Detergent-phospholipid mixed micelles with a crystalline phospholipid core

An unusual micelle was discovered in mixtures of the nonionic detergent oct aethylenegtycol-mono-n-dodecylether with disaturated phospholipids such as 1,2-dimyristoyl-sn-glycero-3-phosphocholine or 1,2-dipatmitoyl-sn-glycero-3 -phosphocholine in water. These mixtures undergo a structural transition up on cooling through the chain-melting temperatures of the respective phospho lipids, resulting in the formation of mixed micelles. Structural features o f the micellar particles were studied here by synchrotron x-ray scattering. The translucent micellar solutions showed characteristic wide-angle reflec tions that were attributed to ordered hydrocarbon chains, whereas the absen ce of small-angle x-ray reflections indicated that there is no long-range o rder in these mixtures. The presence of ordered phospholipid acyl chains wa s confirmed by differential scanning calorimetry and isothermal titration c alorimetry. The endothermic differential scanning calorimetry signals obser ved in the up-scan mode were tentatively ascribed to chain melting and mixi ng of the components. Isothermal titration of the mixed-micellar solutions into an excess of the detergent octaethylenegtycot-mono-n-dodecylether resu lted in sudden uptake of the latent heat by the gel-state phospholipids. Th e heat uptake per mol of phospholipid decreased with increasing detergent/p hospholipid molar ratio. A simple geometric model is presented assuming tha t the dominating particle species in the mixtures is a discoidal phospholip id aggregate with ordered acyl chains, surrounded by a toroidal detergent h oop. The model implies that the fraction of ordered phospholipid chains dec reases with increasing detergent/phospholipid motor ratio, in agreement wit h the calorimetric results and high-resolution NMR spectroscopy.