Phases and phase transitions of the glycoglycerolipids

Ordered Membrane Domain-Forming Properties of the Lipids of Borrelia burgdorferi

Co-existing disordered and ordered (raft) membrane domains exist in Borrelia burgdorferi, the causative agent of Lyme disease. However, although B. burgdorferi contains cholesterol lipids, it lacks sphingolipids-a crucial component of rafts in eukaryotes. To define the principles of ordered lipid domain formation in Borrelia, the domain forming properties of vesicles composed of its three major lipids, acylated cholesteryl galactoside (ACGal), monogalactosyl diacyglycerol (MGalD), and phosphatidylcholine (PC) and/or their mixtures were studied. Anisotropy and fluorescence resonance energy transfer measurements were used to assay membrane order and ordered-domain formation. ACGal had the highest potential to form ordered domains. Interestingly, mixtures of ACGal with B. burgdorferi PC formed ordered domains more readily than mixtures of ACGal with MGalD. This appears to reflect the relatively high level of saturation observed for B. burgdorferi PC, as vesicles containing ACGal and PC, but in which the unsaturated lipid dioleoyl PC was substituted for Borrelia PC, failed to form ordered domains. In addition, the properties of ACGal were compared to those of cholesterol. Depending on what other lipids were present, ordered-domain formation in the presence of ACGal was greater than or equal to that in the presence of cholesterol. Giant unilamellar vesicles formed from ACGal-containing mixtures showed rounded domain shapes similar to those in analogous vesicles containing cholesterol, indicative of liquid-ordered state rather than solid-like gel-state domain formation. Over all, principles of ordered-domain formation in B. burgdorferi appear to be very similar to those in eukaryotes, with saturated PC taking the place of sphingolipids, but with ACGal being the main lipid component inducing ordered-domain formation.

The thermodynamics of simple biomembrane mimetic systems

Insight into the forces governing a system is essential for understanding its behavior and function. Thermodynamic investigations provide a wealth of information that is not, or is hardly, available from other methods. This article reviews thermodynamic approaches and assays to measure collective properties such as heat adsorption / emission and volume variations. These methods can be successfully applied to the study of lipid vesicles (liposomes) and biological membranes. With respect to instrumentation, differential scanning calorimetry, pressure perturbation calorimetry, isothermal titration calorimetry, dilatometry, and acoustic techniques aimed at measuring the isothermal and adiabatic processes, two- and three-dimensional compressibilities are considered. Applications of these techniques to lipid systems include the measurement of different thermodynamic parameters and a detailed characterization of thermotropic, barotropic, and lyotropic phase behavior. The membrane binding and / or partitioning of solutes (proteins, peptides, drugs, surfactants, ions, etc.) can also be quantified and modeled. Many thermodynamic assays are available for studying the effect of proteins and other additives on membranes, characterizing non-ideal mixing, domain formation, bilayer stability, curvature strain, permeability, solubilization, and fusion. Studies of membrane proteins in lipid environments elucidate lipid-protein interactions in membranes. Finally, a plethora of relaxation phenomena toward equilibrium thermodynamic structures can be also investigated. The systems are described in terms of enthalpic and entropic forces, equilibrium constants, heat capacities, partial volume changes, volume and area compressibility, and so on, also shedding light on the stability of the structures and the molecular origin and mechanism of the structural changes.

Influence of the lamellar phase unbinding energy on the relative stability of lamellar and inverted cubic phases

Based on curvature energy considerations, nonbilayer phase-forming phospholipids in excess water should form stable bicontinuous inverted cubic (Q(II)) phases at temperatures between the lamellar (L(alpha)) and inverted hexagonal (H(II)) phase regions. However, the phosphatidylethanolamines (PEs), which are a common class of biomembrane phospholipids, typically display direct L(alpha)/H(II) phase transitions and may form intermediate Q(II) phases only after the temperature is cycled repeatedly across the L(alpha)/H(II) phase transition temperature, T(H), or when the H(II) phases are cooled from T > T(H). This raises the question of whether models of inverted phase stability, which are based on curvature energy alone, accurately predict the relative free energy of these phases. Here we demonstrate the important role of a noncurvature energy contribution, the unbinding energy of the L(alpha) phase bilayers, g(u), that serves to stabilize the L(alpha) phase relative to the nonlamellar phases. The planar L(alpha) phase bilayers must separate for a Q(II) phase to form and it turns out that the work of their unbinding can be larger than the curvature energy reduction on formation of Q(II) phase from L(alpha) at temperatures near the L(alpha)/Q(II) transition temperature (T(Q)). Using g(u) and elastic constant values typical of unsaturated PEs, we show that g(u) is sufficient to make T(Q) > T(H) for the latter lipids. Such systems would display direct L(alpha) --> H(II) transitions, and a Q(II) phase might only form as a metastable phase upon cooling of the H(II) phase. The g(u) values for methylated PEs and PE/phosphatidylcholine mixtures are significantly smaller than those for PEs and increase T(Q) by only a few degrees, consistent with observations of these systems. This influence of g(u) also rationalizes the effect of some aqueous solutes to increase the rate of Q(II) formation during temperature cycling of lipid dispersions. Finally, the results are relevant to protocols for determining the Gaussian curvature modulus, which substantially affects the energy of intermediates in membrane fusion and fission. Recently, two such methods were proposed based on measuring T(Q) and on measuring Q(II) phase unit cell dimensions, respectively. In view of the effect of g(u) on T(Q) that we describe here, the latter method, which does not depend on the value of g(u), is preferable.

