A calorimetric study of the thermotropic behavior of pure sphingomyelin diastereomers

The phase-transition properties of sphingomyelins were investigated in detail with totally synthetic, chemically and stereochemically pure (2S,3R)-(N-stearoylsphingosyl)-1-phosphocholine (D-erythro-C18-SPM) (1) and the corresponding 2S,3S isomer (L-threo-C18-SPM) (2). Heating scans of an unsonicated dispersion of 1 right after hydration showed a main transition (I) at 44.7 degrees C (delta H = 6.8 kcal/mol). Upon incubation at 20-25 degrees C a second transition (II) appeared at 36.0 degrees C (delta H = 5.7 kcal/mol). The two gel phases were designated as G alpha and G beta phases, respectively. The G beta phase was also metastable and relaxed to a third gel phase (G gamma) upon incubation below 10 degrees C. Conversion of the G gamma phase to the liquid-crystalline phase occurred via two new endotherms at 33.4 degrees C (2.6 kcal/mol) (III) and 43.6 degrees C (8.0 kcal/mol) (IV) as well as a main transition at 44.7 degrees C (9.5 kcal/mol). Possible interpretations have been proposed to account for the observed phase transitions. The L-threo isomer 2 showed similar thermotropic behavior to dipalmitoylphosphatidylcholine (DPPC): a "main transition" at 44.2 degrees C (6.0 kcal/mol), a "pretransition" at 43.1 degrees C (1.8 kcal/mol), and upon incubation at 7 degrees C for 2 weeks, a very broad "subtransition" at ca. 35 degrees C. The results are substantially different from previous studies of sphingomyelins using mixtures of stereoisomers. Mixing of 1 with 2, 1 with DPPC, and 2 with DPPC removed the metastability of the gel phase and resulted in a single transition.     

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.     

Thermal behavior of synthetic sphingomyelin-cholesterol dispersions.

The thermotropic behavior of aqueous dispersions of palmitoylsphingomyelin-cholesterol and lignoceryl-sphingomyelin-cholesterol mixtures has been examined by high-sensitivity differential scanning calorimetry. When less than 25 mol % cholesterol is mixed with either sphingomyelin, the calorimetric endotherm is composed of a sharp and a broad component. The sharp-component enthalpy change decreases as the mole percent cholesterol increases with the extrapolated zero enthalpy point being 25 to 30 mol %. With palmitoylsphingomyelin, the temperature of maximum heat capacity of the sharp component decreases monotonically with increasing cholesterol content, while the lignocerylsphingomyelin sharp-component maximum remains constant until more than 20 mol % sterol is present. The broad-component enthalpy change maximizes at 3--4 kcal/mol between 10 and 20 mol % cholesterol and decreases as the ratio of cholesterol is increased or decreased from this range for both sphingomyelins. The results are compared with those from a previous study on dipalmitoylphosphatidylcholine-cholesterol mixtures and are interpreted as evidence for the coexistence of cholesterol-rich and cholesterol-poor phases.     

A calorimetric study of the thermotropic behaviour of 1,2-dipentadecylmethylidene phospholipids

Differential scanning calorimetry was used to study the thermotropic behaviour of 1,2-dipentadecylmethylidene phospholipids with various head groups. The structural variation in the glycerol backbone region leads to a strong restriction of conformational freedom for the first two methylene segments of the chains, so that dipentadecylmethylidene phospholipids show lower transition temperatures, lower enthalpies and lower cooperativity of the transition from the gel to the liquid crystalline phase. The extreme chemical stability of these lipids in the alkaline pH region enables investigations of phosphatidylethanolamine and phosphatidic acid dispersions at high pH values. Both phospholipids show a decrease in the transition temperature and in the transition enthalpy as they become singly and doubly charged, respectively. A complex behaviour of the transition enthalpy of doubly charged 1,2-dipentadecylmethylidene phosphatidic acid was observed when the NaCl concentration of the dispersion was increased.     

