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PubMed2001

Structural phase transitions in CaC2.

Pure CaC2, free of CaO impurities, was obtained by the reaction of elemental calcium with graphite at 1,070 K. By means of laboratory X-ray and synchrotron powder diffraction experiments, the phase diagram was investigated in the temperature range from 10 K to 823 K; this confirmed the literature data that reported the partial coexistence of up to four modifications. Aside from a cubic high-temperature modification CaC2 IV (Fm3m, Z = 4) and the well-known tetragonal modification CaC2 I (I4/mmm, Z = 2), a low-temperature modification CaC2 II (C2/c, Z =4) that crystallizes in the ThC2 structure type and a metastable modification CaC2 III (C2/m, Z = 4) that crystallizes in a new structure type were found. It was shown that phase transition temperatures as well as the relative amounts of the various CaC2 modifications depend upon the size of the crystallites, the thermal treatment. and the purity of the sample, as a comparison with technical CaC2 confirmed.

Journal Article↗
PubMed1975

The effect of chain length and lipid phase transitions on the selective permeability properties of liposomes.

This paper describes experiments showing the importance of the fatty acid chain length on the barrier properties of liposomal bilayers, prepared from saturated lecithins, under conditions of lateral phase separation. 1. Above the gel to liquid crystalline phase transition temperature, liposomes prepared from saturated lecithins with 14 or more carbon atoms per acyl chain exist as stable bilayers, which are practically impermeable to ions. 2. At temperatures well above the transition temperature dilauroyl phosphatidylcholine liposomes exhibited osmotic shrinkage, which was dependent on the ionic size of the solute used to bring about the osmotic gradient, indicating that the permeation through these less stable bilayers takes place mainly via individual diffusion of the permeating ions. 3. An enhanced release of trapped potassium from liposomes was demonstrated in the vicinity of the transition temperature. The extent of the increase, however, depended strongly on the length of the paraffin chain. 4. From measurements of the shrinkage behaviour of liposomes in the vicinity of the transition temperature it is concluded that the increased permeability decreases with increasing diameter of the permeating ion. This finding implies that the increased permeability at the transition temperature cannot be ascribed to "macroscopic" rupture of the liposomal membrane. The maximum permeability in the vicinity of the Tc is discussed in terms of probability and size distribution of statistical pore formation at the boundaries of liquid and solid domains.

Cations, Monovalent↗
PubMed1991

High pressure antagonism of alcohol effects on the main phase-transition temperature of phospholipid membranes: biphasic response.

The combined effects of high pressure (up to 300 bar) and a homologous series of 1-alkanols (ethanol C2 to 1-tridecanol C13) were studied on the main phase-transition temperature of dipalmitoylphosphatidylcholine (DPPC) vesicle membranes. It is known that short-chain alkanols depress and long-chain alkanols elevate the main transition temperature. The crossover from depression to elevation occurs at the carbon-chain length about C10-C12 in DPPC vesicle membranes coinciding with the cutoff chain-length where anesthetic potency suddenly disappears. Alkanols shorter than C8 linearly decreased the transition temperature and high pressure antagonized the temperature depression. Alkanols longer than C10 showed biphasic dose-response curves. High pressure enhanced the biphasic response. In addition, alkanols longer than the cutoff length depressed the transition temperature under high pressure at the low concentration range. These non-anesthetic alkanols may manifest anesthetic potency under high pressure. At higher concentrations, the temperature elevatory effect was accentuated by pressure. This biphasic effect of long-chain alkanols is not related to the 'interdigitation' associated with short-chain alkanols. The increment of the transition temperature by pressure was 0.0242 K bar-1 in the absence of alkanols. The volume change of the transition was estimated to be 27.7 cm3 mol-1. This value stayed constant to the limit of the present study of 300 bar.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed2002

Photon propagator, monopoles, and the thermal phase transition in three dimensional compact QED.

We investigate the gauge boson propagator in the three dimensional compact Abelian gauge model in the Landau gauge at finite temperature. The presence of the monopole plasma in the confinement phase leads to the appearance of an anomalous dimension in the momentum dependence of the propagator. The anomalous dimension as well as an appropriate ratio of photon wave function renormalization constants with and without monopoles is observed to be an order parameter for the deconfinement phase transition. We discuss the relation between our results and the confining properties of the gluon propagator in non-Abelian gauge theories.

