Effects of hydrostatic pressure on bilayer phase behavior and dynamics of dilauroylphosphatidylcholine.

Deuterium nuclear magnetic resonance spectroscopy was used to study the thermotropic phase behavior of dilauroylphosphatidylcholine (DLPC) bilayers at pressures up to 221 MPa. Pressure was found to separate the liquid crystal to gel transition from the gel to ordered crystalline phase transition. The jump in chain order observed on cooling through the transition into the gel phase was found to be small and thus consistent with the trend in longer chain saturated diacyl phosphatidylcholines. On cooling, DLPC was observed to enter an unusual state above the transition into the gel phase. This unusual state displayed fluid-like conformational order but short transverse relaxation times. It was found to be much better pronounced and to span a broader temperature range at elevated pressure than at lower pressures. Transverse relaxation measurements of deuterons on the chain alpha-carbons revealed a substantial slowing of molecular motions within the temperature range of the unusual fluid phase. The observation of such a phase at high pressure appears to be consistent with recent reports of an unusual fluid phase, Lx, in DLPC at ambient pressure.     

Enhanced hydration of dipalmitoylphosphatidylcholine multibilayer by vinblastine sulphate

Vinblastine sulphate, an antimitotic and anti-inflammatory agent, modifies the thermal behaviour of the model membranes: the dipalmitoylphosphatidylcholine DPPC bilayers. The mixed DPPC and vinblastine sulphate multibilayers in the range of DPPC mole fraction 0.4 to 1 display clearly the gel-liquid crystal (chain melting) transition on the thermograms obtained with a differential scanning microcalorimeter. The molar enthalpy of this transition is slightly depressed by vinblastine sulphate (less than 10%). The temperature-composition phase diagram corresponds to a total insolubility of vinblastine sulphate inside the frozen (gel) bilayers and to a solubility of 0.2 (mole fraction) of vinblastine sulphate inside the fluid (liquid crystalline) bilayers. The dissolved vinblastine sulphate depresses the cooperativity number of the frozen ? fluid transition of the bilayers very strongly (4- to 5-times). Up to its solubility concentration, vinblastine sulphate increases the amount of the structural water of the bilayers and modifies the thermal behaviour of this water. The ‘expelled’ vinblastine sulphate molecules are retained by the polar groups of DPPC molecules and screen their electrostatic interactions with the structural water molecules. Below 0°C, the amount of the structural water, which forms the aqueous separation between two bilayers, is enhanced by vinblastine sulphate. However, the drug reduces (screens) the bilayers interaction with the structural water molecules.     

Comparative study on the properties of saturated phosphatidylethanolamine and phosphatidylcholine bilayers: Barrier characteristics and susceptibility to phospholipase A2 degradation

Comparative studies on bilayer systems of saturated phosphatidylcholines and phosphatidylethanolamines revealed a maximum in ionic permeability in phosphatidylcholine bilayers at the temperature of the gel to liquid-crystalline phase transition but such an increase in permeability was not detectable in bilayers of phosphatidylethanolamine. Furthermore, it was found that at the phase transition temperature the phosphatidylcholine bilayers are subject to rapid hydrolysis by pancreatic phospholipase A₂ whereas phosphatidylethanolamine bilayers are not. These differences are discussed in view of detailed information on the molecular organization in the gel and liquid crystalline phases of the two phospholipid classes.     

A Fourier transform infrared spectroscopic study of the interaction of alkaline earth cations with the negatively charged phospholipid 1, 2-dimyristoyl-sn-glycero-3-phosphoglycerol

