The effects of ethylene glycol and dimethyl sulfoxide on cerebroside metastability

Unperturbed hydrocarbon chains and liquid phase bilayer lipid chains: a computer simulation study

In this work, the properties of saturated and unsaturated fatty acid acyl chains 16:0, 18:0, 18:1(n-9)cis, 18:2(n-6)cis, 18:3(n-3)cis, 18:4(n-3)cis, 18:5(n-3)cis, 20:4(n-6)cis, 20:5(n-3)cis and 22:6(n-3)cis in a bilayer liquid crystalline state and similar hydrocarbon chains (with CH[Formula: see text] terminal groups instead of C=O groups) in the unperturbed state characterised by a lack of long-range interaction were investigated. The unperturbed hydrocarbon chains were modelled by Monte Carlo simulations at temperature [Formula: see text] K; sixteen fully hydrated homogeneous liquid crystalline phosphatidylcholine bilayers containing these chains were studied by molecular dynamics simulations at the same temperature. To eliminate effects of the simulation parameters, the molecular dynamics and Monte Carlo simulations were carried out using the same structural data and force field coefficients. From these computer simulations, the average distances between terminal carbon atoms of the chains (end-to-end distances) were calculated and compared. The trends in the end-to-end distances obtained for the unperturbed chains were found to be qualitatively similar to those obtained for the same lipid chains in the bilayers. So, for understanding of a number of processes in biological membranes (e.g., changes in fatty acid composition caused by environmental changes such as temperature and pressure), it is possible to use, at least as a first approximation, the relationships between the structure and properties for unperturbed or isolated hydrocarbon chains.

Phase separation and fluctuations in mixtures of a saturated and an unsaturated phospholipid

We describe quantitatively the interactions in a mixture of a saturated and an unsaturated phospholipid, and their consequences to the phase behavior at macroscopic and microscopic levels. This type of lipid-lipid interaction is fundamental in determining the organization and physical behavior of biological membranes. Mixtures of dipalmitoylphosphatidylcholine (DPPC) and 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) are examined in detail by multiple experimental approaches (differential scanning calorimetry (DSC), fluorescence resonance energy transfer, and confocal fluorescence microscopy) in combination with Monte Carlo simulations in a lattice. The interactions between all possible pairs of lipid species and states are determined by matching the heat capacity calculated through Monte Carlo simulations to that measured experimentally by DSC. Only for one other lipid system, a mixture between two saturated phosphatidylcholines, is a similar quantitative description available. The interactions in the two systems and different representations used to model them are compared. Phase separation occurs in DPPC/POPC at about the center of the phase diagram mapped by DSC, but not at all compositions and temperatures in the coexistence region. Close to the extremes of composition, the phase behavior is best described by large fluctuations. At the heat capacity maxima in the mixtures, the domain size distributions change remarkably; large domains disappear and cooperative fluctuations increase.

Phases and phase transitions of the phosphatidylcholines

LIPIDAT (http://www.lipidat.chemistry.ohio-state.edu) is an Internet accessible, computerized relational database providing access to the wealth of information scattered throughout the literature concerning synthetic and biologically derived polar lipid polymorphic and mesomorphic phase behavior and molecular structures. Here, a review of the data subset referring to phosphatidylcholines is presented together with an analysis of these data. This subset represents ca. 60% of all LIPIDAT records. It includes data collected over a 43-year period and consists of 12,208 records obtained from 1573 articles in 106 different journals. An analysis of the data in the subset identifies trends in phosphatidylcholine phase behavior reflecting changes in lipid chain length, unsaturation (number, isomeric type and position of double bonds), asymmetry and branching, type of chain-glycerol linkage (ester, ether, amide), position of chain attachment to the glycerol backbone (1,2- vs. 1,3-) and head group modification. Also included is a summary of the data concerning the effect of pressure, pH, stereochemical purity, and different additives such as salts, saccharides, amino acids and alcohols, on phosphatidylcholine phase behavior. Information on the phase behavior of biologically derived phosphatidylcholines is also presented. This review includes 651 references.

DMSO produces a new subgel phase in DPPC: DSC and X-ray diffraction study

Equilibrium phases and the kinetics of subgel phase transformation of dipalmitoylphosphatidylcholine (DPPC) hydrated with mixtures of dimethylsulfoxide (DMSO)/water have been studied using differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The rate of gel-to-subgel transformation is decreased with a small increase in X, the DMSO/water mole fraction, but then speeds up and becomes faster than in pure water by X = 0.16. The DSC scans show multiple subgel peaks, some of which can be attributed to impacted domain growth. For X greater than 0.10, XRD shows that there is a new, stable subgel phase, S, which also accounts for some of the multiplicity of DSC peaks. Our electron density profiles show that the thickness of the bilayer in the S phase is greater than in the usual C subgel phase. We suggest that the S subgel phase is characterized by different headgroup ordering and smaller chain tilt angle than in the C subgel phase. Electron density profiles show that increasing X decreases the water space between bilayers in all phases, subgel, gel and fluid (L alpha). For X = 0.20, a different gel phase is also observed that may be due to subtle changes in the orientation of chain tilt first observed in partially dehydrated DMPC. The dehydrating effect of DMSO explains the results of a previous study, confirmed in this study, that increasing the concentration of DMSO raises the main transition temperature and eliminates the ripple phase.

