Interactions of phospholipid monolayers with carbohydrates

Surface pressure studies of phospholipid monomolecular films of dimyristoylphosphatidylcholine (DMPC) and dipalmitoylphosphatidylcholine (DPPC) formed at an air/water interface have been made and the effects on the films studied when various carbohydrates are present in the subphase. The results obtained show that at a given temperature, the area per molecule of DPPC increases with increasing concentration of the carbohydrate in the subphase. The carbohydrate which has the greatest expanding effect on the phospholipid monolayer is glycerol, followed in turn by trehalose, sucrose, glucose, raffinose, and inositol. The mechanism of monolayer expansion by glycerol is different from that observed in other carbohydrates, as the following experiments demonstrate. Below the phase transition temperature of DPPC, the area per molecule of DPPC at a pressure of 12.5 dyn/cm is the same with and without glycerol in the subphase. However, when the monolayer is heated to a temperature above the phase transition temperature for DPPC, the area/molecule on glycerol is considerably greater than the area/molecule on water at the same surface pressure. Cooling the monolayer back to the lower temperature produces an area/molecule of DPPC which is identical on both water and glycerol subphases. Glycerol therefore has no effect on the low-temperature (condensed) monolayers but causes expansion of the high-temperature (expanded) monolayers. By contrast with glycerol, both trehalose and sucrose interact with the DPPC monolayer producing an increased area/molecule over that observed on water, both with low-temperature (condensed) monolayers and with the high-temperature (expanded) monolayers. The efficiency of these carbohydrates at expanding the monolayer films (with the exception of glycerol) shows a strong correlation with their ability to stabilize membrane structure and function at low water contents.     

Interaction of carbohydrates with dry dipalmitoylphosphatidylcholine

Interactions of six carbohydrates (trehalose, sucrose, glucose, raffinose, inositol, and glycerol) with dry dipalmitoylphosphatidylcholine (DPPC) were studied using differential scanning calorimetry (DSC) and infrared spectroscopy (ir) in order to elucidate the mechanism by which some of these carbohydrates preserve structural and functional integrity of dry membranes. Results with DSC showed that trehalose depressed the main transition temperature (Tmid) of dry DPPC below that of fully hydrated DPPC, and raised the enthalpy of that transition more than did addition of water. Results obtained with ir spectroscopy suggested a potential mechanism for this interaction. In the presence of most of the carbohydrates the ir spectrum for DPPC showed changes similar to those seen when water was added to dry DPPC, and the asymmetric P = O stretching band was diminished in intensity. The degree to which the carbohydrates tested affected the integrated intensity of this band and the Tmid was correlated with the ability of those carbohydrates to preserve dry membranes. Also, bands assigned to -OH deformations in the trehalose and other carbohydrates were depressed in the presence of DPPC. Based on these observations, it is suggested that the mechanism of interaction between the carbohydrate and lipid involves hydrogen bonding between -OH groups on the carbohydrate and the phosphate head group of the phospholipid. The only exceptions to this pattern are glycerol, which depresses Tmid of dry DPPC, and myo-inositol, which has no effect on Tmid or the ir spectrum of DPPC; neither carbohydrate can preserve dry membranes. It is suggested, based on ir spectroscopy and previous results with monolayer preparations, that glycerol interacts with phospholipids by a mechanism different from that shown by the other carbohydrates.     

Effect of glycerol on the interfacial properties of dipalmitoylphosphatidylcholine liposomes as measured with merocyanine 540.

Liposomes of dipalmitoylphosphatidylcholine (DPPC) prepared in increasing glycerol/glucose ratios show an increase in the absorbance at 570 nm of merocyanine spectra at temperatures below the phase transition. Since this effect is not observed when liposomes are prepared in solutions containing solely glucose, it is attributed to specific interactions of glycerol with the membrane phase. The increase in the 570 nm absorbance is ascribed to a partial fluidification of the membrane interface and is dependent on the distribution of the dye between the inner and the outer compartments of the liposomes and on their osmotic state. The greatest differences in the absorbance ratio are obtained when merocyanine is added to the external media. In consequence, the changes in the spectra of MC are dependent on the surface state of the liposomes which can be modified by an increase of glycerol or glucose in the external media. The present results are examined in the light of the perturbations that glycerol can induce on the barrier properties of the bilayer.     

