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PubMed1992

Calorimetry of tetraether lipids from Thermoplasma acidophilum: incorporation of alamethicin, melittin, valinomycin, and nonactin.

The development and application of model membrane systems on the basis of tetraether lipids from Thermoplasma acidophilum has been proposed. In this respect incorporation of membrane proteins and ionophores is indispensable and is demonstrated in the case of alamethicin, melittin, nonactin, and valinomycin by calorimetry. Dipalmitoylphosphatidylcholine (DPPC) and dihexadecylmaltosylglycerol (DHMG) were chosen for comparison. Melittin and alamethicin prove to broaden the lipid phase transition and to reduce the melting temperature Tm and enthalpy change (delta H) of the main phospholipid from T. acidophilum (MPL) and DPPC. The decrease in Tm, however, is more pronounced in DPPC than in MPL. Valinomycin shows only a marginal effect on the temperature and width of the transition; delta H is reduced in MPL and remains constant in DPPC and DHMG. With nonactin the phase transition of DPPC is quenched, and delta H and the half-height width are increased. DHMG is affected to a lesser extent and MPL only marginally. The four ionophores exhibit different modulation of the phase transition behavior of the various lipids as expected from their varying molecular structures. Thus, the integral membrane protein alamethicin, the peripheral protein melittin, valinomycin, and nonactin interact primarily with lipid head groups and are readily incorporated into the tetraether lipid structures.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1975

Differential scanning calorimetry and 31P NMR studies on sonicated and unsonicated phosphatidylcholine liposomes.

1. Phase transitions in sonicated (vesicles) and unsonicated liposomes composed of various synthetic phosphatidylcholines are monitored using differential scanning calorimetry and 31P NMR. 2. The temperature (Tc), heat content and width of the phase transition are comparable in both vesicles and liposomes prepared from 1,2-dipalmitoyl phosphatidylcholine and 1,2-dimyristoyl phosphatidylcholine. In vesicles composed of a (1 : 1) mixture of 1,2-dipalmitoyl phosphatidylcholine and 1,2-dioleoyl phosphatidylcholine phase separation occurs as in the bilayers of the unsonicated liposomes. 3. The linewidth of the 31P resonances in vesicles is not greatly dependent upon the fatty acid composition when the lipids are in the disordered liquid crystalline state (above Tc). When the lipids are in the gel state (below Tc), however, there is a marked increase in linewidth, demonstrating a reduction in motion of the phosphate group. 4. The ratio of the amounts of phosphatidylcholine present in the outside and inside monolayter of the vesicle membrane was determined with 31P NMR using Nd3+ as a non-permeating shift reagent. 5. The outside/inside ratio is dependent upon the hydrocarbon chain length. Increasing chain length gives a lower outside/inside ratio and a larger vesicle. Introduction of cis or trans double bonds in the chain influences the outside/inside ratio slightly. 6. The incorporation of cholesterol decreases the outside/inside ratio and increases the size of 1,2-dimyristoyl phosphatidylcholine vesicles. The cholesterol concentration in the outside and inside monolayer is approximately the same. The size of the 1,2-dioleoyl phosphatidylcholine vesicles is also increased by cholesterol incorporation but the outside/inside distribution is also increased, especially between 30 and 50 mol% cholesterol. In these vesicles cholesterol is asymmetrically distributed and strongly prefers the inside monolayer of the vesicle.

Calorimetry↗
PubMed1982

Interactions of glycerol monooleate and dimethylsulphoxide with phospholipids. A differential scanning calorimetry and 31P-NMR study.

1. A comparative study has been made of the effects of the fusogens glycerol monooleate and dimethyl-sulphoxide on the polymorphic phase behaviour of dipalmitoyl phosphatidylcholine and dipalmitoyl phosphatidylethanolamine by differential scanning calorimetry and 31P-NMR techniques. 2. Glycerol monooleate induces a reduction in the temperature, cooperativity and enthalpy of the gel to liquid-crystal transitions of dipalmitoyl phosphatidylcholine and dipalmitoyl phosphatidylethanolamine, whereas dimethylsulphoxide induces an increase in the temperature and enthalpy and a reduction in the cooperativity of the gel to liquid-crystal transitions for those same phospholipids. 3. Glycerol monooleate induces the formation of isotropic and hexagonal (HII) phases when mixed with either dipalmitoyl phosphatidylcholine or dipalmitoyl phosphatidylethanolamine. By contrast, in the presence of dimethylsulphoxide, those same phospholipids retain the lamellar configuration observed in the absence of fusogen. 4. These results are discussed in terms of the mechanisms of chemically induced cell fusion.