Fatty acid composition and thermal behavior of natural sphingomyelins

We found significant differences in the fatty acid composition of several bovine brain, egg yolk and sheep erythrocyte sphingomyelins. These differences in fatty acid composition influence the thermal behavior of hydrated sphingomyelin as recorded by differentail scanning calorimetry. Significant differences were also found in the temperature and complexity of the order-disorder phase transitions of bovine brain sphingomyelin obtained from different sources which, in general, correlate with the relative content of the saturated fatty acids (palmitic (C16:0) and stearic acid (C18:0) acids) and the long unsaturated nervonic acid (C24:1).     

Kinetics of the pretransition of synthetic phospholipids. A calorimetric study

The pretransition in aqueous dispersions of two synthetic phospholipids (dimyristoylphosphatidylcholine and dipalmitoylphosphatidylcholine) has been examined in detail by differential scanning calorimetry. The transition from the high-temperature state (state above pretransition) to the low-temperature state (state below pretransition) is complex and appears to occur via some metastable states. In contrast, the kinetics of the transition from the low- to the high-temperature state is consistent with an activated two-state model. The observed hysteresis is shown to arise mainly from the kinetic nature of the pretransition.     

Differential scanning calorimetric study of the thermotropic phase behavior of a polymerizable, tubule-forming lipid

A comparative study of the polymorphism exhibited by the polymerizable, tubule-forming phospholipid 1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3- phosphocholine (DC23PC) and its saturated analog 1,2-ditricosanoyl-sn-glycero-3-phosphocholine (DTPC) in aqueous suspension is reported. Differential scanning calorimetry (DSC), as well as freeze-fracture electron microscopy and Raman spectroscopy, have been used to study the influence on phase behavior of rigid diacetylene groups in the fatty acyl chains of a phosphatidylcholine. DTPC large multilamellar vesicle (MLV) and small unilamellar vesicle (SUV) suspensions were found to retain liposome morphology after chain crystallization had occurred. In marked contrast, diacetylenic DC23PC suspensions do not maintain liposomal morphology in converting to the low temperature phase. Large MLVs of DC23PC with outer diameters in excess of 1 micron convert to a gel phase with cylindrical or tubular morphology at 38 degrees C, just a few degrees below the lipid's chain melting temperature (TM(H), i.e. temperature of an endothermic event observed during a heating scan) of 43.1 degrees C. Unlike the large MLVs, small MLVs or SUVs of DC23PC, with diameters of 0.4 +/- 0.3 micron and 0.04 +/- 0.02 micron, respectively, exhibit metastability in the liquid-crystalline state for several tens of degrees below the chain melting temperature prior to converting to a gel phase which, by electron microscopy, manifests itself as extended multilamellar sheets. Raman data collected at TM(H) -40 degrees C demonstrate that the gel state formed by DC23PC is very highly ordered relative to that of DTPC, suggesting that special chain packing requirements are responsible for the novel phase behavior of DC23PC.     

Studies on the thermotropic behavior of aqueous phosphatidylethanolamines

Transport of phosphate has been studied in subconfluent monolayers of LLC-PK1 cells. It was found that this transport system shows similar characteristics to those observed in the kidney. Uptake of phosphate is mediated by a Na+-dependent, substrate-saturable process with an apparent Km value for phosphate of 96 +/- 15 mumol/l. Kinetic analysis of the effect of Na+ indicated that at (pH 7.4) two sodium ions are cotransported with one HOP4(2-) ion (Hill coefficient 1.5) with an apparent Km value for sodium of 56 mmol/l. Pi uptake is inhibited by metabolic inhibitors (ouabain and FCCP). In the pH range of 6.6 of 7.4 Pi uptake rate does not change significantly, indicating that both the monovalent and the divalent form of phosphate are accepted by the transport system. It is suggested that phosphate is transported by LLC-PK1 cells together with sodium (2 Na+:1 HPO4(2-) in an electroneutral manner down a favourable sodium gradient.     

