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At least 109 records · Page 6Linked to original sources
PubMed1989

Effect of fatty acyl chain length and structure on the lamellar gel to liquid-crystalline and lamellar to reversed hexagonal phase transitions of aqueous phosphatidylethanolamine dispersions.

The lamellar gel/liquid-crystalline and the lamellar liquid-crystalline/reversed hexagonal phase transitions of aqueous dispersions of a number of synthetic phosphatidylethanolamines containing linear saturated, branched chain, and alicyclic fatty acyl chains of varying length were studied by differential scanning calorimetry, 31P nuclear magnetic resonance spectroscopy, and X-ray diffraction. For any given homologous series of phosphatidylethanolamines containing a single chemical class of fatty acids, the lamellar gel/liquid-crystalline phase transition temperature increases and the lamellar liquid-crystalline/reversed hexagonal phase transition temperature decreases with increases in hydrocarbon chain length. For a series of phosphatidylethanolamines of the same hydrocarbon chain length but with different chemical structures, both the lamellar gel/liquid-crystalline and the lamellar liquid-crystalline/reversed hexagonal phase transition temperatures vary markedly and in the same direction. In particular, at comparable effective hydrocarbon chain lengths, both the lamellar gel/liquid-crystalline and the lamellar liquid-crystalline/reversed hexagonal phase transition temperatures vary in parallel, such that the temperature difference between these two phase transitions is nearly constant. Moreover, at comparable effective acyl chain lengths, the d spacings of the lamellar liquid-crystalline phases and of the inverted hexagonal phases are all similar, implying that the thickness of the phosphatidylethanolamine bilayers at the onset of the lamellar liquid-crystalline/reversed hexagonal phase transition and the diameter of the water-filled cylinders formed at the completion of this phase transition are comparable and independent of the chemical structure of the acyl chain.(ABSTRACT TRUNCATED AT 250 WORDS)

Calorimetry, Differential Scanning↗
PubMedDate not supplied

Thermotropic phase transitions at retinal rod outer segments membrane level.

Thermotropic phase transitions (i.e. phase changes induced by heat) were studied at frog ROS membrane level, with the aid of birefringence (phase difference, T) determination. In both dark and light adapted states, an early phase transition was observed (transition temperature, Tt, from +42 divided by 46 degree C). Apparently this change in the physiological state of the membrane precedes the disorientation of the pigment molecules. Following a period of birefringence stabilization a further decrease in birefringence takes place (from +52 divided by 53 degrees C onwards). During the last part of this phase Rho molecules appear to be completely disoriented, as though in solution. The physical changes underlying the transition phenomena seem related to the unbleached and bleached conditions of the ROS; with rising temperatures, the Tt point does not differ significantly in both conditions.

Animals↗
PubMed1985

The effect of head group structure on phase transition of phospholipid membranes as determined by differential scanning calorimetry.

The phase transition characteristics of cardiolipin and phosphatidylglycerol suspensions were investigated by differential scanning calorimetry. The phase transition temperatures for dimyristoylphosphatidylglycerol, tetramyristoylcardiolipin, dipalmitoylphosphatidylglycerol and tetrapalmitoylcardiolipin were 25.0, 47.0, 40.5 and 62.2 degrees C, respectively. The phase transition temperature for a mixture of two analogous phospholipids was higher than that for phosphatidylglycerol alone, but lower than that for cardiolipin alone. It increased along with cardiolipin content. The phase transition temperature for cardiolipin was increased in the presence of divalent cations, particularly Ca2+. The results indicate that the head group of cardiolipin by itself can increase the phase transition temperature.

Calorimetry, Differential Scanning↗
PubMed2006

Direct observation of the superfluid phase transition in ultracold Fermi gases.

Phase transitions are dramatic phenomena: water freezes into ice, atomic spins spontaneously align in a magnet, and liquid helium becomes superfluid. Sometimes, such a drastic change in behaviour is accompanied by a visible change in appearance. The hallmark of Bose-Einstein condensation and superfluidity in trapped, weakly interacting Bose gases is the sudden formation of a dense central core inside a thermal cloud. However, in strongly interacting gases--such as the recently observed fermionic superfluids--there is no longer a clear separation between the superfluid and the normal parts of the cloud. The detection of fermion pair condensates has required magnetic field sweeps into the weakly interacting regime, and the quantitative description of these sweeps presents a major theoretical challenge. Here we report the direct observation of the superfluid phase transition in a strongly interacting gas of 6Li fermions, through sudden changes in the shape of the clouds--in complete analogy to the case of weakly interacting Bose gases. By preparing unequal mixtures of the two spin components involved in the pairing, we greatly enhance the contrast between the superfluid core and the normal component. Furthermore, the distribution of non-interacting excess atoms serves as a direct and reliable thermometer. Even in the normal state, strong interactions significantly deform the density profile of the majority spin component. We show that it is these interactions that drive the normal-to-superfluid transition at the critical population imbalance of 70 +/- 5 per cent (ref. 12).