Calorimetry, Differential Scanning↗
PubMed1982

Differential scanning calorimetry of milk fat globule membranes.

Differential scanning calorimetry was employed as an aid in examining the structure of the bovine milk fat globule membrane. At least six major endotherms are observed between 10 and 90 degree C, corresponding to order-disorder transitions of discrete structural domains of the membrane. These endothermic transitions occur at 16, 28, 43, 58, 68, and 75 degrees C. The transitions occurring between 10 and 50 degrees C were reversible, suggesting the involvement of lipid. However, the high temperature transitions were irreversible. The calorimetric C transition, center at 43 degree C, was shown to involve neutral lipid, since the endotherm was reversible, insensitive to proteolysis, and similar to the endotherm of the isolated neutral lipid fraction of the milk fat globule membrane. The glycolipid and phospholipid fractions of the milk fat globule membrane yielded endotherms outside of the temperature range of the C transition. Another endotherm, the D transition (58 degree C), was found to involve the denaturation of the major membrane coat protein, butyrophilin (band 12). Evidence for this assignment included the following observations: (i) the nearly selective proteolysis of butyrophilin resulted in the complete removal of the D transition, (ii) the butyrophilin-enriched, Triton X-100-insoluble pellet of milk fat globule membrane yielded a relatively normal D transition, and (iii) the irreversible, disulfide-stabilized aggregation of butyrophilin occurred in the membrane solely at the temperature of the D transition. Furthermore, no other prominent milk fat globule membrane polypeptide formed these non-native disulfide crossbridges during the D transition. The sources of the other major endotherms of the milk fat globule membrane have not yet been assigned.

Animals↗
PubMed1982

Phase separation induced by melittin in negatively-charged phospholipid bilayers as detected by fluorescence polarization and differential scanning calorimetry.

Interactions between melittin and a variety of negatively-charged lipid bilayers have been investigated by intrinsic fluorescence, fluorescence polarization of 1,6-diphenylhexatriene and differential scanning calorimetry. (1) Intrinsic fluorescence of the single tryptophan residue of melittin shows that binding of this peptide to negatively-charged phospholipids is directly related to the surface charge density, but is unaffected by the physical rate of lipids, fluid or gel, single-shell vesicles or unsonicated dispersions. (2) Changes in the thermotropic properties of negatively-charged lipids upon melittin binding allow to differentiate two groups of lipids: (i) A progressive disappearance of the transition, without any shift in temperature, is observed with monoacid C14 lipids such as dimyristoylphosphatidylglycerol and -serine (group 1). (ii) With a second group of lipids (group 2), a transition occurs even at melittin saturation, and two transitions are detected at intermediate melittin content, one corresponding to remaining unperturbed lipids, the other shifted downward by 10-20 degrees C. This second group of lipids is constituted by monoacid C16 lipids, dipalmitoylphosphatidylglycerol and -serine. Phosphatidic acids also enter this classification, but it is the net charge of the phosphate group which allows to discriminate: singly charged phosphatidic acids belong to group 2, whereas totally ionized ones behave like group 1 lipids, whatever the chain length. (3) It is concluded that melittin induces phase separations between unperturbed lipid regions which give a transition at the same temperature as pure lipid, and peptide rich domains in which the stoichiometry is 1 toxin per 8 phospholipids. The properties of such domains depend on the bilayer stability: in the case of C16 aliphatic chains and singly charged polar heads, the lipid-peptide domains have a transition at a lower temperature than the pure lipid. With shorter C14 chains or with two net charges by polar group, the bilayer structure is probably totally disrupted, and the new resulting phase can no longer lead to a cooperative transition.

Anions↗
PubMed1983

X-ray analysis and calorimetry on phosphatidylcholine model membranes. The influence of length and position of acyl chains upon structure and phase behaviour.

