Hydrocarbon chain packing and molecular motion in phospholipid bilayers formed from unsaturated lecithins. Synthesis and properties of sixteen positional isomers of 1,2-dioctadecenoyl-sn-glycero-3-phosphorylcholine.

Isomers of cis-octadecenoic acid, with the double bond in each position in the hydrocarbon chain, were used to synthesize the corresponding 1,2-diacyl-sn-glycero-3phosphorylcholines (lecithins). Differential thermal analysis of the lecithins, as a function of water content, permitted evaluation of the limiting transition temperature (Tc) of each isomer. Values of Tc plotted against double bond position fell on a smooth curve with a minimum at minus 22 degrees for the dioctadec-9'-enoyl compound. The presence of a "pretransition" endotherm in differential thermal analysis of 1,2-dioctadec-15'-and 1,2-dioctadec-16'-enoyl-sn-glycero-3-phosphorylcholine implies the existence of two beta crystalline forms. This was not observed with any of the other lecithins. Enthalpy and entropy data were then obtained from differential scanning calorimetry measurements. Values of delta H were lower (7.6 plus or minus 0.1 kcal mol- minus 1) when the center of unsaturation was near the middle of the hydrocarbon chain than they were (9.6 kcal mol- minus 1) when the center of unsaturation was close to either end of the chain. However, values of delta S showed no consistent variation with double bond position. Four positional isomers of 1-octadec-cis-enoyl-2-octadecanoyl-sn-glycero-3-phosphorylcholine were synthesized. With the double bond near the middle of the chain or close to the terminal group, the Tc values of the mixed acid lecithins were higher than those of the corresponding dioctadecenoyl lecithins. 13-C nuclear magnetic resonance relaxation measurements were used to obtain information about chain motion of selected 1,2-dioctadec-cis-enoyl-sn-glycero-3-phosphorylcholines at a temperature (52 degrees) above the Tc values. Spin-lattice relaxation times of the resolved resonances indicated that location of double bonds near the middle, as compared to either end, of the hydrocarbon chain favors enhanced molecular motion along the length of the chins and especially at the terminal methyl end. In the gel state, the minimum interaction potential energy of hydrocarbon chains in bilayers formed from dioctadecenoyl lipids appears to be minimized by localization of the double bond near the middle of the chains. It is suggested that in the case of homogeneous chains the double bond primarily affects the cooperativity of interactions and has very little steric effect on van der Waals' contacts. By contrast, in bilayers of mixed lecithins, with heterogeneous chains, the steric effect may become dominant, depending on double bond position. These differences in chain packing in the gel state are promulgated beyond the phase transition to the liquid crystalline state as an enhancement of chain motion as the temperature rises above Tc.     

Dilatometric studies of isobranched phosphatidylcholines

Absolute apparent specific volumes have been obtained for phosphatidylcholine lipids with saturated, isobranched hydrocarbon chains with ni = 15 to 20 carbons, with an emphasis upon phase transition behavior, both equilibrium and kinetic. The temperature of the chain-melting transition extrapolates with increasing chain length to the melting temperature of polyethylene with a small odd/even alternation. There are also odd/even alternations in the volume of transition and in the hysteresis of the chain-melting transition, but with the odd and even reversed when compared with the larger odd/even alternation in the lower solid-solid transition that occurs in the longer chain ni lipids. A phenomenological picture is given for the coalescence of the two transitions for shorter ni lipids and this picture is used to sharpen the discussion of the kinetic mechanism of melting. A temperature-reversal experiment shows that the melting from the lowest temperature crystal or C phase to the fluid F phase does not proceed via the metastable gel G phase for 16i. The dilatometric results are combined with recent X-ray structural results for the C and G phases of 17i and 20i to deduce various structural information, including the hydration numbers and the volume of the headgroup, VH = 341 A3, which agrees very well with VH for straight-chain phosphatidylcholines. For the chain-melted F phase the assumption that the methylene volumes of the different ni lipids should be the same at the same temperature is used to obtain the volumes of the methylene and the methyl groups.     

