Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Phase Transition”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 469 records · Page 26Linked to original sources
PubMed1987

Effects of divalent cations on the ultrasonic absorption coefficient of negatively charged liposomes (LUV) near their phase transition temperature.

The ultrasonic absorption coefficient per wavelength (alpha lambda), as a function of temperature and frequency, was determined for large unilamellar vesicles (LUV) in the vicinity of their phospholipid phase transition temperature, using a double crystal acoustic interferometer. (The vesicles were composed of a 4:1 (w/w) mixture of dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylglycerol (DPPG). It has been found that alpha lambda reaches a maximum (alpha lambda)max at the phase transition temperature (tm) of the phospholipids in the bilayer, at an ultrasonic relaxation frequency of 2.1 MHz. Divalent cations (Ca2+ and Mg2+), added to LUV suspensions, shifted (alpha lambda)max to higher temperatures, dependent upon the concentration of divalent cation. It was also found that the shape of the alpha lambda versus t curve was significantly changed, representing changes in the Van't Hoff enthalpy of the transition, and therefore, the cooperative unit of the transition. This suggests that divalent cations interact individually with the negatively charged phospholipid headgroups of DPPG and with DPPC headgroups, thus decreasing the cooperative unit of the transition. The observed upward shift in tm suggests an interaction that increases the activation energy and, therefore, the temperature of the phase transition. However, alpha lambda as a function of frequency did not change with the addition of divalent cations and, thus, the relaxation time of the event responsible for the absorption of ultrasound is not changed by the addition of divalent cations.

Calcium↗
PubMed1976

Phospholipid phase transitions. Effects of n-alcohols, n-monocarboxylic acids, phenylalkyl alcohols and quaternary ammonium compounds.

The interactions of a series of alcohols, acids and quaternary ammonium salts with a phosphatidylcholine-water model biomembrane (dipalmitoyl phosphatidycholine) system have been studied using differential scanning calorimetry. In particular the effects of these molecules upon the lipid endothermic phase transitions were investigated over a range of concentrations. A variety of effects was observed. (a) Those molecules which shift or broaden the main lipid transition can also remove the pretransition endotherm. (b) n-Alcohols and n-monocarboxylic acids containing the same number of carbon atoms have very similar effects at molar concentrations up to 40%. Those molecules containing 12 or more carbon atoms raise the main lipid phase transition whilst those molecules containing 10 or less carbon atoms lower this transition temperature. (c) The phase diagram of stearoyl alcohol in the phosphatidylcholine-water system shows the formation of lipid-alcohol complexes. (d) Alkyl trimethyl ammonium bromides showed behaviour which differs considerably from n-alcohols and n-carboxylic acids of the same chain length. (e) Other alkyltrialkyl and tetraalkylammonium bromides show that a variety of effects on the lipid phase transition can be obtained. (f) With the homologous series of phenyl-alkyl alcohols from benzyl alcohol to 4-phenyl butanol increasing the number of methylenes between the terminal OH and the benzene ring leads to greater interaction between solute and bilayer. The range of different effects obtained with the compounds studied offers a means for introducing various degrees and types of perturbation into membrane systems.

Alcohols↗
PubMed1992

Effect of delta-lysin on the phase transitions of lipid assemblies.

X-ray small-angle diffraction, differential scanning calorimetry (DSC), and temperature scanning densitometry (TSD) were used to study the effect of delta-lysin on the phase transitions of lipid assemblies from 1,2-0-dixehadecyl-sn-glycero-3-phosphocholine (DHPC). The experiments were carried out in excess of water in a temperature range of 0-55 degrees C, and at low peptide concentrations between 10(-4) and 10(-2) moles peptide per mole phospholipid. The incorporation of delta-lysin into lipid assemblies alters the lipid structure without significant changes on the temperatures of phase transition from gel to liquid crystalline phase. The temperature of the main transition was nearly unaffected. A reduction in the transition volume of the lipids with increasing concentrations of delta-lysin was observed. The minor changes in these parameters were interpreted as long-range structural changes caused by the peptide incorporation. The results are discussed in terms of the concept of cooperative phase transition of entire clusters occurring within a membrane implying that relative stable domains of gel phase, and liquid crystalline phase co-exist.

Bacterial Proteins↗
PubMed2004

Phase-transition phenomena of 0.8 microm superfluid 3He slab.

