Permeability of acetic acid across gel and liquid-crystalline lipid bilayers conforms to free-surface-area theory

Solubility-diffusion theory, which treats the lipid bilayer membrane as a bulk lipid solvent into which permeants must partition and diffuse across, fails to account for the effects of lipid bilayer chain order on the permeability coefficient of any given permeant. This study addresses the scaling factor that must be applied to predictions from solubility-diffusion theory to correct for chain ordering. The effects of bilayer chemical composition, temperature, and phase structure on the permeability coefficient (P-m) of acetic acid were investigated in large unilamellar vesicles by a combined method of NMR line broadening and dynamic light scattering. Permeability values were obtained in distearoylphosphatidylcholine, dipalmitoylphosphatidylcholine, dimyristoylphosphatidylcholine, and dilauroylphosphatidylcholine bilayers, and their mixtures with cholesterol, at various temperatures both above and below the gel-->liquid-crystalline phase transition temperatures (T-m). A new scaling factor, the permeability decrement f, is introduced to account for the decrease in permeability coefficient from that predicted by solubility-diffusion theory owing to chain ordering in lipid bilayers. Values of f were obtained by division of the observed P-m by the permeability coefficient predicted from a bulk solubility-diffusion model. In liquid-crystalline phases, a strong correlation (r=0.94) between f and the normalized surface density sigma was obtained: In f=5.3-10.6 sigma. Activation energies (E(a)) for the permeability of acetic acid decreased with decreasing phospholipid chain length and correlated with the sensitivity of chain ordering to temperature, partial derivative sigma/partial derivative(1/T), as chain length was varied. P-m values decreased abruptly at temperatures below the main phase transition temperatures in pure dipalmitoylphosphatidylcholine and dimyristoylphosphatidylcholine bilayers (30-60-fold) and below the pretransition in dipalmitoylphosphatidylcholine bilayers (8-fold), and the linear relationship between In f and sigma established for liquid-crystalline bilayers was no longer followed. However, in both gel and liquid-crystalline phases In f was found to exhibit an inverse correlation with free surface area (In f=-0.31-29.1/a(f), where a(f) is the average free area (in square angstroms) per lipid molecule). Thus, the lipid bilayer permeability of acetic acid can be predicted from the relevant chain-packing properties in the bilayer (free surface area), regardless of whether chain ordering is varied by changes in temperature, lipid chain length, cholesterol concentration, or bilayer phase structure, provided that temperature effects on permeant dehydration and diffusion and the chain-length effects on bilayer barrier thickness are properly taken into account.