Thermal analysis of lipids, proteins and biological membranes. A review and summary of some recent studies

Perforin binding to cells and lipid membranes determined by a simple competition assay

Perforin-mediated lysis consists of at least three steps: perforin binding to the target cell, insertion into the plasma membrane, and polymerization to form pores. Perforin binding, the first step, is critical for pore formation. Accordingly, a competition assay was here established for detecting the perforin-binding activities of nucleated cells and lipid membrane vesicles such as cytoplasts or liposomes. The competition assay has certain advantages over the 51Cr release assay, since no isotope and less perforin are needed for the competition assay, and the perforin-binding activity of liposomes and proteolytic enzyme-treated and fixed nucleated cells can also be detected. The competition assay was used to study the mechanism of resistance of cytolytic T lymphocytes (CTL) to perforin-mediated lysis. The results from this assay indicate that perforin-binding activity is not a function of membrane rigidity, and that there is a direct correlation between the ability of cells to bind perforin and their susceptibility to lysis by perforin, i.e., resistant CTL and their corresponding cytoplasts bind perforin much less effectively than susceptible tumor cells and their cytoplasts. A model is proposed whereby a surface molecule or complex of molecules on CTL interferes with perforin-binding activity, thus protecting CTL from perforin-mediated lysis.

Effect of nicotinic acid on microviscosity in mixed liposomal system of lecithin and sphingomyelin

In rat liver plasma membrane, the molar ratio of sphingomyelin and phospholipid is approximately 1:4, whereas, the molar ratio of phospholipid and cholesterol is 3:1. Considering this ratio to be typical for a real biological membrane, we have studied the effect of anticholesterol and the vasodialatory drug nicotinic acid (NA) on the fluidity profile of a liposomal system of lipids mixed in this ratio using the fluorescence polarization probe 1,6-diphenyl-1-1,3,5-hexatriene. The study reveals that when NA is added to the aqueous dispersion of the mixed lipid system (molar ratio of lipid:NA, 1:1) it creates a more fluid environment for the probe molecule and modifies the fluidity profile of the cholesterol-incorporated liposomal system by eliminating the effect of cholesterol to some extent. The drug also affects the activation energy of diffusion of this system. These results on fluidity have been compared with those in cases of liposomes of individual lipids. The effect of NA on fluidity may be attributed to a mechanical interaction of the drug molecules with the lipid molecules.

A statistical mechanical model of the pre- and subtransitions of lecithin membranes

A statistical mechanical model with experimentally proved facts as starting points is presented. This model explains on molecular level, the pre- and subtransitions appearing in lipid membranes. The model describes the main features of the transitions, the hysteresis of the subtransition and the mobility changes of the heads and chains at these transitions. The model was expanded for phosphatidylcholine homologues with arbitrary chain lengths, and a qualitative agreement in the case of pretransition as far as a quantitative one for the subtransition were found.

Membrane function in mammalian hibernation

For homeotherms the maintenance of a high, uniform body temperature requires a constant energy supply and food intake. For many small mammals, the loss of heat in winter exceeds energy supply, particularly when food is scarce. To survive, some animals have developed a capacity for adaptive hypothermia in which they lower their body temperature to a new regulatory set-point, usually a few degrees above the ambient. This process, generally known as hibernation, reduces the temperature differential, metabolic activity, as well as the energy demand, and thus facilitates survival during winter. Successful hibernation in mammals requires that the enzymatic processes are regulated in such a manner that metabolic balance is maintained at both the high body temperature of the summer-active animal (37 degrees C) and the low body temperature of the winter-torpid animal (approx. 5 degrees C). This means that the cellular membranes have thermal properties capable of maintaining a balanced metabolism at these extreme physiological temperatures. The available evidence indicates that, for some tissues, preparation for hibernation involves an alteration in the lipid composition and thermal properties of cellular membranes. Marked differences in the thermal response of cellular membranes have been observed on a seasonal basis and, in some membranes, differences in lipid composition have been associated with the torpid state. However, to date, no consistent changes in lipid composition which would account for, or explain, the changes in membrane thermal response, have been detected. An important point to emphasize is that the process of 'homeoviscous adaptation', which occurs in procaryotes and some poikilotherms during acclimation to low temperatures, is not a characteristic feature of most membranes of mammalian hibernators.