Effect of n-alcohols and glycerol on the pretransition of dipalmitoylphosphatidylcholine

We have systematically investigated the effect of short-chain n-alcohols and glycerol on the pretransition of 1,2-dipalmitoylphosphatidylcholine (DPPC) by spectrophotometry. It is found that the n-alcohols and glycerol remove the pretransition above a critical concentration for each ligand. In addition, the short-chain n-alcohols below the critical concentration decrease the pretransition temperature. The longer the aliphatic chain length of the n-alcohol (up to butanol) the greater the decrease in the pretransition temperature, and the lower the concentration necessary to remove the pretransition. However, glycerol differs from the short-chain n-alcohols in that it has no significant effect on either the pretransition or the main transition, but it is also capable of removing the pretransition above a critical concentration. It has previously been shown that alcohols have a biphasic effect on the main transition temperature of phosphatidylcholines (Rowe, E.S. (1983) Biochemistry 22, 3299-3305). At high alcohol concentrations, the main transition is not thermodynamically reversible (Rowe, E.S. (1985) Biochim. Biophys. Acta 813, 321-330). Recently, Simon and McIntosh (Biochim. Biophys. Acta (1984) 773, 169-172) have identified that at high ethanol concentration DPPC exists in the interdigitated phase. The critical ligand concentration at which the pretransition disappears coincides with the induction of main transition hysteresis and the biphasic alcohol effect in the main transition. These three effects appear to correlate with the induction of the interdigitated gel state by alcohols and glycerol.     

Pentachlorophenol-induced change of zeta-potential and gel-to-fluid transition temperature in model lecithin membranes

We have determined zeta-potentials for dimyristoylphosphatidylcholine (DMPC) and dipalmitoylphosphatidylcholine (DPPC) membranes by measuring the electrophoretic mobility of multilayered vesicles and the temperatures of the gel-to-ripple-to-fluid phase transitions of sonicated vesicles by a photometric method. Some conclusions are: (1) The zeta-potentials of DMPC and DPPC vesicles become negative due to adsorption of ionized pentachlorophenol (PCP), (2) their magnitude changes, step-like, on gel-to-fluid transition and (3) the temperature of the step-like change in zeta-potential decreases with an increase in PCP concentration. (4) PCP exhibits a large effect on membrane structure: It induces an isothermal phase change from the ordered to disordered state, which is enhanced by monovalent salt in the aqueous phase. (5) Both ionized and unionized PCP decrease the melting phase transition temperature and abolish the pretransition, (6) the unionized species increases the melting transition width and (7) the ionized species is more potent in abolishing the pretransition. (8) The shorter chain lipid (DMPC) is more sensitive to the presence of PCP; the maximum decrease in delta Tt is 13 K (DMPC) and 7 K (DPPC) in the presence of ionized PCP. We have shown experimentally, by comparing the delta Tt from photometric studies with the density of adsorbed PCP derived from zeta-potential isotherms, that (9) the shift of the melting phase transition temperature increases linearly with the density of adsorbed PCP. (10) In contrast to membranes made of negatively charged lipids, the transition temperature of DMPC and DPPC membranes in the presence of PCP further decreases in the presence of monovalent salt. The salt effect is due to screening of the membrane surface leading to enhanced adsorption of ionized PCP and a depression in transition temperature. (11) It is shown that both the adsorption and the changes of gel-to-fluid phase transition temperature can be described in terms of the Langmuir-Stern-Grahame model and (12) proposed that future studies of membrane toxicity of PCP should be focused on its pH dependence.     

