PubMed Health⌕ Search

SEARCH · PubMed Health

Results for “Phase Transition”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

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

At least 1,387 records · Page 77Linked to original sources
PubMed1970

Correlation of in vivo and in vitro phase transitions of membrane lipids in Escherichia coli.

A double mutant of Escherichia coli unable to synthesize or degrade unsaturated fatty acids can incorporate fatty acids with various hydrocarbon chain structures into the membrane phospholipids. The temperature characteristic of three physiological properties of cells grown with different fatty acids (growth, respiration, and efflux of thiomethylgalactoside) is compared with the physical properties of the isolated phosphatidylethanolamines in monolayers at an air-water interface. Breaks in the temperature characteristic of the properties measured in vivo correspond to phase transitions in the lipid films from a liquid-expanded to a condensed form. It is concluded that a liquid-like state of the lipid phase is required for proper membrane function.

Carbon Isotopes↗
PubMed1997

Influence of cis double bonds in the sn-2 acyl chain of phosphatidylethanolamine on the gel-to-liquid crystalline phase transition.

We have semisynthesized 19 species of mixed-chain phosphatidylethanolamines (PEs) in which the sn-1 acyl chain is derived from saturated fatty acids with varying chain lengths and the sn-2 acyl chain has different chain lengths but contains 0, 1, and 2 cis double bond(s). The gel-to-liquid crystalline phase transition temperatures (Tm) of lipid bilayers prepared from these 19 mixed-chain PEs were determined calorimetrically. When the Tm values are compared with those of saturated and monounsaturated counterparts, a common Tm profile is observed in the plot of Tm versus the number of cis double bonds. Specifically, a marked stepwise decrease in Tm is detected as the number of cis double bonds in the sn-2 acyl chain of the mixed-chain PE is successively increased from 0 to 1 and then to 2. The large Tm-lowering effect of the acyl chain unsaturation can be attributed to the increase in Gibbs free energy of the gel-state bilayer as a result of weaker lateral chain-chain interactions. In addition, we have applied molecular mechanics calculations to simulate the molecular structure of dienoic mixed-chain C(X):C(Y:2 delta n,n+3)PE in the gel-state bilayer, thus enabling the three independent structural parameters (N, delta C, and LS) to be calculated in terms of X, Y, and n, which are intrinsic quantities of C(X):C(Y:2 delta n,n+3)PE. When the Tm values and the corresponding N and delta C values of all dienoic mixed-chain PEs under study are first codified and then analyzed statistically by multiple regressions, the dependence of Tm on the structural parameters can be described quantitatively by a simple and general equation. The physical meaning and the usefulness of this simple and general equation are explained.

Calorimetry, Differential Scanning↗
PubMed2001

Sticky spheres, entropy barriers, and nonequilibrium phase transitions.

A sticky spheres model to describe slow dynamics of a nonequilibrium system is proposed. The dynamical slowing down is due to the presence of entropy barriers. An exact steady state analysis of the representative mean field equations, in the case when the clusters are chosen with the same a priori probability, demonstrates a nonequilibrium phase transition from an exponential cluster size distribution to a power law.

Journal Article↗
PubMed2006

Structure and phase transition of the Se-rich variety of antimonpearceite, [(Ag,Cu)6(Sb,As)2(S,Se)7][Ag9Cu(S,Se)2Se2].

The crystal structure of a Se-rich antimonpearceite has been solved and refined by means of X-ray diffraction data collected at temperatures above (room temperature) and below (120 K) an ionic conductivity-induced phase transition. Both structure arrangements consist of the stacking of [(Ag,Cu)(6)(Sb,As)(2)(S,Se)(7)](2-) A (A') and [Ag(9)Cu(S,Se)(2)Se(2)](2+) B (B') module layers in which Sb forms isolated SbS(3) pyramids typically occurring in sulfosalts; copper links two S atoms in a linear coordination, and silver occupies sites with coordination ranging from quasi-linear to almost tetrahedral. In the ionic-conducting form, at room temperature, the silver d(10) ions are found in the B (B') module layer along two-dimensional diffusion paths and their electron densities described by means of a combination of a Gram-Charlier development of the atomic displacement factors and a split-atom model. The structure resembles that of pearceite, except for the presence of both specific (Se) and mixed (S, Se) sites. In the low-temperature ;ordered' phase at 120 K the silver d(10) ions of the B (B') module layer are located in well defined sites with mixed S-Se coordination ranging from quasi-linear to almost tetrahedral. The structure is then similar to that of 222-pearceite but with major differences, specifically its cell metric, symmetry and local arrangement in the B (B') module layer.