Deuterium nuclear magnetic resonance study of the interaction of branched chain compounds (phytanic acid, phytol) with a phospholipid model membrane

Deuterium nuclear magnetic resonance (2H-NMR) spectra have been determined for 50 wt% aqueous dispersions of 1-palmitoyl(stearoyl)-2-[2H31]palmitoyl-sn-glycero-3-phosphocho lin e (PC-d31) containing 20 mol% of the isoprenoid compounds phytol or phytanic acid over the temperature range -5-55 degrees C. Concentration effects of the isoprenoid compounds are also reported. First moments (M1) and order parameters were calculated from the spectra. 20 Mol% of either branched chain compound causes an approximate 9% increase in the mean order parameter SCD. Significant effects are seen on the PC-d31 phase behavior. 20 Mol% of either branched chain compound causes the gel to liquid crystalline onset temperature (Ts) to drop to 28 degrees C from 38 degrees C for PC-d31 alone, as seen from the temperature dependent M1 values. The melting range ([Tl--Ts]) is congruent to 1.5 degrees C for PC-d31 and congruent to 11 degrees C for PC-d31 containing 20 mol% of the branched chain compounds. This is in direct contrast to their straight chain analogues, hexadecanol and palmitic acid, which have been shown to elevate the phase transition temperature. The isoprenoid compounds cause significant disruption of the gel phase, forcing nearest neighbor phospholipid chains apart. Transverse relaxation times (T2e, the time constant for decay of the quandrupolar echo) have been determined over the temperature range -5-50 degrees C. Possible explanation for the effect of the isoprenoid compounds on the dynamic structure of phospholipids in the bilayer are proffered.

Influence of the gel-liquid phase transition on hematoporphyrin triplet deactivation in liposomes

The deactivation of the triplet state of hematoporphyrin and its dimethyl ester in unilamellar liposomes of dipalmitoyl phosphatidylcholine was studied by nanosecond laser flash photolysis. It was found that the rate of deactivation increases abruptly on raising the temperature in the region of the gel-liquid phase transition of the lipid bilayer (41 degrees C). The rate of change has its maximum at 38.4 +/- 0.5 degree C for both porphyrins. This variation is due to the high lateral mobility of the porphyrins in the liquid-crystal bilayer, which enhances the rates of concentration triplet quenching and triplet-triplet annihilation.

Spontaneous vesiculation of large multilamellar vesicles composed of saturated phosphatidylcholine and phosphatidylglycerol mixtures

The influence of temperature and ionic strength on the vesiculation properties of large multilamellar vesicles containing various proportions of dimyristoylphosphatidylglycerol has been investigated. It is shown that at low ionic strengths preformed large multilamellar vesicles composed of dimyristoylphosphatidylcholine and dimyristoylphosphatidylglycerol (7:3) on incubation at the gel to liquid-crystalline transition temperature (Tc approximately 23 degrees C) spontaneously vesiculate to form predominantly unilamellar systems with a mean diameter of 120 nm. Such vesiculation is not observed for incubations at temperatures appreciably above or below Tc, and is also inhibited by higher ionic strengths. Stable large multilamellar vesicles are formed, however, in systems containing the dioleoyl species of phosphatidylcholine or phosphatidylglycerol and also for dimyristoylphosphatidylcholine/dimyristoylphosphatidylserine mixtures. The vesiculation properties of dimyristoylphosphatidylcholine/dimyristoylphosphatidylglycerol mixtures, therefore, appear to reflect an instability in the region of the Tc driven by surface potential effects which are specific for the glycerol headgroup.

Delivery of nitroxide spin label to cultured cells by liposomes

The positively charged nitroxide spin label, 2,2,6,6-tetramethyl-piperidine-N-oxyl-4-trimethylammonium (Cat1), was encapsulated in two types of liposomes, phosphatidylserine/phosphatidylcholine (PS/PC) and phosphatidylserine/distearoylphosphatidylcholine/dipalmitoylphosphatidyl choline (PS/DSPC/DPPC). The liposomes were incubated with mouse thymus-bone marrow (TB) cells to study the uptake and metabolism of nitroxides entrapped in liposomes. The effects of temperature, metabolic inhibitors, and fixation of cells were investigated. The results indicate that different mechanisms are involved in the uptake of these two types of liposomes. PS/PC liposomes interact predominantely with the plasma membrane of TB cells and release Cat1 continuously, whereas the majority of PS/DSPC/DPPC liposomes are taken into the cells intact via endocytosis. These findings suggest that it may be possible to deliver nitroxides selectively, either to the membrane of cells or to their interior by manipulating the lipid composition of the liposomes. This study also found that the rate of reduction of Cat1 delivered using liposomes was increased under hypoxic conditions. Thus, the use of liposomes for in vivo delivery of nitroxides has the potential to provide NMR contrast that reflects different metabolic conditions.