Journal Article↗
PubMed2004

Ferroelastic phase transitions: structure and microstructure.

Landau-type theories describe the observed behaviour of phase transitions in ferroelastic and co-elastic minerals and materials with a high degree of accuracy. In this review, the derivation of the Landau potential G=1/2AthetaS [coth(thetaS/T)-coth(thetaS/TC)]Q2 + 1/4BQ4 + ... is derived as a solution of the general phi4 model. The coupling between the order parameter and spontaneous strain of a phase transition brings the behaviour of many phase transitions to the mean-field limit, even when the atomistic mechanism of the transition is spin-like. Strain coupling is also a common mechanism for the coupling between multiple order parameters in a single system. As well as changes on the crystal structure scale, phase transitions modify the microstructure of materials, leading to anomalous mesoscopic features at domain boundaries. The mesostructure of a domain wall is studied experimentally using X-ray diffraction, and interpreted theoretically using Ginzburg-Landau theory. One important consequence of twin mesostructures is their modified transport properties relative to the bulk. Domain wall motion also provides a mechanism for superelastic behaviour in ferroelastics. At surfaces, the relaxations that occur can be described in terms of order parameters and Landau theory. This leads to an exponential profile of surface relaxations. This in turn leads to an exponential interaction energy between surfaces, which can, if large enough, destabilize symmetrical morphologies in favour of a platelet morphology. Surface relaxations may also affect the behaviour of twin walls as they intersect surfaces, since the surface relaxation may lead to an incompatibility of the two domains at the surface, generating large strains at the relaxation. Landau theory may also be extended to describe the kinetics of phase transitions. Time-dependent Landau theory may be used to describe the kinetics of order-disorder phase transitions in which the order parameter is homogeneous. However, the time-dependent Landau theory equations also have microstructural solutions, explaining the formation of microstructures such as tweed.

Journal Article↗
PubMed2000

A first-order liquid-liquid phase transition in phosphorus

First-order structural phase transitions are common in crystalline solids, whereas first-order liquid-liquid phase transitions (that is, transitions between two distinct liquid forms with different density and entropy) are exceedingly rare in pure substances. But recent theoretical and experimental studies have shown evidence for such a transition in several materials, including supercooled water and liquid carbon. Here we report an in situ X-ray diffraction observation of a liquid-liquid transition in phosphorus, involving an abrupt, pressure-induced structural change between two distinct liquid forms. In addition to a known form of liquid phosphorus--a molecular liquid comprising tetrahedral P4 molecules--we have found a polymeric form at pressures above 1 GPa. Changing the pressure results in a reversible transformation from the low-pressure molecular form into the high-pressure polymeric form. The transformation is sharp and rapid, occurring within a few minutes over a pressure range of less than 0.02 GPa. During the transformation, the two forms of liquid coexist. These features are strongly suggestive of a first-order liquid-liquid phase transition.

Journal Article↗
PubMed1990

Antioxidant activity of probucol and its effects on phase transitions in phosphatidylcholine liposomes.

The effect of probucol on the phase behavior of dimyristoylphosphatidylcholine (DMPC) was examined by fluorescence polarization and differential scanning calorimetry (DSC). Probucol broadens and shifts the temperature of the main phase transition of DMPC liposomes as measured by fluorescence polarization with diphenylhexatriene and trimethyl-ammonium-diphenylhexatrine at concentrations as low as 5 mole%. As measured by DSC, probucol reduces the transition temperature of the gel----liquid-crystalline phase transition of DMPC by approx. 2 C degrees at all concentrations above about 5 mole% probucol and eliminates the pretransition at less than 1 mole%. In addition, the phase transition of DMPC is broadened and the enthalpy of the transition reduced by approx. 50%. Even at high concentrations of probucol, the gel----liquid-crystalline phase transition of DMPC is not eliminated. Similar effects are observed with dipalmitoylphosphatidylcholine liposomes. Based on these DSC measurements, measurements of the melting of probucol in dry mixtures with DMPC and observations of probucol mixtures with DMPC under polarizing optics, the maximum solubility of probucol in DMPC is approx. 10 mole%. This concentration exceeds that required (approx. 0.5 mole%) to prevent peroxidation of 10 mole% arachidonic acid in DMPC liposomes for 30 min in the presence of 0.05 mM Fe(NH4)(SO4)2 at 4 degrees C. Thus, probucol has a limited solubility in saturated phosphatidylcholine bilayers, but is an effective antioxidant at concentrations lower than its maximum solubility.