Effect of n-alcohols and glycerol on the pretransition of dipalmitoylphosphatidylcholine

We have systematically investigated the effect of short-chain n-alcohols and glycerol on the pretransition of 1,2-dipalmitoylphosphatidylcholine (DPPC) by spectrophotometry. It is found that the n-alcohols and glycerol remove the pretransition above a critical concentration for each ligand. In addition, the short-chain n-alcohols below the critical concentration decrease the pretransition temperature. The longer the aliphatic chain length of the n-alcohol (up to butanol) (a) the greater the decrease in the pretransition temperature, and (b) the lower the concentration necessary to remove the pretransition. However, glycerol differs from the short-chain n-alcohols in that it has no significant effect on either the pretransition or the main transition, but it is also capable of removing the pretransition above a critical concentration. It has previously been shown that alcohols have a biphasic effect on the main transition temperature of phosphatidylcholines (Rowe, E.S. (1983) Biochemistry 22, 3299–3305). At high alcohol concentrations, the main transition is not thermodynamically reversible (Rowe, E.S. (1985) Biochim. Biophys. Acta 813, 321–330). Recently, Simon and McIntosh (Biochim. Biophys. Acta (1984) 773, 169–172) have identified that at high ethanol concentration DPPC exists in the interdigitated phase. The critical ligand concentration at which the pretransition disappears coincides with the induction of main transition hysteresis and the biphasic alcohol effect in the main transition. These three effects appear to correlate with the induction of the interdigitated gel state by alcohols and glycerol.     

The phase diagram of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine/sucrose in the dry state. Sucrose substitution for water in lamellar mesophases

The phase diagram of the binary system, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC)/sucrose, was determined by DSC. In contrast to dry DPPC, which exhibits chain melting at 342.5 K, the main feature of the DPPC/sucrose system is eutectic melting at 320 K. This was supported earlier by Crowe, J.H., Crowe, L.M. and Chapman, D. (Science 223 (1984) 701-703), who reported a drastic decrease in the chain-melting temperature of the dry lipid in the presence of some mono- and disaccharides. Electron microscopy suggests that the phase structures on either side of the phase transition are of the lamellar type. Definite sugar saturation concentrations can be derived from this phase diagram. Up to about 17 mol% sucrose, i.e., 1 mol of sucrose per 5 mol of lipid is adopted by DPPC in the low-temperature phase Lc. In the high-temperature phase Lm the saturation concentration is well above 90 mol% sucrose at 320 K (eutectic point) but decreases with increasing temperature. The lower limit of 50 mol% sucrose is reached at 455 K. At this temperature, peritectic melting of sucrose occurs. Because of some similarities in the phase diagrams of DPPC/sucrose and DPPC/water, it is possible to understand the sucrose substitution for water in dry lamellar mesophases.     

The properties of membranes formed from cyclopentanoid analogues of phosphatidylcholine

We have examined the thermal characteristics and barrier properties of vesicles formed from six analogues of $\text{1,2-}\text{dipalmitoyl}\text{-sn-}\text{glycero-3-phosphocholine}$ (DPPC). These analogues differ from DPPC in that the glycerol backbone has been replaced by each of the diastereoisomeric cyclopentane-1,2,3-triols. All of these compounds have main gel to liquid-crystal phase transition temperatures within 5 Kelvin of DPPC and four possess comparable enthalpies and entropies of transition. For two of the analogous, however, the values of the enthalpy and entropy of transition are more than double that of DPPC. The permeability characteristics and organization (as measured by diphenylhexatriene fluorescence depolarization) of vesicles formed from these two compounds suggest that their large transition enthalpy and entropy result from either a reorganization of the polar head group region during the transition or interdigitation of the acyl chains of opposing monolayers.     

The influence of cholesterol on head group mobility in phospholipid membranes.