The effect of the phase transition on the hydration and electrical conductivity of phospholipids.

Adsorption isotherms for various saturated phosphatidylcholines have been obtained. Lipids above and below their phase transition temperature differ only in the amount of water adsorbed and not in the nature of their adsorption isotherms. Cholesterol has an effect similar to that of increasing unsaturation in the hydrocarbon chains. Decreasing the length of the hydrocarbon chains for lipids below their phase transition temperature has no effect on the isotherms. If the chain length is short enough so that the lipids are above their transition temperature, however, a large increase in water adsorption occurs. All of the phospholipids exhibit a rapid increase of electrical conductivity for a few water molecules adsorbed per lipid molecule. All of the phospholipids show a saturation in conductivity at greater amounts of adsorbed water; the shape of the saturation region depends on whether the lipids are above or below their phase transition temperature. The activation energy for the electrical conductivity process depends on whether the hydrated lipids are in the "liquid-like" of the crystalline state, being lower for phospholipids in the liquid-like state. If the lipids are hydrated above their phase transition temperatures, their activation energies are lower than if they are hydrated below the transition temperature. Cholesterol lowers the activation energy. The phosphatidylcholines can be characterized by different activation energies, depending both upon their physical state and the presence of unsaturation in their hydrocarbon chains.     

Temperature and compositional dependence of the structure of hydrated dimyristoyl lecithin.

Differential scanning calorimetry and x-ray diffraction techniques have been used to investigate the structure and phase behavior of hydrated dimyristoyl lecithin (DML) in the hydration range 7.5 to 60 weight % water and the temperature range -10 to +60 degrees C. Four different calorimetric transitions have been observed: T1, a low enthalpy transition (deltaH approximately equal to 1 kcal/mol of DML) at 0 degrees C between lamellar phases (L leads to Lbeta); T2, the low enthalpy "pretransition" at water contents greater than 20 weight % corresponding to the transition Lbeta leads to Pbeta; T3, the hydrocarbon chain order-disorder transition (deltaH = 6 to 7 kcal/mol of DML) representing the transition of the more ordered low temperature phases (Lbeta, Pbeta, or crystal C, depending on the water content) to the lamellar Lalpha phase; T4, a transition occurring at 25--27 degrees C at low water contents representing the transition from the lamellar Lbeta phase to a hydrated crystalline phase C. The structures of the Lbeta, Pbeta, C, and Lalpha phases have been examined as a function of temperature and water content. The Lbeta structure has a lamellar bilayer organization with the hydrocarbon chains fully extended and tilted with respect to the normal to the bilayer plane, but packed in a distorted quasihexagonal lattice. The Pbeta structure consists of lipid bilayer lamellae distorted by a periodic "ripple" in the plane of the lamellae; the hydrocarbon chains are tilted but appear to be packed in a regular hexagonal lattice. The diffraction pattern from the crystalline phase C indexes according to an orthorhombic cell with a = 53.8 A, b = 9.33 A, c = 8.82 A. In the lamellae bilayer Lalpha strucure, the hydrocarbon chains adopt a liquid-like conformation. Analysis of the hydration characteristics and bilayer parameters (lipid thickness, surface area/molecule) of synthetic lecithins permits an evaluation of the generalized hydration and structural behavior of this class of lipids.     

Physical studies of phospholipids. 3. Electron microscope studies of some pure fully saturated 2,3-diacyl-DL-phosphatidyl-ethanolamines and phosphatidyl-cholines

On heating pure, fully saturated 2,3-diacyl-DL-phosphatidyl-ethanolamines and 2,3-diacylphosphatidyl-cholines (lecithins) in water to the transition temperature at which large endothermic heat changes occur, they are observed, by light microscopy, to form myelin figures. This result is discussed in terms of the large difference in the transition temperature for "melting" of the hydrocarbon chains of unsaturated and saturated phospholipids and is illustrated by means of differential thermal analysis (D.T.A.) curves. These structures have been examined by electron microscopy after negative staining and after reaction with osmium tetroxide. Typical phospholipid lamella structures are seen in the phosphatidylcholines after negative staining, and in the phosphatidyl-ethanolamines after both negative staining and osmium fixation. The distances across these lamellae have been measured. Some preliminary investigations of the nature of the osmium tetroxide reaction with the phosphatidyl-ethanolamines have been made.     