Phases and phase transitions of the sphingolipids

LIPIDAT is a computerized database providing access to the wealth of information scattered throughout the literature concerning synthetic and biologically derived polar lipid polymorphic and mesomorphic phase behavior. Herein, we present a review of the LIPIDAT data subset referring to sphingolipids together with an analysis of these data. It includes data collected over a 40-year period and consists of 867 records obtained from 112 articles in 25 different journals. An analysis of these data has allowed us to identify trends in hydrated sphingolipid phase behavior reflecting differences in fatty acyl chain length, saturation and hydroxylation, head group type, and sphingoid base identity. Information on the mesomorphism of biologically-derived and dry sphingolipids is also presented. This review includes 161 references.

Thermotropic behavior of phosphatidylcholine-glucosyl ceramide mixtures: effects of phospholipid acyl chain composition and interaction with water

The thermotropic behavior of multilamellar vesicles composed of mixtures of dimyristoyl phosphatidylcholine-glucosyl ceramide and of egg phosphatidylcholine-glucosyl ceramide was investigated using differential scanning calorimetry. Macroscopic demixing of the lipid components occurred when multilamellar vesicles were prepared from mixtures of glucosyl ceramide and egg phosphatidylcholine by conventional methods. This problem was overcome by a technique based on spray drying of the lipid mixture. The results obtained for the two systems are compared with data available for dipalmitoyl phosphatidylcholine-glucosyl ceramide mixtures (Biochemistry 22 (1983) 3497-3501). All three phosphatidylcholines perturb the complex thermotropic behavior of glucosyl ceramide. The data suggest that the interference with intermolecular interactions among glycosyl ceramide molecules by phospholipid molecules is related to the molecular miscibility of the two components. This is strongly dependent on the acyl chain composition of the phosphatidylcholine and the water activity of the ambient aqueous phase.

Thermotropic properties of dispersions of cholesterol with tetraether lipids from Thermoplasma acidophilum

The main glycophospholipid from Thermoplasma acidophilum is composed of a diisopranol-2,3-glycerotetraether. The fraction of pentane cyclizations of its hydrocarbon chains increases with the growth temperature of the source organism (39-59 degrees C). Hydrated mixtures of these lipids together with cholesterol have been studied by calorimetry. With the reduction of the phase transition temperatures and enthalpy changes of the transitions, cholesterol is readily incorporated into lipid monolayers in the liquid-crystalline and the (metastable) solid-analogue phase. Lipid samples with a high number of acyclic hydrocarbon chains form a stable and a metastable solid-analogue phase. With the increasing concentration of cholesterol the metastable solid-analogue phase is stabilized and the time constant for the formation of the stable solid-analogue phase is prolonged.

Differential scanning calorimetric and Fourier transform infrared spectroscopic investigations of cerebroside polymorphism

Calorimetric and Fourier transform infrared (FTIR) spectroscopic studies have been made of the polymorphism exhibited by bovine brain cerebroside-water systems, and the effect of cholesterol and dipalmitoylphosphatidylcholine (DPPC) upon this polymorphism was investigated. The conversion of the cerebroside from the thermodynamically stable to the metastable form is found to be accompanied by spectral changes, indicating a decrease in cerebroside headgroup hydration and a rearrangement of the hydrogen-bond network. The incorporation of low concentrations of cholesterol and DPPC into cerebroside bilayers broadens the thermal transitions associated with the cerebroside as a result of the disruption of cerebroside-cerebroside interactions. This disruption is evident in the spectra of cerebroside/cholesterol mixtures.

Influence of head-group interactions on the miscibility of synthetic, stereochemically pure glycolipids and phospholipids

Phase diagrams of binary mixtures of the glycoglycerolipids 1,2-di-O-tetradecyl-3-O-beta-D-galactosyl-sn-glycerol (14-Gal) and 1,2-di-O-tetradecyl-3-O-beta-D-glucosyl-sn-glycerol (14-Glc) with the phospholipids L-dimyristoylphosphatidylcholine (DMPC) and L-dimyristoylphosphatidylethanolamine (DMPE) were recorded by high-sensitivity differential scanning calorimetry and used for determination of the glycolipid-phospholipid miscibility in solid and liquid-crystalline states. As a consequence of a metastable behavior of both glycolipids and DMPE, the solid-state glycolipid/phospholipid miscibility was strongly dependent on the temperature prehistory of the samples. While DMPC and 14-Glc mix continuously, the other three binaries display extended regions of solid-solid-phase separation in the equilibrium low-temperature states. The DMPE/glycolipid phase diagrams were of clearly expressed eutectic type. Continuous solutions were formed in the liquid-crystalline and in the metastable solid phases of the mixtures. Simulations of the shape of the phase diagrams using the Bragg-Williams approximation showed certain deviations from ideal mixing in the liquid-crystalline continuous solutions. Since both glycolipids and phospholipids contain fully saturated fatty acids of equal chain length, their mixing properties were predominantly determined by the interactions between the lipid polar moieties, assuming the influence of ester or either linkages of the alkyl chains on the mixing parameters to be negligible. The clearly expressed differences in the mixing of 14-Glc and 14-Gal with phospholipids are most probably due to different hydrogen-bond networks formed by the glucosyl and galactosyl residues.