Journal Article↗
PubMed2001

Chaos-nonchaos phase transitions induced by external noise in ensembles of nonlinearly coupled oscillators.

Nonlinear dynamical behaviors of ensembles of nonlinearly coupled oscillators subjected to external noise are studied on the basis of nonlinear Fokker-Planck equations. The effects of two kinds of noise, the Langevin noise and the noise introduced in the coupling strength, are investigated, and phase transitions involving chaos-nonchaos bifurcations are found to occur as the noise level is changed. An H theorem is proposed for the nonlinear Fokker-Planck equation to ensure stability of the Gaussian type solution that is approached for large times.

Journal Article↗
PubMed2003

Survival-extinction phase transition in a bit-string population with mutation.

A bit-string model for the evolution of a population of haploid organisms, subject to competition, reproduction with mutation, and selection, is studied, using mean-field theory and Monte Carlo simulations. We show that, depending on environmental flexibility and genetic variability, the model exhibits a phase transition between extinction due to random drift and survival. For weak selection the population attains a neutral regime. The mean-field theory describes the infinite-size limit, while simulations are used to study quasistationary properties.

Algorithms↗
PubMed2003

Granular phase transition as a precondition for segregation.

Experimental results are presented on the segregation of a mixture of spheres with two different sizes, rolling on a circularly vibrating table. Beyond a critical density of particles a demixing occurs leading to a clustering of the larger ones. A monodisperse layer of spheres shows a liquid-solid-like phase transition at a slightly lower critical density. These critical particle densities are both found to be independent of the driving frequency, but decrease with increasing vibration amplitude.

Journal Article↗
PubMed1983

An X-ray diffraction study on phase transition temperatures of various membranes isolated from Tetrahymena pyriformis cells grown at different temperatures.

Mitochondrial, microsomal and pellicular membranes were isolated from Tetrahymena cells grown at 39 degrees C or 15 degrees C, and phospholipids, in turn, were separated from total lipids extracted from these membranes. The effect of growth temperature on their solid-to-fluid phase transition temperature was examined by wide-angle X-ray diffraction. The transition temperatures of phospholipids from mitochondria, microsomes and pellicles were 21, 19 and 26 degrees C for cells grown at 39 degrees C and -8, -3 and 6 degrees C for cells grown at 15 degrees C, respectively. All phospholipids were found in a completely fluid state at these growth temperatures. From a comparison between the phospholipids and total lipids from pellicles of cells grown at 39 degrees C, a triterpenoid alcohol, tetrahymanol, caused the transition temperature to increase. The alignment of tetrahymanol in membranes was examined with pellicle'a total lipid oriented in a sample holder.

Cholesterol↗
PubMed2005

Structural phase transitions and out-of-plane dust lattice instabilities in vertically confined plasma crystals.

The formation of plasma crystals confined in an external one-dimensional parabolic potential well is simulated for a normal experimental environment employing a computer code called BOX_TREE. Under appropriate conditions, crystals were found to form layered systems. The system's structural phase transitions, including transitions between crystals with differing numbers of layers and the same number of layers but different intralayer structures, were investigated and found to agree with previous theoretical and experimental research results. One- to two-layer transitions were examined in detail and shown to start at the point where the out-of-plane lattice instability appears. The resulting three layer system caused by this instability was observed at the center of the system. Finally, growth rates for this out-of-plane lattice instability were obtained using the BOX_TREE simulation with these results shown to agree with those obtained from analytical theory.

