Calorimetric studies on phospholipid—water systems

Temperature dependence of elastic properties of the phospholipid vesicles in aqueous suspension probed by Brillouin spectroscopy

The aqueous suspension of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) vesicles with different hydration levelsα(water-to-lipid mass ratio) have been studied by Brillouin spectroscopy in the temperature range from -190 °C to 70 °C. The samples with different hydration levels demonstrate similar temperature behavior of their sound velocity in the temperature range from -190 °C to -25 °C. There is a strong correlation between the hydration level of the sample and the character of the sound velocity temperature dependence at higher temperatures. Nevertheless, all hydrated samples demonstrate a jump in the sound velocity at the gel-fluid phase transition temperature. The amplitude of this jump depends on the hydration levelαof the sample. It has a maximum value in the sample with minimalαnecessary for the phospholipid membrane's full hydration. To evaluate the sound velocity in the phospholipid membrane, we applied the two-component model to analyze the experimental data obtained in the sample withα= 0.25 (close to the minimal necessary value for the full DPPC membrane hydration). It was found that for temperatures higher than 0 °C, the two-component model works well if we consider that sound velocity in water between vesicle layers is approximately a factor of two higher than in bulk water.

Ionic Liquid-Induced Phase-Separated Domains in Lipid Multilayers Probed by X-ray Scattering Studies

A cellular membrane, primarily a lipid bilayer, surrounds the internal components of a biological cell from the external components. This self-assembled bilayer is known to be perturbed by ionic liquids (ILs) causing malfunctioning of a cellular organism. In the present study, surface-sensitive X-ray scattering techniques have been employed to understand this structural perturbation in a lipid multilayer system formed by a zwitterionic phospholipid, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine. The ammonium and phosphonium-based ILs with methanesulfonate anions are observed to induce phase-separated domains in the plane of a bilayer. The lamellar X-ray diffraction peaks suggest these domains to correlate across the bilayers in a smectic liquid crystalline phase. This induced IL-rich lamellar phase has a very low lamellar repeat distance, suggesting the formation of an interdigitated bilayer. The IL-poor phase closely related to the pristine lipid phase shows a decrement in the in-plane chain lattice parameters with a reduced tilt angle. The ammonium and phosphonium-based ILs with a relatively bulky anion, p-toluenemethanesulfonate, have shown a similar effect.

Interactions of dimethylsulfoxide with a dipalmitoylphosphatidylcholine monolayer studied by vibrational sum frequency generation

The interactions between phospholipid monolayers and dimethylsulfoxide (DMSO) molecules were investigated by vibrational sum frequency generation (VSFG) spectroscopy in a Langmuir trough system. Both the head and the tail groups of dipalmitoylphosphatidylcholine (DPPC) as well as DMSO were probed to provide a comprehensive understanding of the interactions between DPPC and DMSO molecules. A condensing effect is observed for the DPPC monolayer on a concentrated DMSO subphase (>20 mol %). This effect results in a well-ordered conformation for the DPPC alkyl chains at very large mean molecular areas. Interactions between DMSO and DPPC headgroups were also studied. DMSO-induced dehydration of the DPPC phosphate group is revealed at DMSO concentration above 10 mol %. The average orientation of DMSO with DPPC versus dipalmitoylphosphate sodium salt (DPPA) monolayers was compared. The comparison revealed that DMSO molecules are perturbed and reorient because of the interfacial electric field created by the charged lipid headgroups. The orientation of the DPPC alkyl chains remains nearly unchanged in the liquid condensed phase with the addition of DMSO. This suggests that DMSO molecules are expelled from the condensed monolayer. In addition, implications for the DMSO-induced permeability enhancement of biological membranes from this work are discussed.

Properties of ternary phospholipid/dimethyl sulfoxide/water systems at low temperatures

X-ray diffraction, neutron diffraction and differential scanning calorimetry were used to investigate phase transitions in the ternary system phospholipid/dimethyl sulfoxide (DMSO)/water under cooling for three homologous phospholipids: dimyristoylphosphatidylcholine (DMPC), dipalmitoylphosphatidylcholine (DPPC), and distearoylphosphatidylcholine (DSPC). Below the temperature of ice formation from -40 to -113 degrees C, a new lamellar phase of DPPC and DSPC was found at and above a DMSO molar fraction of X(DMSO) = 0.05. Below X(DMSO) = 0.05 only a single dehydrated Lc-phase exists after ice formation. The new phase has an increased membrane repeat distance and coexists with a dehydrated Lc-phase. DPPC with a DMSO molar fraction of X(DMSO) = 0.07 shows a membrane repeat distance of the new phase of d = 6.61 +/- 0.03 nm. The value of d increases at the increase of X(DMSO). The new phase was not observed in the ternary system with DMPC. No correlation between the new phase and the glass transition of bound water in the intermembrane space was detected. The new phase was detected only in the systems with excess of water. The creation of the new phase demonstrates the specific DMSO interaction with hydrocarbon chains.

