STAGES OF THE BILAYER-MICELLE TRANSITION IN THE SYSTEM PHOSPHATIDYLCHOLINE-C(12)E(8) AS STUDIED BY DEUTERIUM-NMR AND PHOSPHORUS-NMR, LIGHT-SCATTERING, AND CALORIMETRY

The perturbation of phospholipid bilayer membranes by a nonionic deter gent, octaethyleneglycol mono-n-dodecylether (C(12)E(8)), was investig ated by H-2- and P-31-NMR, static and dynamic light scattering, and di fferential scanning calorimetry. Preequilibrated mixtures of the satur ated phospholipids 1,2-dipalmitoyl-sn-glycero-3-phosphorylcholine (DPP C), 1,2-dimyristoyl-sn-glycero-3-phosphorylcholine (DMPC), and 1,2-dil auroyl-sn-glycero-3-phosphorylcholine (DLPC) with the detergent were s tudied over a broad temperature range including the temperature of the main thermotropic phase transition of the pure phospholipids. Above t his temperature, at a phospholipid/detergent molar ratio 2:1, the memb ranes were oriented in the magnetic field. Cooling of the mixtures bel ow the thermotropic phase transition temperatures of the pure phosphol ipids led to micelle formation. In mixtures of DPPC and DMPC with C(12 )E(8), a narrow calorimetric signal at the onset temperature of the so lubilization suggested that micelle formation was related to the disor der-order transition in the phospholipid acyl chains. The particle siz e changed from 150 nm to similar to 7 nm over the temperature range of the bilayer-micelle transition. The spontaneous orientation of the me mbranes at high temperatures enabled the direct determination of segme ntal order parameters from the deuterium spectra. The order parameter profiles of the phospholipid acyl chains could be attributed to slow f luctuations of the whole membrane and to detergent-induced local pertu rbations of the bilayer order. The packing constraints in the mixed bi layers that eventually lead to bilayer solubilization were reflected b y the order parameters of the interfacial phospholipid acyl chain segm ents and of the phospholipid headgroup. These results are interpreted in terms of the changing average shape of the component molecules. Con sidering the decreasing cross sectional areas in the acyl chain region and the increasing hydration of the detergent headgroups, the bilayer -micelle transition is the result of an imbalance in the chain and hea dgroup repulsion. A neutral or pivotal plane can be defined on the bas is of the temperature dependence of the interfacial quadrupolar splitt ings.