Differential scanning calorimetric study of the effect of sterol side chain length and structure on dipalmitoylphosphatidylcholine thermotropic phase behavior.

We have investigated the thermotropic phase behavior of dipalmitoylphosphatidylcholine (DPPC) bilayers containing a series of cholesterol analogues varying in the length and structure of their alkyl side chains. We find that upon the incorporation of up to approximately 25 mol % of any of the side chain analogues, the DPPC main transition endotherm consists of superimposed sharp and broad components representing the hydrocarbon chain melting of sterol-poor and sterol-rich phospholipid domains, respectively. Moreover, the behavior of these components is dependent on sterol side chain length. Specifically, for all sterol/DPPC mixtures, the sharp component enthalpy decreases linearly to zero by 25 mol % sterol while the cooperativity is only moderately reduced from that observed in the pure phospholipid. In addition, the sharp component transition temperature decreases for all sterol/DPPC mixtures; however, the magnitude of the decrease is dependent on the sterol side chain length. With respect to the broad component, the enthalpy initially increases to a maximum around 25 mol % sterol, thereafter decreasing toward zero by 50 mol % sterol with the exception of the sterols with very short alkyl side chains. Both the transition temperature and cooperativity of the broad component clearly exhibit alkyl chain length-dependent effects, with both the transition temperature and cooperativity decreasing more dramatically for sterols with progressively shorter side chains. We ascribe the chain length-dependent effects on transition temperature and cooperativity to the hydrophobic mismatch between the sterol and the host DPPC bilayer (see McMullen, T. P. W., Lewis, R. N. A. H., and McElhaney, R. N. (1993) Biochemistry 32:516-522).(ABSTRACT TRUNCATED AT 250 WORDS)     

Aggregational behavior of aqueous dispersions of the antifungal lipopeptide iturin A

Iturin A, a lipopeptide isolated from Bacillus subtilis, possesses a strong antifungal activity, and has been devoted to a great deal of attention. Since iturin is an amphiphilic compound with a great propensity to self-associate in solution as well as inside the membrane, the question arises to whether its aggregational behavior is dependent on the concentration of the lipopeptide. In order to test this, the ability of iturin suspensions to encapsulate water-soluble molecules has been examined. Iturin was dispersed at different concentrations above its critical micellar concentration, in a buffer containing the water-soluble dye 5,6-carboxyfluorescein. For iturin A micelles, a Stokes radius of 1.3 nm and an aggregational number of 7 was obtained. The results shown in this work clearly demonstrate that iturin dispersions in water, at concentrations of 0.7, 1.4 and 3 mM, i.e. far above the critical micellar concentration (40 microM), are capable of encapsulating carboxyfluorescein, probably by adopting a type of aggregate different from the micelle. Negative-staining electron microscopy shows the presence of vesicles with an average size of 150 nm. By using (14)C-iturin, it is shown that, at 3 mM concentration, 40 % of the iturin molecules adopt this vesicular state. It is proposed that iturin molecules form a fully interdigitated bilayer, where each hydrocarbon tail span the entire hydrocarbon width of the bilayer, resulting in multilamellar vesicles capable of encapsulating an aqueous compartment. The possible implications of these results to the membrane destabilizing effect of iturin A, are discussed according to the dynamic cone-shape of the iturin molecule.     

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 glycosphingolipids in aqueous dispersions