The dielectric dispersion in the MHz range of the zwitterionic dipolar phosphocholine head groups has been measured from 0--70 degrees C for various mixtures of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and cholesterol. The abrupt change in the derived relaxation frequency f2 observed for pure DPPC at the gel-to-liquid crystalline phase transition at 42 degrees C reduces to a more gradual increase of frequency with temperature as the cholesterol content is increased. In general the presence of cholesterol increases the DPPC head group mobility due to its spacing effect. Below 42 degrees C no sudden changes in f2 are found at 20 or 33 mol% cholesterol, where phase boundaries have been suggested from other methods. Above 42 degrees C, however, a decrease in f2 at cholesterol contents up to 20--30 mol% is found. This is thought to be partly due to an additional restricting effect of the cholesterol on the number of hydrocarbon chain conformations and consequently on the area occupied by the DPPC molecules.     

Kinetics of the lamellar gel–fluid transition in phosphatidylcholine membranes in the presence of sugars

Phase diagrams are presented for dipalmitoylphosphatidylcholine (DPPC) in the presence of sugars (sucrose) over a wide range of relative humidities (RHs). The phase information presented here, determined by small angle X-ray scattering (SAXS), is shown to be consistent with previous results achieved by differential scanning calorimetry (DSC). Both techniques show a significant effect of sucrose concentration on the phase behaviour of this phospholipid bilayer. An experimental investigation into the effect of sugars on the kinetic behaviour of the gel to fluid transition is also presented showing that increasing the sugar content appears to slightly increase the rate at which the transition occurs.     

High pressure effect on the main transition from the ripple gel P'beta phase to the liquid crystal (Lalpha) phase in dipalmitoylphosphatidylcholine. Microcalorimetric study

Scanning microcalorimetry has been used to study the high pressure effect on the main transition from the ripple gel P'(beta) phase to the liquid crystal (L(alpha)) phase in DPPC (dipalmitoylphosphatidylcholine). It has been demonstrated that an increase of the pressure by 200 MPa shifts the transition to higher temperatures by 36.4 degrees. The pressure increase does not affect the cooperativity of transition but reduces noticeably its enthalpy. The changes of the molar partial volume, isothermal compressibility as well as volume thermal expansibility during transition in DPPC suspension have been estimated. It has been shown that monovalent ions (Na(+), Cl(-)) in solution slightly affect the main thermodynamic parameters of the transition. Calcium ions significantly decrease distinction in compressibility and thermal expansibility between liquid-crystal and ripple gel phases of lipid suspension, which in its turn reflects less difference in their volume fluctuations.     

A new crystalline phase of L-alpha-dipalmitoyl phosphatidylcholine monohydrate.

A new phase transition of L-alpha-dipalmitoyl phosphatidylcholine (DPPC) monohydrate from the "biaxial" phase to a crystalline phase (C phase) has been found at 71 degrees C by means of infrared attenuated total reflection (IR-ATR) spectroscopy. The transition is characterized by drastic conformational changes in the glycerophosphorylcholine moiety, which led on the one hand to an alignment of the turn near the ester group in the hydrocarbon chain at glycerol C(2) position. On the other hand a uniform conformation of the glycerophosphorylcholine moiety is found to be typical for the C phase, in contrast to nonuniform head group conformations of DPPC in other regions of the DPPC/water phase diagram investigated so far.     

Effects of hydrostatic pressure on the molecular structure and endothermic phase transitions of phosphatidylcholine bilayers: a Raman scattering study

The temperature dependences of the Raman spectra of aqueous dispersions of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) were monitored at different but constant pressures between 1 and 1210 bar. The changes observed in these Raman spectra are discussed in terms of the effects of high pressure on the phase state and molecular structure of lipid bilayers. It is demonstrated that the temperature of the endothermic gel to liquid-crystal phase transition, as well as the temperature of the pretransition, increases linearly with increasing hydrostatic pressure. The dTm/dP values obtained from a wide range of pressures are 20.8 degrees C X kbar-1 for DPPC and 20.1 degrees C X kbar-1 for DMPC. The dTp/dP value for DPPC is 16.2 degrees C X kbar-1. It is also shown that the volume change that occurs at the gel to liquid-crystal transition is not constant; i.e., d delta Vm/dP decreases by 6.2% (DPPC) or 6.3% (DMPC) per kilobar pressure. The volume change at the pretransition is also pressure dependent; the d delta Vp/dP value of DPPC decreases by 4.7% per kilobar pressure.     