New insights into water–phospholipid model membrane interactions

Modulating the relative humidity (RH) of the ambient gas phase of a phospholipid/water sample for modifying the activity of phospholipid-sorbed water [humidity-controlled osmotic stress methods, J. Chem. Phys. 92 (1990) 4519 and J. Phys. Chem. 96 (1992) 446] has opened a new field of research of paramount importance. New types of phase transitions, occurring at specific values of this activity, have been then disclosed. Hence, it is become recognized that this activity, like the temperature T, is an intensive parameter of the thermodynamical state of these samples. This state can be therefore changed (phase transition) either, by modulating T at a given water activity (a given hydration level), or, by modulating the water activity, at a given T. The underlying mechanisms of these two types of transition differ, especially when they appear as disorderings of fatty chains. In lyotropic transitions, this disordering follows from two thermodynamical laws. First, acting on the activity (the chemical potential) of water external to a phospholipid/water sample, a transbilayer gradient of water chemical potential is created, leading to a transbilayer flux of water (Fick's law). Second, water molecules present within the hydrocarbon region of this phospholipid bilayer interact with phospholipid molecules through their chemical potential (Gibbs-Duhem relation): the conformational state of fatty chains (the thermodynamical state of the phospholipid molecules) changes. This process is slow, as revealed by osmotic stress time-resolved experiments. In thermal chain-melting transitions, the first rapid step is the disordering of fatty chains of a fraction of phospholipid molecules. It occurs a few degrees before the main transition temperature, T(m), during the pretransition and the sub-main transition. The second step, less rapid, is the redistribution of water molecules between the different parts of the sample, as revealed by T-jump time-resolved experiments. Finally, in lyotropic and thermal transitions, hydration and conformation are linked but the order of anteriority of their change, in each case, is probably not the same. In this review, first, the interactions of phospholipid submolecular fragments and water molecules, in the interfacial and hydrocarbon regions of phospholipid/water multibilayer stacks, will be described. Second, the coupling of the conformational states of phospholipid and water molecules, during thermal and lyotropic transitions, will be demonstrated through examples.

Self-association process of a peptide in solution: from beta-sheet filaments to large embedded nanotubes

Lanreotide is a synthetic octapeptide used in the therapy against acromegaly. When mixed with pure water at 10% (w/w), Lanreotide (acetate salt) forms liquid crystalline and monodisperse nanotubes with a radius of 120 A. The molecular and supramolecular organization of these structures has been determined in a previous work as relying on the lateral association of 26 beta-sheet filaments made of peptide noncovalent dimers, the basic building blocks. The work presented here has been devoted to the corresponding self-association mechanisms, through the characterization of the Lanreotide structures formed in water, as a function of peptide (acetate salt) concentration (from 2% to 70% (w/w)) and temperature (from 15 degrees C to 70 degrees C). The corresponding states of water were also identified and quantified from the thermal behavior of water in the Lanreotide mixtures. At room temperature and below 3% (w/w) Lanreotide acetate in water, soluble aggregates were detected. From 3% to 20% (w/w) long individual and monodisperse nanotubes crystallized in a hexagonal lattice were evidenced. Their molecular and supramolecular organizations are identical to the ones characterized for the 10% (w/w) sample. Heating induces the dissolution of the nanotubes into soluble aggregates of the same structural characteristics as the room temperature ones. The solubilization temperature increases from 20 degrees C to 70 degrees C with the peptide concentration and reaches a plateau between 15% and 25% (w/w) in peptide. These aggregates are proposed to be the beta-sheet filaments that self-associate to build the walls of the nanotubes. Above 20% (w/w) of Lanreotide acetate in water, polydisperse embedded nanotubes are formed and the hexagonal lattice is lost. These embedded nanotubes exhibit the same molecular and supramolecular organizations as the individual monodisperse nanotubes formed at lower peptide concentration. The embedded nanotubes do not melt in the range of temperature studied indicating a higher thermodynamic stability than individual nanotubes. In parallel, the thermal behaviors of water in mixtures containing 2-80% (w/w) in peptide have been studied by differential scanning calorimetry, and three different types of water were characterized: 1), bulk water melting at 0 degrees C, 2), nonfreezing water, and 3), interfacial water melting below 0 degrees C. The domains of existence and coexistence of these different water states are related to the different Lanreotide supramolecular structures. All these results were compiled into a binary Lanreotide-water phase diagram and allowed to propose a self-association mechanism of Lanreotide filaments into monodisperse individual nanotubes and embedded nanotubes.