The interaction of aqueous phospholipid dispersions of negatively charged 1,2-dimyristoyl-sn-glycero-3-phosphoglycerol, sodium salt (DMPG) with the divalent cations Mg(2+), Ca(2+) and Sr(2+) at equimolar ratios in 100 mM NaCl at pH 7 was investigated by Fourier transform infrared spectroscopy. The binding of the three cations induces a crystalline-like gel phase with highly ordered and rigid all-trans acyl chains. These features are observed after storage below room temperature for 24 h. When the gel phase is heated after prolonged incubation at low temperature phase transitions into the liquid crystalline phase are observed at 58 degrees C for the DMPG:Sr(2+), 65 degrees C for the DMPG:Mg(2+), and 80 degrees C for the DMPG:Ca(2+) complex. By subsequent cooling from temperatures above T(m) these complexes retain the features of a liquid crystalline phase with disordered acyl chains until a metastable gel phase is formed at temperatures between 38 and 32 degrees C. This phase is characterized by predominantly all-trans acyl chains, arranged in a loosely packed hexagonal or distorted hexagonal subcell lattice. Reheating the DMPG:Sr(2+) samples after a storage time of 2 h at 4 degrees C results in the transition of the metastable gel to the liquid crystalline phase at 35 degrees C. This phase transition into the liquid crystalline state at 35 degrees C is also observed for the Mg(2+) complex. However, for DMPG:Mg(2+) at higher temperatures, a partial recrystallization of the acyl chains occurs and the high temperature phase transition at 65 degrees C is also detected. In contrast, DMPG:Ca(2+) exhibits only the phase transition at 80 degrees C from the crystalline gel into the fluid state upon reheating. Below 20 degrees C, the rate of conversion from the metastable gel to a thermodynamically stable, crystalline-like gel phase decreases in the order Ca(2+)&z. Gt;Mg(2+)>Sr(2+). This conversion into the crystalline gel phase is accompanied by a complete dehydration of the phosphate groups in DMPG:Mg(2+) and by a reorientation of the polar lipid head groups in DMPG:Ca(2+) and in DMPG:Sr(2+). The primary binding sites of the cations are the PO(2)(-) groups of the phosphodiester moiety. Our infrared spectroscopic results suggest a deep penetration of the divalent cations into the polar head group region of DMPG bilayers, whereby the ester carbonyl groups, located in the interfacial region of the bilayers, are indirectly affected by strong hydrogen bonding of immobilized water molecules. In the liquid crystalline phase, the interaction of all three cations with DMPG is weak, but still observable in the infrared spectra of the DMPG:Ca(2+) complex by a slight ordering effect induced in the acyl chains, when compared to pure DMPG liposomes.     

Bilayer phase transitions of N-methylated dioleoylphosphatidylethanolamines under high pressure

The bilayer phase transitions of four kinds of unsaturated phospholipids with different-sized polar head groups, dioleoylphosphatidylethanolamine (DOPE), dioleoylphosphatidyl-N-methylethanolamine (DOMePE), dioleoylphosphatidyl-N,N-dimethylethanolamine (DOMe2PE) and dioleoylphosphatidylcholine (DOPC), were observed by means of differential scanning calorimetry (DSC) and high-pressure light-transmittance. DSC thermogram and light-transmittance curve for each phospholipid vesicle solution exhibited only one phase transition under ambient pressure, respectively. The light-transmittance of DOPC solution at pressure higher than 234 MPa abruptly increased stepwise at two temperatures, which corresponds to the appearance of stable subgel and lamellar gel phases under high pressure in addition to the liquid crystal phase. The constructed temperature (T)-pressure (p) phase diagrams were compared among these phospholipids. The phase-transition temperatures of the phospholipids decreased stepwise by N-methylation of the head group. The slops of the T-p phase boundary (dT/dp) of DOPE, DOMePE and DOMe2PE bilayers (0.127-0.145 K MPa-1) were found to be close to that of the transition from the lamellar crystal (or subgel; Lc) phase to the liquid crystal (Lalpha) phase for DOPC bilayer (0.131 K MPa-1). On the other hand, the dT/dp value of the main transition from the lamellar gel (Lbeta) phase to the Lalpha phase for DOPC bilayer (0.233 K MPa-1) was significantly different from that of the Lc/Lalpha transition, hence both curves intersected with each other at 234 MPa. The thermodynamic quantities associated with the phase transition of DOPE, DOMePE and DOMe2PE bilayers had also similar values to those for the Lc/Lalpha transition of DOPC bilayer. Taking into account of the values of transition temperature, dT/dp and thermodynamic quantities compared with the corresponding results of saturated phospholipids, we identified the phase transitions observed in the DOPE, DOMePE and DOMe2PE bilayers as the transition from the Lc phase to the Lalpha phase although they have been regarded as the main transition in the previous studies. The Lbeta phase is probably unstable for DOPE, DOMePE and DOMe2PE bilayers at all pressures, it exists as a metastable phase at pressures below 234 MPa while as a stable phase at pressures above 234 MPa in DOPC bilayer. The difference in phase stability among the phospholipid bilayers is originated from that in hydration structure of the polar head groups.     