Antioxidants↗
PubMed2003

Phase transition classes in triplet and quadruplet reaction-diffusion models.

Phase transitions of reaction-diffusion systems with site occupation restriction and with particle creation that requires n=3,4 parents, whereas explicit diffusion of single particles (A) is present are investigated in low dimensions by the mean-field approximation and simulations. The mean-field approximation of general nA-->(n+k)A, mA-->(m-l)A type of lattice models is solved and a different kind of critical behavior is pointed out. In d=2 dimensions, the 3A-->4A, 3A-->2A model exhibits a continuous mean-field type of phase transition, that implies d(c)<2 upper critical dimension. For this model in d=1 extensive simulations support a mean-field type of phase transition with logarithmic corrections unlike the recent study of Park et al. [Phys. Rev E 66, 025101 (2002)]. On the other hand, the 4A-->5A, 4A-->3A quadruplet model exhibits a mean-field type of phase transition with logarithmic corrections in d=2, while quadruplet models in one-dimensional show robust, nontrivial transitions suggesting d(c)=2. Furthermore, I show that a parity conserving model 3A-->5A, 2A--> zero in d=1 has a continuous phase transition with different kinds of exponents. These results are in contradiction with the recently suggested implications of a phenomenological, multiplicative noise Langevin equation approach and with the simulations on suppressed bosonic systems by Kockelkoren and Chaté [Phys. Rev. Lett. 90, 125701 (2003)].

Journal Article↗
PubMed2002

Critical fluctuations near the smectic-hexatic phase transition with anticlinic structure.

Layer compression modulus B measurements have been conducted near the transitions between smectic and hexatic phases with synclinic and anticlinic structures. In the synclinic structure, B shows no pretransitional softening near the phase transition. However, in the anticlinic structure, we observed evident critical softening of B near the smectic-hexatic phase transition. These results clearly reveal that the introduction of the in-plane hexatic order in the anticlinic structure is different from the usual smectic-hexatic phase transition.

Journal Article↗
PubMed1994

[Fluorometric analysis of liquid-crystalline to inverted hexagonal phase transition induced by diacylglycerol in liposomes].

In order to delineate the physiological role of diacylglycerol (DAG) to induce phase transition from liquid-crystalline (L alpha) to inverted hexagonal (HII) phase, the minimal concentration of dioleoylglycerol (DOG) required for the initiation of the phase transition was determined by the fluorescence spectroscopy using multi-lamellar vesicles composed of brain phosphatidylcholine (PC), brain phosphatidylethanolamine (PE) and cholesterol (CHOL) = 1:1:1 (mol/mol). DOG reduced the initial temperature of the phase transition at the constant rate of 5.6 degrees C per mol% of DOG and required 1.6 mol% of total lipids for the initial induction of the HII phase in pH 7.4 at 37 degrees C. This suggested that physiological concentration of DAG may induce L alpha to HII phase transition in biomembranes. The molecular motion of lipids in the phase transition was analysed by means of time-resolved fluorescence depolarization spectroscopy using fluorescence-labelled lipids in the same liposomes. Wobbling angle of fluorescence probes increased at the acyl-chain of phospholipid and decreased at the head group in the DOG-induced L alpha to HII phase transition. This corresponds the negative change of mean curvature of the membrane in the phase transition. Wobbling angle of fluorescence probes, 1,6-phenyl-trans-1,3,5-hexatriene (DPH), at the acyl-chain was larger in DPH-glycerol (G) than DPH-phosphatidic acid (PA) and DPH-PC, which suggested that the mobility of the acyl-chain of DAG may be greater than that of PC and PA.

Crystallization↗
PubMed1978

Membrane properties of an extreme thermophile. I. Detection of the phase transition and its dependence on growth temperature.

Phase transition was detected by a fluorescence polarization technique in the membrane of Thermus thermophilus HB8: it was found to be a function of cell growth temperature when the cell growth temperature was varied between 50-80 degrees C. A systematic relation between the phase transition temperature and the growth temperature was observed. Differential scanning calorimetry was also applied. The phase transition was found to take place between 34 and 55 degrees C for cells grown at 50 degrees C and between 52 and 80 degrees C for cells grown at 80 degrees C.

Calorimetry, Differential Scanning↗
PubMed2005

Theory of the dynamics of first-order phase transitions: unstable fixed points, exponents, and dynamical scaling.