Phase behaviour of cord factor and related bacterial glycolipid toxins. A monolayer study.

C-6 esters of methyl alpha-D-glucoside and C-6, C-6' 'diesters of alpha, alpha'-D-trehalose with C18 and C32 threo and erythro mycolic acids (from chemical source) and of C80-erythro-mycolic acid (from natural source) have been synthesized. Esters of a C32 deoxy analogue were prepared as well. Throughout a monolayer study at the air-water interface, these glycolipids are shown to form well organized phases in which the two hydrocarbon chains of mycoloyl residues must be in interaction. Compression isotherms of C32 esters suggested a transition between liquid-expanded and liquid-condensed states. Latent heats Qc and entropy changes delta S associated with these phase transitions as well as the critical temperature at which they occur have been measured. Within the monolayer, the molecular packing of these glycolipids depends on the presence of the hydroxyl group of mycoloyl residues and on its stereochemistry. In particular intermolecular hydrogen bonds between these groups are postulated in the case of the bis(C32-erythro-mycoloyl)-trehalose. On the other hand, short chain C18 esters form fluid phases (t greater than 10 degrees C) whereas very long chain C80 mycoloyl esters of trehalose exist in a condensed state (t = 20 degrees C). These glycolipids were found to interact strongly with dipalmitoylphosphatidylcholine and egg yolk lecithins (3-sn-phosphatidylcholine). Their phase behaviours are discussed in connection with hypotheses concerning the way they can interact with mitochondrial membranes.     

Thermotropic phase behavior of model membranes composed of phosphatidylcholines containing iso-branched fatty acids. 2. Infrared and 31P NMR spectroscopic studies

The polymorphic phase behavior of aqueous dispersions of a number of representative phosphatidylcholines with methyl iso-branched fatty acyl chains was investigated by Fourier transform infrared (FT-IR) and phosphorus-31 nuclear magnetic resonance (31P NMR) spectroscopy. For the longer chain phosphatidylcholines, where two transitions are resolved on the temperature scale, the higher temperature event can unequivocally be assigned to the melting of the acyl chains (i.e., a gel/liquid-crystalline phase transition), whereas the lower temperature event is shown to involve a change in the packing mode of the methylene and carbonyl groups of the hydrocarbon chains in the gel state (i.e., a gel/gel transition). The infrared spectroscopic data suggest that the methyl iso-branched phosphatidylcholines assume a partially dehydrated, highly ordered state at low temperatures, resembling the Lc phase recently described for the long-chain n-saturated phosphatidylcholines. At higher temperatures, some branched-chain phosphatidylcholines appear to assume a fully hydrated, loosely packed gel phase similar to but not identical with the P beta, phase of their linear saturated analogues. Thus, the iso-branched phosphatidylcholine gel/gel transition corresponds, at least approximately, to a summation of the structural changes accompanying both the subtransition and the pretransition characteristic of the longer chain n-saturated phosphatidylcholines. The infrared spectroscopic data also show that, in the low-temperature gel state, there are significant differences between the odd- and even-numbered isoacylphosphatidylcholines with respect to their hydrocarbon chain packing modes as well as to their head group and interfacial hydration states.(ABSTRACT TRUNCATED AT 250 WORDS)     

Structure of gel phase saturated lecithin bilayers: temperature and chain length dependence.