Liposomal nanocapsules in food science and agriculture

Liposomes, spherical bilayer vesicles from dispersion of polar lipids in aqueous solvents, have been widely studied for their ability to act as drug delivery vehicles by shielding reactive or sensitive compounds prior to release. Liposome entrapment has been shown to stabilize encapsulated, bioactive materials against a range of environmental and chemical changes, including enzymatic and chemical modification, as well as buffering against extreme pH, temperature, and ionic strength changes. Liposomes have been especially useful to researchers in studies of various physiological processes as models of biological membranes in both eukaryotes and prokaryotes. Industrial applications include encapsulation of pharmaceuticals and therapeutics, cosmetics, anti-cancer and gene therapy drugs. In the food industry, liposomes have been used to deliver food flavors and nutrients and more recently have been investigated for their ability to incorporate food antimicrobials that could aid in the protection of food products against growth of spoilage and pathogenic microorganisms. In this review we briefly introduce key physicochemical properties of liposomes and review competing methods for liposome production. A survey of non-agricultural and food applications of liposomes are given. Finally, a detailed up-to-date summary of the emerging usage of liposomes in the food industry as delivery vehicles of nutrients, nutraceuticals, food additives, and food antimicrobials is provided.

Ordered 2-D and 3-D nanostructured amphiphile self-assembly materials stable in excess solvent

Amphiphile lyotropic liquid crystalline self-assembly materials are being used for a diverse range of applications. Historically, the most studied lyotropic liquid crystalline phase is probably the one-dimensional (1-D) lamellar phase, which has been employed as a model system for biomembranes and for drug delivery applications. In recent years, the structurally more complex 2-D and 3-D ordered lyotropic liquid crystalline phases, of which reversed hexagonal (H(2)) and reversed cubic phases (v(2)) are two prominent examples, have received growing interest. As is the case for the lamellar phase, these phases are frequently stable in excess water, which facilitates the preparation of nanoparticle dispersions and makes them suitable candidates for the encapsulation and controlled release of drugs. Integral membrane protein crystallization media and templates for the synthesis of inorganic nanostructured materials are other applications for 2-D and 3-D amphiphile self-assembly materials. The number of amphiphiles identified as forming nanostructured reversed phases stable in excess solvent is rapidly growing. In this article, different classes of amphiphiles that form reversed phases in excess solvent are reviewed, with an emphasis on linking phase behavior to amphiphile structure. The different amphiphile classes include: ethylene oxide-, monoacylglycerol-, glycolipid-, phosphatidylethanolamine-, and urea-based amphiphiles.

Lipid composition determines interaction of liposome membranes with Pluronic L61

Triblock copolymers of ethylene oxide (EO) and propylene oxide (PO) of EO(n/2)PO(m)EO(n/2) type (Pluronics) demonstrate a variety of biological effects that are mainly due to their interaction with cell membranes. Previously, we have shown that Pluronics can bind to artificial lipid membranes and enhance accumulation of the anti-tumor drug doxorubicin (DOX) inside the pH-gradient liposomes and transmembrane migration (flip-flop) of NBD-labeled phosphatidylethanolamine in the liposomes composed from one component-lecithin. Here, we describe the effects caused by insertion of other natural lipids in lecithin liposomes and the significance of the lipid composition for interaction of Pluronic L61 with the membrane. We used binary liposomes consisting of lecithin and one of the following lipids: cholesterol, phosphatidylethanolamine, ganglioside GM1, sphingomyelin, cardiolipin or phosphatidic acid. The influence of the additives on (1) membrane microviscosity; (2) binding of Pluronic L61; (3) the copolymer effect on lipid flip-flop and membrane permeability towards DOX was studied. The results showed that insertion of sphingomyelin and cardiolipin did not influence membrane microviscosity and effects of Pluronic on the membrane permeability. Addition of phosphatidic acid led to a decrease in microviscosity of the bilayer and provoked its destabilization by the copolymer. On the contrary, cholesterol increased microviscosity of the membrane and decreased binding of Pluronic and its capacity to enhance flip-flop and DOX accumulation. Analogous tendencies were revealed upon incorporation of egg phosphatidylethanolamine or bovine brain ganglioside GM1. Thus, a reverse dependence between the microviscosity of membranes and their sensitivity to Pluronic effects was demonstrated. The described data may be relevant to mechanisms of Pluronic L61 interaction with normal and tumor cells.

