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At least 919 records · Page 51Linked to original sources
PubMed2003

Nonequilibrium relaxation analysis of Kosterlitz-Thouless phase transition.

A simple and efficient numerical analysis is proposed for the Kosterlitz-Thouless (KT) phase transition. The nonequilibrium relaxation method is applied to it. The two-dimensional ferromagnetic XY models are investigated to show the efficiency. At the KT transition point as well as inside the KT phase, the nonequilibrium relaxation of magnetization from the all-aligned state shows an asymptotic power-law decay, m(t) approximately t(-lambda(T)). Only outside the KT phase, an asymptotic single exponential decay is observed. Using a standard scaling form m(t)=tau(-lambda)(-)m(t/tau) in this regime, where tau is the relaxation time at each temperature, we find a simple and efficient numerical estimation of the KT transition point and dynamical exponent. This method can be applied to various kinds of models which show the KT-like behavior.

Journal Article↗
PubMed2005

Chronoamperometric study of the films formed by 4,4'-bipyridyl cation radical salts on mercury in the presence of iodide ions: consecutive two-dimensional phase transitions.

This paper reports a new mathematical model for consecutive two-dimensional phase transitions that accounts for the chronoamperometric behavior observed in the formation of electrochemical phases by 4,4'-bipyridyl cation radical (BpyH(2)(*)(+)) on mercury in aqueous iodide solutions. Also, a new interpretation for the induction time is proposed.

Journal Article↗
PubMed2000

Phase transitions in the kinetic ising model with competing dynamics

We study the nonequilibrium phase diagram and critical properties of a two-dimensional kinetic Ising model with competing Glauber and Kawasaki dynamics suggested by Tome and de Oliveira [Phys. Rev. A 40, 6643 (1989)]. The role of the Kawasaki dynamics, chosen with probability 1-p, is to simulate a permanent energy flux into the system. The theoretical prediction for the phase diagram is improved significantly by using four- and six-point dynamical mean-field approximations. Monte Carlo simulations support that the ferromagnetic-paramagnetic phase transition changes from second to first order for sufficiently small p. The antiferromagnetic phase is found to be stable for a nonzero value of p even at T=0.

Journal Article↗
PubMed1996

Plasma cholesteryl ester transfer activity is modulated by the phase transition of the lipoprotein core.

Previous studies have shown that lipid transfer protein (LTP) activity is strongly temperature dependent, demonstrating a dramatic rise in activity near 37 degrees C. We have investigated the origin of this rapid rise in LTP activity. LTP-mediated transfers of radiolabeled cholesteryl ester (CE) from LDL to HDL, HDL to LDL, LDL to biotin-LDL, HDL to biotin-HDL, and between liposomes were determined as a function of assay temperature. Only assays containing LDL demonstrated this rapid rise in CE transfer activity. In contrast, TG transfer was almost linear with assay temperature. As human LDL CE undergoes a thermal phase transition near 37 degrees C, we investigated whether the rapid rise in CE transfer was dependent on this transition. Monkey LDL were isolated from animals consuming diets containing cholesterol and enriched in saturated, monounsaturated, or polyunsaturated fatty acids. With these LDL as substrate, the CE transfer between 21 degrees and 49 degrees C could be described by two straight lines, the intersection of which defined the inflection temperature. Among eight LDL samples, the inflection temperature was highly correlated with the CE phase transition determined by differential scanning calorimetry (r2 = 0.86). Both calorimetry and CE transfer activity inflection values were correlated with the saturated + monoene/polyene ratio of the LDL cholesteryl esters (r2 = 0.733 and 0.612, respectively). For LDL with inflection temperatures below 37 degrees C, CE transfer activity at 37 degrees C increased 10-14% for each 1 degree C decrease in the inflection temperature. We conclude that LTP activity is markedly affected by the physical state of the core CE. Diets rich in saturated fatty acids may result in LDL that are poor LTP substrates, which may hinder LTP's ability to promote normal lipoprotein remodeling.

Animals↗
PubMedDate not supplied

[Phase transition in dimorphic fungi].