Modification of dry 1,2-dipalmitoylphosphatidylcholine phase behavior with synthetic membrane-bound stabilizing carbohydrates.

Carbohydrates, particularly disaccharides, have been shown to accumulate in organisms as protective solutes during periods of stress such as freezing and desiccation. Cholesterol and lipid derivatives containing the protective carbohydrates galactose or maltose, O-[11-(1-beta-D-galactosyloxy)-3,6,9-trioxaundecanyl]ol (TEC-GAL), O-[11-(1-beta-D-maltosyloxy)-3,6,9-trioxaundecanyl]ol (TEC-MAL), and 14-(galactosyloxy)-N,N-dimethyl-O-(dipalmitoylphosphatidyl)- 6,9,12-trioxa-3- azoniatetradecanol (DP-GAL), have been synthesized to investigate the interaction of a protective carbohydrate moiety tethered to the 1,2-dipalmitoylphosphatidylcholine (DPPC) bilayer surface. Toward this goal, we have investigated the calorimetric and infrared spectroscopic behavior of mixtures of DPPC codried with these glycolipids. The synthetic glycolipids are shown to decrease significantly the main transition temperature (max Cp) of dry DPPC with a concomitant reduction in the cooperativity of the transition, as evidenced by a decrease in the enthalpy with increasing glycolipid. The decrease in transition temperature is shown to be related to chain melting monitored by the CH2 symmetric stretch frequency through the transition using FTIR. We also present evidence that the glycolipids interact with the interfacial region of DPPC, as shown by the decrease in the phosphate symmetric stretch intensity with increasing concentration of glycolipid. These observed effects are similar to the action of bulk protective sugars with DPPC; however, the concentration of glycolipid and the associated carbohydrate concentration needed to effect the observed changes are reduced compared to the quantity of bulk carbohydrate previously shown to give similar results with DPPC.     

Thermodynamics of alpha-lactalbumin-DL-alpha-dipalmitoylphosphatidylcholine interactions and effect of the antioxidant nicotinamide on these interactions

Differential scanning calorimetry has been used to understand the thermodynamics of the interactions of dl-alpha-dipalmitoylphosphatidylcholine (DPPC) with alpha-lactalbumin and the effect of the antioxidant nicotinamide on these interactions. Nicotinamide decreases the thermal transition temperature of both the lipid and the protein at high concentrations. The thermal unfolding transitions of the protein were two state and calorimetrically reversible. There was no significant change in the shape and thermodynamic parameters accompanying the lipid endotherms, suggesting that nicotinamide did not penetrate the lipid bilayer. The thermal unfoldings of alpha-lactalbumin in the presence of DPPC as cosolute also adhered to two-state reversible mechanism. The changes in the thermodynamic parameters accompanying the thermal transitions were small, indicating no significant interaction of alpha-lactalbumin with DPPC. The changes in the thermodynamic parameters indicate that the lipid bilayer organization, as well as the partitioning of the extrinsic protein alpha-lactalbumin into the bilayer, is not affected in the entire studied concentration range of the lipid. It is observed that the presence of increasing concentration of nicotinamide (as high as 1.0 mol dm(-3)) in the lipid-protein mixture does not affect its partitioning into the lipid bilayer, although nicotinamide preferentially interacts with alpha-lactalbumin. The change in the effect of nicotinamide on lipid transition temperature in the mixture and literature report suggests that nicotinamide may be forming a hydrogen-bonded complex with the protein through its amide functionality. The surface tension data of aqueous nicotinamide in combination with the thermal denaturation results of protein in presence of nicotinamide confirmed that surface tension effect does not have any significant contribution to the effect of nicotinamide on protein.     