Phase transitions are of great importance in a diversity of fields. They are usually classified into continuous phase transitions and first-order phase transitions (FOPTs). Whereas the former has a well-developed theoretical framework of the renormalization-group (RG) theory, no general theory has yet been developed for the latter that appear far more frequently. Focusing on the dynamics of a generic FOPT in the phi4 model below its critical point, we show by a field-theoretic RG method that it is governed by an unexpected unstable fixed point of the corresponding phi3 model. Accordingly, it exhibits a distinct scaling and universality behavior with unstable exponents different from the critical ones.

Journal Article↗
PubMed1984

Modulation of membrane fusion by ionotropic and thermotropic phase transitions.

We have studied the relationship of ionotropic and thermotropic phase transitions to divalent cation induced fusion of large unilamellar phospholipid vesicles. Fusion was monitored by the Tb/dipicolinic acid fluorescence assay for the intermixing of internal aqueous contents of vesicles. The phase behavior of the membranes was followed by differential scanning calorimetry. (1) Sr2+ and Ba2+ shifted the phase transition temperature (Tc) of bovine brain phosphatidylserine vesicles from 6 to 27 and 31.5 degrees C, respectively. These cations induced vesicle fusion at temperatures above or below the Tc of that cation/phospholipid complex, indicating that an isothermal phase change from the liquid-crystalline to the gel phase is not a requirement for membrane fusion. (2) The temperature dependence of the initial rate of fusion of phosphatidylserine/dipalmitoylphosphatidylcholine (1:1) vesicles in the presence of Ca2+ exhibited a pronounced maximum at 17 degrees C, at the lower part of the broad phase transition endotherm whose Tc was about 25 degrees C; fusion was inhibited completely at 30 degrees C when the membrane was in the liquid-crystalline state. These observations suggest that molecular clusters rich in phosphatidylserine, formed when the membrane is in the phase transition region, allow the vesicles to fuse. (3) The fusion of phosphatidylserine/dimyristoylphosphatidylethanolamine (1:1) vesicles, whose Tc was also around 25 degrees C, had a different temperature dependence in that the initial rate increased sharply above the Tc, with a local maximum within the transition region. Phase separation of dimyristoylphosphatidylethanolamine was induced by Ca2+ but not by Mg2+, although both ions induced fusion.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed2005

New findings in phase transitions of spinel Li1.07Mn1.93O4-delta studied by transmission electron microscopy.

Phase transitions and the related phase identifications at different temperatures for spinel Li(1.07)Mn(1.93)O(4-delta) crystals have been studied by transmission electron microscopy. The preliminary results clearly show the existence of at least two types of low-temperature phase: monoclinic and orthorhombic. Oxygen deficiency may raise the phase-transition temperature for the high-temperature (HT) cubic to low-temperature (LT) tetragonal, monoclinic, or orthorhombic phases. When the oxygen deficiency delta is very close to zero, the transition temperature is below room temperature (RT). Therefore, only the HT cubic spinel is observed at RT. When delta=0.182, the transition temperature is higher than RT, so the structures of the LT phases can be studied at RT. This study reveals the structural relationships between the LT phase and the HT phase. These relationships can be summarized as follows: (1) Two orthogonal cubic [440](Cubic)(*) and [440](Cubic)(*) reciprocal vectors are divided into two parts by {220} and {220} super reflections, respectively. This situation resembles the structures caused by correlated tilting and/or distortion of the octahedra in perovskites. (2) One of the cubic <440>Cubic(*) reciprocal vectors is divided into three parts by weak super-reflections. This situation resembles the modulated structure caused by the charge- and orbital-ordering in the perovskite La(1/3)Ca(2/3)MnO(3) of the space group Pnma. (3) One of the cubic <440>(Cubic)(*) reciprocal vectors, e.g. [440](Cubic)(*), is divided into three parts by stronger super-reflections, at the same time some strong reflections of the cubic spinel, e.g. (111)(Cubic)=(320)(Ortho)/2 disappear, indicating that there is a phase transition containing atomic movement. Items (2) and (3) are new findings in the present work.

Journal Article↗
PubMed1977

Role of lipids in the Neurospora crassa membrane. I. Influence of fatty acid composition on membrane lipid phase transitions.