Journal Article↗
PubMed1989

Expression of alpha- and beta-tubulin genes during dimorphic-phase transitions of Histoplasma capsulatum.

Recent investigations have confirmed the presence of one alpha-tubulin gene (TUB1) and one beta-tubulin gene (TUB2) in the dimorphic fungus Histoplasma capsulatum. In the present study, Northern blot (RNA blot) analyses revealed multiple alpha-tubulin transcripts and a single beta-tubulin transcript in the yeast and mycelial phases of the high-virulence 217B strain and low-virulence Downs strain. S1 nuclease protection assays demonstrated one initiation start site and two major stop sites for the TUB1 transcripts, suggesting that variations in 3' processing generate the alpha-tubulin messages of 2.5 and 2.0 kilobases. Dot blot hybridization experiments indicated that tubulin gene expression is developmentally regulated during the dimorphic phase transitions. alpha- and beta-tubulin mRNAs increased six- to eightfold during the yeast-to-mycelium conversion and decreased two- to threefold during the reverse transition. These changes in tubulin mRNA content coincided with major morphological events associated with H. capsulatum development. Western blots (immunoblots) of H. capsulatum yeast-specific proteins resolved by two-dimensional gel electrophoresis demonstrated a single alpha- and a single beta-tubulin isoform. Multiple tubulin polypeptides expressed in mycelia are probably products of posttranslational modifications.

Endonucleases↗
PubMed2003

Collective dynamics near a phase transition in confined fluids.

We performed molecular dynamics simulations in the microcanonical ensemble (MEMD) for a "simple" fluid confined between two solid substrates. From the calculation of the intermediate scattering function F(k( parallel ),t) and through the memory function formalism, we extract material ( i.e. transport and thermodynamics) coefficients in the vicinity of the liquid-gas phase transition. Our results show that approaching the limit of stability ( i.e. the spinodal), the dynamics of the system changes markedly.

Journal Article↗
PubMed2003

Quantum phase transition in quasi-one-dimensional BaRu6O12.

We report the first systematic study of the electrical transport and magnetic properties of BaRu6O12, which has a quasi-one-dimensional (quasi-1D) hollandite structure. We show that BaRu6O12 is quasi-1D electronically as well. Its physical properties were found to be extremely sensitive to disorder. Furthermore, a transition from being metallic with a resistance drop around 2 K to being weakly insulating as the applied magnetic field was increased was also found. We propose that these two features are related to the possible presence of a quantum phase transition in this material system.

Journal Article↗
PubMed2004

G0-G1 cell cycle phase transition as revealed by fluorescence resonance energy transfer: analysis of human fibroblast chromatin.

In the present study, microspectrofluorometry and digital imaging procedures were used to investigate by fluorescence Resonance Energy Transfer (FRET) analysis the changes of chromatin organization during the transition from G0 quiescent stat to G1 phase. G0 transition is a key event in cell cycle progress depending on the activation of specific genes and the concomitant silencing of others, which both entail spatial chromatin rearrangement. Normal human fibroblasts arrested in G0-phase by culture in low-serum containing medium and stimulated to re-enter G1 by serum addition were used as cell model. To investigate the occurrence and timing of these supramolecular chromatin changes, we estimated the relative FRET efficiency in single cells after double-helical DNA. Hoechst 33258 amd propidium iodide were used as a donor-acceptor dye pair since they exhibit particularly favourable spectral characteristics, that allow the calculation procedure to be semplified. The results of FRET analysis were compared to those of the immunocytochemical labelling of two nuclear proteins (i.e., Ki-67 and statin) whose expression is an established marker of potentially proliferating G1 cells or resting G0 cells, respectively. FRET efficiency was lower in G0 than G1 fibroblasts: this is likely due to higher chromatin packaging in quiescent cells which especially hinders the interaction with the donor molecules less favourable, in terms of relative distance and spatial orientation. FRET efficiency significantly increased shortly (1h) after serum stimulation of quiescent fibroblasts, thus indicating that chromatin is rearranged in parallel with activation of cycle-related gene; it is worth noting that these signs largely preceded the occurrence of immunopositivity for Ki-67, which was detectable only 24h after serum stimulation. FRET-based analyses which already proved to be suitable for studying the overall chromatin organization in differentiated cells, may now be envisaged as a powerful tool for detecting, in single cells, more subtle changes linked to the activation of early cycle-related genes.

