PHASE STRUCTURES OF BINARY LIPID BILAYERS AS REVEALED BY PERMEABILITYOF SMALL MOLECULES

The effects of changes in bilayer phase structure on the permeability of acetic acid and trimethylacetic acid were studied in large unilamel lar vesicles (LUVs) composed of dipalmitoylphosphatidylcholine (DPPC)/ cholesterol (CHOL). dihexadecylphosphatidylcholine (DHPC)/CHOL, or DPB C/dimyristoylphosphatidylcholine (DMPC) using an NMR line-broadening m ethod. Phase transitions were induced by changes in temperature and li pid composition (i.e., X-CHOL was varied from 0.0 to 0.5 and X-DMPC fr om 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'(beta)) but significantly reduces P-m in ordered and disordered liquid-crystalline phases (L-o and L-al pha). 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-beta' and L-o or between L-o and L-o phase s. In most respects, phase diagrams generated from the break points in plots of P-m, vs. X-CHOL obtained at various temperatures in DHPC/CHO L and DPPC/CHOL bilayers closely resemble those constructed previously for DPPC/CHOL bilayers using NMR and DSC methods. Above T-m. the phas e diagrams generated from permeability data reveal the presence of bot h the disordered (L-alpha) and the ordered (L-o) liquid-crystalline ph ases. as well as the two-phase coexistence region. In DPPC/DMPC bilaye rs, the addition of DMPC increases P-m dramatically in the gel phase b ut only slightly ill the liquid-crystalline phase. Abrupt changes in s lopes in semi-logarithmic plots of P-m, vs. X(DMPC)also occur at speci fic values of X-DMPC and temperature, from which a phase diagram can b e constructed which closely resembles diagrams obtained previously by other methods. These correlations indicate that trans-bilayer permeabi lity measurements can be used to construct lipid bilayer phase diagram s. 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 o r may be due to the higher lateral compressibility of lipid bilayers i n the two-phase coexistence region. (C) 1998 Elsevier Science B.V.