The thermotropic behavior of 20 chemically related glycosphingolipids (GSLs) of high purity, containing neutral and anionic carbohydrate residues in their oligosaccharide chains, was studied by high-sensitivity differential scanning calorimetry. In general, the polar head group of GSLs appears to be one of the major determinants of their phase behavior. Compared to phospholipids, the presence of the carbohydrate rather than the phosphorylcholine moiety in the polar head group and a sphingosine base in the hydrocarbon portion of GSLs reduces the effect on the transition temperature (Tm) brought about by increasing the number of methylene groups in the amide-linked fatty acyl chains. For simple neutral GSLs, the Tm's were 20-40 degrees C higher than those of phospholipids with comparable hydrocarbon chains. As the oligosaccharide chain of GSLs becomes more complex, the excess heat capacity, Tm, enthalpy (delta Hcal), and entropy of the transition decrease proportionally to the number of carbohydrate residues present in the polar head group. The Tm and delta Hcal for anionic GSLs were 16-25 degrees C and 1-3 kcal mol-1 lower than those of neutral GSLs with comparable oligosaccharide chains. A linear dependence of delta Hcal with Tm was found. However, the slopes of these plots were different for neutral and for anionic GSLs, suggesting different types of intermolecular organizations for the two. The Tm and delta Hcal were linearly dependent on the molecular area of both neutral and anionic GSLs; this indicated that the influence of the complexity of the polar head group in GSLs for establishing the thermodynamic behavior may be mediated by the intermolecular spacings.     

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

The structure of the lectin discoidin I has been studied by circular dichroism and fluorescence spectroscopy. A positive ellipticity band at 224 nm is detected in the CD spectrum of discoidin I. The fluorescence spectra show a defined shoulder at 325 nm that through acrylamide quenching has been associated with a displaced tryptophan residue partly buried in the discoidin I molecule. This tryptophan could also be responsible for the 224 nm positive band of the CD spectrum. These spectroscopic characteristics of discoidin I indicate the existence of structural homologies with fibronectin, where the optical activity of aromatic chromophores has been associated with the positive ellipticity band at 227 nm. The CD adjust parameters and theoretical secondary structure predictions show that discoidin I is a molecule with a low content of alpha-helix and beta-strand and high content of beta-turn structures, similar to other lectins.     

A calorimetric examination of the effect of myotoxin a on the thermotropic phase behavior of model lipid membranes

The effect of myotoxin a on the thermotropic phase behavior of aqueous dispersions of dimyristoyl phosphatidylcholine (DMPC) and dimyristoyl phosphatidylserine (DMPS) was examined using differential scanning calorimetry (DSC). Myotoxin a significantly altered the normal phase behavior of DMPC in a concentration dependent fashion. This effect is perturbed by Ca2+ and is sensitive to ionic strength and pH. High concentrations of toxin eliminate the characteristic pretransition associated with the polar head group of DMPC. They also increase the temperature of the main gel-to-liquid crystal transition from 23 degrees C to 32-35 degrees C. At low concentrations of toxin, the first visible effect is upon the pretransition which is split into two components that diminish with time. The main transition is less affected at low toxin concentrations, although the magnitude of the transition is reduced while it is simultaneously shifted to higher temperatures. The main transition is also split into multiple components. The toxin also had pH specific effects on the phase behavior of DMPS. Above physiological pH (8.5) the normal transition of DMPS at 36-38 degrees C was split in the presence of myotoxin a and new components appeared centered at 31 degrees C and 35 degrees C. These observations are consistent with reports that the skeletal muscle membrane system is the major site of the myonecrotic effect of myotoxin a.     

A comparative calorimetric study of the effects of cholesterol and the plant sterols campesterol and brassicasterol on the thermotropic phase behavior of dipalmitoylphosphatidylcholine bilayer membranes

We present a comparative differential scanning calorimetric study of the effects of the animal sterol cholesterol (Chol) and the plant sterols campesterol (Camp) and brassicasterol (Bras) on the thermotropic phase behavior of dipalmitoylphosphatidylcholine (DPPC) bilayers. Camp and Bras differ from Chol in having a C24 methyl group and, additionally for Bras, a C22 trans-double bond. Camp and especially Bras decrease the temperature, cooperativity and enthalpy of the DPPC pretransition more than Chol, although these effects are attenuated at higher sterol levels. This indicates that they destabilize gel-state DPPC bilayers to a greater extent, but are less soluble, than Chol. Not surprisingly, all three sterols have similar effects on the sterol-poor sharp component of the DPPC main phase transition. However, Camp and especially Bras less effectively increase the temperature and decrease the cooperativity and enthalpy of the broad component of the main transition than Chol. This indicates that at higher sterol concentrations, Camp and Bras are less miscible and less effective than Chol at ordering the hydrocarbon chains of the sterol-enriched fluid DPPC bilayers. Overall, these alkyl side chain modifications generally reduce the ability of Chol to produce its characteristic effects on DPPC bilayer physical properties. These differences are likely due to the less extended and more bent conformations of the alkyl side chains of Camp and Bras, producing sterols with a greater effective cross-sectional area and reduced length than Chol. Hence, the structure of Chol is likely optimized for maximum solubility in, as opposed to maximum ordering of, phospholipid bilayers.     

