Thermotropic Phase Transitions Research Articles

Molecular acoustic as a new tool for the study of biophysical properties of lipoproteins

The method of measurement of velocity and absorption of ultrasound at a fixed frequency (7.2 MHz) and measurement of density were used to study the physical properties of high- (HDL 3) and low- (LDL) density lipoproteins. We found substantial changes in velocity number [ u] and absorption number [αλ] on temperature, which reflect structural changes in the hydrophobic core of LDL at the thermotropic-phase transition. The absorption number revealed broad changes in temperature for both classes of lipoproteins (LP). The density of LP also depends on temperature but in considerably less degree than the acoustic parameters. The values of acoustic parameters were determined, showing that LDL and HDL 3 greatly differ with respect to adiabatic compressibility.

Anisotropic polymeric networks with a controlled nanostructure

AbstractThe synthesis and properties of anisotropically ordered polymer networks with a controlled (molecular) nanostructure are reported. Polymer networks with only a single rigid segment (based on 1, 4‐distyrylbenzene or hydroquinone) between crosslink points were prepared by a simple polyesterification procedure. Unswollen polymer networks containing only one of these rigid segments or a combination of the two were birefringent and exhibited a reversible thermotropic phase transition. The ordering within these networks is attributed to a microphase separation of rigid aromatic and flexible aliphatic segments rather than strictly to a shape‐anisotropy driven liquid crystalline phase. The nature of ordering within these networks when swollen in 1, 2‐dichlorobenzene is also discussed.

Bending elasticities of model membranes: influences of temperature and sterol content

Giant liposomes obtained by electroformation and observed by phase-contrast video microscopy show spontaneous deformations originating from Brownian motion that are characterized, in the case of quasispherical vesicles, by two parameters only, the membrane tension sigma and the bending elasticity k(c). For liposomes containing dimyristoyl phosphatidylcholine (DMPC) or a 10 mol% cholesterol/DMPC mixture, the mechanical property of the membrane, k(c), is shown to be temperature dependent on approaching the main (thermotropic) phase transition temperature T(m). In the case of DMPC/cholesterol bilayers, we also obtained evidence for a relation between the bending elasticity and the corresponding temperature/cholesterol molecular ratio phase diagram. Comparison of DMPC/cholesterol with DMPC/cholesterol sulfate bilayers at 30 degrees C containing 30% sterol ratio shows that k(c) is independent of the surface charge density of the bilayer. Finally, bending elasticities of red blood cell (RBC) total lipid extracts lead to a very low k(c) at 37 degrees C if we refer to DMPC/cholesterol bilayers. At 25 degrees C, the very low bending elasticity of a cholesterol-free RBC lipid extract seems to be related to a phase coexistence, as it can be observed by solid-state (31)P-NMR. At the same temperature, the cholesterol-containing RBC lipid extract membrane shows an increase in the bending constant comparable to the one observed for a high cholesterol ratio in DMPC membranes.

Cerebrosides alter the lyotropic and thermotropic phase transitions of DOPE:DOPC and DOPE:DOPC:sterol mixtures