Dependence of trehalose protective action on the initial phase state of dipalmitoylphosphatidylcholine bilayers

Dipalmitoylphosphatidylcholine (DPPC) bilayers hydrated in the presence of trehalose were equilibrated at various temperatures (4, 20, and 60 degrees C) corresponding to the crystalline Lc, gel L beta', and liquid-crystalline L alpha phases, respectively, and then desiccated at these temperatures or freeze-dried at -80 degrees C to ca. DPPC dihydrate. The thermotropic behavior of the resulting DPPC/trehalose mixtures was investigated by differential scanning calorimetry and found to be dependent not only on the trehalose concentration but also on the phase state of the hydrated bilayers prior to their drying. Trehalose was most effective when the desiccation was carried out from the L alpha phase at 60 degrees C. In this case, one trehalose molecule per two DPPC molecules was sufficient to depress the melting temperature from values typical of DPPC dihydrate to 45 degrees C. Trehalose's influence decreased when dried from the L beta' phase and was significantly less pronounced when dried from the Lc phase. These data show that trehalose's protective influence depends on the initial phase state of the lipid bilayer and reaches its maximum in the liquid-crystalline state. The possible role of this effect in anhydrobiosis is pointed out.

Microviscosity in lecithin liposomes: effect of nicotinic acid

We have studied the effect of nicotinic acid, a drug commonly used as a vasodilatory agent and also for the treatment of hypercholesterolemia, on the fluidity profile of liposomes of egg lecithin and dipalmitoyl lecithin, using a fluorescent polarization probe. In both cases the drug decreases the membrane fluidity and for cholesterol-probed liposomes, it disrupts the "intermediate fluid condition" induced by cholesterol. The drug also affects the activation energy for diffusion in the hydrophobic region of the liposomes.

Specific volumes of lipids in fully hydrated bilayer dispersions

The neutral buoyancy method of obtaining absolute specific volumes of lipid in multilamellar dispersions is critically investigated. Control experiments show that there is no preferential partitioning of 2H2O vs. H2O into the liposomes, and several thermodynamic properties of the samples, such as the enthalpy change and the volume change of the main transition, are changed very little with deuteration of the solvent. The assumption that the molecular volume of the solvent in the interlamellar space is essentially the same as in bulk solution is discussed; and it is shown to introduce rather small corrections. Previous procedures have been modified to avoid possible kinetic limitations in phases with low water permeability. It is concluded that the molecular volume of lipid in bilayers can be obtained to an accuracy better than 0.002 nm3 (2A3) which is less than 0.2% of typical molecular volumes of lipids.

Binding of alkaline-earth metal cations and some anions to phosphatidylcholine liposomes

The dependence of electrophoretic mobility of multilamellar liposomes composed of egg phosphatidylcholine (PtdCho), dimyristoyl-glycerophosphocholine (Myr2Gro-P-Cho) and dipalmitoyl-glycerophosphocholine (Pam2-Gro-P-Cho) on the concentration of several cations and anions has been measured. Values of surface densities of binding sites and intrinsic binding constants of ions to liposome membranes were determined by processing the results in the framework of Gouy-Stern theory. Sharp reductions in the positive surface potential of Myr2Gro-P-Cho and Pam2Gro-P-Cho liposomes have been detected at the thermotropic transition of the lipids from the gel to liquid-crystalline phase. Similar alterations of liposome surface potential were revealed at the temperature of pretransition, as well as at about 50 degrees C, in the case of Pam2Gro-P-Cho. A model is suggested for ion binding to PtdCho membranes, according to which the ion-binding sites are considered as point defects (vacancies) in the structure of lipid head-groups arranged over a trigonal lattice.

The lipoidal permeability barriers of the skin and alimentary tract

The major routes of administration of drugs to humans involve transport either through the intestinal wall or through the skin. Both these barriers are nonpolar in nature and are subserved by membrane lipids. The lipid composition of the brush border of the intestinal wall is unusual, possessing unusually large quantities of glycosylceramide. The lipid composition of the stratum corneum of the skin is also unusual, possessing large quantities of ceramides and free fatty acids. These atypical membrane components are generally more ordered than the other common membrane lipids at body temperature and, thus, are suited for involvement in formation of barriers between the organism and its environment.

Influence of the calcium-induced gel phase on the behavior of small molecules in phosphatidylserine and phosphatidylserine-phosphatidylcholine multilamellar vesicles