The effect of variation of the acyl chain composition of phosphatidylcholines upon thermal behaviour of multilamellar liposomes was evaluated by calorimetry and X-ray studies. A total of thirteen different phosphatidylcholines were examined. They differed from each other in the length as well as in the position of the acyl chains in the glycerol backbone. The experimental results show that the hitherto accepted phase scheme for phosphatidylcholine-water systems is incomplete and has to be extended to include the behaviour of samples that have been stored for long times at low temperatures. The X-ray results show that the structure of the new low-temperature phase is not in agreement with the hexagonal packing of the acyl chains. To explain the X-ray results, a two-dimensional orthorhombic unit cell has to be assumed in order to fit all the observed reflexes in the wide-angle region.

Calorimetry↗
PubMed1985

A calorimetry and deuterium NMR study of mixed model membranes of 1-palmitoyl-2-oleylphosphatidylcholine and saturated phosphatidylcholines.

Binary phase diagrams have been constructed from differential scanning calorimetry (DSC) data for the systems 1-palmitoyl-2-oleylphosphatidylcholine (POPC)/dimyristoylphosphatidylcholine (DMPC), POPC/dipalmitoylphosphatidylcholine (DPPC) and POPC/distearoylphosphatidylcholine (DSPC). Mixtures of POPC with DMPC exhibit complete miscibility in the gel and liquid crystalline states. Mixtures of POPC with DPPC or with DSPC exhibit gel phase immiscibility over the composition range 0-75% DPPC (or DSPC). These results, when taken together with previous studies of mixtures of phosphatidylcholines, are consistent with the hypothesis that PCs whose order-disorder transition temperatures (Tm values) differ by less than 33 deg. C exhibit gel state miscibility. Those whose Tm values differ by more than 33 deg. C exhibit gel state immiscibility. 2H-NMR spectroscopy has been used to further study mixed model membranes composed of POPC and DPPC, in which either lipid has been labeled with deuterium in the 2-, 10- or 16-position of the palmitoyl chain(s) or in the N-methyls of the choline head group. POPC/DPPC mixtures in the liquid crystalline state are intermediate in order between pure POPC and DPPC at the same temperature. The POPC palmitoyl chain is always more disordered than the palmitoyl chains of DPPC in liquid crystalline POPC/DPPC mixtures. This is attributed to the fact that a POPC palmitoyl chain is constrained by direct bonding to have at least one oleyl chain among its nearest neighbors, while a DPPC palmitoyl chain must have at least one neighboring palmitoyl chain. When liquid crystalline POPC, DPPC and POPC/DPPC mixtures are compared at a reduced temperature (relative to the acyl chain order-disorder transition), POPC/DPPC mixtures are more disordered than predicted from the behavior of the pure components, in agreement with enthalpy data derived from DSC studies. Within the temperature range of the broad phase transition of 1:1 POPC/DPPC, a superposition of gel and liquid crystalline spectra is observed for 1:1 POPC/[2H]DPPC, while 1:1[2H]POPC/DPPC exhibits only a liquid crystalline spectrum. Thus, at temperatures within the phase transition region, the liquid crystalline phase is POPC-rich and the gel phase is DPPC-rich. Comparison of the liquid crystalline quadrupole splittings within the thermal phase transition range suggests that mixing of the residual liquid crystalline POPC and DPPC is highly non-ideal.

Calorimetry, Differential Scanning↗
PubMed1985

Studies on protein-lipid interactions in cytochrome c oxidase by differential scanning calorimetry.

The interaction between cytochrome c oxidase and phospholipids was studied by differential scanning calorimetry. The active, lipid-sufficient cytochrome c oxidase undergoes thermodenaturation at 336 K with a relatively broad and concentration dependent endothermic transition. The delipidated enzyme shows an endothermic denaturation temperature at 331.3 K. When the delipidated cytochrome c oxidase was treated with chymotrypsin, a lowered thermodenaturation temperature was observed. When the delipidated cytochrome c oxidase was reconstituted with asolectin to form a functionally active enzyme complex, the thermodenaturation shifted to a higher temperature, with a sharper transition thermogram. The increase in thermotransition temperature and enthalpy change of thermodenaturation of the asolectin-reconstituted enzyme is directly proportionate to the amount of asolectin used, up to 0.5 mg asolectin per mg protein. The thermotransition temperature and enthalpy changes of thermodenaturation for the phospholipid-reconstituted cytochrome c oxidase are affected by the phospholipid headgroup and the fatty acyl groups. Among phospholipids with the same acyl moiety but different head groups, phosphatidylethanolamine was found to be more effective than phosphatidylcholine in protecting cytochrome c oxidase from thermodenaturation. An exothermic transition thermogram was observed for delipidated cytochrome c oxidase embedded in phospholipid vesicles formed with phospholipids containing unsaturated fatty acyl groups. The increase in exothermic transition temperature and exothermic enthalpy change of thermodenaturation of the oxidase-cytochrome c-cytochrome c oxidase complex destabilized cytochrome c but not cytochrome c oxidase toward thermodenaturation.

