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Quantum phase transitions of hard-core bosons in background potentials.

We study the zero temperature phase diagram of hard-core bosons in two dimensions subjected to three types of background potentials: staggered, uniform, and random. In all three cases there is a quantum phase transition from a superfluid (at small potential) to a normal phase (at large potential), but with different universality classes. As expected, the staggered case belongs to the XY universality, while the uniform potential induces a mean field transition. The disorder driven transition is clearly different from both; in particular, we find z approximately 1.4, nu approximately 1, and beta approximately 0.6.

Journal Article↗

Structural stability and phase transitions in WO3 thin films.

Tungsten oxide (WO3) thin films have been produced by KrF excimer laser (lambda = 248 nm) ablation of bulk ceramic WO3 targets. The crystal structure, surface morphology, chemical composition, and structural stability of the WO3 thin films have been studied in detail. Characterization of freshly grown WO3 thin films has been performed using X-ray diffraction (XRD), atomic force microscopy (AFM), energy-dispersive X-ray spectroscopy (EDX), Raman spectroscopy (RS), transmission electron microscopy (TEM), and selected area electron diffraction (SAED) measurements. The results indicate that the freshly grown WO3 thin films are nearly stoichiometric and well crystallized as monoclinic WO3. The surface morphology of the resulting WO3 thin film has grains of approximately 60 nm in size with a root-mean-square (rms) surface roughness of 10 nm. The phase transformations in the WO3 thin films were investigated by annealing in the TEM column at 30-500 degrees C. The phase transitions in the WO3 thin films occur in sequence as the temperature is increased: monoclinic --> orthorhombic --> hexagonal. Distortion and tilting of the WO6 octahedra occurs with the phase transitions and significantly affects the electronic properties and, hence, the electrochemical device applications of WO3.

Journal Article↗

[Effect of phase transitions in microsomal membranes on energetic characteristics of oxidation].

Naphthalene and aniline oxidation by intact and induced enzyme systems of rat liver microsomes, NADPH and O2 has been studied in a broad temperature range. The oxidation of the same substrates has also been studied in the systems "microsomes--cumene hydroperoxide" and "purified cytochrome P-450--cumene hydroperoxide". The temperature curves of catalytic constants for type I substrate (naphthalene-oxidation) in the system with cofactors is characterized by break on Arrenius dependence near 18--24 degrees C and explained by phase transitions of microsomal membrane lipids. The temperature curves of catalytic constants for type II substrate (aniline-oxidation) in all studied systems do not break on Arrenius plots. Catalytic oxidation contants of both substrates in systems "microsomes--hydroperoxide" and "cytochrome P-450--hydroperoxide" are characterized by constant activation energies in broad temperature range. The results obtained are discussed from the point of view of the effect of phospholipids phase transitions in microsomal membrane on a rate limiting step.

Aniline Compounds↗

Effect of the lipid phase transition on the lactose permease from Escherichia coli.

The temperature dependence of lactose active transport, efflux down a concentration gradient, and equilibrium exchange were analyzed in right-side-out membrane vesicles from Escherichia coli containing wild-type lactose permease and mutant Glu325 --> Ala. With respect to uphill transport and efflux down a concentration gradient, both of which involve H(+) symport, Arrhenius plots with wild-type permease exhibit a discontinuity at 18-19 degrees C with a 7-8-fold decrease in activation energy above the phase transition. For equilibrium exchange, which does not involve H(+) symport, the change in activation energy is much less pronounced (2-3-fold) than that observed for active transport or efflux. Strikingly, mutant Glu325 --> Ala, which catalyzes equilibrium exchange as well as wild-type permease but is defective in all translocation reactions that involve net H(+) translocation, exhibits no change whatsoever in activation energy. The findings are consistent with the conclusion that the primary effect of the lipid phase transition is to alter coupling between substrate and H(+) translocation rather than the conformational change(s) responsible for translocation across the membrane.