The behavior of fluorescent and spin-label probes is examined in several fluid and gel phospholipid phases, with particular focus on the Ca2+-induced gel phase in phosphatidylserine (PS). These probes have behavior characteristic of the type of probe and of the type of lipid environment. Anthroyloxy- and doxyl-labeled PS [12-AS-PS and (7,6)PS, respectively] exhibit greatly restricted and/or slow probe motion in Ca(PS)2, even compared to thermotropic gel-phase lipid at the same temperature. In contrast, anthroyloxy- and doxyl-labeled phosphatidylcholine (PC), as well as fluorescent-labeled and spin-labeled fatty acid derivatives, show no apparent change in probe motion in Ca(PS)2 compared to fluid lamellar lipid. Doxyl-labeled phosphatidic acid, phosphatidylethanolamine, and phosphatidylglycerol show restricted motion in Ca(PS)2 relative to fluid-phase lipid, but the electron paramagnetic resonance (EPR) spectra could not be interpreted in terms of simple models for probe ordering. The fluorescent probes diphenylhexatriene (DPH) and trans-parinaric acid methyl ester (tPNA-Me) show motional behavior in Ca(PS)2 that is intermediate between that observed in fluid and in thermotropic gel-phase lipid. When Ca(PS)2 and fluid PS/PC phases coexist, probe molecules distribute between the two phases. Experiments using fluorescence quenching by spin-labeled PC in PS/PC in excess Ca2+ yield the distribution of several fluorophore probes between fluid liquid-crystal and Ca(PS)2 gel phases, expressed as a concentration ratio, RLC/G. The value of RLC/G = 100 in favor of the fluid phase is obtained for 12-AS-PC, 18 for 12-AS-Me, 12 for DPH, 3 for tPnA-Me, and 1 for 12-AS-PS.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of several lipids, fatty acyl chain length, and degree of unsaturation on the motility of bull spermatozoa after cold shock and freezing

Diluents containing sonicated liposomes of purified phosphatidylserine (PS), phosphatidylcholine (PC) with varying fatty acyl chain lengths and double bonds and cholesterol (CH) alone or in combination, or egg yolk lecithin were evaluated for protection of bull sperm during cold shock produced by rapid cooling from 25 to 0 degrees C and during freezing and thawing. Bull semen was washed twice and diluted to 50 X 10(6) sperm/ml in diluents containing no lipid, 0.5 or 5 mM sonicated lipid or 20% egg yolk and plunged into ice water to cold shock the sperm. Sperm so treated were frozen using conventional methods. The percentage of progressively motile sperm (MS) was estimated prior to cooling, after cold shock, and after freezing and thawing. Lipids with fatty acyl chains of less than 12 carbons were toxic to sperm cells. Phosphatidylserine alone or in combination with PC or CH, but not PC or CH alone, protected sperm from cold shock as well as did egg yolk lecithin liposomes or egg yolk. Liposomes of PS/PC or PS/CH were not better than PS in protecting sperm from cold shock. Lipid concentrations of 0.5 mM were more effective than liposomes at 5 mM in protecting sperm during freezing and thawing. During freezing, PS alone or in combination with PC partially protected sperm, but only PS/CH was as effective as egg yolk in protecting sperm from freeze-thaw damage. It is concluded that defined diluents, particularly those containing PS, may be useful in studies of cryobiology of spermatozoa.

Direct imaging of paracrystalline phospholipid structure in the electron microscope

A long chain amphiphilic molecule--the phospholipid 1,2-dihexadecyl sn glycerophosphoethanolamine--has been crystallized epitaxially so that the interlamellar molecular periodicity is parallel to the substrate and hence normal to the electron beam in the electron microscope. This has permitted the direct resolution of the 55.6 A lamellae in unstained crystals at room temperature. The lattice images have shown the presence of line dislocations and lenticular cracks in the crystals. Of significance to their biological properties is that the lattice is undulating with a periodicity of 0.1-0.5 micron. This would also account for the difficulties encountered by X-ray and electron diffraction techniques when examining these crystals.

Combined electron-spin-resonance, X-ray-diffraction studies on phospholipid vesicles obtained from cold-hardened wheats : II. The role of free sterols

The contents of free sterols and phospholipids in leaves of wheat, Triticum aestivum L., cultivars of different frost resistances, as well as the physical state of isolated phospholipids in the presence and absence of sterols, were compared before and after hardening. There was an inverse relationship between the sterol/phospholipid ratio and frost tolerance as a consequence of both a decrease in the free sterol, and an increase in the total phospholipid content. Sterol-sterol interactions were investigated using wide angle X-ray diffraction, while the phase behaviour of phospholipid vesicles was studied using the electron-spin-resonance (ESR) technique. No sterol-sterol interactions at-10° C were detected in vesicles obtained from the hardened most cold-tolerant cultivar (Miranovskaja 808), containing sterols in a ratio (0.08) found in the original lipid extracts. In contrast, when the sterol-phospholipid ratio in the vesicles was set to the level (0.39) found in the extracts of the most sensitive cultivar, Penjamo 62, the appearance of sharp reflexion rings at 4.5·10(-1), 4.8·10(-1) and 5.0·10(-1) nm indicated strong sterol-sterol interactions. The temperatures for the onset of phase separation for vesicles of identical sterol/phospholipid ratios found in lipid extracts of hardened Miranovskaja 808 were almost the same as those measured in purified phospholipids (-15 vs.-16° C). In contrast, the temperature for the onset of phase separation of vesicles with a sterol/phospholipid ratio characteristic of hardened Penjamo 62 was shifted upwards (from-6 to-2° C). Phase separation was not completed in the vesicles of Miranovskaja 808 in the temperature range scanned (-30° C) but was shifted from-22 to-18° C in the presence of sterols in the case of Penjamo 62. The results are discussed in terms of the composition and physical state of membranes in relation to survival at freezing temperatures.