Journal Article↗
PubMed2004

Exact ground states of a frustrated 2D magnet: deconfined fractional excitations at a first-order quantum phase transition.

We introduce a frustrated spin 1/2 Hamiltonian which is an extension of the two dimensional J1-J2 Heisenberg model. The ground states of this model are exactly obtained at a first-order quantum phase transition between two valence bond crystals. At this point, the low energy excitations are deconfined spinons and spin-charge separation occurs under doping in the limit of low concentration of holes. In addition, this point is characterized by the proliferation of topological defects.

Journal Article↗
PubMed2006

n-pentanol at high pressures: rotational isomerism in the liquid phase and the liquid-solid phase transition.

The vibrational spectrum of liquids constituted of chain molecules is difficult to analyze because it may have contributions of different rotational isomers. In turn, with a proper vibrational assignment, this feature allows us to extract information about the effect of temperature or pressure on the molecular conformations in the liquid state. In this regard, the information on the vibrational spectrum in the solid phase greatly simplifies the vibrational analysis of the different rotational conformers existing in the liquid, as the molecules usually present all-trans conformations in the crystalline state. Here we report room-temperature Raman experiments on n-pentanol performed in a sapphire-anvil cell up to 3 GPa. A detailed analysis of the liquid-solid phase transition occurring at 1.3 GPa is provided. The analysis of the Raman spectrum in the solid phase allows the identification of the bands due to the different rotational isomers present in the liquid. The analysis of the spectral region corresponding to skeletal vibrations of the carbon chain (800-1200 cm(-1)) indicates that gauche conformers are promoted by the application of pressure. The analysis of the intensity ratio of those bands assigned to trans and gauge conformations is used to calculate the change in molecular volume ascribed to the trans-gauge isomerization process. We find a value similar to that found in n-alkanes, i.e., -0.88 cm(3) mol(-1). In addition, we find indication that pressure varies the proportions of the different gauge conformers. Thus, it appears that the GTTt to TGTt transition in the carbon chain is favored at high pressures. As expected, a smaller change in the molecular volume accompanies this conformation change.

Journal Article↗
PubMed1988

The relationship between the bilayer to hexagonal phase transition temperature in membranes and protein kinase C activity.

A number of substances affect the activity of protein kinase C. Among uncharged and zwitterionic compounds, those which activate protein kinase C also lower the bilayer to hexagonal phase transition temperature of dielaidoylphosphatidylethanolamine while substances which inhibit protein kinase C raise this transition temperature. Using this criteria, we have identified 3 beta-chloro-5-cholestene, 5 beta-cholan-24-ol and eicosane as new protein kinase C activators and have shown that Z-Ser-Leu-NH2, Z-Gly-Leu-NH2, Z-Tyr-Leu-NH2, cyclosporin A and cholestan-3 beta, 5 alpha, 6 beta-triol are protein kinase C inhibitors.

Cholesterol↗
PubMed1973

Lipid bilayer phase transition: density measurements and theory.

The overall change of density for dipalmitoyl lecithin bilayers agrees with a general order-disorder theory and yields about seven gauche rotations per molecule for the biologically relevant high-temperature phase. The shape of the curve of density against temperature is similar to the result of an exact calculation on a specific model, which gives a 3/2-order phase transition.

Membranes, Artificial↗
PubMed2004

Evolution of the isotropic-to-nematic phase transition in octyloxycyanobiphenyl+aerosil dispersions.