Alanine↗

Effect of lipid phase transition on the binding of anions to dimyristoylphosphatidylcholine liposomes.

Temperature dependence of the electrophoretic mobility of multilamellar liposomes prepared from dimyristoylphosphatidylcholine was measured in the presence of salts with different anions in aqueous solutions. It was established that specific binding of anions to liposome surface induced a pronounced zeta potential (electrostatic potential at the hydrodynamic plane of shear). A combination of Langmuir, Gouy-Chapman, and Boltzmann equations was used to describe the dependence of the zeta potential on the concentration of anions. The values of binding constants (K) and maximum numbers of binding sites per unit area (sigma max) were determined by this method. The sequence for anion affinities to liposome surface was found to be as follows: trinitrophenol greater than ClO-4 greater than I- greater than SCN- greater than Br- greater than NO-3 greater than Cl- approximately or equal to SO2-4. A sharp increase in the negative zeta potential was detected at the temperature of phase transition of the lipid from the gel to liquid-crystalline state. It was found that the parameter K did not change at lipid phase transition and the shifts in zeta potential might be due to alterations of sigma max. The binding sites were considered as defects in the package of lipid molecules in membranes.

Anions↗

Dehydration-induced lamellar-to-hexagonal-II phase transitions in DOPE/DOPC mixtures.

Plasma membranes of protoplasts isolated from non-acclimated rye plants undergo a transition from the bilayer to the inverted hexagonal (HII) phase during freeze-induced dehydration at -10 degrees C. It has been suggested (Bryant, G. and Wolfe, J. (1989) Eur. Biophys. J. 16, 369-372) that the differential hydration of various membrane components may induce fluid-fluid demixing of highly hydrated (e.g., PC) from poorly hydrated (PE) components during dehydration. This could yield a PE-enriched domain more prone to form the HII phase. We have examined the lyotropic phase behavior of mixtures of DOPE and DOPC at 20 degrees C by freeze-fracture electron microscopy, differential scanning calorimetry, and X-ray diffraction. HII phase formation was favored by higher proportions of DOPE and lower water contents. Mixtures of 1:1 and 1:3 DOPE/DOPC had a hydration-dependent appearance of two L alpha phases at water contents just above those at which the HII phase occurred. The hydration-dependence of the lamellar repeat spacings suggested that the DOPE-enriched domains preferentially underwent the L alpha-to-HII phase transition. Mixtures of 3:1 DOPE/DOPC did not separate into two L alpha phases during dehydration. These data suggest that the differential hydration characteristics of various membrane components may induce their lateral fluid-fluid demixing during dehydration.

Chemical Phenomena↗

Antifreeze glycoproteins: relationship between molecular weight, thermal hysteresis and the inhibition of leakage from liposomes during thermotropic phase transition.

Antifreeze glycoproteins (AFGP) were isolated and purified from the blood plasma of rock cod (Gadus ogac), using DEAE-Bio-gel ion exchange chromatography, followed by high performance liquid chromatography (HPLC). The purified proteins were analyzed using polyacrylamide gel electrophoresis (PAGE), and electrospray mass spectrometry. The results indicated that rock cod synthesize seven size classes of glycoproteins, ranging from 2.6 to 24 kDa, with each size class containing multiple isoforms. Antifreeze activity, as determined by thermal hysteresis, indicated that the AFGP could be separated into two groups, with the larger size classes (molecular mass>13 kDa) having approximately 3-4 times the activity of the smaller, proline containing, size classes (molecular mass<10 kDa). All of the AFGP size classes prevented leakage from dielaidoylphosphatidylcholine (DEPC) liposomes as they were cooled through their phase transition temperature, with the larger size classes being approximately 4 times as effective as the smaller ones. It is hypothesized that AFGP prevent liposomes from leaking as they pass through the phase transition temperature by binding to the phospholipid membrane.

Animals↗

Structural evolution in boron nitrides during the hexagonal-cubic phase transition under high pressure at high temperature.