Molecular order, dynamics, and ionization state of phosphatidylethanolamine bilayers as studied by 15N NMR

Dipalmitoylphosphatidylethanolamine (DPPE) and dipalmitoylphosphatidylcholine (DPPC), 15N-labeled in the polar head group, were synthesized. The proton-decoupled 15N spectra of DPPC and DPPE in aqueous dispersion have exactly the form anticipated for powder line shapes governed by an axially symmetric shielding tensor. The chemical shift anisotropy (delta sigma) of DPPC is lower than 0.4 ppm at 30 degrees C and vanished when the temperature or the half-height line width is increased; DPPE always exhibits an asymmetric line shape, and 15N NMR spectra of DPPE are obtained at various temperatures and simulated to measure exactly the chemical shift anisotropy. At each temperature, the order parameter of the C-N bond segment is derivated from delta sigma and reveals that the average orientation of the C-N bond around the axis of rotation is near the "magic angle" (54.7 degrees). Isotropic correlation times are derived from T1, which are higher than values obtained for phosphatidylcholine by other nuclei. Arrhenius plots of T1 and T2 allowed us to calculate the activation energy for the motion of the DPPE and the DPPC C-N bond. The value of this activation energy for the DPPE (53 kJ/mol) is higher than the one found for the DPPC C-N bond (32 kJ/mol). These differences agree with the capacity of the ethanolamine head groups to bind noncovalently to their neighbors in the plane of the membrane surface. A direct titration curve of the amino group is achieved by the variation of the chemical shift with the bulk pH, and the interfacial pKa is calculated to be 11.1.(ABSTRACT TRUNCATED AT 250 WORDS)

The interaction of phosphatidylcholine bilayers with Triton X-100

The interaction of multilamellar phosphatidylcholine vesicles with the non-ionic detergent Triton X-100 has been studied under equilibrium conditions, specially in the sub-lytic range of surfactant concentrations. Equilibrium was achieved in less than 24 h. Estimations of detergent binding to bilayers, using [3H]Triton X-100, indicate that the amphiphile is incorporated even at very low concentrations (below its critical micellar concentration); a dramatic increase in the amount of bound Triton X-100 occurs at detergent concentrations just below those producing membrane solubilization. Solubilization occurs at phospholipid/detergent molar ratios near 0.65 irrespective of lipid concentration. The perturbation produced by the surfactant in the phospholipid bilayer has been studied by differential scanning calorimetry, NMR and Fourier-transform infrared spectroscopy. At low detergent concentration (lipid/detergent molar ratios above 3), a reduction in 2H-NMR quadrupolar splitting occurs, suggesting a decrease in the static order of the acyl chains; the same effect is detected by Fourier-transform infrared spectroscopy in the form of blue shifts of the methylene stretching vibration bands. Simultaneously, the enthalpy variation of the main phospholipid phase transition is decreased by about a third with respect to its value in the pure lipid/water system. For phospholipid/detergent molar ratios between 3 and 1, the decrease in lipid static order does not proceed any further; rather an increase in fluidity is observed, characterized by a marked decrease in the midpoint transition temperature of the gel-to-fluid phospholipid transition. At the same time an isotropic component is apparent in both 31P-NMR and 2H-NMR spectra, and a new low-temperature endotherm is detected in differential scanning calorimetric traces. When phospholipid and Triton X-100 are present at equimolar ratios some bilayer structure persists, as judged from calorimetric observations, but NMR reveals only one-component isotropic signals. At lipid/detergent molar ratios below unity, the NMR lines become narrower, the main (lamellar) calorimetric endotherm tends to vanish and solubilization occurs.

Fluorescence polarization studies on Escherichia coli membrane stability and its relation to the resistance of the cell to freeze-thawing. I. Membrane stability in cells of differing growth phase

Physical properties of Escherichia coli membrane lipids in logarithmic- and stationary-phase cells were studied by measuring the fluorescence polarization change of cis- and trans-parinaric acid as a function of temperature. In aqueous dispersions of phospholipids extracted from cytoplasmic and outer membranes of cells of differing growth phase, a similar polarization increase was observed over the range from physiological temperature to below 0 degrees C, and nearly the same transition ratios were obtained in all samples. The cytoplasmic membrane of both of the growth-phase cells showed a higher polarization ratio above the transition temperatures, compared to that in the aqueous dispersion of phospholipids. The polarization ratios below the transition temperatures of these specimens were lower than the value obtained with the lipids, especially in the stationary-phase specimens. The outer membrane specimens showed a similar polarization change but the transition temperature ranges were considerably higher both in the logarithmic- and the stationary-phase specimens, compared to those in the cytoplasmic membrane specimens. Freeze-thawing of logarithmic-phase cells showed the emergence of activity of certain enzymes which are known to be located in the membranes. The stationary-phase cells did not suffer from any such deleterious effect and maintained a high level of cell viability in a similar treatment. These results indicate that in the stationary-phase cell membranes lipids are in a highly ordered state, and the lipid state causes a membrane stability which results in the high resistance of the cell to freeze-thawing.

