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PubMed1981

A densitometric study of the effects of free fatty acids on the phase transition of dimyristoylphosphatidylcholine bilayers.

The technique of scanning densitometry was applied to a study of the effect of fatty acids on the packing and phase transition properties of dimyristolyphosphaticylcholine (DMPC). It was observed that the three fatty acids studied, i.e. palmitic, palmitoleic and behenic, had the effect of broadening the phase transition and increasing the extent of the change in volume which occurs at the phase transition. Palmitic and behenic acids broaden the phase transition toward higher temperatures while palmitoleic broadens it toward lower temperatures.

Densitometry↗
PubMed1982

Lack of transbilayer coupling in phase transitions of phosphatidylcholine vesicles.

Praseodymium and ytterbium chlorides were used as nuclear magnetic resonance shift reagents to resolve the inner and outer choline methyl resonances of single-walled dimyristoylphosphatidylcholine bilayer vesicles. The gel to liquid-crystalline phase transition of these vesicles was monitored by observing the proton and carbon-13 nuclear magnetic resonance line widths of the choline methyl group nuclei. In the absence of ions the transition occurred at 21.5 degrees C in both halves of the bilayer. With Pr3+ or Yb3+ added to the outside of the vesicles, the phase transition temperature of the outer half of the bilayer was raised several degrees, while the transition temperature of the inner half was unchanged. In vesicles containing 20 mol % cholesterol the phase transition of the outer monolayer was considerably broadened, while the inner half still melted sharply at 21.6 degrees C. By use of dipalmitoylphosphatidylcholine vesicles with UO2(2)+ added to the outside, phase transitions at 41.5 and 44 degrees C were detected by electron spin resonance with the spin-label 2,2,6,6-tetramethylpiperidinyl-l-oxy. These results imply that the two halves of the bilayer in phospholipid vesicles are so weakly coupled that they can undergo the gel to liquid-crystalline phase transition independently.

Dimyristoylphosphatidylcholine↗
PubMed1981

Interfacial preference of anesthetic action upon the phase transition of phospholipid bilayers and partition equilibrium of inhalation anesthetics between membrane and deuterium oxide.

The half-height linewidth (v 1/2) of the 1H-NMR spectra of dipalmitoylphosphatidylcholine vesicles changes abruptly at the phase transition temperature. In the absence of inhalation anesthetics, proton signals from the choline head group (hydrophilic interface) and acyl-chain tails (lipid core) change at the same temperature of 39.6 degrees C. The present study compared the effect of four inhalation anesthetics, i.e., methoxyflurane, chloroform, halothane and enflurane, upon the ligand-induced phase transition of phosphatidylcholine vesicle membranes at 37 degrees C. The anesthetics showed differential action upon the phase transition of the phospholipid vesicle membranes between the lipid core and the hydrophilic interface. The concentrations of anesthetics which induced the phase transition of the lipid core were about 2-fold greater than those required for the phase transition of the interfacial choline head groups. From the area under the proton signals of inhalation anesthetics in the NMR spectra, the maximum solubilities of methoxyflurane, chloroform and halothane in 2H2O at 37 degrees C were determined to be 0.671 . 10(-4), 2.637 . 10(-4) and 1.398 . 10(-4) (expressed as mole fractions), or 3.35, 13.17 and 6.98 mmol/1000 g 2H2O, respectively. The solubilities of the anesthetic vapor in 2H2O expressed as mole fractions according to Henry's law ere 9.586 . 10(-4), 6.432 . 10(-4) and 2.311 10(-4)/atm (1.013 . 10(5) Pa) partial pressure, respectively. The presence of phospholipid vesicles in 2H2O increased the solubility of the inhalation anesthetics. From difference between solubility in 2H2O and a dipalmitoylphosphatidylcholine vesicle suspension, the partition coefficients of methoxyflurane, chloroform and halothane between the phospholipid vesicle membranes and 2H2O were estimated. These values, calculated from the mole fractions, were 3364, 1660 and 3850, respectively at 37 degrees C.

Anesthetics↗
PubMed2005

A phenomenological theory of the isotropic to chiral smectic-C phase transition.