Freezing injury in rye and oat is a consequence of the formation of the inverted hexagonal (H II) phase in regions where the plasma membrane is brought into close proximity with cytoplasmic membranes during freeze-induced dehydration. Susceptibility to plasma membrane destabilization and H II phase formation during freezing is associated with alterations in plasma membrane lipid composition. This paper examines the influence of lipid composition and hydration on the propensity of lipid mixtures of DOPE:DOPC and DOPE:DOPC:sterols with added cerebrosides (CER) to form the H II phase during dehydration. The addition of CER to DOPE:DOPC: β-sitosterol mixtures decreased the water content of the dispersions in a manner suggesting that most or all of the water in the dehydrated mixtures was associated with the phospholipids. The addition of CER significantly decreased the osmotic pressure at which the L α →H II phase transition occurred from an osmotic pressure of 76.1 MPa for DOPE:DOPC (50:50) to 20 MPa in DOPE:DOPC: β-sitosterol:CER (22.5:22.5:50:5) and 8 MPa in DOPE:DOPC: β-sitosterol:CER (15:15:50:20). Experiments examining the effects of CER on the thermally-induced formation of the H II phase in fully hydrated mixtures and examining the influence of CER on the formation of the H II phase in DOPE:DOPC mixtures lacking β-sitosterol suggested that CER facilitated the L α →H II phase transition by effecting a decrease in bilayer hydration and by increased lateral packing pressures within the acyl domain of the bilayer. Taken in sum, these data indicate that the differential propensity of the rye and oat plasma membranes to undergo freeze-induced formation of the L α →H II phase cannot be attributed to one lipid species. Rather, the propensity towards freeze-induced membrane destabilization is a consequence of the summation of physical characteristics of the membrane lipid components that included bilayer hydration, packing pressures within the hydrophobic domain of the membrane, the propensity of the lipid components to demix, and the relative proportions of the various lipid components.

Effect of ionic and neutral surfactants on the properties of phospholipid vesicles: investigation using fluorescent probes

The effect of triton X-100, sodium dodecyl sulphate and cetyl trimethyl ammonium bromide on the physical properties of dimytistoyl phosphatidyl choline liposome was studied in the sublytic concentration range of the detergent, using the different fluorescence probes 8-anilino-1-naphthalene sulphonate, 1,6-diphenylhexatriene and 1-naphthol. Depending on the nature and concentration of the detergent, and the position of the hydrophilic and hydrophobic parts of different detergent in liposomes, the fluidity and permeability of the bilayers were affected. The fluidity decreases at very low concentrations of detergent owing to an improvement in the packing density of phospholipid molecules. The extent of changes in these properties was found to be dependent on the phase state of liposomes, being greater in the solid gel phase than in the fluid liquid crystalline phase. Also, the thermotropic phase transitions of the phospholipid bilayers are affected moderately by neutral surfactants and greatly by charged surfactants owing to an alteration in the water of hydration of the lipid molecules.

Laurdan properties in glycosphingolipid-phospholipid mixtures: a comparative fluorescence and calorimetric study

Laurdan (6-dodecanoyl-2-dimethylamine-naphthalene) is a fluorescent membrane probe of recent characterization. It was shown that this probe discriminates between phase transitions, phase fluctuations and the coexistence of phase domains in phospholipid multilamellar aggregates. We measured the excitation and emission generalized polarization ( GP ex and GP em) of Laurdan in aggregates of complex glycosphingolipids in their pure form and in mixtures with dipalmitoylphosphatidylcholine (DPPC). Our results show that Laurdan detects the broad main phase transition temperature of the neutral ceramide-tetrasaccharide Gg 4Cer (asialo-G M1) and shows a value of GP ex in between that of DPPC and that of ganglioside G M1. In contrast, Laurdan was unable to detect the thermotropic phase transition of G M1. The probe also appears to be unable to detect phase coexistence in both types of pure glycolipid aggregates. Deconvolution of the excess heat capacity vs. temperature curves of pure Gg 4Cer and DPPC/Gg 4Cer mixtures indicates that the thermograms are composed by different transition components. For these cases, Laurdan detects only the high cooperativity component of the transition of the mixture. The peculiar behaviour of Laurdan in aggregates containing complex glycosphingolipids may result from the inherent topological features of the interface that are conferred by the bulky and highly hydrated polar head group of these lipids.

Interactions of thionin from Pyrularia pubera with dipalmitoylphosphatidylglycerol large unilamellar vesicles.

