Phase structures of binary lipid bilayers as revealed by permeability of small molecules

• Published in: Biochimica et biophysica acta 1998-03, Vol.1370 (1), p.64-76
• Main Authors: Xiang, Tian-Xiang, Anderson, Bradley D
• Format: Article
• Language: English
• Subjects: 1,2-Dipalmitoylphosphatidylcholine - chemistry; 1,2-Dipalmitoylphosphatidylcholine - metabolism; Cholesterol; Dimyristoylphosphatidylcholine - chemistry; Dimyristoylphosphatidylcholine - metabolism; Light; Lipid bilayer; Lipid Bilayers - chemistry; Lipid Bilayers - metabolism; Liposomes - chemistry; Liposomes - metabolism; Magnetic Resonance Spectroscopy; Membrane transport; Permeability; Phase diagram; Phospholipid; Phospholipid Ethers - chemistry; Phospholipid Ethers - metabolism; Scattering, Radiation; Temperature
• ISSN: 0005-2736; 0006-3002; 1879-2642
• DOI: 10.1016/S0005-2736(97)00244-7

The effects of changes in bilayer phase structure on the permeability of acetic acid and trimethylacetic acid were studied in large unilamellar vesicles (LUVs) composed of dipalmitoylphosphatidylcholine (DPPC)/cholesterol (CHOL), dihexadecylphosphatidylcholine (DHPC)/CHOL, or DPPC/dimyristoylphosphatidylcholine (DMPC) using an NMR line-broadening method. Phase transitions were induced by changes in temperature and lipid composition (i.e., X CHOL was varied from 0.0 to 0.5 and X DMPC from 0.0 to 1.0). In DPPC/CHOL and DHPC/CHOL bilayers, the addition of CHOL induces only a modest change in the permeability coefficient ( P m) of acetic acid in the gel-phase (P β′) but significantly reduces P m in ordered and disordered liquid-crystalline phases (L o and L α). Abrupt changes in slopes in semi-logarithmic plots of P m vs. X CHOL occur at specific values of X CHOL and temperature corresponding to the boundaries between P β′ and L o or between L α and L o phases. In most respects, phase diagrams generated from the break points in plots of P m vs. X CHOL obtained at various temperatures in DHPC/CHOL and DPPC/CHOL bilayers closely resemble those constructed previously for DPPC/CHOL bilayers using NMR and DSC methods. Above T m, the phase diagrams generated from permeability data reveal the presence of both the disordered (L α) and the ordered (L o) liquid-crystalline phases, as well as the two-phase coexistence region. In DPPC/DMPC bilayers, the addition of DMPC increases P m dramatically in the gel phase but only slightly in the liquid-crystalline phase. Abrupt changes in slopes in semi-logarithmic plots of P m vs. X DMPC also occur at specific values of X DMPC and temperature, from which a phase diagram can be constructed which closely resembles diagrams obtained previously by other methods. These correlations indicate that trans-bilayer permeability measurements can be used to construct lipid bilayer phase diagrams. Positive deviations of P m from predicted values based on the phase lever rule are observed in the two-phase coexistence regions with the degree of the deviation depending on bilayer chemical composition and temperature. These results may reflect a specific contribution of the interfacial region between two phases to higher solute permeability or may be due to the higher lateral compressibility of lipid bilayers in the two-phase coexistence region.