We have investigated the transition phenomena of superfluid 3He in thin 0.8 microm slabs with a cw-NMR method. We found that, just below the phase-transition temperature, only the A phase appeared at any pressure. At lower temperatures, the phase transition to the B phase occurred between 0.3 and 2.74 MPa. We obtained a universal critical thickness delta as a function of pressure. When the reduced slab thickness, d/xi(T), is smaller than delta, only the A phase becomes stable.

Journal Article↗
PubMed1976

The kinetics of biological phase transitions manifested by sigmoid time curves: a review of approaches.

Experimental biological data showing a time course of sigmoid shape are customarily interpreted in terms of mass-action chemical kinetic theory of reactions in free solution. But the reactive sites in biological systems are often located in solid or structured cellular components through which cooperative interactions may occur, resulting in phase transitions. In inorganic systems, the time course of phase transitions often shows experimental curves of sigmoid shape, and this may be predicted theoretically. Therefore it is suggested that when sigmoid time curves are observed in biological systems containing solid or structured components, mathematical analysis based on cooperative interactions and phase transitions is more likely to be valid than analysis based on mass-action concepts.

Animals↗
PubMed1997

The mechanism of lamellar-to-inverted hexagonal phase transitions in phosphatidylethanolamine: implications for membrane fusion mechanisms.

We studied the mechanism of the lamellar-to-inverted hexagonal (L alpha/H[II]) phase transition, using time-resolved cryotransmission electron microscopy (TRC-TEM), 31P-NMR, and differential scanning calorimetry. The transition was initiated in dispersions of large unilamellar vesicles of dipalmitoleoyl phosphatidylethanolamine (DiPoPE). We present evidence that the transition proceeds in three steps. First, many small connections form between apposed membranes. Second, the connections aggregate within the planes of the bilayers, forming arrays with hexagonal order in some projections. Third, these quasihexagonal structures elongate into small domains of H(II) phase, acquiring lipid molecules by diffusion from contiguous bilayers. A previously proposed membrane fusion mechanism rationalizes these results. The modified stalk theory predicts that the L alpha/H(II) phase transition involves some of the same intermediate structures as membrane fusion. The small interbilayer connections observed via TRC-TEM are compatible with the structure of a critical intermediate in the modified stalk mechanism: the trans monolayer contact (TMC). The theory predicts that 1) TMCs should form starting at tens of degrees below TH; 2) when TMCs become sufficiently numerous, they should aggregate into transient arrays like the quasihexagonal arrays observed here by TRC-TEM; and 3) these quasihexagonal arrays can then elongate directly into H(II) phase domains. These predictions rationalize the principal features of our data, which are incompatible with the other transition mechanisms proposed to date. Thus these results support the modified stalk mechanism for both membrane fusion and the L alpha/H(II) phase transition. We also discuss some implications of the modified stalk theory for fusion in protein-containing systems. Specifically, we point out that recent data on the effects of hydrophobic peptides and viral fusion peptides on lipid phase behavior are consistent with an effect of the peptides on TMC stability.

Biophysical Phenomena↗
PubMed1985

Transition phase during hyperventilation therapy for persistent pulmonary hypertension of the neonate.

It is important for the clinician who is hyperventilating infants with persistent pulmonary hypertension (PPHN) to recognize a transition phase during therapy when pulmonary hypertension is no longer the primary cause of hypoxemia, because infants who are hyperventilated develop parenchymal lung disease after 2 to 3 days. This study reports ten infants who showed PaO2 lability early in the course of PPHN, with an inverse relationship between PaO2 and PaCO2. At a mean age of 79 +/- 14 (SEM) there was a transition phase, after which PaO2 lability decreased and the infants did not require hyperventilation. The mean change in PaO2 per change in PaCO2 was significantly (p less than .05) higher pretransition (22.4 +/- 5.2) compared to during transition (5.1 +/- 1.4) or post-transition (1.9 +/- 1.2). Mean alveolar-arterial oxygen gradient was higher (p less than .05) pretransition (495 +/- 36) vs. post-transition (405 +/- 52) and was more labile relative to PaCO2 change pretransition (20.3 +/- 5.9) compared to post-transition (.3 +/- 2.4). When ventilator settings were reduced after the transition phase, PaCO2 rose by 12.2 torr.

Carbon Dioxide↗
PubMed2006

Effects of disorder on a smectic--nematic phase transition.

