Thermotropic Behavior Of Aqueous Dispersions Research Articles

The metastability of glucosyl ceramide in aqueous phase: effect of hydration and phosphatidylcholines of various chain length

The thermotropic behavior of aqueous dispersions of glucosylceraroide and mixtures of (i) dimyristoylphosphatidylcholine-glucosylceramide; (ii) dipalmitoylphosphatidylcholineglucosylceramide; and (iii) egg phosphatidylcholine-glucosylceramide were investigated using differential scanning calorimetry. All these systems form a lamellar array in aqueous phase. Aqueous dispersions of glucocosylceramide exhibit complex thermotropic behavior due to the presence of metastable and stable gel allomorphs. This polymorphism results primarily from hydration-dehydration processes which involve head group-head group interactions. All three phosphatidylcholines disturb the complex thermotropic behavior of glucosylceramide. The data suggest that the interference with the intermolecular interactions among the glycosphingolipid molecules by phospholipid molecules is strongly dependent on the acyl chain of the phosphatidylcholine. This effect is directly related to phospholipid-glycosphingolipid molecular miscibility which is determined from the minimal mole % phosphatidylcholine required to abolish the complex thermotropic behavior of the cerebroside. Based on this criteria we found the following order of efficacy, dipalmitoylphosphatidylcholine > dimyristoylphosphatidylcholine >> egg-phosphatidylcholine (16.5 > 22.5 >> 60 mole %, phosphatidylcholine, respectively). Reducing the level of water below its saturation or replacing 50% of the water by ethylene glycol reduces this effect of the phosphatidylcholine. This supports the assumption that hydration and hydrogen bonding are involved. Phase separation occurs throughout all the phase diagrams of the egg phosphatidylcholine-glucosyl ceramide. This was not the case for the systems of the two disaturated phosphatidylcholines. The system of egg phosphatidylcholine-glucosyi ceramide may be of relevance to Gaucher's disease which is characterized by major accumulation of glucosyi ceramide in various organs.

Eutectic phase behavior of 1-stearoyl-2-caprylphosphatidylcholine and dimyristoylphosphatidylcholine mixtures

The thermotropic behavior of aqueous dispersions of C(18):C(10)PC/diC(14)PC mixtures with different molar ratios has been investigated by high-resolution differential scanning calorimetry. C(18):C(10)PC is a highly asymmetric lipid molecule, whereas diC(14)PC is a symmetric species with the same molecular weight. Their packing properties in the bilayer are known to be similar at T > T m, but very dissimilar at T < T m. Calorimetric results indicate that C(18):C(10)PC and diC(14)PC are completely miscible in the liquid-crystalline state. In the gel state, however, C(18):C(10)PC and diC(14)PC are only partially miscible. The temperature-composition phase diagram for C(18):C(10)PC/diC(14)PC mixtures has the shape characteristic of a typical eutectic system.

Differential scanning calorimetric studies of aqueous dispersions of phosphatidylcholines containing two polyenoic chains

The thermotropic behavior of aqueous dispersions of phosphatidylcholines (PC) that contain two identical unsaturated acyl chains, either linoleoyl, α-linolenoyl, or arachidonoyl chains, has been investigated by differential scanning calorimetry. These dipolyenoic PC displayed very broad thermal transitions, which occurred over a range of approximately 40 C deg, that started between −77°cC (20:4-20:4 PC) and −71° C (18:2-18:2 PC). The enthalpy changes associated with the transitions were small (1 to 2 kcal/mol, 4 to 8 kJ/mol). The broad, low-enthalpy transitions suggest that these lipids might have a very disordered gel phase, or that the transitions from the gel to the liquid crystal might proceed through some intermediate states. The temperatures over which the transitions occurred were influenced only slightly by the number of double bonds between two and four per chain.

Effect of myelin basic protein on the thermotropic behavior of aqueous dispersions of neutral and anionic glycosphingolipids and their mixtures with dipalmitoylphosphatidylcholine.

