Hydrocarbon Chains In Bilayers Research Articles

Effect of cholesterol on the lateral nanoscale dynamics of fluid membranes

Inelastic neutron scattering was used to study the effect of 5 and 40mol% cholesterol on the lateral nanoscale dynamics of phospholipid membranes. By measuring the excitation spectrum at several lateral q (||) values (up to q (||)=3 Å(-1)), complete dispersion curves were determined of gel, fluid and liquid-ordered phase bilayers. The inclusion of cholesterol had a distinct effect on the collective dynamics of the bilayer's hydrocarbon chains; specifically, we observed a pronounced stiffening of the membranes on the nanometer length scale in both gel and fluid bilayers, even though they were experiencing a higher degree of molecular disorder. Also, for the first time we determined the nanoscale dynamics in the high-cholesterol liquid-ordered phase of bilayers containing cholesterol. Namely, this phase appears to be "softer" than fluid bilayers, but better ordered than bilayers in the gel phase.

Transmembrane and Water-Soluble Helix Bundles Display Reverse Patterns of Surface Roughness

Amino acid exposure and surface roughness were calculated for 12 helices from three transmembrane α-helix bundles and 13 helices from seven water-soluble α-helix bundles. Transmembrane helix bundles have relatively rough surfaces exposed to the lipid bilayer hydrocarbon chains and relatively smooth surfaces along helix–helix interfaces. This pattern is the reverse of what occurs in water-soluble helix bundles, where relatively rough surfaces are at the helix–helix interfaces and relatively smooth surfaces are exposed to water. The relatively rough exposed surfaces and buried smooth surfaces of transmembrane helices are likely to contribute to the stability of transmembrane helical bundles in a phospholipid environment.

Effect of vesicle size on their interaction with class A amphipathic helical peptides

Throughout the life span of a lipoprotein particle, the type and number of exchangeable apolipoproteins on its surface varies with particle size, suggesting a role of surface curvature on the lipid-binding properties of these proteins. Peptides 18A, Ac-18A-NH2, Ac-18R-NH2, 37pA, and 37aA have been designed to investigate the lipid-binding properties of the amphipathic alpha-helix structural motif that appears to modulate the lipid-binding properties of the exchangeable plasma apolipoproteins. We report here the results of a quantitative thermodynamic characterization of the effects of modifying helix length and of varying both the location of charged residues about the polar face of the peptides and vesicle size on the lipid affinity and depth of bilayer penetration for model amphipathic alpha-helices. Partition coefficients, Kp, were determined by fluorescence spectroscopy, and binding enthalpies, deltaH, by titration calorimetry. The results indicate that Kp values are on the order of 10(5), with similar deltaG(o) values for the interactions of the peptides with vesicles of various sizes. It appears that a class A motif and increased alpha-helical content optimize binding for 18-residue peptides. The interactions of the model peptides with 20 nm SUV are enthalpically driven with small, negative entropy changes; however, interactions for larger vesicles are entropically driven, likely due to disordering of bilayer hydrocarbon chains. Thermodynamic data indicate that 37pA and 37aA induce greater disordering of bilayer hydrocarbon chains than Ac-18A-NH2. The results of this study suggest that the type of interaction, i.e., enthalpically or entropically driven, may be modulated by the lateral compressibility of the bilayer membrane.

ESR Study on DMPC and DMPG Bilayers in the (Lα + H2O) Phase

The thermal properties of a smectic liquid crystal in the (Lα + H2O) phase of dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylglycerol (DMPG) were investigated as a function of temperature in the region from 24 to 40 °C by the ESR spin probe method using the doxylstearic acids, 5NS, 7NS, and 12NS. The molecular flexibility and homogeneity of these bilayer assemblies were analyzed from the ESR spectra observed at different labeled positions relating to the hydrocarbon chains in bilayers. In the case of a DMPC dispersion, the hyperfine splitting constants for 5NS abruptly increased and then became almost constant above about 29 °C, while the symmetry parameters of 12NS and the b/a parameters for 5NS decreased steeply with increasing temperature up to about 29 °C and then their values were almost constant. On the other hand, in the case of the DMPG dispersion, the hyperfine splitting constants of 5NS decreased with increasing temperature up to about 31 °C, and then they became constant. As a...

Spin-lattice relaxation times of phospholipid aminoxyl spin labels in cardiolipin–cytochrome c bilayers: a pulse saturation–recovery EPR study

A pulsed EPR saturation-recovery method has been used to measure spin-lattice relaxation times (T1, ) for aminoxyl spin labels in cardiolipin bilayers, with and without cytochrome c. The relaxation time for each spin label was determined at various positions in the bilayer, which included the bilayer surface and three positions within the hydrophobic interior in the membrane, as well as a position close to the glycerol backbone of phospholipids in the bilayer. A dynamic profile for the hydrocarbon chains in bilayers was found in agreement with results from other techniques. On addition of cytochrome c to cardiolipin bilayers, changes in the spin-lattice relaxation time and order parameter were observed around the unsaturated region in the cardiolipin acyl chains. Supported by complementary lineshape analysis, these effects were interpreted as changes in the molecular dynamics around the double bonds in the acyl chains of cardiolipin bilayers, induced upon the binding of cytochrome c.