Phase behavior and structure of aqueous dispersions of sphingomyelin

The phase behavior of bovine brain sphingomyelin in water has been determined by polarizing light microscopy, differential scanning calorimetry, and X-ray diffraction. Lamellar phases, in which water is intercalated between sheets of lipid molecules arranged in the classical bilayer fashion, are present over much of the phase diagram. An order-disorder transition separates the high temperature, liquid crystalline, lamellar phase from a more ordered lamellar phase at low temperatures. The hydration characteristics of sphingomyelin are similar to the structurally related lecithin in that only limited amounts of water are incorporated above and below the transition. Above the transition at 47 degrees C, a maximum of 35% by weight of water can be incorporated between the lipid bilayers, the total thickness at maximum hydration being 60.2 A, the lipid thickness 38 A, and the surface area per lipid molecule at the interface 60 A(2). Water in excess of 35% by weight is present as a separate phase. Below the phase transition, at 25 degrees C a maximum of 42% by weight of water may be incorporated between the lipid bilayers. On increasing the hydration, the lamellar repeat distance increases from 63.5 A to a limiting value of 76 A. Within this hydration range the calculated lipid thickness decreases from 63.5 to 42.5 A, and the surface area per lipid molecule increases from 36.1 to 53.6 A(2). Although these changes may be accounted for by a structure in which the hexagonally packed ordered hydrocarbon chains tilt progressively with respect to the normal to the bilayer plane on increasing hydration, it is possible that changes in other more complex lamellar structures may be responsible for these variations in lipid thickness and surface area.     

A differential scanning calorimetric study of the thermotropic phase behavior of model membranes composed of phosphatidylcholines containing linear saturated fatty acyl chains

The thermotropic phase behavior of a series of 1,2-diacylphosphatidylcholines containing linear saturated acyl chains of 10-22 carbons was studied by differential scanning calorimetry. When fully hydrated and thoroughly equilibrated by prolonged incubation at appropriate low temperatures, all of the compounds studied form an apparently stable subgel phase (the Lc phase). The formation of the stable Lc phase is a complex process which apparently proceeds via a number of metastable intermediates after being nucleated by incubation at appropriate low temperatures. The process of Lc phase formation is subject to considerable hysteresis, and our observations indicate that the kinetic limitations become more severe as the length of the acyl chain increases. The kinetics of Lc phase formation also depend upon whether the acyl chains contain an odd or an even number of carbon atoms. The Lc phase is unstable at higher temperatures and upon heating converts to the so-called liquid-crystalline state (the L alpha phase). The conversion from the stable Lc to the L alpha phase can be a direct, albeit a multistage process, as observed with very short chain phosphatidylcholines, or one or more stable gel states may exist between the Lc and L alpha states. For the longer chain compounds, conversions from one stable gel phase to another become separated on the temperature scale, so that discrete subtransition, pretransition, and gel/liquid-crystalline phase transition events are observed.(ABSTRACT TRUNCATED AT 250 WORDS)     

Differential scanning calorimetry of thermotropic phase transitions in vitaminylated lipids: aqueous dispersions of N-biotinyl phosphatidylethanolamines.