Phase behavior of cerebroside and its fractions with phosphatidylcholines: calorimetric studies

Bovine brain cerebroside and its kerasin (beta-D-galactosyl-N-acyl-D-sphingosine) and phrenosin (beta-D-galactosyl-N-(2-D-hydroxyacyl)-D-sphingosine) fractions were mixed with diacylphosphatidylcholines (PCs) to form fully hydrated lamellar phases. These mixtures were examined by differential scanning calorimetry, and phase diagrams for cerebroside/diacylPC mixtures were constructed from the data. Cerebroside was found to be miscible with egg PC at low mole fractions X of cerebroside; the mixture behaves non-ideally for X greater than 0.25. The non-ideal behavior appears to be a superposition of separate interactions of kerasin and phrenosin with egg PC. Strikingly, phrenosin mixes nearly ideally with egg PC. Kerasin mixed with egg PC yields a peritectic phase diagram. Cerebroside and phrenosin were found to be immiscible with dimyristoylphosphatidylcholine (DMPC) in the gel state in low proportions. Both stable and metastable gel phases of kerasin were detected in different endotherms of kerasin/PC mixtures. Kerasin in the stable and metastable gel states exhibits discontinuous and continuous ranges of miscibility, respectively, with DMPC. The stable gel phase of kerasin does not segregate in natural cerebroside. Natural kerasin was found to act isomorphic to semi-synthetic (natural configuration) D-kerasins but not completely to synthetic DL-kerasins of single acyl chain lengths.

Effect of glycerol on the molecular properties of cerebrosides, sulphatides and gangliosides in monolayers

The presence of glycerol, free from surface-active impurities, modifies the molecular area, surface potential/molecule and thermodynamic parameters of compression of monolayers of galactosylceramide, sulphatide and gangliosides GM1, GD1a and GT1b. This may be due to changes of the composition and structural properties of the glycosphingolipid solvation shell with an influence on the intermolecular organization.

The lipoidal permeability barriers of the skin and alimentary tract

The major routes of administration of drugs to humans involve transport either through the intestinal wall or through the skin. Both these barriers are nonpolar in nature and are subserved by membrane lipids. The lipid composition of the brush border of the intestinal wall is unusual, possessing unusually large quantities of glycosylceramide. The lipid composition of the stratum corneum of the skin is also unusual, possessing large quantities of ceramides and free fatty acids. These atypical membrane components are generally more ordered than the other common membrane lipids at body temperature and, thus, are suited for involvement in formation of barriers between the organism and its environment.

Spontaneous vesicularization of myelin lipids is counteracted by myelin basic protein

Hand-vortexed dispersions of several lipids (cerebrosides, sulfatides, PC, PE, PS and sphingomyelin), mixed in the ratios found for these categories of lipids in myelin, exhibit 31P-NMR spectra which have contributions from both isotropic and lamellar resonances. Investigation of this system by freeze-fracture electron microscopy and X-ray diffraction revealed that this lipid mixture has spontaneously formed small unilamellar vesicles (SUVs) (diam. approximately 400 A) and large highly convoluted unilamellar vesicles (LUVs) (diam. approximately 1000 A), the latter possibly resulting from aggregation and fusion of the SUV structures. This vesicularization of the myelin lipids was reversed by the addition of myelin basic protein: only large multilamellar aggregates were formed in the presence of protein, as shown by all three experimental methods. Although no rigorous physical-chemical explanation for these phenomena is yet available, the possibility is suggested that the high concentration of cerebrosides and/or phosphatidylethanolamine in this particular mixture of myelin lipids play pivotal roles in the formation of these unusual vesicles. Spontaneous vesicularization of myelin lipids is discussed as a potential pathway toward destabilization of the myelin sheath.

Effect of ethylene glycol on the phase transition kinetics of gluco- and galactocerebrosides

The effect of different concentrations of ethylene glycol in water on the phase transition (metastable----stable state) of Gaucher's glucocerebroside, of bovine brain cerebroside type II (non hydroxy acyl chains only) and of N-palmitoylgalactocerebroside has been investigated. The phase transition and its kinetics were inferred from the thermograms at different heating and cooling rates and confirmed by FTIR spectra of the cerebrosides in the different states. The significance of the conformational differences of the glucose and of the galactose residues with respect to their solvation, and the subsequent effect on the intermolecular interactions and the phase transition is discussed.