Using x-ray scattering, we have studied the nematic to smectic- phase transition of the liquid crystal butyloxybenzilidene-octylaniline confined in an aerosil network. We find that the disorder introduced by the aerosil network destroys the long-range nature of the phase transition, and that the transition becomes similar to that observed in a finite-size system, with finite low-temperature correlation lengths of the ordered moments and a power-law behavior of the order parameter with respect to the reduced temperature observable in a limited temperature range. We also show evidence for a systematic evolution of the effective order parameter critical exponent beta with increasing disorder.

Journal Article↗
PubMed1991

A dynamical model for phase transitions of lipids induced by calcium ions.

Lipid membranes exist essentially in two different phases. A phase transition can be triggered off either by changing the temperature or, isothermally, by varying an external factor such as ionic concentration, pH, organic solvents, etc. Since the isothermal transition may be induced at physiological temperature, it may play an important regulatory role in diverse cellular functions. Based on the Landau-Ginsburg theory, the thermotropic transitions of lipids has been described by a number of models. In the present work a dynamical model for an isothermal phase transition of phospholipids induced by ionic binding is proposed. The properties of the model show that by ionic binding, phospholipids may form spatial heterogeneous distributions of lipids in fluid and crystalline phases. This heterogeneity possibly being the cause of membrane instabilities, which favour enhanced vesicle fusions observed in the presence of Ca2+.

Calcium↗
PubMed2004

Role of the lattice in the gamma-->alpha phase transition of Ce: a high-pressure neutron and x-ray diffraction study.

The temperature and pressure dependence of the thermal displacements and lattice parameters were obtained across the gamma-->alpha phase transition of Ce using high-pressure, high-resolution neutron and synchrotron x-ray powder diffraction. The estimated vibrational entropy change per atom in the gamma-->alpha phase transition, DeltaS(gamma-alpha)(vib) approximately (0.75+/-0.15)k(B), is about half of the total entropy change. The bulk modulus follows a power-law pressure dependence that is well described using the framework of electron-phonon coupling. These results clearly demonstrate the importance of lattice vibrations, in addition to the spin and charge degrees of freedom, for a complete description of the gamma-->alpha phase transition in elemental Ce.

Journal Article↗
PubMed1996

The effect of increasing membrane curvature on the phase transition and mixing behavior of a dimyristoyl-sn-glycero-3-phosphatidylcholine/ distearoyl-sn-glycero-3-phosphatidylcholine lipid mixture as studied by Fourier transform infrared spectroscopy and differential scanning calorimetry.

The phase transition behavior of a lipid bilayer of dimyristoyl-sn-glycero-3-phosphatidylcholine/distearoyl-sn-glycero-3- phosphatidylcholine (DMPC-d54/DSPC) (1:1) on a solid support with varying curvatures was investigated with differential scanning calorimetry, infrared spectroscopy, and model calculations. With increasing curvature the temperatures of the liquidus and solidus points are shifted to lower values by up to 7 degrees C and 15 degrees C, and the mixing of the two lipid species in the two phase region is altered. With increasing curvature the DSPC dominates the gel phase, whereas the DMPC-d54 is expelled to the fluid phase. Whereas the planar system shows a nearly simultaneous phase transition of DSPC and DMPC-d54, the spherical system with the highest curvature exhibits an almost complete separation of the phase transitions of the two lipids. Model calculations suggest that the shift of the liquidus point can be understood as a reduction of the lateral pressure in the bilayer with increasing curvature. The shift of the solidus line is interpreted as a result of the increased demixing of the two components in the two-phase region with increasing curvature due to lowering of the lateral pressure.

Biophysical Phenomena↗
PubMed1988

Lamellar gel-lamellar liquid crystal phase transition of dipalmitoylphosphatidylcholine multilayers freeze-dried from aqueous trehalose solutions. A real-time X-ray diffraction study.