Systematic low-angle and wide-angle x-ray scattering studies have been performed on fully hydrated unoriented multilamamellar vesicles of saturated lecithins with even chain lengths N = 16, 18, 20, 22, and 24 as a function of temperature T in the normal gel (L beta') phase. For all N, the area per chain Ac increases linearly with T with an average slope dAc/dT = 0.027 A2/degree C, and the lamellar D-spacings also increase linearly with an average slope dD/dT = 0.040 A/degree C. At the same T, longer chain length lecithins have more densely packed chains, i.e., smaller Ac's, than shorter chain lengths. The chain packing of longer chain lengths is found to be more distorted from hexagonal packing than that of smaller N, and the distortion epsilon of all N approaches the same value at the respective transition temperatures. The thermal volume expansion of these lipids is accounted for by the expansion in the hydrocarbon chain region. Electron density profiles are constructed using four orders of low-angle lamellar peaks. These show that most of the increase in D with increasing T is due to thickening of the bilayers that is consistent with a decrease in tilt angle theta and with little change in water spacing with either T or N. Because of the opposing effects of temperature on area per chain Ac and tilt angle 0, the area expansivity alpha A is quite small. A qualitative theoretical model based on competing head and chain interactions accounts for our results.     

Thermotropic phase behavior of model membranes composed of phosphatidylcholines containing iso-branched fatty acids. 1. Differential scanning calorimetric studies

The thermotropic phase behavior of aqueous dispersions of phosphatidylcholines containing one of a series of methyl iso-branched fatty acyl chains was studied by differential scanning calorimetry. These compounds exhibit a complex phase behavior on heating which includes two endothermic events, a gel/gel transition, involving a molecular packing rearrangement between two gel-state forms, and a gel/liquid-crystalline phase transition, involving the melting of the hydrocarbon chains. The gel to liquid-crystalline transition is a relatively fast, highly cooperative process which exhibits a lower transition temperature and enthalpy than do the chain-melting transitions of saturated straight-chain phosphatidylcholines of similar acyl chain length. In addition, the gel to liquid-crystalline phase transition temperature is relatively insensitive to the composition of the aqueous phase. In contrast, the gel/gel transition is a slow process of lower cooperativity than the gel/liquid-crystalline phase transition and is sensitive to the composition of the bulk aqueous phase. The gel/gel transitions of the methyl iso-branched phosphatidylcholines have very different thermodynamic properties and depend in a different way on hydrocarbon chain length than do either the "subtransitions" or the "pretransitions" observed with linear saturated phosphatidylcholines. The gel/gel and gel/liquid-crystalline transitions are apparently concomitant for the shorter chain iso-branched phosphatidylcholines but diverge on the temperature scale with increasing chain length, with a pronounced odd/even alternation of the characteristic temperatures of the gel/gel transition.(ABSTRACT TRUNCATED AT 250 WORDS)     

Thermodynamic and structural characterization of amino acid-linked dialkyl lipids

Using differential scanning calorimetry (DSC), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR), we determined some thermodynamic and structural parameters for a series of amino acid-linked dialkyl lipids containing a glutamic acid-succinate headgroup and di-alkyl chains: C12, C14, C16 and C18 in CHES buffer, pH 10. Upon heating, DSC shows that the C12, C14 and annealed C16 lipids undergo a single transition which XRD shows is from a lamellar, chain ordered subgel phase to a fluid phase. This single transition splits into two transitions for C18, and FTIR shows that the upper main transition is predominantly the melting of the hydrocarbon chains whereas the lower transition involves changes in the headgroup ordering as well as changes in the lateral packing of the chains. For short incubation times at low temperature, the C16 lipid appears to behave like the C18 lipid, but appropriate annealing at low temperatures indicates that its true equilibrium behavior is like the shorter chain lipids. XRD shows that the C12 lipid readily converts into a highly ordered subgel phase upon cooling and suggests a model with untilted, interdigitated chains and an area of 77.2A(2)/4 chains, with a distorted orthorhombic unit subcell, a=9.0A, b=4.3A and beta=92.7 degrees . As the chain length n increases, subgel formation is slowed, but untilted, interdigitated chains prevail.     