Structural evolution during the phase transition from h (hexagonal)- to c (cubic)-boron nitrides (BN) under high pressure (6.5-7.7 GPa) at high temperature (1,700-2,150 degrees C) was examined by using high-resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS). At the initial stage of the evolution, some starting h-BN plates were strongly folded, while others were slightly bent. As a result, a strong texture was formed. HRTEM revealed that the interplanar distance between sp2 sheets became slightly shortened and they were slightly sheared to each other during the folding and bending. As a result, m (monoclinic)-BN was formed near the folding plane with lattice parameters; a = 0.433 nm, b = 0.250 nm, c = 0.32-0.33 nm, and beta = 90-92 degrees. In a succeeding stage, the value of beta increased to 92-95 degrees. c-BN grains appeared with nano-scale twins and sometimes partly included wurtzite-type BN. They started to grow with secondary twins at higher temperature. EELS analysis revealed that the band structure of sp2 sheets changed during the transition from h-BN to m-BN; the density of state for the pi* bond became prominently high in m-BN as compared to that in h-BN.

Animals↗

A database of lipid phase transition temperatures and enthalpy changes.

The systematic study of the mesomorphic phase properties of synthetic and biologically derived lipids began some 30 years ago. In the past decade, interest in this area has grown enormously. As a result, there exists a wealth of information on lipid phase behavior, but unfortunately these data have, until now, been scattered throughout the literature in a variety of books, proceedings and journals. The data have recently been compiled in a centralized database with a view to providing ready access to same and to the appropriate literature. The compilation facilitates review of what has thus far been accomplished and highlights what remains to be done in this active research area. As such, it represents a convenient summary of the existing data which, when evaluated, will enable us to identify where deficits exist in the data, to reveal the fundamental physicochemical principles upon which lipid phase behavior is based and to understand more completely lipid phase relations in biological, reconstituted and formulated systems. The compilation consists of a tabulation of all known mesomorphic and polymorphic phase transition temperatures and enthalpy changes for synthetic and biologically-derived lipids in the dry and in the partially and fully hydrated states. Also included is the effect on these thermodynamic values of pH, and of salt and metal ion concentration and other additives such as proteins, drugs, etc. The methods used in making the measurements and the experimental conditions are reported. Bibliographic information includes complete literature referencing and list of authors. As of this writing, the database is current through June, 1990 and contains in excess of 9500 records. Each record contains 28 fields. Here, we describe how the database originated, its scope and contents, data abstraction procedures, and issues relating to mesophase and lipid nomenclature, data analysis and evaluation, and database maintenance and distribution.

Databases, Factual↗

A database of lipid phase transition temperatures and enthalpy changes.

The systematic study of the mesomorphic phase properties of synthetic and biologically derived lipids began some 30 years ago. In the past decade, interest in this area has grown enormously. As a result, there exists a wealth of information on lipid phase behavior, but unfortunately, these data have, until now, been scattered throughout the literature in a variety of books, proceedings, and journals. The data have recently been compiled in a centralized database with a view to providing ready access to the same and to the appropriate literature. The compilation facilitates review of what has thus far been accomplished and highlights what remains to be done in this active research area. As such, it represents a convenient summary of the existing data which, when evaluated, will enable us to identify where deficits exist in the data, to reveal the fundamental physicochemical principles upon which lipid phase behavior is based, and to understand more completely lipid phase relations in biological, reconstituted, and formulated systems. The compilation consists of a tabulation of all known mesomorphic and polymorphic phase transition temperatures and enthalpy changes for synthetic and biologically derived lipids in the dry and in the partially and fully hydrated states. Also included is the effect on these thermodynamic values of pH, and of salt and metal ion concentration and other additives such as proteins, drugs, etc. The methods used in making the measurements and the experimental conditions are reported. Bibliographic information includes complete literature referencing and list of authors. As of this writing, the database is current through June 1990 and contains 9500 records. Each record contains 28 fields. Here, we describe how the database originated, its scope and contents, data abstraction procedures, and issues relating to mesophase and lipid nomenclature, data analysis, and evaluation, and database maintenance and distribution.