The peptide toxin thionin from Pyrularia pubera binds to dipalmitoylphosphatidylglycerol (DPPG) large unilamellar vesicles as shown by an increase in the intensity and blue-shift of the fluorescence emission spectrum of the single tryptophan residue of the protein. The magnitude of these fluorescence changes increased with temperature near the thermotropic phase transition of DPPG (about 40 degrees C). Fluorescent probes sensitive to the structure and dynamics of the membrane were used to assess the effect of thionin binding on bilayer properties. The fluorescence emission spectra of Prodan, Patman, and Laurdan all showed spectral changes consistent with an increase in bilayer polarity at temperatures below the DPPG phase transition but a decrease in polarity at higher temperatures. Fluorescence polarization experiments and the ratio of monomer-to-excimer fluorescence of the probe 1,3-bis(1-pyrene)propane suggested that thionin increases the bilayer order above the transition temperature. Differential scanning calorimetry revealed that thionin broadens the transition and either increases or decreases the melting temperature depending on the concentration of the peptide. Taken together, the data are consistent with at least three distinct interactions of thionin with the bilayer: (1) thionin bound electrostatically to the bilayer surface; (2) tryptophan of the bound thionin inserted into the bilayer; (3) high-order aggregates of thionin-bound vesicles.

Calorimetric and Spectroscopic Studies of the Thermotropic Phase Behavior of the n-Saturated 1,2-Diacylphosphatidylglycerols

The polymorphic phase behavior of a homologous series of n-saturated 1,2-diacyl phosphatidylglycerols (PGs) was studied by differential scanning calorimetry and Fourier transform infrared and 31P-nuclear magnetic resonance spectroscopy. When dispersed in aqueous media under physiologically relevant conditions, these compounds exhibit two thermotropic phase transitions that are structurally equivalent to the well-characterized pretransitons and gel/liquid-crystalline phase transitions exhibited by bilayers of the corresponding 1,2-diacyl phosphatidylcholines. Furthermore, when incubated at low temperatures, their gel phases spontaneously transform into one or more solid-like phases that appear to be highly ordered, quasicrystalline bilayers that are probably partially dehydrated. The quasicrystalline structures, which form upon short-term, low-temperature annealing of these lipids, are meta-stable with respect to more stable structures, to which they eventually transform upon prolonged low-temperature incubation. The rates of formation of the quasicrystalline phases of the PGs generally tend to decrease as hydrocarbon chain length increases, and PGs whose hydrocarbon chains contain an odd number of carbon atoms tend to be slower than those of neighboring even-numbered homologs. The calorimetric data also indicate that the quasicrystalline phases of these compounds become progressively less stable relative to both their gel and liquid-crystalline phases as the length of the hydrocarbon chain increases and that they decompose either to the liquid-crystalline phase (short- and medium-chain compounds) or to the normal gel phase (long-chain compounds) upon heating. The spectroscopic data indicate that although there is odd-even alternation in the structures of the quasicrystalline phases formed upon short-term low-temperature incubation of these compounds, the structural features of the stable quasicrystalline phases eventually formed are all similar. Furthermore, the degree of hydration and the nature of hydrogen bonding interactions in the headgroup and interfacial regions of these PG bilayers differ significantly from that observed in all other phospholipid bilayers studied so far. We suggest that many of the properties of PG bilayers can be rationalized by postulating that the glycerol moiety of the polar headgroup is directly involved in shielding the negative charges at the surface of the bilayer by means of hydration-like hydrogen bonding interactions with the phosphate moiety.

Conformational order of specific phospholipids in human erythrocytes: correlations with changes in cell shape.

Acyl chain perdeuterated dimyristoylphosphatidylcholine (DMPC-d54) and dimyristoyphosphatidylserine (DMPS-d54) were incorporated into human erythrocytes. Light microscopy demonstrated that erythrocytes incubated with an equimolar mixture of DMPC-d54/DMPS or DMPC/DMPS-d54 remained mostly discocytic whereas cells incubated with either DMPC-d54 or DMPS-d54 alone became echinocytic or stomatocytic, respectively. Cells in which the aminophospholipid translocating protein was inhibited became echinocytic when incubated with DMPS-d54. Fourier transform infrared (FTIR) spectroscopy was used to monitor conformational order in the acyl chains of the incorporated phospholipid, as detected through the asymmetric CD2 stretching vibrations in the intact cells. In cells incubated with equimolar mixtures of DMPC-d54/DMPS or DMPC/DMPS-d54, the deuterated species exhibited no thermotropic phase transitions but revealed chain order intermediate between the gel and liquid-crystal states. In contrast, DMPS-d54 incorporated into the outer leaflet of echinocytic erythrocytes was conformationally ordered while the same species incorporated into the inner leaflet of stomatocytic erythrocytes was highly disordered at all temperatures studied. Finally, DMPC-d54 incorporated into the outer leaflet of echinocytic erythrocytes exhibited a phase transition, suggesting that this species persists in domains. These data indicate that the acyl chain conformational order of specific phospholipids in the intact human erythrocyte is changed with alterations in cell morphology.