In this paper we discuss the direct isotropic to chiral smectic-C phase transition on the basis of a phenomenological theory. The model free energy is written in terms of the coupled order parameters including the spontaneous polarization. We present a detailed analysis of the different phases that can occur and analyze the question under which conditions a direct isotropic to chiral smectic-C phase transition is possible when compared to other phase transitions. On the basis of this model the isotropic-smectic-C* transition is always of first order. The theoretical predictions are compared with the available experimental results.

Journal Article↗
PubMed1999

Photon transmission technique for studying multiple phase transitions in a liquid crystal.

A photon transmission technique was used to monitor the multiple phase transitions in a 4-butoxyphenyl4(')-declyoxybenzoate (BOPDOB) liquid crystal. Drastic decreases in the transmitted photon intensity (I) were attributed to the sequential phase transitions in BOPDOB upon cooling. In this paper, it is assumed that the order parameter rho is proportional to the transmitted photon intensity. The isotropic-nematic and nematic-smectic-A transitions were observed and found to be of first order. It was observed that the smectic-A-smectic-C and smectic-C-smectic-G transitions are second order. It was found that for the smectic-A-smectic-C transition, critical exponent crosses over from beta=0.513+/-0.006, which is consistent with mean-field theory, to beta=0.35+/-0.009, which is consistent with heliumlike behavior, as the Ginzburg criterion predicts. The critical exponent for the smectic-C-smectic-G transition was found to be beta=0.703+/-0.001. Transition temperatures were established at each phase transitions and found to be 84.92 degrees C, 74.85 degrees C, 52.96 degrees C, and 33.03 degrees C for isotropic-nematic, nematic-smectic-A, smectic-A-smectic-C and, smectic-C-smectic-G transitions, respectively.

Journal Article↗
PubMed1974

Kinetics of the crystalline-liquid crystalline phase transition of dimyristoyl L-alpha-lecithin bilayers.

Aqueous dispersions of synthetic dimyristoyl L-alpha-lecithin undergo a sharp decrease in turbidity in the temperature range where the crystalline-liquid crystalline phase transition occurs. Equilibrium transition curves, monitored by the absorbancy change at 300 nm, reproduce all the important features of a calorimetric melting curve [Hinz & Sturtevant (1972) J. Biol. Chem. 247, 6071]. A rapid temperature-jump of the dispersion, measured by the same absorbancy change, has detected several different molecular processes depending on the magnitude of the perturbation. These processes include a phase transition, permeation of ions and water across the membrane, and repture, annealing, or fusion of the bilayer structures. When the temperature-jump is limited to 1 degree, only reactions associated with the phase transition are detectable and no signal is generated after a temperature-jump except for those which end within or extend beyond the transition zones. At least two relaxation times are resolved for the phase transition reactions. The faster one in the millisecond time range is strongly temperature-dependent and has an activation energy close to one million calories per mole. The second one, in the 100-msec time range, appears to have a much smaller activation energy. These observations indicate that strongly cooperative nucleation processes and energy-dependent fast propagation steps occur during the phase transition. Since the kinetics of the transition are complex, intermediate state or transient structures must exist in the transition regions of the bilayer dispersions. These thermal fluctuations of the bilayer structures may have important effects on the lateral diffusion and permeation of molecules and ions in the membrane structure.

Chemical Phenomena↗
PubMed2004

Phospholipid phase transitions in homogeneous nanometer scale bilayer discs.

Nanoscale protein supported phospholipid bilayer discs, or Nanodiscs, were produced for the purpose of studying the phase transition behavior of the incorporated lipids. Nanodiscs and vesicles were prepared with two phospholipids, dipalmitoyl phosphatidylcholine and dimyristoyl phosphatidylcholine, and the phase transition of each was analyzed using laurdan fluorescence and differential scanning calorimetry. Laurdan is a fluorescent probe sensitive to the increase of hydration in the lipid bilayer that accompanies the gel to liquid crystalline phase transition. The emission intensity profile can be used to derive the generalized polarization, a measure of the relative amount of each phase present. Differential scanning calorimetry was used to further quantitate the phase transition of the phospholipids. Both methods revealed broader transitions for the lipids in Nanodiscs compared to those in vesicles. Also, the transition midpoint was shifted 3-4 degrees C higher for both lipids when incorporated into Nanodiscs. These findings are explained by a loss of cooperativity in the lipids of Nanodiscs which is attributable to the small size of the Nanodiscs as well as the interaction of boundary lipids with the protein encircling the discs. The broad transition of the Nanodisc lipid bilayer better mimics the phase behavior of cellular membranes than vesicles, making Nanodiscs a 'native-like' lipid environment in which to study membrane associated proteins.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed2003

Vibration signals from the FT joint can induce phase transitions in both directions in motoneuron pools of the stick insect walking system.

