Detergent-phospholipid mixed micelles with a crystalline phospholipid core

Detergent-phospholipid mixed micelles with a crystalline phospholipid core
approved June 7, 2001 (received for review April 3, 2001)
S?rgio S. Funari*, Brigitte Nuscher, Gert Rapp*, and Klaus Beyer,
PNAS | July 31, 2001
* Max Planck Institute for Colloid and Interface Science, Golm/Potsdam, c/o Hamburger Synchrotronstrahlungslabor at Deutsches Elektronen Synchrotron, Notkestrasse 85, 22603 Hamburg, Germany; and Department of Metabolic Biochemistry, Ludwig-Maximilians-University, Schillerstrasse 44, 80336, Munich, Germany
Edited by Jack Halpern, University of Chicago, Chicago, IL
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.
Membrane solubilization is a key technique in biochemistry. The intermediates encountered upon solubilization of biomembranes by nonionic detergents have been thoroughly studied (1), e.g., cryo-transmission electron microscopy revealed large irregular bilayer sheets and threadlike cylindrical structures (2-4). However, structural details of the small mixed micelles, arising after complete solubilization, are usually beyond direct visualization in the electron microscope. The properties of mixed detergent/lipid micelles recently came into focus when detergent insoluble regions (so-called "rafts") were discovered in cell membranes (5). Improving the understanding of bilayer solubilization and mixed micelle formation is therefore a topic of considerable interest.
Detergents from the poly(oxyethylene) series have found widespread application in membrane biochemistry, owing to their inoffensiveness toward membrane proteins (6). Commonly used members of this class of nonionic amphiphiles are Triton X-100 ([p-(1,1,3,3-tetramethylbutyl)phenyl]poly(oxyethylene)) and C12E8 (octaethyleneglycol-mono-n-dodecyl ether). Biomembrane solubilization using poly(oxyethylene) detergents is remarkable for its temperature dependence (7-9), e.g., the differential solubilization of cell membranes by Triton X-100 far below room temperature has been used to verify the existence of raft-like membrane domains (5).
We previously have shown that mixtures of C12E8 with 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) or 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) undergo a reversible bilayer-micelle transition upon decreasing the temperature below the main-phase transition temperature tm of the respective pure phospholipid (7, 8). The detergent-phospholipid mixed micelles obtained after a sudden temperature change from above tm to 10?C below tm have not been fully characterized so far. Various techniques were combined here to explore the properties of these unusual micellar aggregates. X-ray scattering using synchrotron radiation as well as differential scanning calorimetry (DSC) and isothermal titration calorimetry (ITC) revealed the presence of ordered phospholipid acyl chains in the mixed micellar dispersions below tm. A model will be presented that accounts for the wide-angle x-ray reflections obtained from the micellar solution of mixtures of C12E8 with DMPC or DPPC, and for the decreasing heat consumption observed when mixtures with increasing detergent/phospholipid molar ratio were titrated into an aqueous solution of C12E8.
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