Interaction of the antimicrobial peptide gramicidin S with dimyristoyl--phosphatidylcholine bilayer membranes: a densitometry and sound velocimetry study

We determined changes in the volume and adiabatic compressibility of large multi- and unilamellar vesicles composed of dimyristoylphosphatidylcholine containing various concentrations of the antimicrobial peptide gramicidin S (GS) by applying densitometry and sound velocimetry. Gramicidin S incorporation was found to progressively decrease the phase transition temperature of DMPC vesicles as well as to decrease the degree of cooperativity of the main phase transition and to increase the volume compressibility of the vesicles. GS probably enhanced thermal fluctuations at the region of main phase transition and provide more freedom of rotational movement for the phospholipid hydrocarbon chains. The ability of GS to increase the membrane compressibility and to decrease the phase transition temperature is evidence for regions of distorted membrane structure around incorporated gramicidin S molecules. At relatively high GS concentration (10 mol%), more significant changes of specific volume and compressibility appear. This might suggest changes in the integrity of the lipid bilayer upon interaction with high concentrations of GS.     

Structure and cohesive properties of sphingomyelin/cholesterol bilayers.

Thermal, structural, and cohesive measurements have been obtained for both bovine brain sphingomyelin (BSM) and N-tetracosanoylsphingomyelin (C24-SM) in the presence and absence of cholesterol. A goal of these experiments has been to clarify the mechanisms responsible for the strong interaction between sphingomyelin and cholesterol. Differential scanning calorimetry shows that fully hydrated bilayers of BSM and C24-SM have main endothermic phase transitions at 39 and 46 degrees C, respectively, that reflect the melting of the acyl chains from a gel to a liquid-crystalline phase. For each lipid, the addition of cholesterol monotonically reduces the enthalpy of this transition, so that at equimolar cholesterol the transition enthalpy is zero. The addition of equimolar cholesterol to either BSM or C24-SM coverts the wide-angle X-ray diffraction reflection at 4.15 A to a broad band centered at 4.5 A. Electron density profiles of gel-phase C24-SM bilayers contain two terminal methyl dips in the center of the bilayer, indicating that the lipid hydrocarbon chains partially interdigitate so that the long saturated 24-carbon acyl chains in one monolayer cross the bilayer center and appose the shorter sphingosine chains from the other monolayer. The incorporation of cholesterol adds electron density to the hydrocarbon chain region near the head group and removes the double terminal methyl dip. These wide- and low-angle X-ray data indicate that cholesterol packs into the hydrocarbon chain region near the sphingomyelin head group, fluidizes the methylene chains near the center of the bilayer compared to the gel phase, and reduces the extent of methylene chain interdigitation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Theory of lipid monolayer and bilayer phase transitions: Effect of headgroup interactions

Summary Headgroup and soft core interactions are added to a lipid monolayer-bilayer model and the surface pressure-area phase diagrams are calculated. The results show that quite small headgroup interactions can have biologically significant effects on the transition temperature and the phase diagram. In particular, the difference in transition temperatures of lecithins and phosphatidyl ethanolamines is easy to reproduce in the model. The phosphatidic acid systems seem to require weak transient hydrogen bonding which is also conjectured to play a role in most of the lipid systems. By a simple surface free energy argument it is shown that monolayers under a surface pressure of 50 dynes/cm should behave as bilayers, in agreement with experiment. Although the headgroup interactions are biologically very significant, in fundamental studies of the main phase transition in lipids they are secondary in importance to the hydrocarbon chain interactions (including the excluded volume interaction, the rotational isomerism, and the attractive van der Waals interaction).     