The effect of cholesterol on the structure of phosphatidylcholine bilayers

The effect of cholesterol on the structure of phosphatidylcholine bilayers was investigated by X-ray diffraction methods. Electron density profiles at 5 Å resolution along with chain tilt and chain packing parameters were obtained and compared for phosphatidylcholine/cholesterol bilayers and for pure phosphatidylcholine bilayers in both the gel and liquid crystalline states. The cholesterol in the bilayer was localized by noting the position of discrete elevations in the electron density profiles. Cholesterol can either increase or decrease the width of the bilayer depending on the physical state and chain length of the lipid before the introduction of cholesterol. For saturated phosphatidylcholines containing 12–16 carbons per chain, cholesterol increases the width of the bilayer as it removes the chain tilt from gel state lipids or increases the trans conformations of the chains for liquid crystalline lipids. However, cholesterol reduces the width of 18 carbon chain bilayers below the phase transition temperature as the long phospholipid chains must deform or kink to accomodate the significantly shorter cholesterol molecule. Although cholesterol has a marked effect on hydrocarbon chain organization, it was found that, within the resolution limits of the data, the phosphatidylcholine head group conformation is unchanged by the addition of cholesterol to the bilayer. The head group is oriented parallel to the plane of the bilayer for phosphatidylcholine in the gel and liquid crystalline states and this orientation is not changed by the addition of cholesterol.     

Crystallization of phosphatidylserine bilayers induced by lithium

Utilizing differential scanning calorimetry and x-ray diffraction, 1,2-dimyristoyl-L-glycero-3-phospho-L-serine (DMPS) was shown to form hydrated bilayer membrane structures exhibiting a gel leads to liquid crystalline transition at 39 degrees C (delta H = 7.2 kcal/mol). Addition of up to molar concentrations of the alkali halides NaCl, KCl, Rl Cl, and CsCl produced relatively minor changes in DMPS bilayer structure or stability. For example, in the presence of 0.5 M NaCl, the transition temperature (Tc = 42 degrees C) and transition enthalpy (delta H = 7.0 kcal/mol) show only minor changes. In marked contrast, addition of LiCl results in "'crystallization" of the DMPS bilayer membrane structure. At 0.5 M LiCl, the crystalline DMPS exhibits a bilayer gel leads to liquid crystal transition at 89 degrees C accompanied by a high enthalpy change, delta H = 16.0 kcal/mol. Thus, Li+ induces an isothermal crystallization of DMPS bilayers, the hydrocarbon chains adopting a more ordered packing mode than the "hexagonal" arrangement of the gel state. In view of the widespread use of lithium in the treatment of manic-depressive illness, we also raise the possibility that interaction of Li+ with anionic membrane phospholipids could play a role in its pharmacological action.     

Analysis of the defect structure of gel-phase lipid.

The partitioning of the spin label 2,2,6,6-tetramethylpiperidinyl-1-oxy (Tempo) into phosphatidylcholine bilayers and the monomer-aggregate equilibrium for chlorophyll a incorporated into phosphatidylcholine bilayers have been interpreted in terms of the formation of defects in the gel-phase lipid, starting some 20 degrees C below the temperature of the main gel to liquid crystalline phase transition. By contrast, defects seem to be largely absent from bilayers of dipalmitoylphosphatidylethanolamine in the gel phase. The defect structure accounts for the continuous nature of the phase transition for phosphatidylcholines, and also for the increase in width of the transition caused by the addition of alcohols.     

