title: Acyl-chain methyl distributions of liquid-ordered and -disordered membranes.
abstract: A central feature of the lipid raft concept is the formation of cholesterol-rich lipid domains. The introduction of relatively rigid cholesterol molecules into fluid liquid-disordered (L(d)) phospholipid bilayers can produce liquid-ordered (L(o)) mixtures in which the rigidity of cholesterol causes partial ordering of the flexible hydrocarbon acyl chains of the phospholipids. Several lines of evidence support this concept, but direct structural information about L(o) membranes is lacking. Here we present the structure of L(o) membranes formed from cholesterol and dioleoylphosphatidylcholine (DOPC). Specific deuteration of the DOPC acyl-chain methyl groups and neutron diffraction measurements reveal an extraordinary disorder of the acyl chains of neat L(d) DOPC bilayers. The disorder is so great that >20% of the methyl groups are in intimate contact with water in the bilayer interface. The ordering of the DOPC acyl chains by cholesterol leads to retraction of the methyl groups away from the interface. Molecular dynamics simulations based on experimental systems reveal asymmetric transbilayer distributions of the methyl groups associated with each bilayer leaflet.
authors.affiliation: Department of Physiology and Biophysics, University of California, Irvine, California, USA.
authors.forename: Mihaela
authors.initials: M
authors.lastname: Mihailescu
authors.forename: Rishi G
authors.initials: RG
authors.lastname: Vaswani
authors.forename: Eduardo
authors.initials: E
authors.lastname: Jardón-Valadez
authors.forename: Francisco
authors.initials: F
authors.lastname: Castro-Román
authors.forename: J Alfredo
authors.initials: JA
authors.lastname: Freites
authors.forename: David L
authors.initials: DL
authors.lastname: Worcester
authors.forename: A Richard
authors.initials: AR
authors.lastname: Chamberlin
authors.forename: Douglas J
authors.initials: DJ
authors.lastname: Tobias
authors.forename: Stephen H
authors.initials: SH
authors.lastname: White
mesh_terms: D002462:Cell Membrane / Q000737:chemistry*; D002784:Cholesterol / Q000737:chemistry; D008051:Lipid Bilayers / Q000737:chemistry; D008968:Molecular Conformation; D056004:Molecular Dynamics Simulation; D010713:Phosphatidylcholines / Q000737:chemistry
chemical_list: D008051:Lipid Bilayers; D010713:Phosphatidylcholines; D002784:Cholesterol; C017251:1,2-oleoylphosphatidylcholine