The morphogenetic process, the transition from the micelial to fungal growth phase in imperfect microscopic dimorphic fungi Phaeococcomyces de Hoog (strain Ch49), induced by high concentrations of transition metal ions, was considered in terms of the phase transition theory. It was shown that, although microscopic dimorphic fungi developing in the culture represent a system far from equilibrial, the transitions from the micelial to fungal phase under determined external influences have features characteristic for a common phase transition.

Fungi↗
PubMed2004

Altered gene expression in the transition phase by disruption of a Na+/H+ antiporter gene (shaA) in Bacillus subtilis.

The shaA gene (sodium-hydrogen antiporter gene A, identical to mrpA) is largely responsible for Na+ extrusion in Bacillus subtilis. The disruption of shaA combined with a low concentration of NaCl completely abolishes sporulation but allows normal growth. To investigate the role of shaA and shaA-mediated sodium ion homeostasis in sporulation, we performed a comprehensive study of expression profiles of eight alternative sigma factors, sigmaB and the seven extracytoplasmic function sigma factors (sigmaM, sigmaV, sigmaW, sigmaX, sigmaY, sigmaZ, and sigmaYlaC) in an attempt to determine the global change of gene expression that results from a disturbance of Na+ homeostasis caused by shaA disruption. Induction of sigmaB activity in the transition phase was impaired in the shaA mutant, and this effect was enhanced in the presence of 30 mM NaCl. Salt stress activation of sigmaB occurred normally in the shaA mutant. sigmaM-, sigmaW-, sigmaX-dependent transcription and sigZ transcription was also induced in the transition phase of the wild-type, which was modulated by shaA disruption. The induction of sigmaM-dependent transcription was enhanced in the shaA mutant, while that of sigmaX-dependent transcription and sigZ transcription was decreased. sigmaW-dependent transcription was increased throughout the growth phase of the shaA mutant, which was consistent with the result of proteome analysis. We conclude that shaA disruption resulted in the modulated induction of alternative sigma factor activities, which would be problematic for the cell upon entering the sporulation stage.

Artificial Gene Fusion↗
PubMed2005

Phase transition in vector spin glasses.

We first give an experimental and theoretical introduction to spin glasses, and then discuss the nature of the phase transition in spin glasses with vector spins. Results of Monte Carlo simulations of the Heisenberg spin glass model in three dimensions are presented. A finite size scaling analysis of the correlation length of the spins and chiralities shows that there is a single, finite-temperature transition at which both spins and chiralities order.

Glass↗
PubMed1985

Phase transitions and fatty acid spin label behavior in interdigitated lipid phases induced by glycerol and polymyxin.

Glycerol and polymyxin have been shown by X-ray diffraction to induce interdigitated bilayers in phosphatidylcholine (PC) and phosphatidylglycerol (PG), respectively (McDaniel, R.V., et al. (1983) Biochim. Biophys. Acta 731, 97-108; Ranck, J.-L. and Tocanne, J.-F. (1982) FEBS Lett. 143, 175-178). In the present study we have investigated the phase behavior of PC and PG in the presence of glycerol and polymyxin by differential scanning calorimetry and the use of fatty acid spin labels. Interdigitation causes a large increase in the order parameter of a fatty acid spin labeled near the terminal methyl, 16-doxylstearate, so that it was similar to that of a fatty acid labeled much closer to the polar head group region, 5-doxylstearate. Thus interdigitation abolishes the fluidity gradient found in a non-interdigitated bilayer. 16-Doxylstearate may be useful in detecting interdigitation of lipid bilayers caused by other substances. The different samples all went through two transitions on heating or cooling, or both. However, use of the fatty acid spin label showed that the molecular events during these transitions varies for different samples. The results suggested that PC-glycerol freezes from the liquid-crystalline phase into a non-interdigitated gel phase. This subsequently becomes interdigitated upon lowering the temperature a few degrees, in a low enthalpy transition. PG-polymyxin shows a similar behavior except that the enthalpy of the non-interdigitated gel to interdigitated phase transition is greater and the transition is reversible on heating. Thus on heating PG-polymyxin first goes through a transition from the interdigitated phase to a non-interdigitated gel phase and then, in a separate transition, to the liquid-crystalline phase. This occurs because the fatty acid chains in the presence of polymyxin become too disordered with increase in temperature to maintain the interdigitated state. PG-glycerol goes into the interdigitated state less readily than the other mixtures. If cooled rapidly, PG-glycerol freezes into a metastable phase which is more disordered than the interdigitated phase. It goes into the interdigitated phase in an exothermic transition on heating. An increase in fatty acid chain length causes greater steric hindrance to interdigitation but also increases the stabilizing energy gained by interdigitation.