An X-ray diffraction study of alterations in bovine lung surfactant bilayer structures induced by albumin

Lung surfactant (LS) is an extra-cellular lipid-protein system responsible for maintaining low surface tension in the lung and alveolar stability. Serum proteins cause dysfunction of this material, e.g. in adult respiratory distress syndrome (ARDS). BLES is a clinically used LS consisting of most of the lipids and associated proteins from bovine lung lavage. Aqueous phases of BLES at 30% and 70% hydration, with and without 5% by weight of bovine serum albumin (BSA), calculated on the amount of lipids, were studied using X-ray diffraction during cooling from 42 to 5 degrees C. The diffraction curves are consistent with a transition from a lamellar liquid crystalline phase to a gel phase transition at cooling in the interval 30-20 degrees C. The long-spacings correspond to a reduction of the bilayer thickness during this transition. The wide-angle region shows a peak at 4.1 A below 25 degrees C, which is characteristic of the hexagonal chain packing of the gel phase. The perturbation of the bilayers by the presence of BSA seems to induce a significant decrease of the bilayer thickness. Calculations on the observed limits of swelling (taking place in the range 50-60%) indicate that BSA is closely associated with the BLES bilayers, probably due to electrostatic interaction with the cationic surfactant proteins SP-B and SP-C. This study show that the LS lipid structural organizations are extremely susceptible to small amounts of serum albumin, which may have implications in surfactant related lung disease and clinical applications of surfactant therapy.     

Structure and thermotropic properties of phosphatidylethanolamine and its N-methyl derivatives

The structure and thermotropic properties of hydrated bilayers of 1,2-dimyristoyl-sn-glycero-3-phospho-ethanolamine (DMPE) and its N-monomethyl (mmDMPE) and N,N-dimethyl (dmDMPE) derivatives have been investigated by differential scanning calorimetry and X-ray diffraction. For DMPE, mmDMPE, and dmDMPE, multilamellar dispersions (approximately 50 wt % water) show chain melting bilayer gel----bilayer liquid-crystal transitions (onset) at 49.2, 42.3, and 30.7 degrees C, respectively, with the corresponding value for 1,2-dimyristoyl-sn-glycero-3-phosphocholine occurring at 23 degrees C. Thus, the bilayer chain melting transition decreases with increasing N-methylation, as originally reported for the corresponding palmitoyl series [Vaughan, D.J., & Keough, K.M. (1974) FEBS Lett. 47, 158-161]. This transition is reversible on cooling, and DMPE, mmDMPE, and dmDMPE form the original bilayer gel phase with the rotationally disordered hydrocarbon chains packed in a hexagonal lattice. Following prolonged incubation at -4 degrees C, the bilayer gel phase is shown to be metastable, and conversion to a low-temperature "crystalline" phase occurs with the hydrocarbon chains adopting a specific packing mode. For DMPE, mmDMPE, and dmDMPE, either a single or a double endothermic transition occurs as the "crystal" bilayer phase converts to the bilayer gel phase. A similar pattern of behavior is observed for the palmitoyl series. The relatively slow kinetic conversion of the metastable bilayer gel phase with hexagonally packed hydrocarbon chains to a bilayer phase in which the chains have "crystallized" appears to be a general property of membrane phospholipids and sphingolipids.     