Animals↗
PubMed1985

Morphology and phase behavior of two types of unilamellar vesicles prepared from synthetic phosphatidylcholines studied by freeze-fracture electron microscopy and calorimetry.

Differential scanning calorimetry and freeze-fracture electron microscopy have been used to characterize the phase behavior and morphology of two types of unilamellar vesicles composed of synthetic phosphatidylcholines. The first type displayed an average diameter of roughly 100 nm and was formed by slow dilution and dialysis of octylglucoside-solubilized lipid. These large, unilamellar vesicles were termed dialyzed, octylglucoside vesicles and could be obtained as a fairly well defined and uniform population of vesicles. The second vesicle type was prepared by a unique procedure involving dialysis of deoxycholate-solubilized lipid at its pre-transition temperature. This procedure produced a much more heterogeneous distribution of vesicle sizes (500 to 4000 nm in diameter) and left some dilamellar and oligolamellar species which could not be conveniently separated from the giant, unilamellar vesicles constituting the major portion of the sample. Both populations of vesicles displayed phase behavior similar, but not identical to that of large, multilamellar vesicles (LMV). Fracture-face morphology of the gel phase was also observed to differ between the two unilamellar and the multilamellar species. LMV have previously been shown to have clear undulated or banded fracture-faces in the P beta phase, while octylglucoside vesicles are shown here to have facetted fracture-faces. Giant, unilamellar vesicles displayed a faint banded morphology similar to but less distinct than that of the LMV P beta phase. These results have demonstrated that bilayer apposition is not required to support the banded fracture-face morphology characteristic of the P beta phase but that a limiting curvature is necessary.

Calorimetry↗
PubMed1986

Phospholipid structure determines the effects of peptides on membranes. Differential scanning calorimetry studies with pentagastrin-related peptides.

The effect of phospholipid structure on the interaction between small peptides and phospholipid membranes has been studied by high-sensitivity differential scanning calorimetry. The peptides used, N-Boc-beta-Ala-Trp-Met-Arg-Phe-NH2 and N-Boc-beta-Ala-Trp-Met-Lys-Phe-NH2, are basic analogs of the hormone pentagastrin. These peptides split the gel-to-liquid crystalline phase transition of synthetic phosphatidylcholines into two components. For dimyristoyl (DMPC), dipalmitoyl (DPPC) and 1-stearoyl-2-oleoyl (SOPC) phosphatidylcholines, one component remains at the temperature corresponding to that of pure lipid and the other one is shifted towards higher temperatures. With increasing peptide concentration there is a gradual increase in the enthalpy of the high-temperature component at the expense of the low-temperature one, and there is also an increase in the total enthalpy of the transition. A mixture of the peptide with distearoylphosphatidylcholine (DSPC) behaves differently, with the transition occurring at a temperature below that of the pure lipid increasing with peptide concentration. The susceptibility of various phosphatidylcholines to perturbation by the peptides increases in the order DMPC greater than SOPC greater than DPPC greater than DSPC. The effect of these peptides on the phase transitions of acidic phosphatidylglycerols is generally greater than with the corresponding phosphatidylcholines, but the dependence on the length of lipid hydrocarbon chains is similar. Perturbation of the thermotropic phase transition is strongest for dimyristoylphosphatidylglycerol, followed by the dipalmitoyl and the distearoyl analogs. The effect of the peptides on the phase transition of dimyristoylphosphatidylserine is significantly smaller compared to that observed with dimyristoylphosphatidylglycerol and it is further reduced for dimyristoylphosphatidic acid. The phase transition of this latter lipid remains virtually unchanged, even in the presence of high concentrations of the peptide. Similar resistance to the perturbation of the phase transitions by the peptides is observed for synthetic phosphatidylethanolamine. The different susceptibility of various phospholipids to perturbation by the peptides is suggested to be related to different degrees of intermolecular interaction between phospholipid molecules, and particularly to different abilities of phospholipids to form intermolecular hydrogen bonding.