Phase Transition Temperature and Chilling Sensitivity of Bovine Oocytes

A limiting factor for achieving cryopreservation of oocytes is direct chilling injury (DCI), which occurs during cooling. DCI, or cold shock, is defined as an irreversible damage expressed shortly after exposure to low, but not freezing, temperatures. The primary target of DCI is thought to be the plasma membrane. Recently, an association between DCI in sperm and the thermotropic phase transition of their membrane lipids was demonstrated. In the present study, we examined the phase transition of the membrane lipids of immature andin vitro-matured bovine oocytes during cooling, using Fourier transform infrared spectroscopy (FTIR). The phase transition of the membrane lipids of oocytes at the germinal vesicle (GV) stage occurred between 13 and 20°C, while a very broad phase transition, which centered around 10°C, was observed for mature oocytes (MII) stage. Thermotropic phase transitions were demonstrated to be related to the temperature at which DCI affected the integrity of the oocyte membranes. When immature oocytes were cooled to 13°C, fewer oocytes (40%) retained their membrane integrity than after exposure to 4°C (51%) or holding them at 38°C (78%), (as determined by the Fluorescein Diacetate-FDA test). This finding might suggest that holding immature oocytes at the phase transition temperature is more damaging to their membranes than exposure to lower temperatures. By contrast, no significant differences in membrane integrity were observed whenin vitro-matured oocytes were cooled to the same temperatures. Subsequently, GV oocytes were cooled to 4°C, and 26% underwent maturation and 19% underwent fertilizationin vitro. In vitro-matured oocytes that were cooled to 4°C displayed a slightly decreased rate of fertilization; the overall fertilization was 60% with 24% polyspermy, rather than the 76% fertilization rate with 12% polyspermy obtained with those not subjected to cooling. The high rate of polyspermy indicates that a site(s) other than the plasma membrane is affected during cooling of bovine oocytes. Nucleated bovine GV oocytes were electrofused within vitro-matured and enucleated oocytes, and then cooled to 4°C. Evaluation of the membrane integrity of the fused oocytes showed that these oocytes are chilling resistant, which strongly suggests that alteration of the membrane composition of an oocyte can change the cell's susceptibility to low temperatures. This finding led to an improvement in the survival of oocytes after cryopreservation.

Antifreeze glycoproteins inhibit leakage from liposomes during thermotropic phase transitions.

Antifreeze glycoproteins (AFGPs), found in the blood of polar fish at concentrations as high as 35 g/liter, are known to prevent ice crystal growth and depress the freezing temperature of the blood. Previously, Rubinsky et al. [Rubinsky, B., Mattioli, M., Arav, A., Barboni, B. & Fletcher, G. L. (1992) Am. J. Physiol. 262, R542-R545] provided evidence that AFGPs block ion fluxes across membranes during cooling, an effect that they ascribed to interactions with ion channels. We investigated the effects of AFGPs on the leakage of a trapped marker from liposomes during chilling. As these liposomes are cooled through the transition temperature, they leak approximately 50% of their contents. Addition of less than 1 mg/ml of AFGP prevents up to 100% of this leakage, both during chilling and warming through the phase transition. This is a general effect that we show here applies to liposomes composed of phospholipids with transition temperatures ranging from 12 degrees C to 41 degrees C. Because these results were obtained with liposomes composed of phospholipids alone, we conclude that the stabilizing effects of AFGPs on intact cells during chilling reported by Rubinsky et al. may be due to a nonspecific effect on the lipid components of native membranes. There are other proteins that prevent leakage, but only under specialized conditions. For instance, antifreeze proteins, bovine serum albumin, and ovomucoid all either have no effect or actually induce leakage. Following precipitation with acetone, all three proteins inhibited leakage, although not to the extent seen with AFGPs. Alternatively, there are proteins such as ovotransferrin that have no effect on leakage, either before or after acetone precipitation.