Interaction of the antimicrobial peptide gramicidin S with dimyristoyl–phosphatidylcholine bilayer membranes: a densitometry and sound velocimetry study

We determined changes in the volume and adiabatic compressibility of large multi- and unilamellar vesicles composed of dimyristoylphosphatidylcholine containing various concentrations of the antimicrobial peptide gramicidin S (GS) by applying densitometry and sound velocimetry. Gramicidin S incorporation was found to progressively decrease the phase transition temperature of DMPC vesicles as well as to decrease the degree of cooperativity of the main phase transition and to increase the volume compressibility of the vesicles. GS probably enhanced thermal fluctuations at the region of main phase transition and provide more freedom of rotational movement for the phospholipid hydrocarbon chains. The ability of GS to increase the membrane compressibility and to decrease the phase transition temperature is evidence for regions of distorted membrane structure around incorporated gramicidin S molecules. At relatively high GS concentration (10 mol%), more significant changes of specific volume and compressibility appear. This might suggest changes in the integrity of the lipid bilayer upon interaction with high concentrations of GS.     

Effects of headgroup methylation and acyl chain length on the volume of melting of phosphatidylethanolamines.

The change in volume associated with the gel to liquid-crystalline phase transition for phosphatidylethanolamines of various chain lengths and headgroup methylation was determined by measuring the pressure dependence of the phase transition temperature and computing the volume change by using the Clausius-Clapyron equation. The volumes thus obtained were comparable to those previously obtained by using scanning dilatometry. The melting volume was larger for lipids with longer acyl chains, as found previously. The melting volume for a series of N-methylated dipalmitoylphosphatidylethanolamines (DPPEs) did not increase monotonically with increasing headgroup methylation. Instead, the melting volume increased in the order N,N-dimethyl-DPPE less than N-methyl-DPPE less than DPPE less than dipalmitoylphosphatidylcholine. This unanticipated result is hypothesized to result from the competing effects of headgroup methylation on molecular volume and hydrogen bonding on the volume of melting.     

Structure and polymorphism of the hydrocarbon chains of lipids: a study of lecithin-water phases

This work describes the structure of a variety of lecithin-water phases observed below the “melting” temperature of the hydrocarbon chains, with special emphasis on the conformation of the chains. The lecithins studied in this work are the homologous series dioctanoyl to distearoyl, 2-decanoyl-1-stearoyl, and a preparation from hen eggs. The hydrocarbon chains are found to adopt a variety of conformations in addition to type α, the liquid-like organization observed above the melting temperature. Type β: the chains are stiff and parallel, oriented at right angles to the plane of the lamellae and packed with rotational disorder in a two-dimensional hexagonal lattice ($\text{a ~ 4.85}\text{A}\text{?}\text{)}$. Type β′: similar to β, but with the chains tilted with respect to the normal to the lamellae. Type δ: the chains are probably coiled into helices, whose axes are perpendicular to the plane of the polar groups and are packed with rotational disorder in a two-dimensional square lattice ($\text{a ~ 4.80}\text{A}\text{?}\text{)}$, α is the predominant conformation, common to most lipids in the presence of water and at sufficiently high temperature, and the one more relevant to membranes; β is observed at lower temperatures in lipids whose chains are heterogeneous and in the presence of very small amounts of water; β′ is found in synthetic lecithins with identical chains, in the presence of variable amounts of water; δ is observed in dry lecithins. A highly ordered crystalline phase, yet displaying rotational disorder of the chains, is observed in almost dry lecithins. Most of the phases are lamellar, and contain one lipid bilayer per repeat unit. Two phases display two-dimensional lattices: Pδ, formed by ribbon-like elements with the chains in the δ conformation; Pβ′, formed by lamellae of type β′ distorted by periodic ripples. The results emphasize the clear-cut difference between the liquid-like and the other types of partly ordered conformations, as well as the correlations which exist between the chemical composition and the structure of the lipids below the melting temperature of the chains.     