Calorimetry, Differential Scanning↗
PubMed1986

Studies on the interaction of human erythrocyte band 3 with membrane lipids using deuterium nuclear magnetic resonance and differential scanning calorimetry.

Human erythrocyte band 3, reconstituted into large unilamellar phospholipid vesicles, has been used as a model system for studying the interactions between membrane lipids and large transmembrane glycoproteins. Both 2H-nuclear magnetic resonance (2H-NMR) and differential scanning calorimetric techniques have been used to probe dimyristoylphosphatidylcholine-band 3 interactions over the temperature range 4-32 degrees C. Analysis of 2H-NMR spectra allowed the assignment of liquid crystal, gel phase and two-phase regions for several protein/lipid mole fractions in the range (1-20) X 10(-4). Sample size was limited by the amount of available glycoprotein and this precluded exact determination of the phase boundaries for this system. The sharp discontinuity in the spectral first moment, M1, seen at the phase transition of the pure phospholipid is progressively diminished by addition of protein, and at the highest protein concentration the first moment varies smoothly between the two phases. For T greater than 26 degrees C or less than 16 degrees C, the moments are relatively insensitive to protein concentration, while between 20 and 26 degrees C the moments increase with protein concentration up to the boundary of the two-phase region. Beyond this boundary, they remain constant or decrease slightly with increasing amount of protein. A preliminary phase diagram for band 3 in this lipid system is presented, based on 2H-NMR data. Differential scanning calorimetry (DSC) showed that addition of glycoprotein dramatically alters the scan shape and tends to extend the coexistence of two phases to higher temperatures.

Anion Exchange Protein 1, Erythrocyte↗
PubMed1987

A differential scanning calorimetry study of the interaction of alpha-tocopherol with mixtures of phospholipids.

When alpha-tocopherol was included in multibilayer vesicles of dimyristoylphosphatidylcholine, dipalmitoylphosphatidylcholine and distearoylphosphatidylcholine it induced a broadening of the main transition and a displacement of this transition to lower temperatures, as seen by differential scanning calorimetry. This effect was quantitatively more important in the samples of distearoylphosphatidylcholine than in those of the other phosphatidylcholines. Alpha-Tocopherol when present in equimolar mixtures of dimyristoylphosphatidylcholine and diastearoylphosphatidylcholine, which show monotectic behaviour, preferentially partitions in the most fluid phase. The effect of alpha-tocopherol on the phase transition of dilauroylphosphatidylethanolamine and dipalmitoylphosphatidylethanolamine is qualitatively different of that observed on phosphatidylcholines, and several peaks are observed in the calorimetric profile, probably indicating the formation of separated phases with different contents in alpha-tocopherol. The effect was more apparent in dipalmitoylphosphatidylethanolamine than in dilauroylphosphatidylethanolamine. The inclusion of alpha-tocopherol in equimolar mixtures of dilauroylphosphatidylethanolamine and dipalmitoylphosphatidylcholine, which show cocrystallization, only produced a broadening of the phase transition and a shift to lower temperatures. However, in the case of equimolar mixtures of dipalmitoylphosphatidylcholine which also show cocrystallization, the effect was to cause lateral phase separation with the formation of different mixtures of phospholipids and alpha-tocopherol. Alpha-Tocopherol was also included in equimolar mixtures of phosphatidylethanolamine and phosphatidylcholine showing monotectic behaviour, and in this case alpha-tocopherol preferentially partitioned in the most fluid phase, independently of whether this was composed mainly of phosphatidylcholine or of phosphatidylethanolamine.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1988

The effect of alpha-lactalbumin on the thermotropic phase behaviour of phosphatidylcholine bilayers, studied by fluorescence polarization, differential scanning calorimetry and Raman spectroscopy.

