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At least 667 records · Page 37Linked to original sources
PubMed2006

Phase transition of a DPPC bilayer induced by an external surface pressure: from bilayer to monolayer behavior. a molecular dynamics simulation study.

Understanding the lipid phase transition of lipid bilayers is of great interest from biophysical, physicochemical, and technological points of view. With the aim of elucidating the structural changes that take place in a DPPC phospholipid bilayer induced by an external isotropic surface pressure, five computer simulations were carried out in a range from 0.1 to 40 mN/m. Molecular dynamics simulations provided insight into the structural changes that took place in the lipid structure. It was seen that low pressures ranging from 0.1 to 1 mN/m had hardly any effect on the structure, electrical properties, or hydration of the lipid bilayer. However, for pressures above 40 mN/m, there was a sharp change in the lipid-lipid interactions, hydrocarbon lipid fluidity, and electrostatic potential, corresponding to the mesomorphic transition from a liquid crystalline state (L(alpha)) to its gel state (P'(beta)). The head lipid orientation remained almost unaltered, parallel to the lipid layer, as the surface pressure was increased, although a noticeable change in its angular distribution function was evident with the phase transition.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1998

[Phase transition in the matrix as a regulator of enzymatic activity of proteinases].

The kinetic behavior of proteolytic enzymes immobilized in thermosensitive hydrogels was studied at the phase transition (collapse) of the carriers. The dependence of the activity of immobilized enzymes on the state of the matrix allows the use of the hydrogel phase transition to regulate the activity of immobilized enzymes. Several cases of such regulation were demonstrated.

Acrylic Resins↗
PubMed2001

Phase transitions and postural deviations during bimanual kinesthetic tracking.

Upper limb coordination was studied by examining pattern stability of between-hand rhythmical coordination. In the first of two experiments, relative phase of rhythmical wrist flexion-extension was examined within a kinesthetic tracking paradigm. Eight right-handed subjects actively tracked a driven hand being flexed and extended by a computer-controlled AC servo-motor. Hand movements were constrained in flexion or extension. The simultaneous contraction of wrist flexors and extensors was defined as inphase (IP) and the alternating contraction of wrist flexors and extensors as antiphase (AP). Phase transitions (from AP to IP) were observed in 16% of trials prepared in AP. Fewer phase transitions occurred when the right wrist was constrained in flexion, and also when the left wrist was constrained in extension. IP patterns were performed with greater stability than AP patterns. These effects were explored further in a second experiment with the addition of a secondary probe reaction time task to assess demands on central capacity, and the analysis of wrist flexor and extensor electromyographic activity. Subjects returned longer reaction times for AP than IP movement, suggesting the AP movement pattern placed a greater demand on central capacity than the IP movement pattern. During this kinesthetic tracking task, similar dynamic principles emerged as those observed during bilaterally active bimanual rhythmical coordination. The greater stability of the hand-posture combination where the driven left hand was constrained in extension and the active right hand was constrained in flexion may be a demonstration of unique central control of coupled activity.

Adult↗
PubMed2002

Observation of the soft-mode condensation in the Sm-A-Sm-C(*)(alpha) phase transition by nonlinear dielectric spectroscopy.

We have theoretically and experimentally studied the nonlinear dielectric response under large orientational fluctuations near the Sm-A-Sm-C(*)(alpha) phase transition point in a chiral smectic liquid crystal. The third-order nonlinear dielectric response due to the fluctuations was observed in the vicinity of the transition point. By using it, we have successfully observed the soft-mode condensation inducing the Sm-A-Sm-C(*)(alpha) phase transition.

Journal Article↗
PubMedDate not supplied

[Propagation of spikes in statistical neuron ensembles. I. Concept of phase transitions].

A system of two coupled integro-differential equations for the propagation of sipkes is presented. A qualitative consideration of the system shows possibility of concentrational phase transition in the neuron ensemble from the state of spontaneous firing to the strong periodic oscilltation activity. Naer the point of the phase transition the neuron ensemble becomes labile, which maintains appropriate conditons for the existence of mosaic structures in the neuron network.

Models, Neurological↗
PubMed2004

Rigorous analysis of singularities and absence of analytic continuation at first-order phase-transition points in lattice-spin models.