Metastable behaviour of saturated phosphatidylethanolamines: a densitometric study

The specific volumes of saturated phosphatidylethanolamines having 12-18 carbon atoms per chain and the volume changes resulting from their temperature-induced phase transitions have been investigated using differential scanning densitometry. No dependence on acyl-chain length has been found for the molar volume changes associated with the Lc----L alpha transition, while the L beta----L alpha transition showed delta V-changes that increase with the length of the acyl-chains. The independence of acyl-chain length of the molar volume changes concomitant with the Lc----L alpha transition can be rationalized by the assumption that the volume changes result from the combined effects of hydration and acyl chain melting. An interesting corollary of the present studies is the disappearance of metastability at chain lengths of about 20-22 carbon atoms.     

Tris buffer causes acyl chain interdigitation in phosphatidylglycerol

The structure of the gel phase and the properties of the acyl chain disordering transition of dipalmitoyl phosphatidylglycerol (DPPG) have been studied using differential scanning calorimetry, differential scanning dilatometry, and X-ray diffraction. In the presence of small, monovalent cations, DPPG at 22 degrees C exists in a lamellar phase in which the hydrocarbon chains are tilted from the perpendicular to the bilayer surface. Around 34 degrees C, there is a small pretransition (delta H less than 1 kcal/mol) followed by the main transition at 40.4 degrees C (delta H = 8.3 kcal/mol; delta V = 0.0381 ml/g). If DPPG is suspended in Tris-HCl buffer in the absence of other monovalent cations, X-ray diffraction data show that at 22 degrees C, the gel phase consists of interdigitated acyl chains perpendicular to the plane of the bilayer. No pretransition is observed and the main transition occurs at 41.3 degrees C with delta H = 9.1 kcal/mol and delta V = 0.0514 ml/g. If sufficient Na+ or K+ ions are added to the Tris-buffered DPPG, the phase behavior reverts to what is observed in the absence of Tris. Analysis of the energetics of the main transition shows that the increase in van der Waals interaction energy resulting from the larger delta V in Tris can be compensated by the favorable energetics of removing terminal methyl groups from the bilayer surface. The amount of disordering, i.e. formation of gauche rotamers, is likely to be the same in Tris as it is in buffers without amphiphilic cations.     

Thermodynamics of carbohydrate-lipid interactions.

Thermodynamic analyses of carbohydrate-lipid interactions were performed by investigating the effects of a series of carbohydrates, including monosaccharides, disaccharides, and trisaccharides, on the phase-transition properties of aqueous dispersions of 1,2-dipalmitoyl phosphatidylcholine (DPPC). The temperature of the lipid's main phase transition from the gel to liquid-crystalline phase is essentially unchanged in the presence of carbohydrate. The change in the free energy (delta G) of the transition is zero when a carbohydrate is added to aqueous dispersions of DPPC, while the enthalpy (delta H) and the entropy of the melting of DPPC are decreased. The thermodynamic information was used to examine carbohydrate-lipid interactions. Such interactions were elucidated according to our knowledge of the specific properties of carbohydrates in aqueous solutions and the previously proposed hydrophobic interaction involving hydrocarbon tails of the lipid in aqueous dispersions.     

A theoretical model of the temperature- and pressure-induced phase transition of phospholipid bilayers

A statistical thermodynamic model of phospholipid bilayers is developed. In the model, a new concept of a closely packed system is applied, i.e., a system of hard cylinders of equal radii, the radius being a function of the average number of gauche rotations in a hydrocarbon chain. Using this concept of a closely packed system, reasonable values are obtained for the change in specific volume at the order-disorder transition of lecithin bilayers. In addition to interactions between the lipid matrix and water molecules, between the head groups themselves and between hydrocarbon chains, as well as the intramolecular energy associated with chain conformation, the Hamiltonian of the membrane also includes the energy of the pressure field. Thus, the phase transition of phospholipid membranes induced not only by temperature hut also by hydrostatic pressure is described by this model simultaneously. In accordance with the experimental results, a linear relationship is obtained between the phase transition temperature and phase transition pressure. The other calculated phase transition properties of lecithin homologues. e.g., changes in enthalpy, surface area. thickness and gauche number per chain are in agreement with the available experimental data. The ratio of kink to interstitial conduction of bilayers is also estimated.