permeated three to four times faster than bromide. The dependence of the halide permeability coefficients on bilayer thickness and on ionic size were consistent with permeation of hydrated ions by a solubility-diffusion mechanism rather than through transient pores. Halide permeation therefore differs from that of a monovalent cation such as potassium, which has been accounted for by a combination of the two mechanisms depending on bilayer thickness.

Electroformation in a flow chamber with solution exchange as a means of preparation of flaccid giant vesicles

“…While the exchange of glycerol was considered rate-limiting during the solute transport through the tether tube, the situation is reversed here. The membrane is much more permeable to glycerol than to anions (permeability of DOPC bilayer for Cl − is 1.2 · 10 −8 cm/s, [33]), and the membrane permeability to cations is even lower [34]. Even lower is the membrane permeability for sugars (permeability of DMPC bilayer for glucose is 1.4 · 10 −10 cm/s, [35]).…”

How did cells get their size?

“…For prebiotic short-chain amphiphile liposomes, chloride might have equilibrated by solubility-diffusion (Paula et al, 1998), but what of the various counterions, for which solubility-diffusion is demonstrably insignificant (Deamer, 1997)? An intriguing possibility is that the liposomes had abiotic cation pores fashioned of polyphosphate and polyhydroxybutyrate (PHB), since these simple materials self-assemble as cation pores in lipid bilayers (Das et al, 1997).…”

Remote Loading of 64Cu2+ into Liposomes without the Use of Ion Transport Enhancers

“…[15][16][17][18] From these studies, low ion permeability of phospholipid bilayers, such as liposomes, is expected to result in highly unfavorable loading kinetics of charged ion species. Common practice is therefore to use ionophores to increase the trans-membrane ion diffusion rate, and thereby improve the loading kinetics of charged ions such as radionuclides into liposomes.…”