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Phase diagram of ternary cholesterol/palmitoylsphingomyelin/palmitoyloleoyl-phosphatidylcholine mixtures: spin-label EPR study of lipid-raft formation.

ABSTRACT: For canonical lipid raft mixtures of cholesterol (chol), N-palmitoylsphingomyelin (PSM), and 1-palmitoyl-2-oleoylphosphatidylcholine (POPC), electron paramagnetic resonance (EPR) of spin-labeled phospholipids--which is insensitive to domain size--is used to determine the ternary phase diagram at 23°C. No phase boundaries are found for binary POPC/chol mixtures, nor for ternary mixtures with PSM content <24 mol %. EPR lineshapes indicate that conversion from the liquid-disordered (L(?)) to liquid-ordered (L(o)) phase occurs continuously in this region. Two-component EPR spectra and several tie lines attributable to coexistence of gel (L(?)) and fluid phases are found for ternary mixtures with low cholesterol or low POPC content. For PSM/POPC alone, coexistence of L(?) and L(?) phases occurs over the range 50-95.5 mol % PSM. A further tie line is found at 3 mol % chol with endpoints at 50 and ?77 mol % PSM. For PSM/chol, L(?)-L(o) coexistence occurs over the range 10-38 mol % chol and further tie lines are found at 4.5 and 7 mol % POPC. Two-component EPR spectra indicative of fluid-fluid (L(?)-L(o)) phase separation are found for lipid compositions: 25%POPC>10%, and confirmed by nonlinear EPR. Tie lines are identified in the L(?)-L(o) coexistence region, indicating that the fluid domains are of sufficient size to obey the phase rule. The three-phase triangle is bounded approximately by the compositions 40 and 75 mol % PSM with 10 mol % chol, and 60 mol % PSM with 25 mol % chol. These studies define the compositions of raft-like L(o) phases for a minimal realistic biological lipid mixture.

SUBMITTER: Ionova IV

PROVIDER: S-EPMC3328704 | biostudies-literature | 2012 Apr

REPOSITORIES: biostudies-literature

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Phase diagram of ternary cholesterol/palmitoylsphingomyelin/palmitoyloleoyl-phosphatidylcholine mixtures: spin-label EPR study of lipid-raft formation.

Ionova Irina V IV Livshits Vsevolod A VA Marsh Derek D

Biophysical journal 20120401 8

For canonical lipid raft mixtures of cholesterol (chol), N-palmitoylsphingomyelin (PSM), and 1-palmitoyl-2-oleoylphosphatidylcholine (POPC), electron paramagnetic resonance (EPR) of spin-labeled phospholipids--which is insensitive to domain size--is used to determine the ternary phase diagram at 23°C. No phase boundaries are found for binary POPC/chol mixtures, nor for ternary mixtures with PSM content <24 mol %. EPR lineshapes indicate that conversion from the liquid-disordered (L(α)) to liquid ...[more]

PMID: 22768941

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Project description:Molecular dynamics simulations of mixtures of the ceramide nonhydroxy-sphingosine (NS), cholesterol, and a free fatty acid are performed to gain molecular-level understanding of the structure of the lipids found in the stratum corneum layer of skin. A new coarse-grained force field for cholesterol was developed using the multistate iterative Boltzmann inversion (MS-IBI) method. The coarse-grained cholesterol force field is compatible with previously developed coarse-grained force fields for ceramide NS, free fatty acids, and water and validated against atomistic simulations of these lipids using the CHARMM force field. Self-assembly simulations of multilayer structures using these coarse-grained force fields are performed, revealing that a large fraction of the ceramides adopt extended conformations, which cannot occur in the single bilayer in water structures typically studied using molecular simulation. Cholesterol fluidizes the membrane by promoting packing defects, and an increase in cholesterol content is found to reduce the bilayer thickness due to an increase in interdigitation of the C24 lipid tails, consistent with experimental observations. Using a reverse-mapping procedure, a self-assembled coarse-grained multilayer system is used to construct an equivalent structure with atomistic resolution. Simulations of this atomistic structure are found to closely agree with experimentally derived neutron scattering length density profiles. Significant interlayer hydrogen bonding is observed in the inner layers of the atomistic multilayer structure that are not found in the outer layers in contact with water or in equivalent bilayer structures. This work highlights the importance of simulating multilayer structures, as compared to the more commonly studied bilayer systems, to enable more appropriate comparisons with multilayer experimental membranes. These results also provide validation of the efficacy of the MS-IBI derived coarse-grained force fields and the framework for multiscale simulation.

Dataset Information

Phase diagram of ternary cholesterol/palmitoylsphingomyelin/palmitoyloleoyl-phosphatidylcholine mixtures: spin-label EPR study of lipid-raft formation.

Publications

Phase diagram of ternary cholesterol/palmitoylsphingomyelin/palmitoyloleoyl-phosphatidylcholine mixtures: spin-label EPR study of lipid-raft formation.

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