The relationship between lipid composition and phase transition was investigated by differential scanning calorimetry for intact and membrane phospholipid extracts of wild-type (w/t) and the cel-(Tw 40) mutant of Neurospora crassa. The cel-(Tw 40) mutant (grown on minimal, sucrose medium supplemented with Tween 40 at approximately 34 degrees C) had approximately twice the saturated fatty acid content of w/t organisms grown at approximately 22 degrees C. The gel-liquid crystal phase transitions of ergosterol-free extracts derived from w/t and cel-(Tw 40) occur at -31 and -11 degrees C, respectively. The heats of transition (delta H) of these extracts were 1 and 13 cal/g, respectively. The addition of ergosterol (the predominant sterol in Neurospora) to the phospholipid extracts decreased the observed heats of transition, but did not alter the transition temperature. Intact Neurospora, whether w/t or cal-(Tw 40) did not manifest similar gel-liquid crystal phase transitions in the differential scanning calorimeter. However, an endothermic peak at approximately 30 degrees C was observed in intact cells and extracted phospholipids of both w/t and cel-(Tw 40) organisms. This peak was insensitive to the addition of ergosterol, had a low heat content (delta H congruent to 1 cal/g), and was reversible.

Calorimetry, Differential Scanning↗
PubMed2007

Quantitative CARS spectroscopy using the maximum entropy method: the main lipid phase transition.

The maximum entropy method for phase retrieval of multiplex coherent anti-Stokes Raman scattering (CARS) spectra is described in detail and applied to the time-resolved measurement of the main lipid phase transition of small, unilamellar 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) vesicles subject to a 3 min temperature sweep. Since the--thus derived--imaginary part of the third-order CARS susceptibility can be directly related to the linear vibrational spectrum, the multiplex CARS spectral data can be analyzed quantitatively and without prior knowledge of the sample. It is shown that the maximum entropy model provides an exact description of the original data, including the noise, if all available autocorrelation functions are used. Our findings confirm the acyl-chain order-disorder phase-transition behavior of small, unilamellar lipid vesicles.

Computer Simulation↗
PubMed2004

Structural phase transitions from and to the quasicrystalline state.

Phase transitions from and to the quasicrystalline state show a typical signature due to some structural peculiarities. Both quasicrystals and most of their translationally periodic transformation products, the approximants, consist of the same basic structural units ('clusters'). This is the cause of low-energy interfaces between the newly forming phase and the parent phase. It is also the origin of the rather high stability of the frequently resulting orientationally twinned nanodomain structures. Owing to topological incompatibilities between quasiperiodic and periodic structures, purely displacive phase transitions are impossible. Diffusion of a significant fraction of atoms, at least on the scale of the cluster diameters, always has to take place. Locally similar icosahedral structural ordering between parent phase and nucleating phase is also responsible for the frequently occurring formation of icosahedral quasicrystals from undercooled liquid alloys or during devitrification of metallic glasses. The different types of experimentally observed phase transitions are discussed, from amorphous to quasicrystalline, quasicrystalline to ordered/disordered quasicrystalline and quasicrystalline to crystalline as a function of temperature, pressure, irradiation and high-energy ball milling, respectively.

Journal Article↗
PubMed2005

Pressure-induced shape change of phospholipid vesicles: implication of compression and phase transition.

A microscopic study has allowed the analysis of modifications of various shapes acquired by phospholipid vesicles during a hydrostatic pressure treatment of up to 300 MPa. Giant vesicles of dimyristoylphosphatidylcholine / phosphatidylserine (DMPC/PS) prepared at 40 degrees C mainly presented a shape change resembling budding during pressure release. This comportment was reinforced by the incorporation of 1,2-dioleyl-sn-glycero-3-phosphatidylethanolamine (DOPE) or by higher temperature (60 degrees C) processing. The thermotropic main phase transition (L alpha to P beta') of the different vesicles prepared was determined under pressure through a spectrofluorimetric study of 6-dodecanoyl-2-dimethylamino-naphtalene (Laurdan) incorporated into the vesicles' bilayer. This analysis was performed by microfluorescence observation of single vesicles. The phase transition was found to begin at about 80 MPa and 120 MPa for DMPC/PS vesicles at, respectively, 40 degrees C and 60 degrees C. At 60 degrees C the liquid-to-gel transition phase was not complete within 250 MPa. Addition of DMPE at 40 degrees C does not significantly shift the onset boundary of the phase transition but extends the transition region. At 40 degrees C, the gel phase was obtained at, respectively, 110 MPa and 160 MPa for DMPC/PS and DMPC/PS/DOPE vesicles. In comparing volume data obtained from image analysis and Laurdan signal, we assume the shape change is a consequence of the difference between lateral compressibility of the membrane and bulk water. The phase transition contributes to the membrane compression but seems not necessary to induce shape change of vesicles. The high compressibility of the L alpha phase at 60 degrees C allows induction on DMPC/PS vesicles of a morphological transition without phase change.

Compressive Strength↗