Binding Sites↗
PubMed2003

Intramolecular coupling as a mechanism for a liquid-liquid phase transition.

We study a model for water with a tunable intramolecular interaction J(sigma), using mean-field theory and off-lattice Monte Carlo simulations. For all J(sigma)> or =0, the model displays a temperature of maximum density. For a finite intramolecular interaction J(sigma)>0, our calculations support the presence of a liquid-liquid phase transition with a possible liquid-liquid critical point for water, likely preempted by inevitable freezing. For J=0, the liquid-liquid critical point disappears at T=0.

Journal Article↗
PubMed2005

Rapid heating of a strongly coupled plasma near the solid-liquid phase transition.

Between 10(4) and 10(6) 9Be+ ions were trapped in a Penning trap and laser cooled to approximately 1 mK, where they formed a crystalline plasma. We measured the ion temperature as a function of time after turning off the laser cooling and observed a rapid temperature increase as the plasma underwent the solid-liquid phase transition at T approximately 10 mK (Gamma approximately 170). We present evidence that this rapid heating is due to a sudden release of energy from weakly cooled degrees of freedom involving the cyclotron motion of trapped impurity ions. This equilibration of cyclotron motion with motion parallel to the magnetic field is more than 10 orders of magnitude faster than that predicted by currently available theory, which is valid only in the absence of correlations (Gamma<<1).

Journal Article↗
PubMed1977

Effects of lipid phase transition of the freeze-cleaved envelope of Escherichia coli.

We studied the fine structure of the envelope of Escherichia coli auxotroph K1060 after the cells were grown in the presence of one of the following fatty acids; oleic, palmitelaidic, or elidic acid. The cells were freeze-fractured after exposure to temperatures above and below the lipid phase transition range. As judged by freeze-etching methods, we observed that below the transition range the fracture plane of the inner membrane showed the typical aggregation of intramembranous particles (IMP) and concomitant development of areas devoid of IMP. In these areas we found a regular arrangement of equally spaced ridges, often intersected at 90 degrees by arrays of similar ridges. The ridges were composed of spherical particles measuring 4 to 5 nm in diameter. Formation and melting of these arrays took place within 15 to 30s after temperature shift-down or shift-up, respectively. Fixation in glutaraldehyde prevented these changes. The outer-membrane fracture plane revealed ordered areas to a lesser degree; these were discernible only by the regular arrangement of the IMP of the concave fracture plane. We interpret the data by suggesting that the pattern of ridges in E. coli K1060 is analogous to the band patterns described for artificial liposomes, and that the particles, possibly proteins, are lined up or extruded along the ridges during membrane lipid crystallization.

Cell Membrane↗
PubMed2006

Bifurcations and phase transitions in spatially extended two-member hypercycles.

Mounting theoretical and experimental evidence indicates that the success of molecular replicators is strongly tied to the local nature of their interactions. Local dispersal in a given spatial domain, particularly on surfaces, might strongly enhance the growth and selection of fit molecules and their resistance to parasites. In this work the spatial dynamics of a simple hypercycle model consisting of two molecular species is analysed. In order to characterize it, both mean field models and stochastic, spatially explicit approaches are considered. The mean field approach predicts the presence of a saddle-node bifurcation separating a phase involving stable hypercycles from extinction, consistently with spatially explicit models, where an absorbing first-order phase transition is shown to exist and diffusion is explicitly introduced. The saddle-node bifurcation is shown to leave a ghost in the phase plane. A metapopulation-based model is also developed in order to account for the observed phases when both diffusion and reaction are considered. The role of information and diffusion as well as the relevance of these phases and the underlying spatial structures are discussed, and their potential implications for the evolution of early replicators are outlined.

Animals↗