Phase behavior of mixed aqueous dispersions of DPPC and dodecyl glycosides: Aggregation states implicated in the micelle-to-vesicle transition

The aggregation behavior of dipalmitoylphosphatidylcholine (DPPC) mixed with dodecyl beta-D-glucopyranoside (DG) and/or dodecyl beta-D-maltoside (DM) was studied in excess buffer. From high-performance liquid chromatography on a gel exclusion column and quasi-elastic light scattering, ultrasonic irradiation of DPPC-DG mixtures yields closed unilamellar vesicles for DG-to-DPPC molar ratios smaller than 1.8, above which some discoid structures are seen by cryofracture electron microscopy. Solubilization by DM of pure DPPC and DPPC-DG vesicles was followed at 37 degrees C. Turbidity recordings reveal seven significant break points corresponding to the limits separating distinct aggregation states and corresponding to a precise partitioning of DM between lipid aggregates and buffer. Three of these Limits coincide with the boundaries of four distinct phase domains revealed by SAXS performed on more concentrated samples over the same composition range. At 37 degrees C, with increasing DM-to-DPPC ratios, (1) fully hydrated lamellar structures, (2) coexisting lamellar and micellar assemblies, (3) mixed micelles separating from the aqueous phase, and (4) isotropic micellar solutions successively appear. The sequence of these phases is sensitive to temperature variations; bilayer formation is favored by heating. Domain 3 is attributed to a cloud point phenomenon occurring in the micellar region at temperatures higher than 30 or 34 degrees C, with or without DG, respectively.