The thermotropic behavior of the natural glycosphingolipids galactosylceramide, asialo-Gal beta 1-3GalNAc beta 1-4Gal(3-2 alpha NeuAc)beta 1-4Glc beta 1-Cer (GM1), sulfatide, GM1, NeuAc alpha 2-3Gal beta 1-3GalNAc beta 1-4Gal(3-2 alpha NeuAc)beta 1-4Glc beta 1-1Cer (GD1a), and NeuAc alpha 2-3Gal beta 1-3GalNAc beta 1-4Gal(3-2 alpha NeuAc8-2 alpha NeuAc)beta 1-4Glc beta 1-1 Cer (GT1b), and their mixtures with dipalmitoylphosphatidylcholine (DPPC) in the presence of myelin basic protein (MBP) was studied by high sensitivity differential scanning calorimetry. The transition temperature of DPPC, galactosylceramide, and asialo-GM1 is affected little by MBP while their transition enthalpy is decreased in proportion to the amount of protein in the mixture. The thermotropic behavior of anionic glycosphingolipids is considerably perturbed by MBP. The transition temperature of gangliosides increases in the presence of MBP, whereas that of sulfatide decreases. The enthalpy of the transition of anionic glycosphingolipids increases markedly in the presence of MBP. The excess heat capacity function of these systems can be resolved into two independent phase transitions. Phase separation of enriched lipid/protein domains occurs in a magnitude that depends on the amount of MBP; the rest of the lipid phase exhibits some altered thermodynamic properties. In mixtures of glycosphingolipids with DPPC, phase separation is also present but no phase transition with the characteristic of pure DPPC is found. MBP is changing the properties of the lipid mixture as a whole and does not interact exclusively with the glycosphingolipids. The proportion of MBP required to produce the maximal changes is greater the greater the complexity of the glycosphingolipids polar head group. Relatively small variations of the amount of MBP induce large shifts in the proportion of the different phases present.

Thermotropic behavior of aqueous dispersions of glucosylceramide-dipalmitoylphosphatidylcholine mixtures

Thermotropic behavior of aqueous dispersions of glucosylceramide-dipalmitoylphosphatidylcholine mixtures

Thermal behavior of stearoylsphingomyelin-cholesterol dispersions.

The thermotropic behavior of aqueous dispersions of stearoylsphingomyelin-cholesterol mixtures was examined by high-sensitivity differential scanning calorimetry. When less than 20 mol % cholesterol was mixed with the sphingomyelin and the samples were held at room temperature for 7-9 days before the initiation of calorimetric measurements, a sharp endotherm at 56-57 degrees C and a broad endotherm at 35-50 degrees C were observed. In addition, samples containing 15-20 mol % stearol exhibited a sharp endotherm at 43-45 degrees C. If samples were held at room temperature for less than 2 h before the initiation of calorimetric analysis, the 56-57 degrees C endotherm was usually not seen. Instead, a combination of broad and sharp endotherms over the range of 35-50 degrees C was observed. Occasionally, exotherms were also observed within this temperature range. These results, along with those from previous studies, imply that a cholesterol-rich phase coexists with a cholesterol-poor phase in which the sphingomyelin molecules may exist in two distinctly different gel states.

Thermotropic behavior of phosphatidylethanolamine-cholesterol and phosphatidylethanolamine-phosphatidylcholine-cholesterol mixtures.

The thermotropic behavior of aqueous dispersions of phosphatidylethanolamine-cholesterol and phosphatidyl-ethanolamine-phosphatidylcholine-cholesterol mixtures has been studied by high-sensitivity differential scanning calorimetry. The gel to liquid-crystalline phase transition of phosphatidylethanolamines is broadened and shifted to lower temperature when cholesterol is incorporated into the bilayer. When the cholesterol content is below 25 mol %, the calorimetric endotherms seem to consist of two components, a broad one at considerably lower temperature than the original transition and another component at only slightly lower temperature. This thermotropic behavior can be explained by the assumption of a homogenous distribution of cholesterol in phosphatidylethanolamine bilayers. Scanning calorimetry of equimolar mixtures of phosphatidylethanolamines with phosphatidylcholines, which show either ideal or nonideal mixing properties, reveals that when cholesterol is added to these mixtures it shows no preferential affinity for either of the phospholipids.