Freeze/thaw effects on lipid-bilayer vesicles investigated by differential scanning calorimetry

Differential scanning calorimetry (DSC) has been used to study the effects of repeated freezing and thawing on dipalmitoylphosphatidylcholine (DPPC) vesicles. Aqueous suspensions of both multilamellar vesicles (MLVs) and large unilamellar vesicles (LUVs) were cycled between -37 and 8 degrees C, and for each thawing event, the enthalpy of ice-melting was measured. In the case of MLVs, the enthalpy increased each time the vesicles were thawed until a steady state was attained. In contrast, the enthalpies measured for LUV suspensions were independent of the number of previous thawing events. It was concluded that MLVs in terms of freezing characteristics contain two pools of water, namely bulk water and interlamellar water. Interlamellar water does not freeze under the conditions employed in the present study, and the MLVs therefore experience freeze-induced dehydration, which is the reason for the observed increase in ice-melting enthalpy. Furthermore, the thermodynamic results suggest that the osmotic stress resulting from the freeze-induced dehydration changes the lamellarity of the MLVs.

Thermal and structural behavior of anhydrous milk fat. 2. Crystalline forms obtained by slow cooling

The crystallization behavior of milk fat has been examined on slow cooling at 0.1 degrees C/min from 50 to -15 degrees C, to determine the variations of triacylglycerol organizations as a function of temperature. The experiments have been conducted with an instrument allowing coupled X-ray diffraction (XRD) at both small and wide angles and high-sensitivity differential scanning calorimetry (DSC) recordings from the same sample by taking advantage of the high-energy flux of a synchrotron. On slow cooling, milk fat triacylglycerols sequentially crystallize in four different lamellar structures with double-chain length of 41.5, 48.3, and 39.2 A and a triple-chain length of 62.2 A stackings. Simultaneous wide-angle XRD has shown that initial nucleation occurs in a packing of beta' type at about 24 degrees C. For temperature < 13 degrees C, triacylglycerols crystallize in an hexagonal subcell of alpha type, leading to the coexistence of the beta' + alpha polymorphic forms, which is recorded until -15 degrees C. Thermal analysis allowed to correlate the formation of the different crystalline species monitored by XRDT (XRD as a function of temperature) to the exothermal events recorded simultaneously by differential scanning calorimetry. The evolution of the species formed during crystallization was also monitored on heating at 2 degrees C/min. The absence of polymorphic evolution on heating, as well as the high final melting point observed, about 40 to 41 degrees C, confirmed that cooling at 0.1 degrees C/min leads to quasi equilibrium.

Thermal and Structural Behavior of Milk Fat

Milk fat crystallization was studied using X-ray diffraction as a function of temperature (XRDT) and differential scanning calorimetry (DSC) analysis considering crystals formed during slow cooling of natural milk fat globules of cream. During cooling at |dT/dt|=0.15 degrees C/min from 55 to -8 degrees C, the crystalline varieties formed in fat globules by triacyglycerols (TGs) correspond to two double-chain-length organizations (2L) of 46.5 and 40 Å and to two triple-chain-length stackings (3L) of 71.3 and 65 Å. Nucleation occurs in the alpha form; then the alpha+beta' polymorphic forms coexist until the end of the cooling. The four crystalline varieties start to form within a 10 degrees C range, from about 21 degrees C, preventing separation of overlapped peaks by DSC recording. In a second step, the sample of cream was heated at 2 degrees C/min in the range -8 to +60 degrees C to follow the melting behavior of the crystals. XRDT measurements show the progressive transformations of the crystalline varieties correlated with endotherms and exotherms recorded by DSC. The 40-Å structure takes advantage of the melting of the other species to grow until its melting. The comparison made with anhydrous milk fat behavior under the same conditions shows that crystallization is different in emulsion and in bulk. Copyright 2001 Academic Press.

Thermal and Structural Behavior of Milk Fat

The thermal and structural properties of unstable varieties of triacylglycerols (TGs) crystallizing in milk fat globules of cream are examined in the range -8- +50 degrees C using a new instrument allowing simultaneously time-resolved synchrotron X-ray diffraction at both wide and small angles as a function of temperature (XRDT) and high sensitivity differential scanning calorimetry (DSC). Small angle X-ray diffraction shows that the unstable alpha form first formed by cream quenching to -8 degrees C corresponds in fact to two different lamellar phases corresponding to 2L (47 Å) and 3L (70.4 Å) arrangements of TGs. The bilayered structure is very unstable since it disappears during the course of a 20-min isothermal conditioning at -8 degrees C. On fast heating, the crystalline evolution of cream TGs demonstrates the monotropic character of their polymorphism. The structural and thermal behaviors of cream which are compared to that of its anhydrous milk fat isolated from the cream (C. Lopez et al., J. Dairy Sci., submitted) show that the crystallization occurring in emulsion droplets is similar to bulk. However, the comparison of XRD peak widths indicates that the TG crystallization is more disordered in emulsion. This disorder is attributed to the constraints due to the interface curvature in emulsion droplets. Copyright 2000 Academic Press.