Viscosity of rat adrenocortical lipids in different functional states: morphological characteristics.

Impaired adrenocortical steroidogenic activity is often concomitant with morphologically and physiologically altered lipids in the cells of the adrenal cortex. The physical state of these lipid droplets and the morphological characteristics of crystal-shaped bodies were studied in different functional states of adrenocortical cells. In the perinatal period when steroidogenesis is suppressed by a negative feedback mechanism, crystal-shaped bodies (i.e. rectangular, electron-lucent formations, either alone or in clusters, surrounded by lysosomal matrix or in close proximity of lysosomes) were frequently observed in the inner zona fasciculata and zona reticularis. In experimentally suppressed adrenocortical activity, following the administration of dexamethasone, aminoglutethimide or cycloheximide, almost identical crystal-shaped bodies were frequently observed in adrenocortical cells. These crystal-shaped bodies appear to be cholesterol, as revealed by the digitonin reaction at the electron microscopic level. Following stimulation of the zona fasciculata by ACTH treatment for 14 days, a marked increase in the fluidity of the lipid droplets was observed in the thermotropic phase transitions with the polarizing microscope. In contrast, following aminoglutethimide treatment, the fluidity of the lipid droplets decreased. The thermotropic phase transitions of normal and neoplastic human adrenal cells, namely adrenocortical tumours causing Conn's or Cushing's syndrome, were also investigated. When hormone biosynthesis was enhanced, the appearance of birefringence and multiple phase transitions of lipid droplets was demonstrable in the low-temperature range.

The pathology of cornea in Tangier disease (familial high density lipoprotein deficiency).

To clarify the underlying causes of corneal opacification in Tangier disease. Both corneas were removed at death from a 62 year old man with Tangier disease, and were examined by direct and transmission electron microscopy, histochemistry, biochemical analysis by thin-layer and gas-liquid chromatography after extraction, and by differential scanning calorimetry. Membranous inclusions in the stroma were seen on transmission electron microscopy. Direct analysis confirmed enrichment with phospholipids and cholesterol, with acyl patterns and proportions as ester broadly similar to those of normal cornea. Tangier cornea showed major thermotropic phase transitions in the range 28-37 degrees C, peak 30-33 degrees C, extending above profiles of normal clear cornea and without the complexity of those seen with cornea with heavy arcus involvement. Lipid accumulation underlies corneal opacification in Tangier disease. The excess material is mainly phospholipid and cholesterol esters. As at other sites which are below body core temperature, notably tonsil, accumulation may be enhanced by local impaired mobilisation of material as the phase transitions of the excess lipid present extend above ambient corneal temperatures.

Laurdan fluorescence spectroscopy reveals a single liquid-crystalline lipid phase and lack of thermotropic phase transitions in the plasma membrane of living human sperm

Membrane lipid phase(s), phase coexistence, and thermotropic phase transitions have been investigated in viable human spermatozoa using Laurdan fluorescence spectroscopy. Generalized polarization (GP) values derived from Laurdan excitation and emission spectra confirm that the sperm plasma membrane is a low polar, highly rigid (liquid-ordered) structure, and give evidence that, in the range from 10°C to 42°C, membrane lipids are in a single liquid-crystalline phase. The absence of phase transitions in the same thermal range argues against the hypothesis that the lipid domains previously detected on the sperm surface are produced by lipid lateral phase separation. The above findings are likely accounted for by the high cholesterol to phospholipid molar ratio found in the human sperm membrane. This is the first time that membrane lipid phase and polarity have been detected and quantified in living mammalian spermatozoa.

