1-2-oleoylphosphatidylcholine and n-hexadecane

The Amber Lipid14 force field is expanded to include cholesterol parameters for all-atom cholesterol and lipid bilayer molecular dynamics simulations. The General Amber and Lipid14 force fields are used as a basis for assigning atom types and basic parameters. A new RESP charge derivation for cholesterol is presented, and tail parameters are adapted from Lipid14 alkane tails. 1,2-Dimyristoyl-sn-glycero-3-phosphocholine (DMPC), 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayers are simulated at a range of cholesterol contents. Experimental bilayer structural properties are compared with bilayer simulations and are found to be in good agreement. With this parameterization, another component of complex membranes is available for molecular dynamics with the Amber Lipid14 force field.

Topics: Alkanes; Cholesterol; Dimyristoylphosphatidylcholine; Glycerylphosphorylcholine; Lipid Bilayers; Molecular Dynamics Simulation; Naphthalenes; Neutron Diffraction; Phosphatidylcholines; Temperature; X-Ray Diffraction

Here, we examine the different mechanisms of poly(ethylene glycol)-mediated fusion of small unilamellar vesicles composed of dioleoylphosphatidylcholine/dioleoylphosphatidylethanolamine (DOPE)/sphingomyelin/cholesterol in a molar ratio of 35:30:15:20 at pH 7.4 versus pH 5. In doing so, we test the hypothesis that fusion of this lipid mixture should be influenced by differences in hydration of DOPE at these two pH values. An examination of the literature reveals that DOPE should be less hydrated at pH 5 (where influenza virus particles fuse with endosome membranes) than at pH 7.4 (where synaptic vesicles or HIV virus particles fuse with plasma membrane). Ensemble kinetic experiments revealed substantial differences in fusion of this plasma membrane mimetic system at these two pH values. The most dramatic difference was the observation of two intermediates at pH 5 but loss of one of these fusion intermediates at pH 7.4. Analysis of data collected at several temperatures also revealed that formation of the initial fusion intermediate (stalk) was favored at pH 7.4 due to increased activation entropy. Our observations support the hypothesis that the different negative intrinsic curvature of DOPE can account for different fusion paths and activation thermodynamics in steps of the fusion process at these two pH values. Finally, the effects of 2 mol % hexadecane on fusion at both pH values seemed to have similar origins for step 1 (promotion of acyl chain or hydrocarbon excursion into interbilayer space) and step 3 (reduction of interstice energy leading to expansion to a critical stalk radius). Different hexadecane effects on activation thermodynamics at these two pH values can also be related to altered DOPE hydration. The results support our kinetic model for fusion and offer insight into the critical role of phosphatidylethanolamine in fusion.

Topics: Alkanes; Animals; Cattle; Cell Membrane; Cholesterol; Hydrogen-Ion Concentration; Kinetics; Membrane Fusion; Phosphatidylcholines; Phosphatidylethanolamines; Polyethylene Glycols; Sphingomyelins; Thermodynamics; Unilamellar Liposomes

This study compares the intrinsic permeability coefficients of 40 drug molecules, obtained by three popular variants of the PAMPA assay, based on: (a) n-hexadecane, (b) 2% w/v dioleyoylphosphatidylcholine in n-dodecane, and (c) 20% w/v lecithin in n-dodecane, the HDM-, DOPC-, DS-PAMPA models, respectively. It was shown that PAMPA permeability values consistently rank in magnitude according to: DS>DOPC>HDM, with molecules like metoprolol showing 1000-fold greater permeability in DS than in HDM. Abraham descriptors were used to rationalize these observations. Water-solubilized polar molecules form very strong H-bonds with the solvent. Such molecules need to break these bonds in order to enter the pure alkane phase, which, in turn, offers no compensating H-bond solvation. Thus, more energy appears to be needed for a polar molecule to penetrate a pure alkane barrier, compared to a barrier possessing some H-bond interactions. The 20% phospholipid content of the DS-PAMPA lipid may be thought to ease the permeation process, by offering a compensating source of H-bonding within the membrane phase.