The thermotropic phase behavior of a homologous series of saturated diacyl phosphatidylethanolamines in which the headgroup is N-derivatized with biotin has been investigated by differential scanning calorimetry. In 1 M NaCl, derivatives with acyl chainlengths from C(12:0) to C(20:0) all exhibit sharp chain-melting phase transitions, which are reversible with a hysteresis of 1.5 degrees or less, except for the C(12:0) lipid which has a transition temperature below 0 degree C. The transition enthalpy and the transition entropy depend approximately linearly on the lipid chainlength, with incremental values per CH2 group that are very similar to those obtained for the corresponding underivatized phosphatidylethanolamines in aqueous dispersion. The chainlength-independent contribution to the transition enthalpy is significantly smaller than that for the underivatized phosphatidylethanolamines, and that for the transition entropy is much smaller; the latter suggesting that the N-biotinylated phosphatidylethanolamine headgroups are differently hydrated from those of the underivatized lipids. The gel-to-fluid phase transition temperatures of the N-biotinylated lipids are lower than those of the parent phosphatidylethanolamines, and their chainlength dependence conforms well with that predicted by assuming that the transition enthalpy and entropy are linearly dependent on chainlength. Although the chain-melting phase behavior is generally similar to that of the parent phosphatidylethanolamines, the gel phases (and the fluid phases in the case of chainlengths C(12:0) to C(16:0)) have a different lyotropic structure in the two cases, and this is reflected in the chainlength-independent contributions to the thermodynamic parameters. In the absence of salt, the thermotropic phase behavior of aqueous dispersions of the N-biotinyl phosphatidylethanolamines is considerably more complex.(ABSTRACT TRUNCATED AT 250 WORDS)     

A calorimetric study of the thermotropic behaviour of mixtures of brain cerebrosides with other brain lipids

We have used a computer-controlled differential scanning calorimeter to determine the phases present in mixtures of the brain galactocerebrosides with other representative brain lipids. There are two types of brain galactocerebroside, those which possess an alpha-hydroxy substituent on the acyl chain (HFA) and those that do not (NFA). In the liquid crystalline state both cerebrosides were miscible with all the lipids studied, but in the gel state they were immiscible with cholesterol and the brain phosphatidylcholines. However, cholesterol mixtures in which the cholesterol mole fraction exceeded one third formed homogeneous metastable gel states on cooling from above the melting point of the cerebroside. Relaxation to the stable two phase state took place slowly over several hours. The solubilities of the galactocerebrosides in the other main brain sphingolipid, sphingomyelin, were much higher. Only in the case of the NFA galactocerebroside and at low mole fractions of sphingomyelin was immiscibility detected. Ternary mixtures of the two cerebrosides with sphingomyelin/cholesterol and phosphatidylcholine/cholesterol (PC/Chol) showed different miscibility characteristics. On cooling from 80 degrees C all mixtures formed homogeneous gel states. However, on standing the cerebrosides separated into discrete gel phases in all mixtures but one, that in which HFA galactocerebrosides were mixed with sphingomyelin and cholesterol. The cerebroside in the mixture with the composition closest to that of myelin, HFA/PC/Chol, melted at 38 degrees C. On scanning guinea pig CNS myelin which had been equilibrated at 5 degrees C a transition was detected with Tmax 33 degrees C. On the basis of comparison with the HFA/PC/Chol mixture we propose that the transition in myelin at this temperature is due to the melting of a galactocerebroside gel phase.     

Kinetics of the pretransition of synthetic phospholipids a calorimetric study

The pretransition in aqueous dispersions of two synthetic phospholipids (dimyristoylphosphatidylcholine and dipalmitoylphosphatidylcholine) has been examined in detail by differential scanning calorimetry. The transition from the high-temperature state (state above pretransition) to the low-temperature state (state below pretransition) is complex and appears to occur via some metastable states. In contrast, the kinetics of the transition from the low- to the hightemperature state is consistent with an activated two-state model. The observed hysteresis is shown to arise mainly from the kinetic nature of the pretransition.