Databases, Factual↗

Evolution of behavioral attractors with learning: nonequilibrium phase transitions.

Learning a bimanual coordination task (synchronization to a visually specified phasing relation) was studied as a dynamical process over 5 days of practicing a required phasing pattern. Systematic probes of the attractor layout of the 5 Ss' coordination dynamics (expressed through a collective variable, relative phase) were conducted before, during, and after practice. Depending on the relationship between the initial coordination dynamics (so-called intrinsic dynamics) and the pattern to be learned (termed behavioral information, which acts as an attractor of the coordination dynamics toward the required phasing), qualitative changes in the phase diagram occurred with learning, accompanied by quantitative evidence for loss of stability (phase transitions). Such effects persisted beyond 1 week. The nature of change due to learning (e.g., abrupt vs. gradual) is shown to arise from the cooperative or competitive interplay between behavioral information and the intrinsic dynamics.

Adult↗

A theory of the chain melting phase transition of aqueous phospholipid dispersions.

A model for the chain melting phase transition in dilute aqueous phospholipid bilayer dispersions is presented. This model includes interactions between head groups, between hydrocarbon chains, and within the chains. The head groups are modeled as hard disks which are constrained to lie on a two-dimensional surface separating the aqueous and hydrocarbon regions. The chain statistics problem is treated in an approximate manner using an approach motivated by scaled particle theory to describe the inter-chain steric repulsions in a mathematically tractable way. In this approach the whole system interacts with any given chain through an average lateral pressure which is proportional to the hard disk pressure. Following Nagle, we assume that the steric repulsions between chains and between head groups and the trans-gauche rotation energies are the dominant interactions in determining the transition and we describe the effect of the other interactions with a mean field approximation. Using the known transition temperature of a series of 1,2-diacyl phosphatidyl cholines to adjust two parameters in the theory, the model gives enthalpy and area changes that are in quite reasonable agreement with experiment. Moreover, the curvature observed in the plot of the transition temperature against acyl chain length is reproduced.

Binding Sites↗

Gauge cell method for simulation studies of phase transitions in confined systems

A method for Monte Carlo studies of phase equilibrium in confined systems is presented using an example of vapor-liquid equilibrium (capillary condensation and evaporation) in cylindrical pores. The method, named the gauge cell method, allows one to construct the full phase diagram of a confined fluid in the form of a van der Waals loop, which includes stable, metastable, and unstable equilibrium states. The phase coexistence is then determined by thermodynamic integration along the metastable and unstable regions of the phase diagram employing Maxwell's rule of equal areas. The simulation results agree with experimental data on the capillary condensation of nitrogen at its boiling temperature on mesoporous molecular sieves. The method can be applied to other phase transitions in confined systems such as fluid-fluid separation, layering, and freezing.

Journal Article↗

Effect of sodium cholate on the phase transition temperature of dipalmitoyl phosphatidylcholine.

On treatment with sodium cholate, the phase transition temperature (Tc) of multilamellar liposomes derived from dipalmitoyl phosphatidylcholine (DPPC) progressively decreased with increasing cholate: lipid-P molar ratio. A molar ratio of 4.5 caused a one degree depression in Tc. The unilamellar cholate vesicles of DPPC exhibited the same Tc as that of the multilamellar ones. The formation of unilamellar vesicles from the multilamellar ones by cholate treatment was investigated by electron microscopy.

Cholic Acid↗

Evaluation of phase transition errors in heat capacity calorimeters using SPICE simulated RC models.

