Stages of the bilayer-micelle transition in the system phosphatidylcholine-C12E8 as studied by deuterium- and phosphorous-NMR, light scattering, and calorimetry

Abstract

The perturbation of phospholipid bilayer membranes by a nonionic detergent, octaethyleneglycol mono-n-dodecylether (C12E8), was investigated by 2H- and 31P-NMR, static and dynamic light scattering, and differential scanning calorimetry. Preequilibrated mixtures of the saturated phospholipids 1,2-dipalmitoyl-sn-glycero-3-phosphorylcholine (DPPC), 1,2-dimyristoyl-sn-glycero-3-phosphorylcholine (DMPC), and 1,2-dilauroyl-sn-glycero-3-phosphorylcholine (DLPC) with the detergent were studied over a broad temperature range including the temperature of the main thermotropic phase transition of the pure phospholipids. Above this temperature, at a phospholipid/detergent molar ratio 2:1, the membranes were oriented in the magnetic field. Cooling of the mixtures below the thermotropic phase transition temperatures of the pure phospholipids led to micelle formation. In mixtures of DPPC and DMPC with C12E8, a narrow calorimetric signal at the onset temperature of the solubilization suggested that micelle formation was related to the disorder-order transition in the phospholipid acyl chains. The particle size changed from 150 nm to approximately 7 nm over the temperature range of the bilayer-micelle transition. The spontaneous orientation of the membranes at high temperatures enabled the direct determination of segmental order parameters from the deuterium spectra. The order parameter profiles of the phospholipid acyl chains could be attributed to slow fluctuations of the whole membrane and to detergent-induced local perturbations of the bilayer order. The packing constraints in the mixed bilayers that eventually lead to bilayer solubilization were reflected by the order parameters of the interfacial phospholipid acyl chain segments and of the phospholipid headgroup. These results are interpreted in terms of the changing average shape of the component molecules. Considering 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 headgroup repulsion. A neutral or pivotal plane can be defined on the basis of the temperature dependence of the interfacial quadrupolar splittings.

Substrate Efflux Propensity Plays a Key Role in the Specificity of Secretory A-type Phospholipases

“…This line of reasoning is supported by recent molecular dynamics simulations providing transversal distribution of the entropy and free energy of hexane (which mimics an acyl chain segment) in phospholipid bilayers (60). It is also supported by the fact that acyl chain length has much less influence with octyl glucoside micelles, since these are far more disordered than lipid bilayers (61), and the orientational order of acyl chains of incorporated phospholipids (when minor components, as in this study) also changes little with depth as indicated by data obtained with similar systems (62).…”

Nanoporous Microbead Supported Bilayers: Stability, Physical Characterization, and Incorporation of Functional Transmembrane Proteins

“…[18][19][20][21][22][23][24][25][26] These results provide a physicochemical basis for the purification and functional reconstitution of complex transmembrane proteins into unilamellar vesicles. There are hundreds of studies on this subject, but a few noteworthy examples pertinent to this study include the proteotypical G-protein coupled receptor, bacteriorhodopsin (bR), 27 and a ligandgated ion channel responsible for fast signal transduction, the serotonin receptor (5HT3R).…”

Temperature-induced micelle to vesicle transition: kinetic effects in the DMPC/NaC system

“…The partitioning of the surfactant molecules between the lipidic assemblies and the aqueous continuum can be modified by three categories of procedures: surfactant extraction, dilution and temperature variation. With respect to the last approach, it has been shown that temperature-induced transition from micelles to bilayer structures occurs in phospholipid containing systems involving octylglucoside (Miguel 1989), C 12 E 8 (Otten 1995) and bile salts (Polozova, 1993;Polozova, 1995;Forte, 1998) for appropriate lipid / detergent ratios. The temperature variation procedure allows to perform MVT without changing the total lipid and surfactant concentrations in the sample.…”

“…Various methods are applied to study the aggregation behaviour of mixed lipid-detergent systems and MVT processes: time-resolved fluorescence (Das, 1996;Seras 1994), cryotransmission electron microscopy (Meyuhas, 1997;Silvanter, 1996;Seras, 1996), static and dynamic light scattering (Miguel, 1989;Polozova, 1995;Forte, 1998;Egelhaaf 1994), gel exclusion chromatography (Lesieur, 1990), differential scanning calorimetry (Otten, 1995;Forte 1998), NMR (Otten, 1995;Polozova, 1993). Compared to light or neutron scattering (Long, 1994;Pedersen, 1995;Hjelm, 1992;Kiselev, 1999) small-angle X-ray scattering (SAXS) is generally not considered as a convenient technique for phospholipid vesicle characterisation (Van Zanten, 1996).…”