Calorimetry, Differential Scanning↗
PubMed2005

Molecular dynamics simulation of discontinuous volume phase transitions in highly-charged crosslinked polyelectrolyte networks with explicit counterions in good solvent.

The volumetric properties of highly-charged defect-free polyelectrolyte networks with tetrafunctional crosslinks are studied through molecular dynamics simulations in the canonical ensemble. The network backbone monomers, which are monovalent, and the counterions, which are mono-, di-, or trivalent, are modeled explicitly in the simulations, but the solvent is treated implicitly as a dielectric medium of good solvation quality. The osmotic pressure of the network-solvent system is found to depend greatly on the strength of electrostatic interactions. Discontinuous volume phase transitions are observed when the electrostatic interactions are strong, and the onset of these transitions shifts to higher solvent dielectricity as the counterion valency increases. The roles of the various virial contributions to the osmotic pressure are examined. The network elasticity entropy is found to behave nearly classically. As the network contracts and collapses with increasing strength of electrostatic interactions, the loss of counterion entropy leads to increased counterion osmotic pressure contributions via two mechanisms. The reduction in available configurational space increases the counterion translational entropy contribution to the ideal part of the osmotic pressure, and the greater number of counterion-monomer contacts formed due to counterion condensation and confinement increases the counterion excluded-volume entropy contribution to the excess part of the osmotic pressure. These observations contrast the decrease in the single ideal-gas-like counterion translational entropy contribution to the osmotic pressure predicted by the counterion condensation-charge renormalization theory. An accompanying decrease in the total electrostatic energy balances the loss of counterion excluded-volume entropy as the polyelectrolyte networks collapse in low-dielectric solvents. This interplay between the electrostatic energy and the counterion excluded-volume entropy appears to be responsible for the discontinuous volume phase transitions that are observed in polyelectrolyte networks. The structure of the polyelectrolyte network is also found to be affine in the swollen state, with constituent chains nearly fully extended, and nonaffine in the collapsed state, with the chains adopting a Gaussian conformation.

Journal Article↗
PubMed1992

Triplet radical ion pair state of the Zn-porphyrin-viologen dyad as a magnetic field sensitive probe of phase transitions in small unilamellar vesicles.

The magnetic field effect on the recombination kinetics of the triplet radical ion pair state (RIPS) of the Zn-porphyrin-viologen dyad (P-Ph-Vi2+) in the small unilamellar vesicles (SUV) of D,L-dipalmitoyl-alpha-phosphatidylcholine has been studied by the nanosecond laser flash photolysis technique at 5-60 degrees C. The increase in temperature from 25 to 40 degrees C enhances the rate constant (kr) of the RIPS recombination in zero magnetic field from 0.9 x 10(6) to 1.6 x 10(6) s-1, while kr is temperature insensitive at 5-25 and 40-60 degrees C. The typical break in the kr temperature dependence is observed in the temperature range of the phase transition of the SUV bilayers from the solid to the fluid state. The kr value in a strong magnetic field (B = 0.24 T) is equal to 2.7 x 10(5) s-1 and it is independent of temperature at 5-60 degrees C. The shape of the magnetic field dependence of kr is unaffected by the phase transition of the SUV bilayers and is characterized by the existence of an initial plateau of kr at B = 0 to 0.5 mT.

Electromagnetic Fields↗
PubMed2004

Detection of molecular interactions at membrane surfaces through colloid phase transitions.