Liquid ordered and gel phases of lipid bilayers: fluorescent probes reveal close fluidity but different hydration

Hydration and fluidity of lipid bilayers in different phase states were studied using fluorescent probes selectively located at the interface. The probe of hydration was a recently developed 3-hydroxyflavone derivative, which is highly sensitive to the environment, whereas the probe of fluidity was the diphenylhexatriene derivative, 1-[4-(trimethylamino)phenyl]-6-phenylhexa-1,3,5-triene. By variation of the cholesterol content and temperature in large unilamellar vesicles composed of sphingomyelin or dipalmitoylphosphatidlycholine, we generated different phases: gel, liquid ordered (raft), liquid crystalline, and liquid disordered (considered as liquid crystalline phase with cholesterol). For these four phases, the hydration increases in the following order: liquid ordered < gel approximately liquid disordered < liquid crystalline. The membrane fluidity shows a somewhat different trend, namely liquid ordered approximately gel < liquid disordered < liquid crystalline. Thus, gel and liquid ordered phases exhibit similar fluidity, whereas the last phase is significantly less hydrated. We expect that cholesterol due to its specific H-bonding interactions with lipids and its ability to fill the voids in lipid bilayers expels efficiently water molecules from the highly ordered gel phase to form the liquid ordered phase. In this study, the liquid ordered (raft) and gel phases are for the first time clearly distinguished by their strong difference in hydration.     

Interdigitated lipid bilayers of long acyl chain species of cerebroside sulfate. A fatty acid spin label study

The metastable phase behavior of semi-synthetic species of cerebroside sulfate (CBS), with hydroxy and non-hydroxy fatty acids from 16 to 26 carbons in length, was compared in Li+ and K+ using differential scanning calorimetry. The structure of the metastable and various stable phases formed in the presence of these two cations was investigated using a fatty acid spin label, 16-doxylstearate. A number of stable phases with successively higher phase transition temperatures and enthalpies occur in the presence of K+ (see the preceding paper). Li+ prevents formation of the most stable phases with the highest transition temperatures and enthalpies for all species of CBS. However, it does not prevent a transition from the metastable phase to the first stable phase of the longer chain C24 and C26 species. Furthermore, it allows C24:0h-CBS to undergo a similar transition, in contrast to a high K+ concentration, which prevents it. The spin label has anisotropic motion in the metastable gel phase formed by all species of CBS on cooling from the liquid crystalline phase. The spectra resemble those in gel phase phospholipids. The spin label is partially insoluble in the most stable phases formed by all the lipids, including the unsaturated C24:1 species, preventing further elucidation of their structure using this technique. However, the spin label is soluble in the first stable phase formed on cooling by the longer chain C24:0 and C26:0-CBS in Li+ and K+ and by C24:0h-CBS in Li+, and is motionally restricted in this phase. The motional restriction is similar to that observed in the mixed interdigitated bilayers of asymmetric species of phosphatidylcholine and fully interdigitated bilayers formed by symmetric phospholipids. It strongly suggests that the highly asymmetric long chain species of CBS form a mixed interdigitated bilayer in their first stable gel phases while the metastable phase of these and the shorter chain lipids may be partially interdigitated. The metastable phase of C24:1-CBS is more disordered suggesting that it may not be interdigitated at all. Thus the results suggest that (i) the hydroxy fatty acid inhibits but does not prevent formation of a mixed interdigitated bilayer by long chain species of CBS, (ii) an increase in non-hydroxy fatty acid chain length from 24 to 26 carbons promotes it, and (iii) a cis double bond probably prevents any form of interdigitation. These results may be relevant to the physiological and pathological roles of these structural modifications of CBS.     

Temperature dependence of the repulsive pressure between phosphatidylcholine bilayers.