We report about two new rigorous results on the nonanalytic properties of thermodynamic potentials at first-order phase transition. For lattice models (d>or=2) with arbitrary finite state space, finite-range interactions which have two ground states, we prove that the pressure has no analytic continuation at the first-order phase-transition point, under the only further assumptions that the Peierls condition is satisfied for the ground states and that the temperature is sufficiently low. For Ising models with Kac potentials J(gamma)(x)=gamma(d)phi(gammax), where 0 0) and analyticity in the mean field limit (gamma SE pointing arrow 0).

Journal Article↗
PubMed2000

Dynamic phase transition, universality, and finite-size scaling in the two-dimensional kinetic Ising model in an oscillating field.

We study the two-dimensional kinetic Ising model below its equilibrium critical temperature, subject to a square-wave oscillating external field. We focus on the multidroplet regime, where the metastable phase decays through nucleation and growth of many droplets of the stable phase. At a critical frequency, the system undergoes a genuine nonequilibrium phase transition, in which the symmetry-broken phase corresponds to an asymmetric stationary limit cycle for the time-dependent magnetization. We investigate the universal aspects of this dynamic phase transition at various temperatures and field amplitudes via large-scale Monte Carlo simulations, employing finite-size scaling techniques adopted from equilibrium critical phenomena. The critical exponents, the fixed-point value of the fourth-order cumulant, and the critical order-parameter distribution all are consistent with the universality class of the two-dimensional equilibrium Ising model. We also study the cross-over from the multidroplet regime to the strong-field regime, where the transition disappears.

Journal Article↗
PubMed2005

Evaluation of rapidly disintegrating tablets manufactured by phase transition of sugar alcohols.

The aim of the present study was to assess the properties of rapidly disintegrating (RD) tablets manufactured by the phase transition method. RD tablets were produced by compressing powder containing erythritol (melting point: 122 degrees C) and xylitol (melting point: 93 approximately 95 degrees C), and then heating at about 93 degrees C for 15 min. The hardness and oral disintegration time of the heated tablets increased with an increase of the xylitol content. These results suggested that the heating process and xylitol content might influence the properties of RD tablets. Then we evaluated the physicochemical properties of the RD tablets, including the median pore size, crystallinity, hardness, and oral disintegration time of tablets made with and without heating. After heating, the median pore size of the tablets was increased and tablet hardness was also increased. The increase of tablet hardness with heating and storage did not depend on the crystal state of the lower melting point sugar alcohol. It is concluded that a combination of low and high melting point sugar alcohols, as well as a phase transition in the manufacturing process, are important for making RD tablets without any special apparatus.

Delayed-Action Preparations↗
PubMed1976

Fluorescence depolarization studies of phase transitions and fluidity in phospholipid bilayers. 1. Single component phosphatidylcholine liposomes.

The fluorescence depolarization associated with the hydrophobic fluorescent probe 1,6-diphenyl-1,3,5-hexatriene is used to monitor the changes in fluidity accompanying the gel-liquid crystalline phase transition in synthetic phosphatidycholine dispersions. The parameters of the phase transition are determined for both large, multilamellar liposomes and small, single-lamellar vesicles. These parameters are compared with those obtained using other techniques. In addition, the data are interpreted in terms of two limiting molecular models, which in turn offer insight into the structural differences between multilammelar liposomes and small vesicles.

Binding Sites↗
PubMed2004

Evidence of a phase transition of RbHCO3 from high-resolution solid-state 13C and 87Rb NMR by comparison with KHCO3.

A variable-temperature high-resolution 13C and 87Rb solid-state NMR study of powder rubidium hydrogencarbonate, RbHCO3, is presented for the first time. At ambient temperature, RbHCO3 is formed by centrosymmetric dimers linked by hydrogen bonds, but almost no information is available on this compound concerning proton disorder and the low-temperature phase. However, potassium hydrogencarbonate, KHCO3, which has an isomorphic structure for the high temperature phase, was well studied: it undergoes a non-ferroic, non-ferroelectric phase transition at Tc = 318 K between two monoclinic structures. The protons are disordered in an asymmetric double-well potential in the low-temperature phase, and the double-well potential becomes symmetric in the high-temperature phase. By comparison with recent solid-state NMR experimental results on KHCO3, we show that RbHCO3 undergoes a phase transition at Tc approximately 245 K, and give evidence that the proton dynamic disorder in both compounds is very similar.