The mechanism of the phase transition of dipalmitoylphosphatidylcholine multilayers freeze-dried from fully hydrated gel phase (L beta') in the presence of trehalose has been investigated by real-time X-ray diffraction methods. Sequential diffraction patterns were recorded with an accumulation time of 3 s during heating and 1.2 s during cooling between about 20 and 80 degrees C. A transition is observed in the range 47-53 degrees C that involves structural events typical of a lamellar gel-lamellar liquid-crystal (L beta--L alpha) transformation. This transition is completely reversible with a temperature hysteresis of 2-3 degrees C and thereby resembles the main phase transition of fully hydrated dipalmitoylphosphatidylcholine multilayers. The mechanism of the transition from L beta to L alpha as seen in the wide-angle scattering profiles show that the sharp peak at about 0.41 nm, characteristic of the gel phase, broadens and shifts progressively to about 0.44 nm towards the end of the transition. A temperature jump of 6C degrees/s through the phase transition region of a freeze-dried dipalmitoylphosphatidylcholine: trehalose mixture (molar ratio 1:1) showed that the phase transition had a relaxation time of about 2 s which is similar to that of the main transition in the fully hydrated lipid. X-ray diffraction studies of the melting of dipalmitoylphosphatidylcholine freeze-dried from the lamellar-gel phase in the absence of trehalose showed a transition at above 70 degrees C. The low-angle diffraction data of phospholipid/trehalose mixtures are consistent with an arrangement of trehalose molecules in a loosely packed 'monolayer' separating bilayers of phospholipid. Trehalose appears to reduce the direct interbilayer hydrogen bond coupling thereby modifying the thermal stability and the phase transition mechanism of the bilayers.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1994

Manipulation of inhibin during the luteal-follicular phase transition of the primate menstrual cycle fails to affect FSH secretion.

The pattern of inhibin concentrations in blood during the menstrual cycle in primates has suggested an endocrine role of inhibin in the negative feedback control of FSH secretion during the luteal phase. Conversely, the fall in inhibin during the late luteal phase may play a role in the rise in serum FSH during the luteal-follicular phase transition. This hypothesis was examined by determining the effects of manipulation of inhibin on FSH secretion in stumptailed macaques. During the mid-luteal phase the putative inhibin feedback was inhibited by i.v. administration of 20 ml of ovine antiserum to human recombinant inhibin in 4 macaques. FSH secretion was unaffected during the initial 24 h period post-treatment and the timing of the rise in FSH which occurred during the subsequent luteal-follicular phase transition was normal. To determine whether the elevated serum concentrations of FSH observed during the early follicular phase could be reduced by administration of inhibin, 5 cyclic macaques were treated with 200 micrograms of recombinant human inhibin i.v. Serum FSH concentrations were unaltered. These results suggest that inhibin does not play a major role in modulating FSH secretion during the luteal-follicular phase transition.

Animals↗
PubMed2006

Condensation phase transitions of symmetric conserved-mass aggregation model on complex networks.

We investigate condensation phase transitions of the symmetric conserved-mass aggregation (SCA) model on random networks (RNs) and scale-free networks (SFNs) with degree distribution P(k) approximately k(-gamma). In the SCA model, masses diffuse with unit rate, and unit mass chips off from mass with rate omega. The dynamics conserves total mass density rho. In the steady state, on RNs and SFNs with gamma > 3 for omega is not equal to infinity, we numerically show that the SCA model undergoes the same type of condensation transitions as those on regular lattices. However, the critical line rho(c)(omega) depends on network structures. On SFNs with gamma < or = 3, the fluid phase of exponential mass distribution completely disappears and no phase transitions occurs. Instead, the condensation with exponentially decaying background mass distribution always takes place for any nonzero density. For the existence of the condensed phase for gamma < or = 3 at the zero density limit, we investigate one lamb-lion problem on RNs and SFNs. We numerically show that a lamb survives indefinitely with finite survival probability on RNs and SFNs with gamma > 3, and dies out exponentially on SFNs with gamma< or = 3. The finite lifetime of a lamb on SFNs with gamma < or = 3 ensures the existence of the condensation at the zero density limit on SFNs with gamma < or = 3, at which direct numerical simulations are practically impossible. At omega = infinity, we numerically confirm that complete condensation takes place for any rho > 0 on RNs. Together with the recent study on SFNs, the complete condensation always occurs on both RNs and SFNs in zero range process with constant hopping rate.

Journal Article↗
PubMed2005

In situ studies of phase transitions in thin discotic films.

The crystalline to liquid crystalline (Cr-LC) phase transition in thin films of zone-cast hexa-peri-hexabenzocoronene sixfold substituted with dodecyl side chains (HBC-C12H25) has been studied in detail using grazing incidence X-ray diffraction (GID), electron diffraction (ED), and variable angle spectroscopic ellipsometry (VASE), When heating the material, a first minor transition is observed around 42 degrees C. This change is attributed to alterations of the crystalline alkyl chain packing, which only slightly changes the electronic properties of the material. At higher temperatures of about 90 degrees C, but still significantly below the previously reported transition temperature in bulk, the Cr-LC transition is observed. An accompanying large increase in optical anisotropy is compatible with the X-ray data, showing a transition from the as-cast herringbone-like crystalline state to a highly ordered discotic hexagonal columnar LC phase. The structural transition has the macroscopic effect of increasing the film thickness. The high structural order of the as-cast low-temperature phase is only partly recovered after cooling, and the phase transition exhibits a large hysteresis. From the ellipsometry data, the dielectric tensor of HBC-C12H25 was refined to unprecedented detail.