Bilayer structure and interbilayer repulsive pressure were measured from 5 to 50 degrees C by the osmotic stress/x-ray diffraction method for both gel and liquid crystalline phase lipid bilayers. For gel phase dibehenoylphosphatidylcholine (DBPC) the bilayer thickness and pressure-distance relations were nearly temperature-independent, and at full hydration the equilibrium fluid spacing increased approximately 1 A, from 10 A at 5 degrees C to 11 A at 50 degrees C. In contrast, for liquid crystalline phase egg phosphatidylcholine (EPC), the bilayer thickness, equilibrium fluid spacing, and pressure-distance relation were all markedly temperature-dependent. As the temperature was increased from 5 to 50 degrees C the EPC bilayer thickness decreased approximately 4 A, and the equilibrium fluid spacing increased from 14 to 21 A. Over this temperature range there was little change in the pressure-distance relation for fluid spacings less than approximately 10 A, but a substantial increase in the total pressure for fluid spacings greater than 10 A. These data show that for both gel and liquid crystalline bilayers there is a short-range repulsive pressure that is nearly temperature-independent, whereas for liquid crystalline bilayers there is also a longer-range pressure that increases with temperature. From analysis of the energetics of dehydration we argue that the temperature-independent short-range pressure is consistent with a hydration pressure due to polarization or electrostriction of water molecules by the phosphorylcholine moiety. For the liquid crystalline phase, the 7 A increase in equilibrium fluid spacing with increasing temperature can be predicted by an increase in the undulation pressure as a consequence of a temperature-dependent decrease in bilayer bending modulus.     

Effect of insulin on the lytic action of lysophosphatidylcholine in lipid bilayers

The effect of insulin on the bilayer properties of dimyristoylphosphatidylcholine liposomes at the gel and the liquid crystalline state was measured by differential scanning calorimetry and absorbance at 450 nm. It is found that insulin promotes a decrease in the enthalpy of the gel-liquid crystalline transition without displacing the transition temperature. Under these conditions the lytic action of monomyristoylphospatidylcholine is enhanced, decreasing the critical lytic concentrations to values comparable to the bilayer at the gel state. The effect of the lysoderivate on liposomes in contact with increasing concentrations of insulin promotes a reorganization of the lipids into smaller particles as inferred from fluorescence dequenching, turbidity and exclusion chromatography assay. It is concluded that the action of lysoderivates can be enhanced, at temperatures above the transition temperature, by proteins that without spanning the lipid bilayers can perturb the bilayer interface.     

The molecular basis of mesomorphic phase transitions in phospholipid systems

The demonstrated existence and possible physiological relevance of mesomorphic phase transitions in cellular membranes suggests that a theoretical understanding of lipid phase behavior is biologically relevant. As a step in this direction, the gel to liquid crystal phase transition of phospholipid bilayers is examined. A qualitative mechanism involving configurational coupling of the lipid hydrocarbon chains is proposed to explain the transition. The predictions of the mechanism which pertain to the structure of the liquid crystal are explored and found to be in accord with the present experimental view of this phase.     

Effects of cyclosporine A on biomembranes. Vibrational spectroscopic, calorimetric and hemolysis studies.

Cyclosporine A (CSA)-dipalmitoylphosphatidylcholine (DPPC) interactions were investigated using scanning calorimetry, infrared spectroscopy, and Raman spectroscopy. CSA reduced both the temperature and the maximum heat capacity of the lipid bilayer gel-to-liquid crystalline phase transition; the relationship between the shift in transition temperature and CSA concentration indicates that the peptide does not partition ideally between DPPC gel and liquid crystalline phases. This nonideality can be accounted for by excluded volume interactions between peptide molecules. CSA exhibited a similar but much more pronounced effect on the pretransition; at concentrations of 1 mol % CSA the amplitude of the pretransition was less than 20% of its value in the pure lipid. Raman spectroscopy confirmed that the effects of CSA on the phase transitions are not accompanied by major structural alterations in either the lipid headgroup or acyl chain regions at temperatures away from the phase changes. Both infrared and Raman spectroscopic results demonstrated that CSA in the lipid bilayer exists largely in a beta-turn conformation, as expected from single crystal x-ray data; the lipid phase transition does not induce structural alterations in CSA. Although the polypeptide significantly affects DPPC model membrane bilayers, CSA neither inhibited hypotonic hemolysis nor caused erythrocyte hemolysis, in contrast to many chemical agents that are believed to act through membrane-mediated pathways. Thus, agents, such as CSA, that perturb phospholipid phase transitions do not necessarily cause functional changes in cell membranes.     