A technique is presented which allows the development of extremely complex (> 5000 components) RC models of calorimeters and provides simulation under a wide variety of inputs. A commercially available circuit simulation program (SPICE) is used to 'build' RC models of relaxation and scanning calorimeter designs used to measure heat capacity. The instrument models are constructed using subcircuits to represent small elements of the materials used in the designs. Only simple linear equations are needed to evaluate the RC values in the subcircuit. The subcircuits also use voltage controlled switches to simulate phase transitions where lambda, C, and delta h can change value instantaneously at predetermined temperatures. The resulting simulations of the two designs provide an ability to predict instrument sensitivity to lambda and C independently. Simulated outputs agree with measured outputs within 10%. Model simulations show serious errors in the heat capacity determinations from both designs during phase transitions. Interactions between lambda, C, and delta h are shown for both designs. The technique provides a means to construct, evaluate, and optimize a calorimeter design completely in software.

Calorimetry, Differential Scanning↗

Observation of surface and bulk phase transitions in nematic liquid crystals.

The behaviour of liquid crystal (LC) molecules near a surface is of both fundamental and technological interest: it gives rise to various surface phase-transition and wetting phenomena, and surface-induced ordering of the LC molecules is integral to the operation of LC displays. Here we report the observation of a pure isotropic-nematic (IN) surface phase transition-clearly separated from the bulk IN transition-in a nematic LC on a substrate. Differences in phase behaviour between surface and bulk are expected, but have hitherto proved difficult to distinguish, owing in part to the close proximity of their transition temperatures. We have overcome these difficulties by using a mixture of nematic LCs: small, surface-induced composition variations lead to complete separation of the surface and bulk transitions, which we then study independently as a function of substrate and applied magnetic field. We find the surface IN transition to be of first order on surfaces with a weak anchoring energy and continuous on surfaces with a strong anchoring. We show that the presence of high magnetic fields does not change the surface IN transition temperature, whereas the bulk IN transition temperature increases with field. We attribute this to the interaction energy between the surface and bulk phases, which is tuned by magnetic-field-induced order in the surface-wetting layer.

Journal Article↗

Viscosity of a liquid crystal near the nematic-smectic A phase transition.

We report the results of an x-ray scattering study where both the dynamic and the static properties of a liquid crystal (8OCB) near the nematic-smectic A phase transition were probed. The static, time-averaged data show the gradual formation of smectic layers in the nematic phase, and we find that the smectic order correlation length parallel to the molecular axis diverges with the critical exponent nu( parallel )=0.70(4) at the transition. The literature value is nu( perpendicular )=0.58 for the perpendicular direction. By x-ray photon correlation spectroscopy, we find that the viscosity coefficient eta(3) shows critical, diverging behavior at the phase transition with a critical exponent x=0.95(5). This contradicts previous light scattering work (x=0.50), but is in good agreement with the theoretical prediction x=3nu( parallel )-2nu( perpendicular ) by Hossain et al.

Journal Article↗

Regulation of NIH-3T3 cell G1 phase transit by serum during exponential growth.

The proliferation rate of mammalian cells is regulated normally in the G1 phase of the cell cycle. During this phase, it is convenient to assign positive and negative roles to the molecular programs that regulate the duration of G1 and the phase transition from G1 to S phase. Density-dependent inhibition of cellular proliferation results in an increase in the duration of G1. This form of regulation is due to both secreted factors and cell-cell contact. Serum is mitogenic to a variety of mammalian cell types. Because quiescent cells enter S phase as a result of serum addition to culture media, serum is usually regarded as a source of positive regulatory growth factors. We have measured the length of the G1, S and G2+M phases of NIH 3T3 cells during exponential growth as a function of cell density and serum concentration. The G1 length increases during exponential growth as a function of density while S and G2+M are relatively constant. Further, this increase in G1 phase time, or density mediated negative regulation, is inhibited by increasing serum concentration. This phenotype is saturable between 10% to 20% serum. Serum concentrations above 2.5% are able to increase the rate of cell cycling (decrease the G1 phase time) by inhibiting density dependent negative regulation of NIH 3T3.

3T3 Cells↗