Area per molecule and distribution of water in fully hydrated dilauroylphosphatidylethanolamine bilayers

The area per lipid molecule for fully hydrated dilauroylphosphatidylethanolamine (DLPE) has been obtained in both the gel and liquid-crystalline states by combining wide-angle X-ray diffraction, electron density profiles, and previously published dilatometry results [Wilkinson, D. A., & Nagle, J. F. (1981) Biochemistry 20, 187-192]. The molecular area increases from 41.0 +/- 0.2 to 49.1 +/- 1.2 A2 upon melting from the gel to liquid-crystalline phase. The thickness of the bilayer, as measured from the electron density profiles, decreases about 4 A upon melting, from 45.2 +/- 0.3 to 41.0 +/- 0.6 A. A somewhat unexpected result is that the fluid layer between fully hydrated bilayers is the same in both gel and liquid-crystalline phases and is only about 5 A thick. From these data, plus the volume of the anhydrous DLPE molecule, it is possible to determine the number of water molecules per lipid and their approximate distribution relative to the lipid molecule. Our analysis shows that there are about 7 and 9 waters per DLPE molecule in the gel and liquid-crystalline phases, respectively. About half of the water is located in the fluid space between adjacent bilayers, and the remaining waters are intercalated into the bilayer, presumably in the head group region. There are significantly fewer water molecules in the fluid spaces between DLPE bilayers than in the fluid spaces in gel- or liquid-crystalline-phase phosphatidylcholine bilayers. This small fluid space in PE bilayers could arise from interbilayer hydrogen bond formation through the water molecules or electrostatic interactions between the amine and phosphate groups on apposing bilayers.

Synthesis, calorimetry, and X-ray diffraction of lecithins containing branched fatty acid chains

Lecithins with branched fatty acid chains were synthesized and characterized by differential scanning calorimetry and X-ray diffraction. The influence of three chemical alterations on the phase transition parameters were investigated: length of the branches in 2-position of the acyl chains, position of the branches in the acyl chains, and position of the branched fatty acid chains in the glycerol backbone. The results show that the branched phosphatidylcholines (PCs) have a reduced gel-to-liquid-crystalline phase transition temperature (Tm) compared to the corresponding straight-chain PCs. Depending on both the length of the branches in 2-position of the acyl chains and the position of the branches in the acyl chains, the Tm-values pass through a minimum. The systematic change of the main-transition temperatures Tm is connected with a modified structural polymorphism. If the length of the branches increases three types of polymorphism were observed.

The role of interfacial structured water on the glycoprotein arrangement in liposomes

The effect of perturbing the interfacial water structure in liposomes on the glycoprotein arrangement in the bilayer was investigated. This perturbation was achieved by a series of reagents called structure makers and breakers. The glycoprotein arrangement in the liposomes was determined by fluorescence measurement with 1-anilino-2-naphthalene sulphonate (ANS). A dependence of (n) (number of binding sites for ANS on the glycoprotein molecule) with concentration of structure maker and breaker reagents was observed. The results have been interpreted as a possible new arrangement of membrane-bound glycoprotein, due to the effect of perturbing the interfacial water structure in the liposomes.

Effects of polyamines on the thermotropic behaviour of dipalmitoylphosphatidylglycerol

Using differential scanning calorimetry the thermotropic behaviour of the acidic phospholipid dipalmitoylphosphatidylglycerol (DPPG) was studied as a function of pH, concentration of NaCl and the concentration of organic polyvalent cations, polyamines. Although changes in hydration forces and hydrophobic effect may be involved the measured pH/[NaCl]/Tm phase diagram for DPPG suggests that, even at pH 8.5, DPPG is incompletely deprotonated. In terms of energetics however, the observed changes of about 1 degree C are relatively minor corresponding to a bilayer free energy change of 30 cal/mol. The effects of spermine and spermidine on the phase transition of DPPG were strongly concentration dependent even in the micromolar range. The observed effects correlated to the number of positive charges in these cations and thus the greatest effect was observed for spermine. Specific and nonspecific interactions between DPPG and spermine are suggested to exist which are both reversed by high concentrations of NaCl. These results indicate that under proper conditions changes in the intracellular concentrations of polyamines may be capable of triggering an isothermal phase change in membrane acidic phospholipids.

