Fusion of vesicles with the air-water interface: the influence of polar head group, salt concentration, and vesicle size.
(41/927)
Fusion of vesicles with the air-water interface and consequent monolayer formation has been studied as a function of temperature. Unilamellar vesicles of DMPC, DPPC, and DODAX (X=Cl(-), Br(-)) were injected into a subphase containing NaCl, and the surface pressure (tension) was recorded on a Langmuir Balance (Tensiometer) using the Wilhelmy plate (Ring) method. For the zwitterionic vesicles, plots of the initial surface pressure increase rate (surface tension decrease rate) as a function of temperature show a peak at the phase transition temperature (T(m)) of the vesicles, whereas for ionic ones they show a sharp rise. At high concentrations of NaCl, ionic DODA(Cl) vesicles seem to behave like zwitterionic ones, and the rate of fusion is higher at the T(m). The influence of size was studied comparing large DODA(Cl) vesicles with small sonicated ones, and no significant changes were found regarding the rate of fusion with the air-water interface. (+info)
Effect of free fatty acids on the permeability of 1,2-dimyristoyl-sn-glycero-3-phosphocholine bilayer at the main phase transition.
(42/927)
We measured the influence of saturated and unsaturated free fatty acids on the permeability and partition of ions into 1, 2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) bilayers. The bilayer permeability was measured using the depletion of N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-1, 2-dihexadecanoyl-sn-glycero-3-phosphatidylethanolamine (N-NBD-PE) fluorescence as a result of its reduction by dithionite. We observed a distinct increase of dithionite permeability at the main gel-fluid phase transition of DMPC. When vesicles were formed from a mixture of DMPC and oleic acid, the membrane permeability at the phase transition was reduced drastically. Stearic acid and methyl ester of oleic acid have little effect. Similar results in the quenching of pyrene-PC in DMPC vesicles by iodide were obtained. Again, the increase of iodide partition into the lipid phase at the main phase transition of DMPC was abolished by the addition of unsaturated free fatty acids. Free fatty acids, in concentrations up to 5 mol%, do not abolish DMPC phase transition when measured by differential scanning calorimetry. It seems that unsaturated, but not saturated, free fatty acids reduce the lipid bilayer permeability to dithionite and iodide ions at the main phase transition of DMPC, without altering the thermodynamic properties of the bilayer. (+info)
A model for the lipid pretransition: coupling of ripple formation with the chain-melting transition.
(43/927)
Below the thermotropic chain-melting transition, lipid membrane c(P) traces display a transition of low enthalpy called the lipid pretransition. It is linked to the formation of periodic membrane ripples. In the literature, these two transitions are usually regarded as independent events. Here, we present a model that is based on the assumption that both pretransition and main transition are caused by the same physical effect, namely chain melting. The splitting of the melting process into two peaks is found to be a consequence of the coupling of structural changes and chain-melting events. On the basis of this concept, we performed Monte Carlo simulations using two coupled monolayer lattices. In this calculation, ripples are considered to be one-dimensional defects of fluid lipid molecules. Because lipids change their area by approximately 24% upon melting, line defects are the only ones that are topologically possible in a triangular lattice. The formation of a fluid line defect on one monolayer leads to a local bending of the membrane. Geometric constraints result in the formation of periodic patterns of gel and fluid domains. This model, for the first time, is able to predict heat capacity profiles, which are comparable to the experimental c(P) traces that we obtained using calorimetry. The basic assumptions are in agreement with a large number of experimental observations. (+info)
Protein-induced membrane disorder: a molecular dynamics study of melittin in a dipalmitoylphosphatidylcholine bilayer.
(44/927)
A molecular dynamics simulation of melittin in a hydrated dipalmitoylphosphatidylcholine (DPPC) bilayer was performed. The 19, 000-atom system included a 72-DPPC phospholipid bilayer, a 26-amino acid peptide, and more than 3000 water molecules. The N-terminus of the peptide was protonated and embedded in the membrane in a transbilayer orientation perpendicular to the surface. The simulation results show that the peptide affects the lower (intracellular) layer of the bilayer more strongly than the upper (extracellular) layer. The simulation results can be interpreted as indicating an increased level of disorder and structural deformation for lower-layer phospholipids in the immediate vicinity of the peptide. This conclusion is supported by the calculated deuterium order parameters, the observed deformation at the intracellular interface, and an increase in fractional free volume. The upper layer was less affected by the embedded peptide, except for an acquired tilt relative to the bilayer normal. The effect of melittin on the surrounding membrane is localized to its immediate vicinity, and its asymmetry with respect to the two layers may result from the fact that it is not fully transmembranal. Melittin's hydrophilic C-terminus anchors it at the extracellular interface, leaving the N-terminus "loose" in the lower layer of the membrane. In general, the simulation supports a role for local deformation and water penetration in melittin-induced lysis. As for the peptide, like other membrane-embedded polypeptides, melittin adopts a significant 25 degree tilt relative to the membrane normal. This tilt is correlated with a comparable tilt of the lipids in the upper membrane layer. The peptide itself retains an overall helical structure throughout the simulation (with the exception of the three N-terminal residues), adopting a 30 degree intrahelical bend angle. (+info)
Cholesterol effects on the phosphatidylcholine bilayer polar region: a molecular simulation study.