Effet de l'addition de la colipase pancréatique de porc sur la perméabilité au glucose et la transition de phase de liposomes de phosphatidyl choline

An interaction between porcine pancreatic coli-pase and lecithin liposomes is demonstrated by gel filtration assays. The extent of the colipase penetration into the phospholipid bilayer was assessed by permeability and calorimetry studies carried out on the liposome colipase complex. The addition of colipase to liposomes induces a three fold increase in the permeability to [6-H3] glucose. This result reflects a perturbation in the bilayer which may be the consequence of the colipase interaction. The phase transition temperature is not modified by the added colipase. This observation suggests that the perturbation brought by the protein does not affect the acyl chain packing of the bulk lipid. On the other hand the enthalpy of transition (delta H) is decreased from 8.9 to 7.8 kcal/mole by the addition of colipase to the lipid. This could be explained by the interaction of the colipase with neighbouring acyl chains which do not participate in the cooperative melting of the bulk lipid. In agreement with previous spectrophotometric observations, the present results are indicative of hydrophobic interactions between colipase and bilayer hydrocarbon chains.

The molecular organization of asymmetric lipid bilayers and lipid-peptide complexes

Oriented fatty acid bilayers with asymmetric distributions of lipid head group types, hydrocarbon chain lengths, and associated polypeptides have been analyzed by a combined use of high resolution electron microscopy and X-ray diffraction techniques. The exclusion of fixatives, stains, and embedding materials has made it possible to relate unequivocally microscopic images to molecular composition. The ultrastructure of asymmetric bilayers has been determined by a novel analysis in which one half of the bilayer serves as a structural reference for the entire bilayer. Absolute electron density profiles at 7 Å resolution have been computed for bilayers formed from long and short chain length lipids either segregated to opposite sides or mixed together in both sides of the bilayer. The data indicate that the two lipids self organize in a specific paired configuration. Detailed analysis of bilayers associated with poly- l-lysine shows that although this hydrophilic peptide resides near the lipid head group region, its presence alters the arrangement of the bilayer hydrocarbon chains.

Hydrocarbon chain packing and molecular motion in phospholipid bilayers formed from unsaturated lecithins. Synthesis and properties of sixteen positional isomers of 1,2-dioctadecenoyl-sn-glycero-3-phosphorylcholine.

Isomers of cis-octadecenoic acid, with the double bond in each position in the hydrocarbon chain, were used to synthesize the corresponding 1,2-diacyl-sn-glycero-3phosphorylcholines (lecithins). Differential thermal analysis of the lecithins, as a function of water content, permitted evaluation of the limiting transition temperature (Tc) of each isomer. Values of Tc plotted against double bond position fell on a smooth curve with a minimum at minus 22 degrees for the dioctadec-9'-enoyl compound. The presence of a "pretransition" endotherm in differential thermal analysis of 1,2-dioctadec-15'-and 1,2-dioctadec-16'-enoyl-sn-glycero-3-phosphorylcholine implies the existence of two beta crystalline forms. This was not observed with any of the other lecithins. Enthalpy and entropy data were then obtained from differential scanning calorimetry measurements. Values of delta H were lower (7.6 plus or minus 0.1 kcal mol- minus 1) when the center of unsaturation was near the middle of the hydrocarbon chain than they were (9.6 kcal mol- minus 1) when the center of unsaturation was close to either end of the chain. However, values of delta S showed no consistent variation with double bond position. Four positional isomers of 1-octadec-cis-enoyl-2-octadecanoyl-sn-glycero-3-phosphorylcholine were synthesized. With the double bond near the middle of the chain or close to the terminal group, the Tc values of the mixed acid lecithins were higher than those of the corresponding dioctadecenoyl lecithins. 13-C nuclear magnetic resonance relaxation measurements were used to obtain information about chain motion of selected 1,2-dioctadec-cis-enoyl-sn-glycero-3-phosphorylcholines at a temperature (52 degrees) above the Tc values. Spin-lattice relaxation times of the resolved resonances indicated that location of double bonds near the middle, as compared to either end, of the hydrocarbon chain favors enhanced molecular motion along the length of the chins and especially at the terminal methyl end. In the gel state, the minimum interaction potential energy of hydrocarbon chains in bilayers formed from dioctadecenoyl lipids appears to be minimized by localization of the double bond near the middle of the chains. It is suggested that in the case of homogeneous chains the double bond primarily affects the cooperativity of interactions and has very little steric effect on van der Waals' contacts. By contrast, in bilayers of mixed lecithins, with heterogeneous chains, the steric effect may become dominant, depending on double bond position. These differences in chain packing in the gel state are promulgated beyond the phase transition to the liquid crystalline state as an enhancement of chain motion as the temperature rises above Tc.

Chain ordering in liquid crystals: II. Structure of bilayer membranes

The ordering of the hydrocarbon chain interior of bilayer membranes has been calculated using the molecular field approximation developed in previous work on liquid crystals. Different statistical averages are evaluated by exact summation over all conformations of a single chain in the field due to neighboring molecules. The internal energy of each conformation, as well as contributions arising from interaction with the molecular field and from a lateral pressure on the chain have been included. The results describe properties of both lipid monolayers and bilayers. For monolayers, the calculated pressure-area relationships are in good agreement with experimental observations. The order parameter for hydrocarbon chains in bilayers (or monolayers) as a function of temperature, lateral pressure and position along the chain, is shown and compared with the available NMR data. Combining the results of calculation and NMR measurements we obtain the value for intrinsic lateral pressure within bilayer membranes, in excellent agreement with direct measurements on surface monolayers. The calculation also gives average length of hydrocarbon chains, thermal expansion coefficient and fraction of bonds in gauche conformations. The effect of cholesterol and proteins within the bilayer is qualitatively described, and the contribution of the bilayer interior to membrane elasticity is determined.