Role of interfacial structured water in membrane: osmotic properties of L-alpha-egg lecithin liposomes

The role of large amounts of membrane-bound water in regulating various functions of the membrane is not clear at present. We have investigated the effect of perturbing the interfacial water structure on the osmotic shrinkage properties, such as water permeability and extent of shrinkage of egg lecithin liposomes. Water structure was perturbed by a series of reagents which have been earlier reported to affect phase transition of dipalmitoyl phosphatidylcholine liposomes by perturbing interfacial water structure. Anomalous variations of osmotic shrinkage properties with concentration of structure maker and breaker reagents have been interpreted to arise from concentration-dependent structural transitions of the ordered water at the membrane-aqueous interface. Various modes of interaction of these reagents on interfacial structured water have been suggested. Influence of molecular size and functional groups on the molecule in actions of some structure makers and breakers were also observed.

The binding of the radioprotective agent cysteamine with the phospholipidic membrane headgroup-interface region

The interaction of the aminothiol radioprotector cysteamine (beta-mercaptoethylamine) (CYST) with dipalmitoylphosphatidylcholine (DPPC) artificial membranes has been studied by differential scanning calorimetry (DSC), turbidimetry and spin labeling. This hydrophilic molecule displays a biphasic, concentration-dependent binding to the phospholipidic head groups at neutral pH. In the CYST/DPPC molar ratio 1:160-1:2 (mole/mole) an increasing ordering effect is observed. At high concentrations (over 3:1 ratio), this ordering effect decreases. With the symmetric disulfide dimer cystamine, the biphasic effect is not shown and the membrane rigidity decrease is obtained only at concentration ratio higher than 1:1. The charge repartition of the cysteamine molecule has been shown to be disymmetric, +0.52 e on the NH3 group and +0.19 e on the SH extremity, [38] whereas the cystamine molecule is electrostatically symmetrical. These properties could be related to their membrane effects. With cysteamine, at a low concentration, an electrostatic bridging between the negatively charged phosphate groups of the polar heads induces the increase in membrane stability: the molecules behave like a divalent cation. At high concentrations a displacement of the slightly charged SH extremity by the amine disrupts the bridges and induces the decrease in rigidity: the drug behaves like a monovalent cation. Due to its symmetric charge and its double length, such an effect is not observed with cystamine. This study could bring further information about the interactions between cysteamine and polyelectrolytic structures (ADN for example) and about the radioprotective properties of this drug.

Phosphatidylinositol transfer proteins: structure, catalytic activity, and physiological function

Among the diverse lipid transfer proteins which are found in tissues and biological fluids are those which exhibit a specificity toward phosphatidylinositol and phosphatidylcholine, with a preference for the former. Phosphatidylinositol transfer proteins (PI-TPs) have been purified from several eukaryotic sources; those present in bovine brain and heart have been extensively studied. This review examines the tissue distribution of PI-TPs and the means by which transfer activity is measured using natural and artificial membranes. The interaction of these proteins with lipid monolayers and bilayers is discussed in terms of phospholipid fatty acyl and polar head group compositions. The inhibition of transfer activity by sulfhydryl agents and amphiphilic amines is summarized. The metabolism of the phosphoinositides is considered and a role for PI-TPs is proposed.

Phase behaviour of a diglyceride prodrug: spontaneous formation of unilamellar vesicles

A prodrug (Fig. 1(IV)) is synthesized consisting of the beta-blocker bupranolol which is covalently linked to 1, 3-dipalmitoyl-2-succinyl-glycerol. The resulting lipid-like prodrug is amphipathic and surface active. It disperses readily in H2O above 30 degrees C forming a smectic lamellar phase. This prodrug bears one positive charge at neutral pH and hence the swelling behaviour of dispersions in H2O is similar to that of charged phospholipids: the dispersions show continuous swelling with increasing water content and consequently in the excess H2O region of the phase diagram the thermodynamically most stable structure is the unilamellar vesicle. This includes oligomeric vesicles which may be defined as unilamellar vesicles containing smaller, also unilamellar vesicles entrapped in their internal aqueous compartment. The prodrug dispersions in H2O are polydisperse with vesicle sizes ranging from 0.1 micron to several micron. Sonication of these dispersions produce small unilamellar vesicles of an average size and size distribution similar to sonicated egg phosphatidylcholine dispersions. Unsonicated dispersions of the prodrug in H2O undergo reversibly sharp order-disorder transitions at 32 degrees C with an enthalpy change of delta H = 10 kcal/mol. In sonicated aqueous dispersions this phase transition is asymmetric and significantly broadened indicating that the cooperativity is markedly reduced. The peak temperature and enthalpy change of this broad transition are reduced compared to the transition observed with unsonicated dispersions. The temperature dependence of the electron spin resonance (ESR) hyperfine splitting and order parameter also reflects the order-disorder transition. From ESR spin labeling it is concluded that in sonicated dispersions the prodrug molecule is more mobile and its anisotropy of motion is reduced compared to unsonicated dispersions. This result indicates that the molecular packing in the highly curved bilayers of small unilamellar prodrug vesicles is significantly perturbed compared to bilayers of unsonicated dispersions.