The influence of vibratory signals from the femoral chordotonal organ fCO on the activities of muscles and motoneurons in the three main leg joints of the stick insect leg, i.e., the thoraco-coxal (TC) joint, the coxa-trochanteral (CT) joint, and the femur-tibia (FT) joint, was investigated when the animal was in the active behavioral state. Vibration stimuli induced a switch in motor activity (phase transition), for example, in the FT joint motor activity switched from flexor tibiae to extensor tibiae or vice versa. Similarly, fCO vibration induced phase transitions in both directions between the motoneuron pools of the TC joint and the CT joint. There was no correlation between the directions of phase transition in different joints. Vibration stimuli presented during simultaneous fCO elongation terminated the reflex reversal motor pattern in the FT joint prematurely by activating extensor and inactivating flexor tibiae motoneurons. In legs with freely moving tibia, fCO vibration promoted phase transitions in tibial movement. Furthermore, ground vibration promoted stance-swing transitions as long as the leg was not close to its anterior extreme position during stepping. Our results provide evidence that, in the active behavioral state of the stick insect, vibration signals can access the rhythm generating or bistable networks of the three main leg joints and can promote phase transitions in motor activity in both directions. The results substantiate earlier findings on the modular structure of the single-leg walking pattern generator and indicate a new mechanism of how sensory influence can contribute to the synchronization of phase transitions in adjacent leg joints independent of the walking direction.

Animals↗
PubMed2006

Structure determination and phase transition behaviour of dimethyl sulfate.

The crystal structures of phase I and phase II of dimethyl sulfate, (CH3O)2SO2, have been determined using complementary high-resolution neutron powder and single-crystal X-ray diffraction techniques. Below its melting point of 241 K dimethyl sulfate crystallizes in an orthorhombic structure (I) in the space group Fdd2. On cooling below approximately 175 K the crystal transforms to a monoclinic structure (II) in the space group I2/a. The molecule is located on a twofold axis (Z' = 1/2) in both structures. The phase transition is of first order with strong hysteresis. The phase transition results in changes to both the intra- and the intermolecular coordination environment.

Journal Article↗
PubMed2005

Effects of intramolecular dipolar coupling on the isotropic-nematic phase transition of a hard spherocylinder fluid.

The thermodynamics of a simple model, containing the minimum set of features required to provide liquid crystal-like phase behavior and the dipolar coupling observable in the NMR spectrum of orientationally ordered fluids, are presented within the framework of Onsager theory. The model comprises a fluid of hard spherocylinders with a pair of embedded freely rotating magnetic dipoles. The behavior of the isotropic-nematic phase transition is explored as a function magnetic field strength and of the relative orientation between the nematic director and the external magnetic field. When the field and director are aligned the phase diagram is similar to those predicted for a hard rod fluid in flow fields, electric fields, and magnetic fields, with the field promoting orientational order in the fluid and the isotropic-nematic phase transition being replaced by a paranematic-nematic phase transition. In contrast, when the field and director are perpendicular, the field destabilizes the nematic phase and the phase transition is shifted to higher densities. The variation of the mean magnetic moment and the dipolar coupling are examined as a function of the orientational structure of the fluid. The model is used to support the hypothesis that dipolar couplings observed in the spectra of human leg muscle originate from nematic-like liquid crystal phases in relatively small metabolite molecules. The fitted theoretical predictions of the dependence of the dipolar coupling on the orientation of the field with respect to the nematic director are shown to provide a good description of the experimental data.

Journal Article↗
PubMed2004

Magnetic resonance elastography and diffusion-weighted imaging of the sol/gel phase transition in agarose.