High-resolution ac calorimetry has been carried out on dispersions of aerosils in the liquid crystal octyloxycyanobiphenyl (8OCB) as a function of aerosil concentration and temperature spanning the crystal to isotropic phases. The liquid crystal 8OCB is elastically stiffer than the previously well studied octylcyanobiphenyl (8CB)+aerosil system and so general quenched random-disorder effects and liquid crystal specific effects can be distinguished. A double heat capacity feature is observed at the isotropic to nematic phase transition with an aerosil independent overlap of the heat capacity wings far from the transition and having a nonmonotonic variation of the transition temperature. A crossover between low and high aerosil density behavior is observed for 8OCB+aerosil. These features are generally consistent with those on the 8CB+aerosil system. Differences between these two systems in the magnitude of the transition temperature shifts, heat capacity suppression, and crossover aerosil density between the two regimes of behavior indicate a liquid crystal specific effect. The low aerosil density regime is apparently more orientationally disordered than the high aerosil density regime, which is more translationally disordered. An interpretation of these results based on a temperature dependent disorder strength is discussed. Finally, a detailed thermal hysteresis study has found that crystallization of a well homogenized sample perturbs and increases the disorder for low aerosil density samples but does not influence high-density samples.

Journal Article↗
PubMed2005

Structural phase transitions and unusual melting behavior in a classical two-dimensional Coulomb bound cluster.

The melting properties of a cluster with N equally charged particles confined by a Coulomb potential are studied. The system exhibits a structural phase transition before it melts. The melting process is not dominated by the usual thermal hops between stable states. We also show that the symmetry of the ground state configuration is a dominant factor in determining the melting temperature and that more confined particles in the cluster do not necessarily have a higher melting temperature.

Journal Article↗
PubMed1998

FLD interacts with genes that affect different developmental phase transitions to regulate Arabidopsis shoot development.

A new fld mutant allele, fld-2, which significantly delayed flowering, was isolated and characterized in Arabidopsis thaliana. Even under long-day conditions after more than 100 days in the greenhouse, the majority of fld-2 mutant plants had not bolted. In addition, mutant inflorescences produced more than 10 co-florescences that were subtended by a high number of rosette-like leaves before giving rise to flowers. The late-flowering phenotype of the fld-2 mutation could be partially overcome by both vernalization and GA treatment but it was not influenced by 5-azaC treatment. Phenotypic analyses of double mutants indicated that fld-2 is epistatic to early flowering mutants elf1, elf2 and elf3. In addition, fld-2 could enhance vegetative characteristics in embryonic flower 1 (emf1) mutants by causing many small sessile leaves in fld-2 emf1 double mutants. The relief of the terminal flower 1 (tfl1) mutant phenotype in fld-2 tfl1 double mutants, and the enhancement of leafy (lfy) and apetala1 (ap1) mutant phenotypes in fld-2 lfy and fld-2 ap1 double mutants, suggest that FLD is also likely to be involved in the floral transition. Our results strongly suggest that the FLD gene plays a key role in regulating the reproductive competence of the shoot and results in different developmental phase transitions in Arabidopsis.

Arabidopsis↗
PubMed2005

Hydrogen-bond topology and the ice VII/VIII and ice Ih/XI proton-ordering phase transitions.

The existence of an ice Ih/XI proton-ordering transition to a low-temperature ferroelectric phase has sparked considerable debate in the literature. Electronic density functional theory calculations, extended using graph invariants, confirm that a transition to a low-temperature ferroelectric phase should occur. The predicted transition at 98 K is in qualitative agreement with the observed transition at 72 K, and the low-temperature phase is the ferroelectric phase determined in diffraction experiments. The theoretical methods used to predict the phase transition are validated by comparing their prediction to the well-characterized ice VII/VIII proton-ordering transition.

Journal Article↗
PubMed2003

Collective dynamics near fluid phase transitions.

By means of molecular dynamics simulations, we calculate the intermediate scattering function F(k(axially),t) where k(||) is the wave number and t is the time. We focus on thermodynamic states in the vicinity of a fluid phase transition in bulk and confined systems which we locate in parallel Monte Carlo simulations in the grand canonical ensemble. As one approaches the limit of stability of the fluid (i.e., its spinodal) from either low- or high-density branches of a subcritical isotherm, F(k(axially),t) becomes increasingly long-range. The apparent lack of decorrelation in the metastable regime can be understood within the framework of a simple mean-field theory that links the long-range nature of F(k(axially),t) to a divergence of the ratio of isostress and isochoric heat capacities gamma. Our results suggest that as one approaches the spinodal the dynamic structure factor S(k(axially),omega) (omega frequency), which is related to F(k(axially),t) through a Laplace transformation, should undergo a qualitative change from the usual triplet of Brillouin and Rayleigh lines to a singlet (delta-function-like peak) centered at omega=0 for states directly at the spinodal. This qualitative change in S(k(axially),omega) should be measurable in scattering experiments thereby promoting more detailed insight into the phase behavior and thermodynamic stability of confined and bulk fluids.