Thermotropic and lyotropic properties of long chain alkyl glycopyranosides: part III: pH-sensitive headgroups

As part of a series of papers, the influence of carbohydrate headgroups and aliphatic chains on the mesogenic properties of glycolipids was investigated. Alkyl glycosides with different types of aliphatic chains were synthesised. Neutral glycolipids were oxidized to their uronic acid derivatives, using the well established TEMPO-oxidation. For comparison a 6-deoxy-6-amino alkylglucopyranoside was synthesised. In addition, the thermotropic and lyotropic phase behaviour of the synthesised compounds were investigated. The thermotropism was characterised by polarising microscopy, the lyotropism by the contact preparation method.

Synthesis and mesogenic properties of glycosyl diacylglycerols

We synthesised glycosyl diacylglycerols bearing unsaturated or chiral methyl branched fatty acid chains. The thermotropism was measured with polarising microscopy and additionally the lyotropism with the contact preparation method. The synthesised compounds displayed thermotropic S(A) (lamellar), cubic and columnar phases and investigation of the lyotropic phase behaviour led to the observation of inverted bicontinuous cubic V(II) phases, lamellar L(alpha) phases and normal bicontinuous cubic V(I) phases. The phases are discussed with respect to the chemical structures that have been varied systematically to derive structure--property relationships.

The enthalpy of acyl chain packing and the apparent water-accessible apolar surface area of phospholipids

The energetics of phospholipid aggregation depend on the apparent water-accessible apolar surface area (ASAap), ordering effects of the chains, and headgroup interactions. We quantify the enthalpy and entropy of these interactions separately. For that purpose, the thermodynamics of micelle formation of lysophosphatidylcholines (LPCs, chains C10, C12, C14, and C16) and diacylphosphatidylcholines (DAPCs, chains C5, C6) and C7) are studied using isothermal titration calorimetry. The critical micelle concentration (CMC) values are 90, 15, and 1.9 mM (C5-C7-DAPC) and 6.8, 0.71, 0.045, and 0.005 mM (LPCs). The group contributions per methylene of DeltaDeltaG(0) = -3.1 kJ/mol and DeltaDeltaC(P) = -57 J/(mol. K) for LPCs agree with literature data on hydrocarbons and amphiphiles. An apparent deviation of DAPCs (-2.5 kJ/mol, 45 J/(mol. K)) is due to an intramolecular interaction between the two chains, burying 20% of the surface. The chain/chain interaction enthalpies in a micelle core are by approximately -2 kJ/(mol) per methylene group more favorable than in bulk hydrocarbons. We conclude that the impact of the chain conformation and packing on the interaction enthalpy is very pronounced. It serves to explain a variety of effects reported on membrane binding. Interactions within the water-accessible region show considerable DeltaH, but almost no DeltaG(0). The heat capacity changes suggest about three methylene groups (ASAap approximately 100 A2) per LPC remain exposed to water in a micelle (DAPC: 2 CH2/70 A2).

Thermotropic and lyotropic properties of long chain alkyl glycopyranosides. Part II. Disaccharide headgroups

We have investigated the thermotropic and lyotropic properties of some long chain alkyl glycosides with disaccharide headgroups. The thermotropism was measured with polarising microscopy and additionally the lyotropism with the contact preparation method, Fourier-transform Infrared (FTIR) spectroscopy, X-ray diffraction and small angle neutron scattering. A broad thermotropic as well as lyotropic polymorphism was found. The compounds displayed thermotropic S(A) (lamellar) and cubic phases, and the investigation of the lyotropic phase behaviour led to the observation of inverted bicontinuous cubic V(II) phases, lamellar L(alpha) phases, normal bicontinuous cubic V(I) phases, normal columnar H(I) phases, normal discontinuous cubic I(I) phases and lyotropic cholesteric phases. The phases are discussed with respect to the chemical structures that have been varied systematically to derive structure-property relationships.