Ice premelting during differential scanning calorimetry

Premelting at the surface of ice crystals is caused by factors such as temperature, radius of curvature, and solute composition. When polycrystalline ice samples are warmed from well below the equilibrium melting point, surface melting may begin at temperatures as low as -15 degrees C. However, it has been reported (Bronshteyn and Steponkus, 1993. Biophys. J. 65:1853-1865) that when polycrystalline ice was warmed in a differential scanning calorimetry (DSC) pan, melting began at about -50 degrees C, this extreme behavior being attributed to short-range forces. We show that there is no driving force for such premelting, and that for pure water samples in DSC pans curvature effects will cause premelting typically at just a few degrees below the equilibrium melting point. We also show that the rate of warming affects the slope of the DSC baseline and that this might be incorrectly interpreted as an endotherm. The work has consequences for DSC operators who use water as a standard in systems where subfreezing runs are important.

Phase diagram of 1,2-dioleoylphosphatidylethanolamine (DOPE):water system at subzero temperatures and at low water contents

The phase behavior of partially hydrated 1, 2-dioleoylphosphatidylethanolamine (DOPE) has been studied using differential scanning calorimetry and X-ray diffraction methods together with water sorption isotherms. DOPE liposomes were dehydrated in the H(II) phase at 29 degrees C and in the L(alpha) phase at 0 degrees C by vapor phase equilibration over saturated salt solutions. Other samples were prepared by hydration of dried DOPE by vapor phase equilibration at 29 degrees C and 0 degrees C. Five lipid phases (lamellar liquid crystalline, L(alpha); lamellar gel, L(beta); inverted hexagonal, H(II); inverted ribbon, P(delta); and lamellar crystalline, L(c)) and the ice phase were observed depending on the water content and temperature. The ice phase did not form in DOPE suspensions containing <9 wt% water. The L(c) phase was observed in samples with a water content of 2-6 wt% that were annealed at 0 degrees C for 2 or more days. The L(c) phase melted at 5-20 degrees C producing the H(II) phase. The P(delta) phase was observed at water contents of <0.5 wt%. The phase diagram, which includes five lipid phases and two water phases (ice and liquid water), has been constructed. The freeze-induced dehydration of DOPE has been described with the aid of the phase diagram.

Hydration of phospholipid bilayers in the presence and absence of cholesterol

Differential scanning calorimetry (DSC) was used for determining the number of unfreezable water molecules per molecule of phosphatidylserine from bovine spinal cord (PS) or dimyristoyl phosphatidylserine (DMPS) and dimyristoyl phosphatidylcholine (DMPC), alone or in mixtures with cholesterol. It was assumed that the unfreezable water molecules are tightly bound to the phospholipid. It was found that when the phospholipids are in the gel state and in the absence of cholesterol, PS binds 2.5 water molecules, DMPS 3.8 water molecules and DMPC 3.5 water molecules. In the presence of cholesterol the number of water molecules bound increases in the region where phase separation of cholesterol takes place [D. Bach, Chem. Phys. Lipids 35 (1984) 385-392; E.J. Wachtel, N. Borochov, D. Bach, Biochim. Biophys. Acta 1066 (1991) 63-69; D. Bach, N. Borochov, E. Wacktel, Chem. Phys. Lipids, submitted].

Calorimetric and thermodielectrical measurements of water interactions with some food materials

A new thermoanalytical method, which allows the measurement of complex dielectrical permittivities as a function of temperature in the microwave frequency domain, is described and compared to Differential Scanning Calorimetry (DSC) with respect to the characterization of water in food materials. Ice crystallization temperatures and melting enthalpies, measured by DSC dynamically on the same sample, allowed precise determination of the amount of frozen water and its enthalpy as a function of total water content, especially at low water contents near the unfrozen water limit. Thermal and Dielectrical Analysis (TDA) measurements provide immediate information about water interactions in food materials, even down to the lowest water contents, which are generally difficult to assess by other techniques. Dielectrical behaviour of eight glucose-water melts, containing from 0 to 24% water, has been examined as a function of temperature. The dependence of the observed dielectric relaxations on water content and temperature are discussed, and the results obtained by TDA are compared to those from conventional frequency sweeping determinations. The influence of temperature, hydration, and state of the material on dielectrical relaxation determinations are also discussed, with reference to glucose and sorbitol behaviour. The variations in dielectric constant during starch heating and dehydration are presented and analysed, with the aim of understanding the microwave drying process.