The effects of bovine alpha-lactalbumin on the thermotropic properties of dimyristoylphosphatidylcholine liposomes are studied by Raman spectroscopy, fluorescence polarization and differential scanning calorimetry. The Raman spectrum reveals the drastic effects of the protein on the phospholipid structure. The transition temperature shifts downwards and the inter- and intrachain order in the lipid matrix progressively diminish with increasing protein concentration. Up to a lipid to protein molar ratio R = 25, the bilayer structure however is maintained. From fluorescence polarization data we conclude that the protein restricts the mobility of the DPH probe. In view of the Raman results, the lower probe mobility obviously cannot be associated with a more rigid lipid matrix. Nevertheless the transition temperatures of the alpha-lactalbumin-phospholipid complex increases. DSC measurements give no decisive way out for this discrepancy. These results confirm that different types of lipid order are involved in lipid-protein interactions. Compared to the free protein, the alpha-helicity of the protein has increased in the complex.

Calorimetry, Differential Scanning↗
PubMed1990

Differential scanning calorimetry and Fourier transform infrared analysis of lipid-protein interactions involving the nicotinic acetylcholine receptor.

Lipid-protein interactions were studied using Torpedo californica acetylcholine receptor (AChR) as a model system by reconstituting purified AChR into dielaidoylphosphatidylcholine (DEPC, 18:1 trans-9,10) membranes. The structural and thermodynamic behavior of lipids in the vicinity of the protein were studied by differential scanning calorimetry and Fourier transform infrared spectroscopy. The effects of AChR on the thermodynamic parameters associated with lipid phase transitions were to reduce the enthalpy change, lower the transition temperature and reduce the cooperative behavior of the lipid molecules. A stoichiometry of approx. 95 lipids per AChR molecule was found by simulating the decrease in enthalpy in terms of a simple model in which a fixed number of lipid molecules are prevented from undergoing a cooperative phase transition. In parallel, the vibrational spectra of pure DEPC and AChR reconstituted in DEPC membranes at various lipid to protein ratios were examined. Profiles of the 3000-2800 cm-1 C-H stretching region and 1350-950 cm-1 characteristic of the headgroup region of the lipid exhibit little sensitivity to protein/lipid ratio reflecting weak interaction of AChR with DEPC. The lipid carbonyl on the other hand appear to be increasingly hydrogen bonded in the presence of AChR. The results provide new information about the size and physical state of the motionally restricted lipid environment that surrounds the acetylcholine receptor. The results are discussed in the context of lipid-mediated alterations in acetylcholine receptor function.

Animals↗
PubMed1990

The dissimilar interactions of the calcium antagonist flunarizine with different phospholipid classes and molecular species: a differential scanning calorimetry study.

The influence of the class IV calcium antagonist flunarizine on the phase behaviour of different species of the major phospholipid classes of mammalian plasma membranes has been examined using differential scanning calorimetry. We show that it has the ability to substantially influence the phase behaviour of phospholipids. Flunarizine significantly influences the gel to liquid-crystalline transition temperature of phosphatidylserines whilst having little effect on those of the phosphatidylethanolamines tested. The liquid-crystalline to inverted hexagonal phase transition of phosphatidylethanolamines is, however, strongly induced by the presence of flunarizine. Examination of the effect of flunarizine on the phase behaviour of different phosphatidylcholine species revealed an acyl-chain dependent influence. Dissimilar results with phosphatidylcholines, phosphatidylethanolamines and phosphatidylserines reveal different locations and ionization states for the drug in the different phospholipid bilayers. These results not only indicate an essential role for the ionization state of the drug in determining drug-phospholipid interactions but also the role of the phospholipid in determining the ionization state of the drug and have important implications for drug-membrane interactions demonstrating that drug interaction with one phospholipid may bear no relation whatsoever to its interaction with another.

Animals↗
PubMed1992

Interaction of retinol and retinoic acid with phospholipid membranes. A differential scanning calorimetry study.