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.

Thermotropic properties of bipolar lipids of Sulfolobus solfataricus and of their mixtures with dipalmitoylphosphatidylcholine

The thermotropic properties of the bipolar lipids, glycerol dialkylglycerol tetraether (GDGT) and glycerol dialkylnonitol tetraether (GDNT), were determined at different degrees of hydration and in mixtures with dipalmitoylphosphatidylcholine (DPPC). The number of water molecules rendered unfreezable by the GDNT molecule is 10+/-1.5 and that by the GDGT molecule 2.8+/-0.7 or about 1.1-1.5 H2O molecules per OH group. Binding of water molecules causes randomization of the two polar heads from the oriented form prevailing in the dry state. The hydration seems to be a cooperative process extending over a whole lipid domain. DPPC added in small amounts to GDNT interacts preferentially with the nonitol halves of the molecules separating them from the glycerol half molecules. In the cooperative interaction domain each DPPC molecule is surrounded by up to six GDNT molecules. Cooperative domains formed during the interaction of DPPC with GDGT are less pronounced. In both cases they affect the thermotropic properties of the system.

A lipid-phase separation model of low-temperature damage to biological membranes

An hypothesis is proposed to explain the damage caused to biological membranes exposed to low temperatures. The thesis rests on the general observation that the lipid components of most membranes are heterogeneous and undergo phase transitions from gel-phase lamellae to liquid-crystalline lamellae and some to a non-lamellar, hexagonal-II phase over a wide range of temperatures. As a consequence of these phase transitions the lateral distribution of the lipids characteristic of the growth temperature is disturbed and redistribution takes place on the basis of the temperature at which phase transitions occur. When membranes are cooled, first the non-lamellar forming lipids pass through a transition to a fluid lamellar phase and are miscible with bilayer-forming lipids into which they diffuse. On further cooling the high-melting-point lipids begin to crystallize and separate into a lamellar gel phase, in the process excluding the low-melting point lipids and intrinsic proteins. The lipids in these remaining regions form a gel phase at the lowest temperature. It is suggested that, because the non-lamellar lipids tend to undergo a liquid-crystalline to gel-phase transition at higher temperatures than lamellar-forming lipids, these will tend to phase separate into a gel phase domain rich in these lipids. Damage results when the membrane is reheated, whereupon the hexagonal-II-forming lipids give rise to non-lamellar structures. These probably take the form of inverted micelles sandwiched within the lipid bilayer and they completely destroy the permeability barrier properties of the membrane. The model is consistent with the phase behavior of membrane lipids and the action of cryoprotective agents in modifying lipid phase properties.

General and local anaesthetics perturb the fusion of phospholipid vesicles

The effects of general and local anaesthetics on Ca2+-induced fusion of negatively charged lipid vesicles have been investigated. Vesicles composed of phosphatidylcholine and phosphatidic acid (2:1 molar ratio) were induced to fuse using 5 mM free Ca2+. Fusion, assessed by an increase in size using gel filtration techniques and confirmed by electron microscopy, displayed a dependence on Ca2+ and Mg2+ concentration and on temperature. The inhalational anaesthetics halothane, methoxyflurane and diethyl ether enhanced fusion as did the uncharged local anaesthetic benzocaine. In contrast, the charged local anaesthetics lignocaine and bupivacaine inhibited the fusion process. It is suggested that the enhancement observed with the inhalational anaesthetics and benzocaine was mediated by an effect on lipid fluidity and the inhibition observed with the charged tertiary amine anaesthetics was due to an antagonism towards Ca2+.

Influence of sterol structure on yeast plasma membrane properties

Fluorescence anisotropy measurements indicated that physical changes occurred in the lipids of plasma membranes of yeast sterol mutants but not in the plasma membrane of an ergosterol wild-type. Parallel experiments with model membrane liposomes verified that the physical changes in lipids observed in the sterol mutants are dependent on the sterol present and not the phospholipid composition. In addition, the physical changes in lipids observed in liposomes derived from wild-type phospholipids were eliminated by addition of ergosterol but persisted in the presence of cholesterol, cholestanol, ergostanol, or sterols from the sterol mutants. No physical changes in lipids were observed, however, in plasma membranes from a sterol auxotroph, even when the auxotroph was grown on cholesterol or cholestanol. The lack of physical changes in lipids in the sterol auxotroph may reflect the ability of the auxotroph to modify its phospholipid composition with respect to its sterol composition. These results indicate that high specificity 'sparking' sterol is not required for the regulation of overall bulk lipid properties of the plasma membrane.