Topics: Algorithms; Alkanes; Cell Membrane Permeability; Chemical Phenomena; Chemistry, Physical; Intestinal Absorption; Membranes, Artificial; Models, Biological; Phosphatidylcholines; Regression Analysis; Solubility

The fusion peptide of the HIV fusion protein gp41 is required for viral fusion and entry into a host cell, but it is unclear whether this 23-residue peptide can fuse model membranes. We address this question for model membrane vesicles in the presence and absence of aggregating concentrations of poly(ethylene glycol) (PEG). PEG had no effect on the physical properties of peptide bound to membranes or free in solution. We tested for fusion of both highly curved and uncurved PC/PE/SM/CH (35:30:15:20 mol %) vesicles and highly curved PC/PE/CH (1:1:1) vesicles treated with peptide in the presence and absence of PEG. Fusion was never observed in the absence of PEG, although high peptide concentrations led to aggregation and rupture, especially in unstable PC/PE/CH (1:1:1) vesicles. When 5 wt % PEG was present to aggregate vesicles, peptide enhanced the rate of lipid mixing between curved PC/PE/SM/CH vesicles in proportion to the peptide concentration, with this effect leveling off at peptide/lipid (P/L) ratios approximately 1:200. Peptide produced an even larger effect on the rate of contents mixing but inhibited contents mixing at P/L ratios >1:200. No fusion enhancement was seen with uncurved vesicles. The rate of fusion was also enhanced by the presence of hexadecane, and peptide-induced rate enhancement was not observed in the presence of hexadecane. We conclude that gp41 fusion peptide does not induce vesicle fusion at subrupturing concentrations but can enhance fusion between highly curved vesicles induced to fuse by PEG. The different effects of peptide on the rates of lipid mixing and fusion pore formation suggest that, while gp41 fusion peptide does affect hemifusion, it mainly affects pore formation.

Topics: Alkanes; Amino Acid Sequence; Animals; Cattle; Cholesterol; HIV Envelope Protein gp41; Humans; Kinetics; Lipid Bilayers; Membrane Fusion; Molecular Sequence Data; Peptide Fragments; Phosphatidylcholines; Phosphatidylethanolamines; Polyethylene Glycols; Sphingomyelins; Viral Fusion Proteins

Helical instability induced by gly residues in the transmembrane domain (TMD) of G protein, the fusion protein of vesicular stomatitis virus (VSV), was speculated to aid in the later steps of the fusion process, because G protein with ala's substituted for the two TMD gly's was inactive (Cleverley, D. Z., and Lenard, J. (1998) Proc. Natl. Acad. Sci. U. S. A. 95, 3425-30). Here we examine the conformations of synthetic peptides corresponding to fusion-active (GGpep) and inactive (AApep; G's replaced by A's) TMDs by CD spectroscopy, and then their effects on the kinetics of poly (ethyleneglycol) (PEG)-mediated fusion of small unilamellar vesicles. GGpep and AApep both assumed history-dependent, non-interconvertible ordered structures. Both peptides were largely helical under all conditions if derived from trifluoroethanol solutions, and aggregated in a beta-sheet form if derived from acetonitrile solutions. In solvent, detergents or lipid bilayers, GGpep showed a greater range of secondary structural features than did AApep. The two peptides had large but different effects on PEG-mediated fusion. Both enhanced the rate but not the extent of lipid mixing. AApep significantly inhibited the extent of fusion pore formation while GGpep had no effect. The initial rate of fusion was enhanced 6-fold by GGpep and less than 2-fold by AApep. Addition of 5 mol % hexadecane overrode all peptide-induced effects. We suggest that both GGpep and hexadecane promote pore formation by stabilizing the nonlamellar structures in fusion intermediates or initial small pores. AApep, which had fewer nonhelical features in its CD spectrum than GGpep, actually inhibited fusion pore formation.

Topics: Alkanes; Amino Acid Sequence; Amino Acid Substitution; Carbohydrate Conformation; Cholesterol; Circular Dichroism; Kinetics; Liposomes; Membrane Fusion; Membrane Glycoproteins; Molecular Sequence Data; Peptide Fragments; Phosphatidylcholines; Phosphatidylethanolamines; Polyethylene Glycols; Protein Conformation; Protein Structure, Secondary; Sphingomyelins; Vesicular stomatitis Indiana virus; Viral Envelope Proteins; Viral Fusion Proteins