The molecular architecture of-and biochemical processes within--cell membranes play important roles in all living organisms, with many drugs and infectious disease agents targeting membranes. Experimental studies of biochemical reactions on membrane surfaces are challenging, as they require a membrane environment that is fluid (like cell membranes) but nevertheless allows for the efficient detection and characterization of molecular interactions. One approach uses lipid membranes supported on solid substrates such as silica or polymers: although the membrane is trapped near the solid interface, it retains natural fluidity and biological functionality and can be implanted with membrane proteins for functional studies. But the detection of molecular interactions involving membrane-bound species generally requires elaborate techniques, such as surface plasmon resonance or total internal reflection fluorescence microscopy. Here we demonstrate that colloidal phase transitions of membrane-coated silica beads provide a simple and label-free method for monitoring molecular interactions on lipid membrane surfaces. By adjusting the lipid membrane composition and hence the pair interaction potential between the membrane-supporting silica beads, we poise our system near a phase transition so that small perturbations on the membrane surface induce dramatic changes in the macroscopic organization of the colloid. We expect that this approach, used here to probe with high sensitivity protein binding events at membrane surfaces, can be applied to study a broad range of cell membrane processes.

Antibodies↗
PubMed1996

The native-like tertiary fold in molten globule alpha-lactalbumin appears to be controlled by a continuous phase transition.

On account of its ability to discriminate between secondary, loop and sidegroup structure and its special sensitivity to conformational mobility, vibrational Raman optical activity (ROA) has provided new insights into the complexity of order within the molten globule state from measurements on alpha-lactalbumin at pH 2.0 over the temperature range 2 to 45 degrees C. Thus while much of the secondary structure present in the native protein persists with only a small gradual decrease with increasing temperature, the tertiary backbone fold changes dramatically, being almost complete and native-like at 2 degrees C and almost completely disordered at 35 degrees C. The change of the tertiary fold with temperature is cooperative but has no latent heat, and so has the approximate characteristics of a continuous phase transition, being of the order-disorder type since it involves the interconversion of rigid, locally-ordered loop structure with disordered mobile backbone structure. This has implications for protein folding because the long-range correlations that exist in the critical region of a continuous (but not in a first-order) phase transition could resolve, in principle, the problem of how the protein finds its native-like folding pattern at the molten globule stage.

Lactalbumin↗
PubMed2004

Surface-induced orientational phase transition in a lyotropic liquid crystal observed by nonlinear optical techniques.

We have observed a phase transition from a uniaxial to a biaxial nematic phase in a lyotropic liquid crystal as a function of decreasing film thickness. The results, obtained by optical second-harmonic generation experiments in a wedged cell geometry, are supported by additional Z -scan measurements and can be interpreted by wall-induced ordering effects.

Journal Article↗
PubMed1986

Atomic and Raman spectroscopy of the dipalmitoylphosphatidic acid-calcium complex and phase transitions.

Calcium binding measurements by atomic absorption spectroscopy and temperature-dependent phase transitions studies by Raman spectroscopy were combined in order to investigate the effect of Ca2+ binding on dipalmitoylphosphatidic acid (DPPA) dispersed in CaCl2 solutions of varying concentration at pH 7. The peak heights for the Raman CH stretch bands observed at 2885 cm-1 and 2935 cm-1 were used as a measure of hydrocarbon chain randomization and aggregate ultrastructure. Two transitions were observed for both pure DPPA and DPPA-Ca2+ mixtures. Ca2+ binding caused greatly increased DPPA chain rigidity in the melted state above Tm, but had much less effect on the solid phases below Tm. The increase in rigidity in the fluid state was observed to vary linearly with the molar ratio of bound Ca2+ to total DPPA throughout the range 0 to 1. The results of the Raman and Ca2+ binding measurements are explained by a model in which two populations of DPPA co-exist in the fluid state when Ca2+ binding has not reached saturation. One population consists of the Ca2+-bound DPPA complex with stoichiometric 1:1 binding ratio (determined from an atomic absorption Ca2+ binding study), and the second population is free DPPA. We propose that Ca2+-induced clustering and separation of the two components occurs chiefly because of differences in chain fluidity of the two components.

Calcium↗
PubMed1997

The phosphatase inhibitor okadaic acid stimulates the TSH-induced G1-S phase transition in thyroid cells.