Thermotropic and lyotropic properties of long chain alkyl glycopyranosides. Part I: monosaccharide headgroups

A systematic structure variation of a classical amphiphile (dodecyl-beta-D-glucopyranoside) is performed, demonstrating the influence of anomeric linkage, configuration, ring size and flexibility as well as electric charges on the mesophase behaviour. In addition, we have investigated the thermotropic and lyotropic properties of some long chain alkyl glycosides with monosaccharide headgroups. The thermotropism was measured with polarizing microscopy and differential scanning calorimetry, and additionally the lyotropism with FTIR spectroscopy and X-ray diffraction.

Phase behavior of fluorocarbon and hydrocarbon double-chain hydroxylated and galactosylated amphiphiles and bolaamphiphiles. Long-term shelf-stability of their liposomes

This paper describes the morphological characterization, by freeze-fracture electron microscopy, and the thermotropic phase behavior, by differential scanning calorimetry and/or X-ray scattering, of aqueous dispersions of various hydroxylated and galactosylated double-chain amphiphiles and bolaamphiphiles, several of them containing one or two hydrophobic fluorocarbon chains. Colloidal systems are observed in water with the hydroxylated hydrocarbon or fluorocarbon bolaamphiphiles only when they are dispersed with a co-amphiphile such as rac-1,2-dimyristoylphosphatidylcholine (DMPC) or rac-1,2-distearoylphosphatidylcholine (DSPC). Liposomes are formed providing the relative content of bolaamphiphiles does not exceed 20% mol. Most of these liposomes can be thermally sterilized and stored at room temperature for several months without any significant modification of their size and size distribution. The hydrocarbon galactosylated bolaamphiphile HO[C24][C12]Gal forms in water a lamellar phase (the gel to liquid-crystal phase transition is complete at 45 degrees C) and a Im3m cubic phase above 47 degrees C. The fluorocarbon HO[C24][F6C5]Gal analog displays a more complex and metastable phase behavior. The fluorinated non-bolaform galactosylated [F8C7][C16]AEGal and SerGal amphiphiles form lamellar phases in water. Low amounts (10% molar ratio) of the HO[C24][F6C5]Gal or HO[C24][C12]Gal bolaamphiphiles or of the single-headed [F8C7][C16]AEGal improve substantially the shelf-stability of reference phospholipon/cholesterol 2/1 liposomes. These liposomes when co-formulated with a single-headed amphiphile from the SerGal series are by far less stable.

Self-assembly of synthetic glycolipid/water systems

Glycolipids (amphiphiles that bear oligosaccharides as their hydrophilic headgroups) are of importance both scientifically and technically. This review describes recent advances in our understanding of the molecular correlations in phase behavior in aqueous glycolipids over the past several years. In the first part, we discuss how headgroup stereochemistry affects the phase behavior of glycolipids both in two- and three-dimensional systems. In the second part, we discuss the effects of alkyl chain structure and phase behavior of phytanyl-chained glycolipid/water systems. The physical properties of glycolipid/water systems depend strongly on the inter-headgroup interactions that are related to such factors as stereochemistry (conformation) and size of headgroups, type of sugar residues involved, alkyl chain structure, etc. Thus, apart from the conventional concept like 'hydrophilic/lipophilic balance', explicit accounts of headgroup interactions are crucial to control the particular glycolipid/water system concerned. This is in marked contrast to the conventional amphiphile/water systems where the inter-headgroup interactions are in most cases simply repulsive.

Infrared spectroscopy of glycolipids

The application of infrared spectroscopy to the physicochemical characterization of glycolipids as biologically important membrane constituents is described. In this contribution, the analysis of diacyl- and sphingosine-based (cerebroside, ganglioside) oligosaccharides and the class of lipid A-anchored lipooligo- and polysaccharides is reviewed. Furthermore, interaction of glycolipids with various agents as well as of sugars with membranes are discussed. The reviewed data prove the capacity of FTIR spectroscopy to monitor intra- and intermolecular interactions under physiologically relevant conditions refering to pH, temperature, and ion concentrations. This includes the characterization of acyl chain order and the beta<-->alpha chain melting transition, cation and drug binding to charged groups, and supramolecular organization of the glycolipid aggregates. In various examples, the biological relevance of IR data analysis are presented.

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.