Journal Article↗
PubMed2004

Structures, phase transitions and orientational properties of the C60 monomer and polymers.

The high symmetry and round shape of the fullerene molecules induce orientational phenomena that play an important role in the properties of the fullerene compounds. This is illustrated in this review which is focused on the orientational properties of C60 monomer and polymer structures. First, the orientational state of C60 at room temperature, its peculiar orientational ordering phase transition and the associated glassy state are described, together with the corresponding temperature-pressure phase diagram. Then, a summary of the polymerization processes and of the varied structures obtained under high-pressure high-temperature conditions or via doping by alkali metals is proposed. Finally, some information is given on the orientational properties of C60 layers and surfaces.

Journal Article↗
PubMed2002

The effect of liposomes on thermotropic membrane phase transitions of bovine spermatozoa and oocytes: implications for reducing chilling sensitivity.

We have examined the effects of combinations between egg-phosphatidylcholine (EPC) or dipalmitoylphosphatidylcholine (DPPC) liposomes with either bovine spermatozoa or oocytes on cellular chilling sensitivity, lipid phase transition temperature (T(m)), and the ability of the oocytes to develop to the blastocyst stage. Spermatozoa and oocytes were exposed to EPC and DPPC liposomes at various temperatures (spermatozoa: 4, 12, 16, and 25 degrees C; oocytes: 4, 16, and 32 degrees C). The membrane integrity of the spermatozoa-control group decreased significantly following exposure to 16 or 12 degrees C, compared to ambient temperature (25 degrees C). In contrast, the EPC-sperm group had a greater resistance to chilling at each temperature and showed a decline in membrane integrity only at the lowest temperatures investigated. However, the DPPC-sperm group was injured significantly at all temperatures tested. Similar to the sperm, oocytes from the control group that were exposed to 16 degrees C were injured more severely than oocytes that were electrofused with EPC or DPPC liposomes. The membrane integrity of the oocytes at 16 degrees C that were electrofused with either EPC or DPPC liposomes was approximately the same as the control group held at 32 degrees C (normalized to 100%), compared to 46% in the control group at 16 degrees C (P<0.01). The transition temperatures of the sperm and oocyte membranes revealed different T(m) for the different liposome treatments. All groups had a significantly higher cleavage rate, as well as increased blastocyst formation when oocytes were exposed to temperatures above or below their T(m). We suggest that the T(m) of spermatozoa or oocytes can be changed by spontaneous association or electrofusion of liposomes with cellular membranes and, consequently, the chilling sensitivity can be altered. The resulting possibility is that embryo development after cryopreservation could be improved with such a method.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1991

Empirical estimation of the gel to liquid-crystalline phase transition temperatures for fully hydrated saturated phosphatidylcholines.

Phospholipids are a major component of biological membranes. In excess water, phospholipids may self-assemble into fully hydrated lamellae which, upon heating, may undergo the gel to liquid-crystalline phase transition at the characteristic temperature, Tm. Our present knowledge about the Tm values for various phospholipids is far from complete, although it is necessary to know the Tm value for preparing liposomes. In this study, we have derived empirically a general expression of Tm = 154.2 + 2.0(delta C) - 142.8(delta C/CL) - 1512.5(1/CL) in which two apparent structural parameters, delta C and CL, of a phosphatidylcholine molecule and their ratio, delta C/CL, are applied to estimate the Tm value of the phosphatidylcholine bilayer in excess water. The parameter delta C is the effective chain-length difference, in C-C bond lengths, between the two acyl chains for the phosphatidylcholine molecule in the gel-state bilayer, and CL is the effective length of the longer of the two acyl chains, also in C-C bond lengths. A figure containing 163 calculated Tm values is presented, and this information will be useful as a guide for designing experiments.

Gels↗