Journal Article↗
PubMed2005

Lysolipid incorporation in dipalmitoylphosphatidylcholine bilayer membranes enhances the ion permeability and drug release rates at the membrane phase transition.

The enhanced permeability of lipid bilayer membranes at their gel-to-liquid phase transition has been explained using a "bilayer lipid heterogeneity" model, postulating leaky interfacial regions between still solid and melting liquid phases. The addition of lysolipid to dipalmitoylphosphatidylcholine bilayers dramatically enhances the amount of, and speed at which, encapsulated markers or drugs are released at this, already leaky, phase transition through these interfacial regions. To characterize and attempt to determine the mechanism behind lysolipid-generated permeability enhancement, dithionite permeability and doxorubicin release were measured for lysolipid and non-lysolipid, containing membranes. Rapid release of contents from lysolipid-containing membranes appears to occur through lysolipid-stabilized pores rather than a simple enhancement due to increased drug solubility in the bilayer. A dramatic enhancement in the permeability rate constant begins about two degrees below the calorimetric peak of the thermal transition, and extends several degrees past it. The maximum permeability rate constant coincides exactly with this calorimetric peak. Although some lysolipid desorption from liquid state membranes cannot be dismissed, dialyzation above T(m) and mass spectrometry analysis indicate lysolipid must, and can, remain in the membrane for the permeability enhancement, presumably as lysolipid stabilized pores in the grain boundary regions of the partially melted solid phase.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1988

Interaction of surfactants with bilayer of negatively charged lipid: effect on gel-to-liquid-crystalline phase transition of dilauroylphosphatidic acid vesicle membrane.

The interaction of surfactants with the vesicle membrane of the negatively charged lipid, dilauroylphosphatidic acid, was investigated through their effect on the gel-to-liquid-crystalline phase transition of the lipid bilayer. Three types of surfactants (anionic, cationic and non-ionic) with different hydrocarbon chain length were examined. (i) Anionic sodium alkylsulfates affected the phase transition temperature, Tm, only weakly. (ii) Non-ionic alkanoyl-N-methylglucamides decreased Tm monotonously with increasing concentration. The depression of Tm induced by these surfactants was analyzed by applying the van't Hoff model for the freezing-point depression, and the partition coefficients of the surfactants between bulk water and lipid membrane were estimated. (iii) Cationic alkyltrimethylammonium bromides affected Tm in a complex manner depending on the hydrocarbon chain length of the surfactants. Octyl-/tetradecyl-trimethylammonium bromide depressed/elevated Tm monotonously with increasing concentration, whereas the change in Tm induced by decyl- and dodecyltrimethylammonium bromides was not monotonous but biphasic. This complex behavior of the phase transition temperature was well explained, based on the statistical mechanical theory presented by Suezaki et al. (Biochim. Biophys. Acta, 818 (1985) 31-37), which takes into account the interaction between surfactant molecules incorporated in the lipid membrane.

Anions↗
PubMed1999

Are lipid phase transitions responsible for chilling damage in human platelets?

In previous studies we have proposed that the well-known chilling-induced activation of human blood platelets can be ascribed at least in part to a thermotropic phase transition in membrane lipids. The evidence that this is the case is reviewed and amplified in this review, followed by an examination of the available physical data concerning phase transitions in lipid mixtures that mimic the mixture found in platelet membranes. Assuming complete mixing at all temperatures and equal contributions of the members of the mixture to the phase transition, the lipid mixture found in platelets should give values for Tm ranging from about 1 degrees C to about 16 degrees C, depending on the isomers present in the mixture. (The former value is not in agreement with the observed Tm, but the latter is in excellent agreement.) However, examination of the phase diagram for a binary pair of lipids found in platelet membranes shows that ideal mixing almost certainly does not occur; instead of a linear phase diagram, a convex one was obtained. This shape for the phase diagram, which would displace Tm to an unexpectedly elevated temperature, is in agreement with previously published phase diagrams for mixtures of this type. The prediction, based on thermodynamic properties of lipids found in the platelets, is that Tm will be displaced upward in more complex mixtures of the composition found in platelets, a prediction that requires experimental testing.

Blood Platelets↗