Algorithms↗
PubMed2005

Observation of the main phase transition of dinervonoylphosphocholine giant liposomes by fluorescence microscopy.

The phase heterogeneity of giant unilamellar dinervonoylphosphocholine (DNPC) vesicles in the course of the main phase transition was investigated by confocal fluorescence microscopy observing the fluorescence from the membrane incorporated lipid analog, 1-palmitoyl-2-(N-4-nitrobenz-2-oxa-1,3-diazol)aminocaproyl-sn-glycero-3-phosphocholine (NBDPC). These data were supplemented by differential scanning calorimetry (DSC) of DNPC large unilamellar vesicles (LUV, diameter approximately 0.1 and 0.2 microm) and multilamellar vesicles (MLV). The present data collected upon cooling reveal a lack of micron-scale gel and fluid phase coexistence in DNPC GUVs above the temperature of 20.5 degrees C, this temperature corresponding closely to the heat capacity maxima (T(em)) of DNPC MLVs and LUVs (T(em) approximately 21 degrees C), measured upon DSC cooling scans. This is in keeping with the model for phospholipid main transition inferred from our previous fluorescence spectroscopy data for DMPC, DPPC, and DNPC LUVs. More specifically, the current experiments provide further support for the phospholipid main transition involving a first-order process, with the characteristic two-phase coexistence converting into an intermediate phase in the proximity of T(em). This at least macroscopically homogenous intermediate phase would then transform into the liquid crystalline state by a second-order process, with further increase in acyl chain trans-->gauche isomerization.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed2001

Phase transition in an asymmetric generalization of the zero-temperature q-state Potts model.

An asymmetric generalization of the zero-temperature q-state Potts model on a one-dimensional lattice, with and without boundaries, has been studied. The dynamics of the particle number, and especially the large time behavior of the system, has been analyzed. In the thermodynamic limit, the system exhibits two kinds of phase transitions, a static and a dynamic phase transition.

Journal Article↗
PubMed1986

The influence of cholesterol on the main phase transition of unilamellar dipalmytoylphosphatidylcholine vesicles. A differential scanning calorimetry and iodine laser T-jump study.

The influence of cholesterol (CHOL) on the main phase transition in single shell dipalmytoylphosphatidylcholine (DPPC) vesicles was investigated in equilibrium and kinetic experiments. CHOL increases the optical density and causes a slight hysteresis in turbidity transition curves. Static fluorescence anisotropy measurements showed interesting differences for three probes sensing different parts in the hydrophobic region of the phospholipid bilayer. Differential scanning calorimetry (DSC) peaks can be separated into a narrow and a broad component. The narrow component, which decreases linearly with increasing CHOL content and disappears at 20 mol %, is attributed to the transition of free phospholipid, while the broad component, being associated with the transition of CHOL-lipid units, increases monotoniously from 0 to 20%. Kinetic experiments were performed on our iodine-laser T-jump arrangement with turbidity detection. Three cooperative relaxation signals in the microsecond and millisecond time range were detected for pure DPPC vesicles as well as vesicles containing 7.5 and 16.5 mol % CHOL. All three relaxation processes were changed by CHOL: the superposition of the three relaxation amplitudes can be separated into a narrow and a broad component, as in DSC experiments. A speculative model is presented which assumes an inhomogeneous CHOL distribution fluctuating on a millisecond time scale in the temperature region of the main phase transition.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1993

Kinetics of MPF and histone H1 kinase activity differ during the G2- to M-phase transition in mouse oocytes.

Maturation promoting factor (MPF) is universally recognized as the biological entity responsible for driving the cell cycle from G2- to M-phase. Histone H1 kinase activity is widely accepted as a biochemical indicator of p34cdc2 protein kinase complex activity and therefore MPF activity. In this paper we present results which indicate that during the G2- to M-phase transition in mouse oocytes the dynamic of p34cdc2 related histone H1 kinase activity differs markedly from the biological activity of MPF as measured by classical cell fusion procedures. MPF is activated just before germinal vesicle breakdown (GVBD) whereas histone H1 kinase is activated 5-7 h later coincident with the formation of the definitive first metaphase plate. The biological activity of MPF is merely reduced to about 50% of control levels by a short period of protein synthesis inhibition (1-2 h) and completely suppressed after a prolonged period of inhibition (4-5 h). By contrast, inhibition of protein synthesis in mouse oocytes results in a rapid and complete suppression of histone H1 kinase activity. Therefore, biological MPF and histone H1 kinase activity should not be used in an interchangeable manner during the G2- to M-phase transition in mouse oocytes.