The dynamics of the sol/gel phase transition in agarose was analyzed with magnetic resonance elastography (MRE) and diffusion-weighted imaging, providing complementary information on a microstructural as well as on a macroscopic spatial scale. In thermal equilibrium, the diffusion coefficient of agarose is linearly correlated with temperature, independent of the sol/gel phase transition. In larger agarose samples, the transition from the sol to the gel state was characterized by a complex position and temperature dependency of both MRE shear wave patterns and apparent diffusion coefficients (ADC). The position dependency of the temperature was experimentally found to be qualitatively similar to the behavior of the ADC maps. The dynamics of the temperature could be described with a simplified model that described the heat exchange between sol and gel compartments. The experiments supported the approach to derive temperature maps from the ADC maps by a linear relationship. The spatially resolved dynamics of the temperature maps were therefore employed to determine the elasticities. For this reason, experimental MRE data were simulated using a model of coupled harmonic oscillators. The calculated images agreed well with the experimentally observed MRE wave patterns.

Elasticity↗
PubMed2003

Low-energy electron diffraction study of the phase transition of Si(001) surface below 40 K.

The phase transition of Si(001) surface below 40 K was studied by low-energy electron diffraction (LEED). The temperature dependence of the intensities and widths of the quarter order diffraction spots and LEED intensity versus electron energy curves (I-V curves) were obtained in the temperature region from 20 to 300 K. While the spot intensities of the quarter order spots decrease and the widths broaden, the I-V curves do not change so much below 40 K. This clearly shows that a phase transition occurs from an ordered phase above 40 K to a disordered phase below 40 K.

Journal Article↗
PubMed2005

Absorbing phase transition with a conserved field.

We study a lattice gas model where the number of particles is conserved during dynamical process. Our model shows a continuous phase transition from a fluctuating phase to two symmetric absorbing states at the critical point in one dimension. We conjecture the values of the critical exponents characterizing the phase transition of our model. We show that the obtained values are in good agreement with those estimated from computer simulations. The critical exponents indicate that our model exhibits an absorbing phase transition which is different from the known ones.

Journal Article↗
PubMed1999

Ultrasound interaction with large unilamellar vesicles at the phospholipid phase transition: perturbation by phospholipid side chain substitution with deuterium.

The ultrasonic absorption, alpha lambda, as a function of temperature and frequency was determined in large unilamellar vesicles (LUVs) in which specific phospholipid side chains were deuterated. Deuteration significantly altered the temperature and frequency dependence of alpha lambda. The frequency change was especially marked, with decreased frequency and broadening of the ultrasound relaxation, even with only minor changes in the phase transition temperature. Deuteration decreased the Tm and enthalpy of the lipid phase transition, as shown by differential scanning calorimetry, whereas electron spin resonance showed that at and above the lipid phase transition, no differences in the mobility as a function of temperature were observed. These results show that the observed increase in ultrasonic absorption in LUVs at the phospholipid phase transition arises from the interaction of ultrasound with the hydrophobic side chains, probably coupling with structural reorganization of small domains of molecules, a process which is maximized at the phase transition temperature.

1,2-Dipalmitoylphosphatidylcholine↗
PubMedDate not supplied

Effect of bovine serum on the phase transition temperature of cholesterol-containing liposomes.

The phase transition temperature of liposomes composed of dipalmitoylphosphatidylcholine (DPPC)/hydrogenated soy phosphatidylcholine (HSPC) at a 2:1 molar ratio was estimated in buffer, 30% and 50% bovine serum by monitoring the leakage of encapsulated self quenched doxorubicin (Dox) from the vesicles when exposed to a temperature increasing from 30-52 degrees C. The results showed that bovine serum caused a slight decrease in the phase transition temperature from 44 to 41 degrees C in 50% serum. Addition of 50% cholesterol to this liposomal composition resulted in the disappearance of transition temperature in buffer and 30% serum, whereas 50% bovine serum resulted in the reappearance of the transition temperature at 46 degrees C. The data suggest that bovine serum affects the transition temperature of liposomes in a concentration-dependent manner, and this effect is more pronounced in cholesterol-rich liposomes. The time course for the release of Dox from both kinds of liposomes (cholesterol rich and cholesterol free) during incubation in 50% bovine serum, at temperatures close to the transition temperature (42 degrees, 45 degrees C), was followed. The results showed an increased leakage of Dox from both kinds of liposomes, at both temperatures. However, liposomes with high cholesterol content released more drug at 42 degrees than at 45 degrees C. The size of these liposomes was monitored in 10% bovine serum at 25 degrees C for a period of 1 h using photon correlation spectroscopy. The data showed no variation in the size of both cholesterol-rich and cholesterol-free liposomes for this period, which indicates that bovine serum does not affect the size of either cholesterol-rich or cholesterol-free liposomes.