The influence of retinol and retinoic acid, two retinoids of major interest, on the main gel to liquid-crystalline phase transition of different phospholipid membranes has been studied by means of differential scanning calorimetry. Both compounds exerted perturbing effects on the phase transition of membranes composed of dipalmitoylphosphatidylcholine or dipalmitoylphosphatidylethanolamine. At concentrations up to 42.5 mol% of retinoid in the membrane, the delta H was not much affected with respect to the pure phospholipid, indicating a rather slight interaction. As the concentration of retinol was increased the Tc transition temperature decreased. A fluid-phase immiscibility was observed for the system DPPC/retinol at concentrations between 0 and 33 mol%. Almost ideal phase diagrams were obtained for the mixture DPPE/retinol. At concentrations of 33 mol% and higher retinol was able to induce phase separations in DPPC membranes, but not in DPPE. The effect of retinoic acid was much weaker, the Tc and delta H remaining almost unaltered and equal to that of the pure phospholipid up to concentrations of 30 mol%, at neutral pH. Retinoic acid exerted a pH-dependent effect. As the pH decreased, and therefore increased the extent of protonation of retinoic acid, the pertubation of the membrane induced by this compound was less. A strong effect, both on Tc and delta H, was observed at pH 10, where the retinoic acid moiety will be mainly unprotonated and the negative charge will generate repulsive forces thus destabilizing the membrane. The mixture DPPC/retinoic acid presents a region of fluid-phase immiscibility. At low pH, when the retinoic acid moiety was fully protonated, this fluid-immiscibility region extended from 0 to 36 mol% of retinoic acid, but its size decreased with increasing pH, and at pH 10 it was only found from 0 to 3 mol%. These results are discussed in terms of the possible retinoid/phospholipid interactions and the disposition of the retinoid moiety in the bilayer.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1992

Amphipathic interactions of cannabinoids with membranes. A comparison between delta 8-THC and its O-methyl analog using differential scanning calorimetry, X-ray diffraction and solid state 2H-NMR.

The effects of (-)-delta 8-tetrahydrocannabinol (delta 8-THC) and its biologically inactive O-methyl ether analog on model phospholipid membranes were studied using a combination of differential scanning calorimetry (DSC), small angle X-ray diffraction and solid state 2H-NMR. The focus of this work is on the amphipathic interactions of cannabinoids with membranes and the role of the free phenolic hydroxyl group which is the only structural difference between these two cannabinoids. Identically prepared aqueous multilamellar dispersions of phosphatidylcholines in the absence and presence of cannabinoids were used. The DSC thermograms and X-ray diffraction patterns of these preparations allowed us to detect the strikingly different manners in which these two cannabinoids affect the thermotropic properties and the thickness of the bilayer. In order study the effects of the cannabinoids on different regions of the bilayer, we used solid state 2H-NMR with four sets of model membranes from dipalmitoylphosphatidylcholine deuterated in different sites, viz., the choline trimethylammonium head group, or one of the following three groups in the acyl chains; the 2'-methylene, 7'-methylene, 16'-methyl groups. Analysis of quadrupolar splittings indicated that delta 8-THC resides near the bilayer interface and the inactive analog sinks deeper towards the hydrophobic region. The temperature dependence of the solid state 2H-NMR spectra showed that, during the bilayer phase transition, the disordering of the choline head groups is a separate event from the melting of the acyl chains, and that amphipathic interactions between delta 8-THC and the membrane separate these two events further apart in temperature. The inactive analog lacks the ability to induce such a perturbation.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1993

Perturbation of the chain melting transition of DPPC by galactose, agarose and Laurdan as determined by differential scanning calorimetry.

Differential scanning calorimetry was used to determine the effects of agarose hydrogel, galactose and the fluorophore Laurdan on the thermal behavior of multilamellar liposomes of DPPC. Although the effect of agarose on the phase transition was found to be negligible at low concentrations (< or = 2.5%), higher concentrations result in an endotherm that broadens and splits into two calorimetric events, one of which is at a higher temperature than that of hydrated DPPC. Equal weight fractions of galactose produce similar effects, although both fractions have raised melting temperatures. The higher melting components may be produced by osmotically-driven dehydration of the inner liposomal monolayers, a physical interaction between the carbohydrates and lipid headgroups, or a combination of both. Laurdan has little effect on the phase transition of DPPC vesicles at the concentration used in the sensor (0.67 mol%); concentrations up to 5.4 mol% only slightly lowered the melting temperature.

1,2-Dipalmitoylphosphatidylcholine↗