The permeability enhancing mechanism of DMSO in ceramide bilayers simulated by molecular dynamics

The lipids of the topmost layer of the skin, the stratum corneum, represent the primary barrier to molecules penetrating the skin. One approach to overcoming this barrier for the purpose of delivery of active molecules into or via the skin is to employ chemical permeability enhancers, such as dimethylsulfoxide (DMSO). How these molecules exert their effect at the molecular level is not understood. We have investigated the interaction of DMSO with gel-phase bilayers of ceramide 2, the predominant lipid in the stratum corneum, by means of molecular dynamics simulations. The simulations satisfactorily reproduce the phase behavior and the known structural parameters of ceramide 2 bilayers in water. The effect of DMSO on the gel-phase bilayers was investigated at various concentrations over the range 0.0-0.6 mol fraction DMSO. The DMSO molecules accumulate in the headgroup region and weaken the lateral forces between the ceramides. At high concentrations of DMSO (> or =0.4 mol fraction), the ceramide bilayers undergo a phase transition from the gel phase to the liquid crystalline phase. The liquid-crystalline phase of ceramides is expected to be markedly more permeable to solutes than the gel phase. The results are consistent with the experimental evidence that high concentrations of DMSO fluidize the stratum corneum lipids and enhance permeability.     

The effect of chain length and lipid phase transitions on the selective permeability properties of liposomes.

This paper describes experiments showing the importance of the fatty acid chain length on the barrier properties of liposomal bilayers, prepared from saturated lecithins, under conditions of lateral phase separation. 1. Above the gel to liquid crystalline phase transition temperature, liposomes prepared from saturated lecithins with 14 or more carbon atoms per acyl chain exist as stable bilayers, which are practically impermeable to ions. 2. At temperatures well above the transition temperature dilauroyl phosphatidylcholine liposomes exhibited osmotic shrinkage, which was dependent on the ionic size of the solute used to bring about the osmotic gradient, indicating that the permeation through these less stable bilayers takes place mainly via individual diffusion of the permeating ions. 3. An enhanced release of trapped potassium from liposomes was demonstrated in the vicinity of the transition temperature. The extent of the increase, however, depended strongly on the length of the paraffin chain. 4. From measurements of the shrinkage behaviour of liposomes in the vicinity of the transition temperature it is concluded that the increased permeability decreases with increasing diameter of the permeating ion. This finding implies that the increased permeability at the transition temperature cannot be ascribed to "macroscopic" rupture of the liposomal membrane. The maximum permeability in the vicinity of the Tc is discussed in terms of probability and size distribution of statistical pore formation at the boundaries of liquid and solid domains.     

Polymorphic phases of galactocerebrosides: spectroscopic evidence of lamellar crystalline structures

Fourier transform infrared spectroscopy was applied to study the structural and thermal properties of bovine brain galactocerebroside (GalCer) containing amide linked non-hydroxylated or alpha-hydroxy fatty acids (NFA- and HFA-GalCer, respectively). Over the temperature range 0-90 degrees C, both GalCer displayed complex thermal transitions, characteristic of polymorphic phase behavior. Upon heating, aqueous dispersions of NFA- and HFA-GalCer exhibited high order-disorder transition temperatures near 80 and 72 degrees C, respectively. En route to the chain melting transition, the patterns of the amide I band of NFA-GalCer were indicative of two different lamellar crystalline phases, whereas those of HFA-GalCer were suggestive of lamellar gel and crystalline bilayers. Cooling from the liquid-crystalline phase resulted in the formation of another crystalline phase of NFA-GalCer and a gel phase of HFA-GalCer, with a phase transition near 62 and 66 degrees C, respectively. Prolonged incubation of GalCer bilayers at 38 degrees C revealed conversions among lamellar crystalline phases (NFA-GalCer) or between lamellar gel and crystalline bilayer structures (HFA-GalCer). Spectral changes indicated that the temperature and/or time induced formation of the lamellar crystalline structures of NFA- and HFA-GalCer was accompanied by partial dehydration and by rearrangements of the hydrogen bonding network and bilayer packing mode of GalCer.