Membrane perturbing properties of sucrose polyesters

Sucrose polyester (SPE), in the form of sucrose octaesters and sucrose hexaesters of palmitic (16:0), stearic (18:0), oleic (18:1cis), and linoleic (18:2cis) acids, have many uses. Applications include: a non-caloric fat substitute, detoxification agent, and oral contrast agent for human abdominal (MRI) magnetic resonance imaging. However, it has been shown that the ingestion of SPE was shown to generate a depletion of physiologically important lipidic vitamins and other lipophilic molecules. In order to better understand, at the molecular level, the type of interaction between SPE and lipid membrane, we have, first synthesized different type of labelled and non-labelled SPEs. Secondly, we have studied the effect of SPEs on multilamellar dispersions of dielaidoylphosphatidylethanolamine (DEPE) and dipalmitoylphosphocholine (DPPC) as a function of temperature, SPE composition and concentration. The effects of SPEs were studied by differential scanning calorimetry (DSC), X-ray diffraction, 2H and 31P NMR spectroscopy. At low concentration (< 1 mol%) all of the SPEs lowered the bilayer to the inverted hexagonal phase transition temperature of DEPE and induced the formation of a cubic phase in a composition dependent manner. At the same low concentration, SPEs in DPPC induce the formation of a non-bilayer phase as seen by 31P NMR. Order parameter measurements of DPPC-d62/SPE mixtures show that the SPE effect on the DPPC monolayer thickness is dependent on the SPE, concentration, chains length and saturation level. At higher concentration (> or = 10 mol%) SPE are very potent DEPE bilayer to HII phase transition promoters, although at that concentration the SPE have lost the ability to form cubic phases. SPEs have profound effects on the phase behaviour of model membrane systems, and may be important to consider when developing current and potential industrial and medical applications.     

Electric field increases the phase transition temperature in the bilayer membrane of phosphatidic acid

The effect of the electric field on the phase transition temperature (Tc) of acidic 1,2-dipalmitoyl-sn-glycero-3-phosphate (DPPA) and 1,2-dipalmitoyl-sn-glycero-3-thionphosphate (thion-DPPA) and zwitterion, i.e. 1,2-dipalmitoyl-rac-3-phosphocholine and 1,2-distearoyl-rac-glycero-3-phosphocholine (DPPC and DSPC), lipids has been investigated. The phase transition was detected using the jump-like increase effect in the conductance of the planar bilayer membrane. A voltage increase to 150 mV has been shown to increase the phase transition temperature in a bilayer lipid membrane (BLM) of phosphatidic acids (DPPA and thion-DPPA) by 8-12 degrees C while the transition temperature in the bilayer of zwitterion lipids (DPPC and DSPC) increases insignificantly. The increasing of Tt in BLM of acidic lipids is attributed to the voltage-induced changes in the molecule packing density.     

Raman spectroscopic study of an interdigitated lipid bilayer. Dipalmitoylphosphatidylcholine dispersed in glycerol

Dipalmitoylphosphatidylcholine (DPPC) dispersed in perdeuterated glycerol was investigated in order to determine the effects on the Raman spectra of hydrocarbon chain interdigitation in gel-phase lipid bilayers. Interdigitated DPPC bilayers formed from glycerol dispersions in the gel phase showed a decrease in the peak height intensity $\text{I}{}_{2850}\text{/I}{}_{2880}$ ratio, for the symmetric and asymmetric methylene CH stretching modes, respectively, as compared to non-interdigitated DPPC/water gel-phase dispersions. The decrease in this spectral ratio is interpreted as an increase in chain-chain lateral interactions. Spectra recorded in the 700–740 cm^?1 CN stretching mode region, the 1000–1200 cm^?1 CC stretching mode region and the 1700–1800 cm^? CO stretching mode region were identical for both the interdigitated and non-interdigitated hydrocarbon chain systems. At low temperatures the Raman peak height intensity ratios $\text{I}{}_{2935}\text{/I}{}_{2880}$ were identical for the DPPC/glycerol and DPPC/water dispersions, indicating that this specific index for monitoring bilayer behavior is insensitive to acyl chain interdigitation. The increase, however, in the change of this index at the gel-liquid crystalline phase transition temperature for the DPPC/glycerol dispersions implies a larger entropy of transition in comparison to the non-interdigitated DPPC/water bilayer system.     