(45/927)
A molecular dynamics (MD) simulation of a fully hydrated, liquid-crystalline dimyristoylphosphatidylcholine (DMPC)-Chol bilayer membrane containing approximately 22 mol% Chol was carried out for 4.3 ns. The bilayer reached thermal equilibrium after 2.3 ns of MD simulation. A 2.0-ns trajectory generated during 2.3-4.3 ns of MD simulation was used for analyses to determine the effects of Chol on the membrane/water interfacial region. In this region, 70% of Chol molecules are linked to DMPC molecules via short-distance interactions, where the Chol hydroxyl group (OH-Chol) is 1) charge paired to methyl groups of the DMPC choline moiety ( approximately 34%), via the hydroxyl oxygen atom (Och); 2) water bridged to carbonyl ( approximately 19%) and nonester phosphate ( approximately 14%) oxygen atoms, via both Och and the hydroxyl hydrogen atom (Hch); and 3) directly hydrogen (H) bonded to carbonyl ( approximately 11%) and nonester phosphate ( approximately 5%) oxygen atoms, via Hch ( approximately 17% of DMPC-Chol links are multiple). DMPC's gamma-chain carbonyl oxygen atom is involved in 44% of water bridges and 51% of direct H bonds formed between DMPC and Chol. On average, a Chol molecule forms 0.9 links with DMPC molecules, while a DMPC molecule forms 2.2 and 0.3 links with DMPC and Chol molecules, respectively. OH-Chol makes hydrogen bonds with 1.1 water molecules, preferentially via Hch. The average number of water molecules H bonded to the DMPC headgroup is increased by 7% in the presence of Chol. These results indicate that inclusion of Chol decreases interlipid links and increases hydration in the polar region of the membrane. (+info)
Molecular and mesoscopic properties of hydrophilic polymer-grafted phospholipids mixed with phosphatidylcholine in aqueous dispersion: interaction of dipalmitoyl N-poly(ethylene glycol)phosphatidylethanolamine with dipalmitoylphosphatidylcholine studied by spectrophotometry and spin-label electron spin resonance.
(46/927)
Spin-label electron spin resonance (ESR) spectroscopy, together with optical density measurements, has been used to investigate, at both the molecular and supramolecular levels, the interactions of N-poly(ethylene glycol)-phosphatidylethanolamines (PEG-PE) with phosphatidylcholine (PC) in aqueous dispersions. PEG-PEs are micelle-forming hydrophilic polymer-grafted lipids that are used extensively for steric stabilization of PC liposomes to increase their lifetimes in the blood circulation. All lipids had dipalmitoyl (C16:0) chains, and the polymer polar group of the PEG-PE lipids had a mean molecular mass of either 350 or 2000 Da. PC/PEG-PE mixtures were investigated over the entire range of relative compositions. Spin-label ESR was used quantitatively to investigate bilayer-micelle conversion with increasing PEG-PE content by measurements at temperatures for which the bilayer membrane component of the mixture was in the gel phase. Both saturation transfer ESR and optical density measurements were used to obtain information on the dependence of lipid aggregate size on PEG-PE content. It is found that the stable state of lipid aggregation is strongly dependent not only on PEG-PE content but also on the size of the hydrophilic polar group. These biophysical properties may be used for optimized design of sterically stabilized liposomes. (+info)
Structural determinants in the C-terminal domain of apolipoprotein E mediating binding to the protein core of human aortic biglycan.
(47/927)
Apolipoprotein (apo) E-containing high density lipoprotein particles were reported to interact in vitro with the proteoglycan biglycan (Bg), but the direct participation of apoE in this binding was not defined. To this end, we examined the in vitro binding of apoE complexed with dimyristoylphosphatidylcholine (DMPC) to human aortic Bg before and after glycosaminoglycan (GAG) depletion. In a solid-phase assay, apoE.DMPC bound to Bg and GAG-depleted protein core in a similar manner, suggesting a protein-protein mode of interaction. The binding was decreased in the presence of 1 m NaCl and was partially inhibited by either positively (0.2 m lysine, arginine) or negatively charged (0.2 m aspartic, glutamic) amino acids. A recombinant apoE fragment representing the C-terminal 10-kDa domain, complexed with DMPC, bound as efficiently as full-length apoE, whereas the N-terminal 22-kDa domain was inactive. Similar results were obtained with a gel mobility shift assay. Competition studies using a series of recombinant truncated apoEs showed that the charged segment in the C-terminal domain between residues 223 and 230 was involved in the binding. Overall, our results demonstrate that the C-terminal domain contains elements critical for the binding of apoE to the Bg protein core and that this binding is ionic in nature and independent of GAGs. (+info)
Conformation and dynamics of melittin bound to magnetically oriented lipid bilayers by solid-state (31)P and (13)C NMR spectroscopy.
(48/927)
The conformation and dynamics of melittin bound to the dimyristoylphosphatidylcholine (DMPC) bilayer and the magnetic orientation in the lipid bilayer systems were investigated by solid-state (31)P and (13)C NMR spectroscopy. Using (31)P NMR, it was found that melittin-lipid bilayers form magnetically oriented elongated vesicles with the long axis parallel to the magnetic field above the liquid crystalline-gel phase transition temperature (T(m) = 24 degrees C). The conformation, orientation, and dynamics of melittin bound to the membrane were further determined by using this magnetically oriented lipid bilayer system. For this purpose, the (13)C NMR spectra of site-specifically (13)C-labeled melittin bound to the membrane in the static, fast magic angle spinning (MAS) and slow MAS conditions were measured. Subsequently, we analyzed the (13)C chemical shift tensors of carbonyl carbons in the peptide backbone under the conditions where they form an alpha-helix and reorient rapidly about the average helical axis. Finally, it was found that melittin adopts a transmembrane alpha-helix whose average axis is parallel to the bilayer normal. The kink angle between the N- and C-terminal helical rods of melittin in the lipid bilayer is approximately 140 degrees or approximately 160 degrees, which is larger than the value of 120 degrees determined by x-ray diffraction studies. Pore formation was clearly observed below the T(m) in the initial stage of lysis by microscope. This is considered to be caused by the association of melittin molecules in the lipid bilayer. (+info)