Microviscosity of cucumber (Cucumis sativus L.) fruit protoplast membranes is altered by triacontanol and abscisic acid

Cucumber (Cucumis sativus L.) fruit protoplast membranes were probed with diphenylhexatriene (DPH) and pyrene, and also with two different plant growth regulators, triacontanol (TRIA) and abscisic acid (ABA). Fluorescence anisotropies of DPH and pyrene were measured after incorporating them into the membranes. The fluorescence lifetime of membrane-bound pyrene was also measured by using neodymium-doped yttrium aluminium garnet (Nd:YAG) laser of 35 ps pulses. The microviscosities of the membranes were calculated using the values of fluorescence anisotropy and lifetime. In the presence of TRIA and ABA, there was a sharp decrease in the fluorescence lifetime of pyrene. Similarly, there was also a decrease in the microviscosities of the membranes and increase in the rate of rotation of membrane-bound fluorophore, induced by the plant growth regulators. Furthermore, TRIA or TRIA + ABA could reduce the fluorescence anisotropy of both the fluorophores whereas, ABA decreased the anisotropy of only pyrene. This property of ABA may be due to its confinement to a specific spacial facet in the membrane. Fatty acid analysis indicated that membrane microviscosity fluctuations were not due to altered fatty acid composition alone as it is known that change in lipid-protein interaction would also alter the physical status of the membrane.

Enthalpy is a proper criterion for comparability of monolayer and bilayer studies: isobaric temperature scanning measurements on glycolipid monolayers

The thermotropic behaviour of glycolipid monolayers has been studied by isobaric temperature scanning measurements to elucidate conditions under which monolayers exhibit thermodynamic and structural properties comparable to those observed in bilayers. A selection of synthetic, stereochemically pure, glyceroglycolipids with identical, ether-linked alkyl chains of 12, 14, or 16 CH2-groups has been investigated. The head groups of the glycolipids consisted of glucose, galactose, maltose, lactose or maltotriose moieties with beta-configuration of the glycosidic bond. These glycolipids were chosen to permit a quantitative characterization of three effects, (i) the role of the length of the aliphatic chains, (ii) the influence of the size of the head group, and (iii) the influence of the stereochemistry of the sugar moieties on the structure and stability of the monolayers. To probe the effects of stereochemical alterations in the glycerol moiety 2,3-O-ditetradecyl-1-O-beta-D-glucosyl-sn-glycerol (14-2,3-Glc) was compared with 1,2-O-ditetradecyl-3-O-beta-D-glucosyl-sn-glycerol (14-1,2-Glc). It has been shown that in general several features of bilayers can be obtained from monolayer studies with reasonable accuracy, provided the proper parameters are chosen. The monolayer is stabilized by elongation of the aliphatic chains of the lipids and destabilized when the monosaccharide read groups is replaced by a di-, or trisaccharide, in a similar manner as in the bilayer. The stabilizing effect that has been observed in bilayer studies, when galactose instead of glucose is introduced as head group, has also been established in the monolayer studies. This stabilizing effect is even retained in the lipids having disaccharide head groups. On the basis of these monolayer studies in connection with WAXS and SAXS measurements on multilamellar systems, we suggest that identity of transition enthalpies of the chain melting L beta-L alpha transition is an appropriate criterion for estimating molecular areas and area changes of bilayers from monolayer measurements and vice versa. However, estimates of transition temperatures are poor using the enthalpy criterion. If identity of transition temperature is introduced as criterion, glycolipid monolayers must be compressed to about 43 +/- 3 mN m-1. Under these conditions the agreement between the calculated enthalpies and structural properties of monolayers and multilayers is poor. As a general conclusion it can be emphasized that for monolayer and bilayer systems of glycolipids there exists no such parameter as a universal pressure or a universal temperature that automatically renders monolayer data identical to bilayer data. Depending on which property (transition temperatures, transition enthalpies, lateral areas and transitional area changes) one wants to extrapolate from monolayer to bilayer different lateral pressures have to be applied.

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.

The aggregation behaviour of two structurally isomeric glycolipids

The two structurally isomeric glycolipids N-octyl-(1-hexadecylamido)-beta-D-glucopyranoside (Glc-N-[8/16]) and N-hexadecyl-(1-octylamido)-beta-D-glucopyranoside (Glc-N[16/8]) were synthesized. The lyotropic phase behaviour of these isomers was investigated in order to determine the extent to which the relative disposition of the alkyl chains with respect to the amide unit affects the aggregation properties of the amphiphiles. Both isomers exhibit lamellar and inverse topology cubic mesophases. The extent and polymorphism of the non-lamellar phases appear to be more pronounced in the Glc-N-[8/16] isomer relative to the Glc-N-[16/8] isomer. A molecular mechanics study was undertaken in an attempt to rationalize this behaviour.