Properties of ganglioside GM1 in phosphatidylcholine bilayer membranes.

Gangliosides have been shown to function as cell surface receptors, as well as participating in cell growth, differentiation, and transformation. In spite of their multiple biological functions, relatively little is known about their structure and physical properties in membrane systems. The thermotropic and structural properties of ganglioside GM1 alone and in a binary system with 1,2-dipalmitoyl phosphatidylcholine (DPPC) have been investigated by differential scanning calorimetry (DSC) and x-ray diffraction. By DSC hydrated GM1 undergoes a broad endothermic transition TM = 26 degrees C (delta H = 1.7 kcal/mol GM1). X-ray diffraction below (-2 degrees C) and above (51 degrees C) this transition indicates a micellar structure with changes occurring only in the wide angle region of the diffraction pattern (relatively sharp reflection at 1/4.12 A-1 at -2 degrees C; more diffuse reflection at 1/4.41 A-1 at 51 degrees C). In hydrated binary mixtures with DPPC, incorporation of GM1 (0-30 mol%; zone 1) decreases the enthalpy of the DPPC pretransition at low molar compositions while increasing the TM of both the pre- and main transitions (limiting values, 39 and 44 degrees C, respectively). X-ray diffraction studies indicate the presence of a single bilayer gel phase in zone 1 that can undergo chain melting to an L alpha bilayer phase. A detailed hydration study of GM1 (5.7 mol %)/DPPC indicated a conversion of the DPPC bilayer gel phase to an infinite swelling system in zone 1 due to the presence of the negatively charged sialic acid moiety of GM1. At 30-61 mol % GM1 (zone 2), two calorimetric transitions are observed at 44 and 47 degrees C, suggesting the presence of two phases. The lower transition reflects the bilayer gel --> L alpha transition (zone 1), whereas the upper transition appears to be a consequence of the formation of a nonbilayer, micellar or hexagonal phase, although the structure of this phase has not been defined by x-ray diffraction. At > 61 mol % GM1 (zone 3) the calorimetric and phase behavior is dominated by the micelle-forming properties of GM1; the presence of mixed GM1/DPPC micellar phases is predicted.     

Thermotropic behavior of dipalmitoylphosphatidylcholine vesicles reconstituted with the glycoprotein of vesicular stomatitis virus

The vesicular stomatitis virus glycoprotein reconstituted into dipalmitoylphosphatidylcholine (DPPC) vesicles exerts a profound effect upon the DPPC gel to liquid-crystalline phase transition. The glycoprotein was reconstituted into DPPC vesicles by octyl glucoside dialysis. The gel to liquid-crystalline phase transition of these vesicles was monitored by differential scanning calorimetry. Vesicles formed in the absence of glycoprotein (600--2100-A diameter) underwent the phase transition at 41.0 degrees C and had an associated enthalpy change of 8.0 +/- 1.6 kcal/mol. Increasing the mole ratio of glycoprotein to DPPC in the vesicles to 0.15 mol % reduced both the transition temperature and the transition enthalpy change. The enthalpy change as a function of the mole percent glycoprotein could be fit to a straight line by a least-squares procedure. Extrapolation of the results to the glycoprotein concentration where the enthalpy change was zero indicated one glycoprotein molecule bound 270 +/- 150 molecules of DPPC.