Calorimetric investigation of the complex phase behavior of glucocerebroside dispersions.

The thermotropic behavior of aqueous dispersions of glucocerebroside from Gaucher's spleen has been investigated by different scanning calorimetry. These results indicate that glucocerebroside undergoes two distinct phase transitions centered at 47 and 83 degrees C, respectively. The high-temperature transition is associated with the main gel-liquid crystalline transition and has an enthalpy change of 13.6 kcal/mol of lipid; this transition is not rapidly reversible and the liquid crystalline phase supercools to a metastable gel phase. The low-temperature transition is exothermic with an enthalpy change of -6.3 kcal/mol and involves a transformation of the metastable gel phase to a more highly ordered gel conformation, without involving changes in the conformational state of the hydrocarbon chains. The behavior of these transitions as a function of the amount of water suggests that the origin of the metastability is related to a hydration-dehydration process of the cerebroside molecule. Experiments with synthetic-D-erythro-N-palmitoylglucocerebroside revealed the same thermotropic behavior. These glucocerebroside transitions are irreversible and together define a unidirectional cycle in which each state of the molecule can only be reached by completing the entire cycle.

Studies on the anomalous thermotropic behavior of aqueous dispersions of dipalmitoylphosphatidylcholine-cholesterol mixtures.

Examination of the thermotropic behavior of aqueous dispersions of dipalmitoylphosphatidylcholine-cholesterol mixtures by high-sensitivity scanning calorimetry has revealed that the phospholipid gel to liquid-crystalline phase transition consists of two components. One, a relatively sharp transition centered at 39.6-40.7 degrees C, exhibits a transition enthalpy change which decreases linearly with increasing cholesterol content, approaching zero at a cholesterol content of about 25 mol %. The other, a broad, lower intensity transition centered at approximately 41.5 degrees C for cholesterol concentrations of 20 mol %, displays an enthalpy change which is maximal at about 20-25 mol % cholesterol and which decreases as the cholesterol content decreases to zero or increases above 25 mol %. The origin of these two transitions is discussed in terms of a separation of these lipid mixtures into cholesterol-rich and cholesterol-poor domains.

A calorimetric study of the thermotropic behavior of aqueous dispersions of natural and synthetic sphingomyelins.

A recently developed differential scanning calorimeter has been used to characterize the thermotropic behavior of aqueous dispersions of liposomes containing sphingomyelin. Liposomes derived from sheep brain sphingomyelin exhibit a broad gel-liquid crystalline phase transition in the temperature range of 20-45 degrees C. The transition is characterized by maxima in the heat capacity function at 31.2 and 37.1 degrees C and a total enthalpy change of 7.2 +/-0.4 kcal/mol. Beef brain sphingomyelin liposomes behave similarly but exhibit heat capacity maxima at 30, 32, and 38 degrees C and a total enthalpy change of 6.9 kcal/mol. The thermotropic behavior of four pure synthetic sphingomyelins is reminiscent of multilamellar lecithin liposomes in that a single, sharp, main transition is observed. Results obtained for liposomes containing mixtures of different sphingomyelins are complex. A colyophilized mixture of N-palmitoylsphingosinephosphorylcholine, N-stearoylsphingosinephosphorylcholine, and N-lignocerylsphingosinephosphorylcholine in a 1 : 1 : 1 mol ratio exhibits a single transition with a Tm below that observed for the individual components. On the other hand a 1 : 1 mixture of N-stearoylsphingosinephosphorylcholine and 1-palmitoyl-2-oleylphosphatidylcholine exhibits three maxima in the heat capacity function. It is clear from these results that the thermotropic behavior of sphingomyelin-containing liposomes is a complex function of the exact composition. Furthermore, it appears that the behavior of the liposomes derived from natural sphingomyelins cannot be explained in terms of phase separation of the individual components.