We have studied the phase behavior of binary mixtures of long- and short-chain lipids, namely, dimyristoyl phosphatidylcholine (DMPC) and dihexanoyl phosphatidylcholine (DHPC), using optical microscopy and small-angle neutron scattering. Samples with a total lipid content of 25 wt %, corresponding to ratios Q ([DMPC]/[DHPC]) of 5, 3.2, and 2, are found to exhibit an isotropic (I) --> chiral nematic (N) --> lamellar phase sequence on increasing temperature. The I-N transition coincides with the chain melting transition of DMPC at Q = 5 and 3.2, but the N phase forms at a higher temperature for Q = 2. All three samples form multilamellar vesicles in the lamellar phase. Our results show that disklike "bicellar" aggregates occur only in the lower temperature isotropic phase and not in the higher temperature magnetically alignable N phase, where they were previously believed to exist. The N phase is found to consist of long, flexible wormlike micelles, their entanglement resulting in the very high viscosity of this phase.

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Mixtures of long- and short-chain phospholipids, specifically 14:0 and 6:0 phosphatidylcholines (DMPC and DHPC), have been used successfully in NMR studies as magnetically alignable substrates for membrane-associated proteins. However, recent publications have shown that the phase behavior of these mixtures is much more complex than originally thought. Using polarized light microscopy and small-angle neutron scattering, phase diagrams of DMPC/DHPC mixtures at molar ratios of 2, 3.2, and 5 have been determined. Generally, at temperatures below the main-chain melting transition of DMPC (T(M) = 23 degrees C), an isotropic phase of disk-like micelles is found. At high temperatures (T > 50 degrees C), a lamellar phase consisting of either multilamellar vesicles (MLV) or extended lamellae is formed, which at low lipid concentrations (e.g., MLV) coexists with an excess of water. At intermediate temperatures and lipid concentrations, a chiral nematic phase made up of worm-like micelles was observed.

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We observe the spontaneous formation of path-dependent monodisperse and polydisperse phospholipid unilamellar vesicles (ULV) from two different equilibrium morphologies specifically, disklike micelles and extended lamellae, respectively. On heating beyond a temperature Tc, low temperature disklike micelles, or so-called bicelles, transform into lamellae. Dilution of the lamellar phase, at a fixed temperature, results in a complete unbinding transition and the formation of polydisperse ULV, demonstrating the instability of the lamellar phase. On the other hand, heating of a dilute bicellar phase above Tc results in monodisperse ULV, which on cooling revert back to bicelles for lipid concentrations phi > or = 0.5 wt % and transform into oblate ellipsoids for phi = 0.1 wt %, a morphology not previously seen in "bicellar" lipid mixtures. Monodisperse ULV reform on heating of the oblate ellipsoids.

We present electron density maps (EDMs) of the ripple phase formed by phosphorylcholine lipids such as dimyristoyl phosphatidylcholine (DMPC), palmitoyl-oleoyl phosphatidylcholine (POPC), dihexadecyl phosphatidylcholine, and dilauroyl phosphatidylcholine (DLPC). With the exception of DLPC, the rippled bilayers have a sawtooth shape in all the systems, with one arm being almost twice as long as the other. For DMPC and POPC bilayers, EDMs have been obtained at different temperatures at a fixed relative humidity, and the overall shape of the ripples and the ratio of the lengths of the two arms are found to be insensitive to temperature. EDMs of all the systems with saturated hydrocarbon chains suggest the existence of a mean chain tilt along the ripple wave vector. In the literature it is generally assumed that the asymmetry of the rippled bilayers (absence of a mirror plane normal to the ripple wave vector) arises from a sawtoothlike height profile. However, in the case of DLPC, the height profile is found to be almost symmetric and the asymmetry results mainly from different bilayer thicknesses in the two arms of the ripple. We also present EDMs of the metastable ripple phase of dipalmitoyl phosphatidylcholine, formed on cooling from the L(alpha) phase.

We have determined the phase behavior of disaturated phosphatidylglycerols (PGs) of chain lengths n(CH2) = 14-18 at high pH and ionic strength using calorimetry, dilatometry, as well as x-ray diffraction. PGs with n(CH2) = 14 and 16 show thermotropic behavior similar to that of phosphatidylcholines (PCs). The area/lipid obtained in the gel phase is smaller than that reported for PCs despite the expected larger effective headgroup size. This can be explained by the tilting of the PG headgroup out of the bilayer plane, and we provide experimental evidence for a headgroup tilt transition. For distearoyl PG, we further find that the "usual" gel phase coexists with an interdigitated phase, which exhibits a transition from an orthorhombic into a hexagonal chain packing. The total amount of the interdigitated phase depends significantly on the temperature but is found to be largely independent of temperature equilibration time and different sample preparation protocols. Thus, the development of the interdigitated phase appears to be kinetically trapped. The formation of interdigitated phases in PGs at much smaller chain lengths than in PCs is of high relevance to interaction studies with antimicrobial peptides, as it provides a mechanism for the discrimination of membranes composed of different lipid species.

Using small-angle neutron scattering and dynamic light scattering, we have constructed partial structural phase diagrams of lipid mixtures composed of the phosphatidylcholines dimyristoyl and dihexanoyl doped with calcium ions (Ca2+) and/or the negatively charged lipid, dimyristoyl phosphatidylglycerol (DMPG). For dilute solutions (lipid concentration < or =1 wt %), spontaneously forming unilamellar vesicles (ULVs) were found, and their polydispersity was determined to be approximately 20%. The stability of the Ca2+- or DMPG-doped ULVs was monitored over a period of 4 days and their structural parameters (e.g., average outer radius, ) were found to be insensitive to the lipid concentration (Clp). However, doping the dimyristoyl/dihexanoyl system with both Ca2+ and DMPG resulted in ULVs whose was found to be Clp dependent. The of DMPG-doped ULVs remained unchanged over an extended period of time (at least 4 days), a good indication of their stability.