Animals↗
PubMed2003

Nonequilibrium phase transitions of vortex matter in three-dimensional layered superconductors.

Large-scale simulations on the three-dimensional (3D) frustrated anisotropic XY model have been performed to study the nonequilibrium phase transitions of vortex matter in weak random pinning potential in layered superconductors. The first-order phase transition from the moving Bragg glass to the moving smectic is clarified, based on thermodynamic quantities. A washboard noise is observed in the moving Bragg glass in 3D simulations for the first time. It is found that the activation of the vortex loops plays the dominant role in the dynamical melting at high drive.

Journal Article↗
PubMed2003

Application of the optical waveguide lightmode spectroscopy to monitor lipid bilayer phase transition.

An instrument for optical waveguide lightmode spectroscopy (OWLS) was designed and developed for measurements at different and controlled temperatures in a range of 15 degrees C around room temperature. The instrument allows to scan the waveguide modes at different wavelengths on the same optical chip using different lasers. This instrument was used to monitor DMPC lipid bilayer main phase transition around the critical temperature. The main problem in these experiments is that the OWLS measurements do not give enough information about an optically anisotropic system like a lipid bilayer. Experimental OWLS data at two different wavelengths can however approximately solve the problem. The temperature dependence of the thickness and the refractive indices (ordinary and extraordinary) for the lipid bilayer around the phase transition is presented. (A theoretical derivation of the extraordinary refractive index is given in.)

Computer Simulation↗
PubMed2003

Critical behavior of thermal parameters at the smectic-A-hexatic-B and smectic-A-smectic-C phase transitions in liquid crystals.

High temperature resolution measurements of thermal conductivity, thermal diffusivity, and specific heat, with simultaneous polarized light visual inspection of the sample, have been performed at two different liquid crystal phase transitions: the SmA-SmC (Smectic-A-Smectic-C) and the SmA-HexB (Smectic-A-hexatic-B) in racemic A7 [4-(3-methyl-2-chlorobutanoyloxy)-4(')-heptyloxybiphenyl] and 65OBC (n-hexyl-4(')-n-pentyloxybiphenyl-4-carboxylate) compounds, respectively. In the past, anomalies in the thermal conductivity at the transitions have been reported. Our results indicate a nonsingular behavior of the thermal conductivity at both transitions, similarly to what has been previously reported for the smectic-A-nematic phase transition. It is also shown how, in several cases, the nature of the transition can be affected by the sample thermal history due to the presence of strain annealing phenomena.

Journal Article↗
PubMed1999

A continuous topological change during phase transitions in amphiphile/water systems.

Amphiphiles are molecules such as surfactants or lipids that have a polar head group (hydrophilic) attached to nonpolar hydrophobic alkyl chains. Because of this characteristic they self-assemble in water and give rise to a wide range of phases with different structures and properties. Aqueous dispersions of amphiphiles are present in every aspect of day-to-day life-e.g., forming biological cell membranes, stabilizing emulsified food, or being used as soap. Time-resolved x-ray diffraction has been used to study the hexadecylhexa(oxyethylene glycol) ether (C16EO6)/water system, which shows an intermediate phase whose structure depends on the thermal path between lamellar and hexagonal structures. Heating the hexagonal phase from room temperature leads to a lamellar phase via an Ia3d cubic structure. Cooling from the lamellar phase initially leads epitaxially to an intermediate Rm before the hexagonal phase is reached. Both cubic and Rm phases are formed by very similar rod units, but the overall structures differ because of their spatial distribution and they both bridge morphologically the hexagonal and lamellar phases. The Ia3d does so on heating, whereas the Rm does on cooling. The structural path during the phase transitions is determined by topological similarities between the forming phase and the one from which it originates. Although the estimated curvature energies for these two phases are similar, on cooling, kinetics and topology are initial factors determining the path for the phase transitions, whereas on heating energy is the dominant factor.

Cell Membrane↗