Protein phosphorylation plays an essential role in regulating many cellular processes in eukaryotes. Signal transduction mechanisms that are reversibly controlled by protein phosphorylation require also protein phosphatases (PPs). Okadaic acid (OA), which is a potent inhibitor of protein phosphatase 2A (PP2A) and protein phosphatase 1, elicits phosphorylation of many proteins in unstimulated cells and induces different cellular responses, including transcriptional activation, shape changes, and pseudomitotic state. In this study, the effects of OA on rat thyroid cells (FRTL-5 strain) were analyzed to evaluate the role of serine/threonine phosphatases in hormone-induced thyroid cell proliferation. OA at a concentration range between 0.1 and 1 nM stimulated thyroid cell growth. Furthermore, 0.25 nM OA increased about 3.5-fold the thyrotropin (TSH)-induced DNA synthesis in quiescent cells. OA treatment also stimulated cell proliferation induced by drugs that mimic TSH effect, such as 8Br-cAMP and cholera toxin, suggesting that PP2A activity was relevant in the cAMP pathway activated by the hormone. Flow cytometry experiments showed that OA significantly increased the fraction of TSH-stimulated quiescent cells entering the S phase. In order to define the mechanisms underlying the observed stimulatory effect of OA on thyroid cell growth, expression of genes relevant in the G1-S phase transition was evaluated. A 2-fold increase in the level of cyclin D1 mRNA expression was found by Northern blot analysis in OA-treated cells. Although cdk2 gene expression was not modulated by the same OA treatment, an increase in Cdk2 protein was revealed by immunoprecipitation experiments. Moreover, OA modifies the phosphorylation pattern of the tumor suppressor retinoblastoma protein, a key event in the G1-S phase transition. Therefore, these experiments reveal that PP2A phosphatases play an important role in thyroid cell growth and can act at multiple sites in the TSH pathways driving cells to S phase.

8-Bromo Cyclic Adenosine Monophosphate↗
PubMed1981

Interaction of phorbol esters with lipid bilayers : thermotropic changes in fluorescence polarization, phase transition and calcium ionophoresis.

The influence of phorbol esters upon the thermotropic behaviour of multilamellar liposomes formed of dipalmitoylphosphatidylcholine (DPPC) or dimyristoylphosphatidylcholine (DMPC) was investigated, as a model for possible interferences of the phorbol esters with the phospholipid domain of biological membranes. Both biologically active (TPA, 12-O-tetradecanoylphorbol-13-acetate, PDD, phorbol-12,13-didecanoate) and inactive (4 alpha-PDD, 4 alpha-phorbol-12,13-didecanoate) phorbol esters lowered the temperature required to cause a fall in fluorescence polarization of a fluorescent probe inserted in the lipid matrix of the DMPC or DPPC liposomes and facilitated the process of calcium exchange-diffusion in DPPC liposomes containing the ionophore A23187. Both of these effects could be due to a decrease in viscosity of the liposomal matrix. However, differential scanning calorimetry revealed that the thermotropic changes evoked by each of these phorbol esters were not identical. In most cases, the phorbol esters decreased both the main phase transition temperature and enthalpy of melting. However, when TPA was incorporated in DMPC liposomes, i.e. when a myristoyl chain was present in both the phorbol ester and phospholipid, no change in the enthalpy of melting could be detected, whereas the main phase transition temperature decreased in proportion to the TPA content of the liposomes. These findings emphasize the view that phorbol esters indeed interact with phospholipids and that the characteristics of such an interaction may tightly depend on the precise chemical structure of both the phorbol ester and phospholipid under consideration.

Calcimycin↗
PubMed2002

First- and second-order phase transitions in scale-free networks.

We study first- and second-order phase transitions of ferromagnetic lattice models on scale-free networks, with a degree exponent gamma. Using the example of the q-state Potts model we derive a general self-consistency relation within the frame of the Weiss molecular-field approximation, which presumably leads to exact critical singularities. Depending on the value of gamma, we have found three different regimes of the phase diagram. As a general trend first-order transitions soften with decreasing gamma and the critical singularities at the second-order transitions are gamma dependent.

Journal Article↗
PubMed2005

Pressure induced phase transitions of TTAB-micellar solutions studied by SANS and Raman spectroscopy.

Using small angle neutron scattering in combination with optical Raman spectroscopy, phase transitions in the micellar system tetradecyl-trimethylammonium bromide (TTAB) have been investigated in a wide range of temperatures, pressures and surfactant concentrations. Allowing for the sluggish kinetics of crystallisation, the phase diagram in the p-T-c space could be determined that is dominated by a pronounced two-phase region of coexisting liquid and solid phases at elevated pressures.

Biophysical Phenomena↗