Animals↗
PubMed1992

A study of the phase transition behaviours of cholesterol and saturated egg lecithin, and their interaction by differential scanning calorimetry.

The thermal behaviours of hydrogenated egg lecithin (PC) and cholesterol (C) in anhydrous form and in aqueous dispersions were studied by differential scanning calorimetry. The role of C in phase transitions of C-PC mixtures prepared by physical mixing or coprecipitating from chloroform has been examined. C underwent a phase transition at 34 degrees C and a second one at 76-78 degrees C which depended on the thermal history of the samples. C lowered the gel to liquid crystalline phase transition temperature (Tt) of the aqueous dispersions prepared from C-PC coprecipitates, broadened the endothermic peak at the Tt, and at 1:1 molar ratio the C-PC system did not undergo the phase transition. The C-PC interaction was observed below the Tt. In aqueous dispersions prepared from C-PC physical mixtures, C did not influence the phase transition and acted like an inert diluent. Pellets prepared from C-PC mixtures formed myelin bodies as a result of their erosion in 0.185 M borate buffer (pH 7.4) at below Tt, but the thermal behaviours of these myelin bodies were different for pellets prepared differently. Addition of bovine serum albumin (BSA) (10%, w/w) to the physical mixtures or coprecipitates of C-PC, or to PC alone did not show any effect on the thermal behaviours of their aqueous dispersions at the Tt.

Buffers↗
PubMed1997

Compressibility, Phase Transitions, and Oxygen Migration in Zirconium Tungstate, ZrW2O8

In situ neutron diffraction experiments show that at pressures above 2 kilobars, cubic zirconium tungstate (ZrW2O8) undergoes a quenchable phase transition to an orthorhombic phase, the structure of which has been solved from powder diffraction data. This phase transition can be reversed by heating at 393 kelvin and 1 atmosphere and involves the migration of oxygen atoms in the lattice. The high-pressure phase shows negative thermal expansion from 20 to 300 kelvin. The relative thermal expansion and compressibilities of the cubic and orthorhombic forms can be explained in terms of the "cross-bracing" between polyhedra that occurs as a result of the phase transition.

Journal Article↗
PubMed1992

Kinetics of the lamellar-inverse hexagonal phase transition determined by time-resolved X-ray diffraction.

The kinetics of the lamellar (L alpha)-inverse hexagonal (HII) phase transition in diacylphosphatidylethanolamine (PE)--water systems were probed with time-resolved X-ray diffraction. Transition kinetics in the fast time regime (approximately 100 ms) were studied by initiating large temperature jumps (up to 30 degrees C) with a 50-ms electrical current pulse passed through a lipid-salt water dispersion, resulting in ohmic heating of the sample. Diffraction with a time resolution to 10 ms was acquired at the National Synchrotron Light Source. The time constant for the phase transition for 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) was on the order of 100 ms for the largest temperature jumps recorded. Faster transition behavior was found for a 1,2-dielaidoyl-sn-glycero-3-PE mixture. The HII lattice parameters for both systems were seen to swell from an initial value commensurate with the lamellar lattice to the final equilibrium value. The rate of swelling was seen to be independent of the magnitude of the temperature jump. For small temperature jumps (less than 10 degrees C), the phase transition kinetics slow dramatically, and transition studies can readily be performed on a conventional rotating anode X-ray source. At 4 degrees C, a DOPE sample was observed to slowly convert to the hexagonal phase over the course of a week, with the decay in the lamellar intensity fitting a power law behavior over four decades of time. This power law behavior is shown to have interesting consequences to the determination of the phase transition temperature of lipid-water dispersions by conventional methods such as calorimetry.

Kinetics↗