Detergent-phospholipid mixed micelles with a crystalline phospholipid core.

An unusual micelle was discovered in mixtures of the nonionic detergent octaethyleneglycol-mono-n-dodecylether with disaturated phospholipids such as 1,2-dimyristoyl-sn-glycero-3-phosphocholine or 1,2-dipalmitoyl-sn-glycero-3-phosphocholine in water. These mixtures undergo a structural transition upon cooling through the chain-melting temperatures of the respective phospholipids, resulting in the formation of mixed micelles. Structural features of the micellar particles were studied here by synchrotron x-ray scattering. The translucent micellar solutions showed characteristic wide-angle reflections that were attributed to ordered hydrocarbon chains, whereas the absence of small-angle x-ray reflections indicated that there is no long-range order in these mixtures. The presence of ordered phospholipid acyl chains was confirmed by differential scanning calorimetry and isothermal titration calorimetry. The endothermic differential scanning calorimetry signals observed in the up-scan mode were tentatively ascribed to chain melting and mixing of the components. Isothermal titration of the mixed-micellar solutions into an excess of the detergent octaethyleneglycol-mono-n-dodecylether resulted in sudden uptake of the latent heat by the gel-state phospholipids. The heat uptake per mol of phospholipid decreased with increasing detergent/phospholipid molar ratio. A simple geometric model is presented assuming that the dominating particle species in the mixtures is a discoidal phospholipid aggregate with ordered acyl chains, surrounded by a toroidal detergent hoop. The model implies that the fraction of ordered phospholipid chains decreases with increasing detergent/phospholipid molar ratio, in agreement with the calorimetric results and high-resolution NMR spectroscopy.