Coupling between Thermodynamics and Conformations in Wormlike Polymer Nematics

We adapt the Gupta-Edwards theory of wormlike main-chain nematics to the case of finite-length polymers. The thermodynamic parameters of lyotropic and thermotropic phase transitions are obtained for arbitrary polymer lengths and we find that the crossover between long- and short-chain regimes takes place for polymers of relatively low chain length (shorter than the persistence length). We show that this is related to the fact that in the nematic phase there are two qualitatively different length scales, which are analogous to the persistence length in the isotropic phase. One scale, ξ⊥, defines the contour length associated with independent fluctuations of chain segments about the nematic director and decreases with the nematic field. Another scale, ξ∥, determines the correlation length of the longitudinal component of the tangent vector. This size is larger than ξ⊥ and increases exponentially with the strength of the nematic interaction. This allows one to identify an intermediate range of chain length ξ⊥ << L << ξ∥, in which the isotropic-nematic phase transition is determined by the long-chain limit but the single-molecule conformation is practically a straight line. The implications of our results for the crystallization of short polymers, such as normal alkanes, are discussed.

Thermotropic Phase Transition of Phosphatidylinositol 4,5-Bis(phosphate) Aggregates in Aqueous Solution

Thermotropic Phase Transition of Phosphatidylinositol 4,5-Bis(phosphate) Aggregates in Aqueous Solution

Effects of Temperature and Glycerides on the Enhancement of Agkistrodon piscivorus piscivorus Phospholipase A2 Activity by Lysolecithin and Palmitic Acid

The effect of temperature and various glycerides to modulate the ability of lysolecithin and fatty acid to promote high phospholipase A2 activity was studied using dipalmitoylphosphatidylcholine large unilamellar vesicles as substrate. The length of the lag phase prior to the accumulation of sufficient hydrolysis products (lysolecithin and fatty acid) to support high phospholipase activity was shortest at temperatures near the thermotropic phase transition of the phospholipid substrate. A reduction in the lag phase correlated with a reduction in the requirement for hydrolysis products at the phase transition temperature, where the bilayer exists in a state of fluctuating domains of gel and liquid crystal. Dipalmitoylglycerol and tripalmitoylglycerol also reduced the length of the lag phase. This reduction was both concentration-dependent and temperature-dependent relative to the phase transition in the presence of the glycerides. As with the effect of temperature, the ability of di- and triglycerides to decrease the lag time correlated with a decrease in the amount of reaction products necessary to promote high phospholipase activity. This effect coincided with the tendency of the glycerides to form domains in the bilayer. Glycerides that did not form domains either had no effect (monopalmitoylglycerol) or increased the length of the lag phase (dicaprylglycerol). These data suggest that the effect of the reaction products to increase phospholipase A2 activity is aided by the presence of fluctuations in lipid domains within the bilayer.

N-alkylammonium intercalated 2-d hydrous titanates and their thermotropic phase transition

The long chain alkylammonium (C nH 2n+1NH 3, n = 8∼18) titanates have been prepared by reacting the 2-dimensional layered hydrous titanate (H 2Ti 4O 9 · 1.2H 2O) with the corresponding alkylammonium salt in an aqueous solutions and their thermotropic phase transition was also observed. The intercalation reaction could be carried out successfully by the exchange reaction between guest molecules, where the layered titanantes were intercalated in advance with a smaller butylammonium guest before reacting with a larger and longer alkylammonium one in order to reduce the steric hindrance during the intercalation reaction. The interlayer distances of the alkylammonium derivatives increase almost linearly with the number of carbon atom ( Δ d/Δ n = 1.19 A ̊ ) and the interlayer alkylammonium ions form bilayer with average tilting angle (θ) of ≈ 70°. Thermotropic phase transition of the intercalated compounds were also investigated with DSC and high temperature XRD.

Stages of the bilayer-micelle transition in the system phosphatidylcholine-C12E8 as studied by deuterium- and phosphorous-NMR, light scattering, and calorimetry

The perturbation of phospholipid bilayer membranes by a nonionic detergent, octaethyleneglycol mono-n-dodecylether (C12E8), was investigated by 2H- and 31P-NMR, static and dynamic light scattering, and differential scanning calorimetry. Preequilibrated mixtures of the saturated phospholipids 1,2-dipalmitoyl-sn-glycero-3-phosphorylcholine (DPPC), 1,2-dimyristoyl-sn-glycero-3-phosphorylcholine (DMPC), and 1,2-dilauroyl-sn-glycero-3-phosphorylcholine (DLPC) with the detergent were studied over a broad temperature range including the temperature of the main thermotropic phase transition of the pure phospholipids. Above this temperature, at a phospholipid/detergent molar ratio 2:1, the membranes were oriented in the magnetic field. Cooling of the mixtures below the thermotropic phase transition temperatures of the pure phospholipids led to micelle formation. In mixtures of DPPC and DMPC with C12E8, a narrow calorimetric signal at the onset temperature of the solubilization suggested that micelle formation was related to the disorder-order transition in the phospholipid acyl chains. The particle size changed from 150 nm to approximately 7 nm over the temperature range of the bilayer-micelle transition. The spontaneous orientation of the membranes at high temperatures enabled the direct determination of segmental order parameters from the deuterium spectra. The order parameter profiles of the phospholipid acyl chains could be attributed to slow fluctuations of the whole membrane and to detergent-induced local perturbations of the bilayer order. The packing constraints in the mixed bilayers that eventually lead to bilayer solubilization were reflected by the order parameters of the interfacial phospholipid acyl chain segments and of the phospholipid headgroup. These results are interpreted in terms of the changing average shape of the component molecules. Considering the decreasing cross sectional areas in the acyl chain region and the increasing hydration of the detergent headgroups, the bilayer-micelle transition is the result of an imbalance in the chain and headgroup repulsion. A neutral or pivotal plane can be defined on the basis of the temperature dependence of the interfacial quadrupolar splittings.

Further Studies on Physicochemical Properties of Mitochondrial Membranes during the Formation Process of Megamitochondria in the Rat Liver by Hydrazine

Physicochemical properties of lipids extracted from hepatic mitochondria of rats fed with 1% hydrazine diet for 3 days (3H-mitochondria) and 7 days (7H-mitochondria) were studied using differential scanning calorimetry and the fluorescence polarization measurements. Results obtained from lipids extracted from 3H-mitochondria were as follows: (1) Thermotropic phase transition temperatures ( T m) were elevated with a distinct increase in transition enthalpy (Δ H) compared to those of the control due to increases in relative amounts of phosphatidylethanolamine (PE) and phosphatidylserine (PS) in phospholipid species. (2) T m in lipids and phospholipids extracted from 3H-mitochondria was elevated with decreases in Δ H after the addition of Ca 2+, whereas Δ H of the control was increased after the addition of Ca 2+. (3) Phosphatidylcholine (PC) liposomes containing PE or PS at concentrations corresponding to PE/PC or PS/PC ratio in 3H-mitochondria showed elevations of transition temperatures with decreases in Δ H values after the addition of Ca 2+, whereas those containing PE or PS ratio at concentrations corresponding to PE/PC or PS/PC in the control mitochondria showed increases in Δ H values after the addition of Ca 2+. (4) There were essentially no changes in membrane fluidity between lipids extracted from 3H-mitochondria and those extracted from the control mitochondria. Addition of Ca 2+ to lipids extracted from 3H-mitochondria, however, caused elevation of membrane fluidity while those extracted from the control mitochondria showed no changes in membrane fluidity after the addition of Ca 2+. (5) Increases in the ratio of unsaturated fatty acids to saturated fatty acids ( U/S ratio) in phospholipids extracted from 3H-mitochondria were found to be mainly due to those in PS. These data may suggest that hydrazine modifies the membrane of mitochondria favorable for the membrane fusion via enhanced acidic phospholipid-Ca 2+ interactions.