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At least 631 records · Page 35Linked to original sources
PubMed2004

Structural phase transitions, and Br.N and Br.Br interactions in 1-phenyl-2-methyl-4-nitro-5-bromoimidazole.

Crystals of C(10)H(8)N(3)O(2)Br undergo two reversible phase transitions between 295 and 100 K. The first, of an order-disorder nature, is a second-order transition and takes place continuously over a wide temperature range. This transition is connected with the doubling of the length of the c axis of the unit cell and with the change of the space group from P2(1)/m with Z' = 1/2 (room-temperature alpha-phase) to P2(1)/c, Z' = 1 (beta-phase, 200-120 K). During this transition the molecule loses the C(s) symmetry of the alpha-phase. The second transition takes place between 118 and 115 K, and is accompanied by a change of the crystal symmetry to the triclinic space group P1; (low-temperature gamma-phase). This second phase transition is accompanied by the twinning of the crystal. Neither the molecular geometry nor the crystal packing shows any dramatic changes during these phase transitions. Halogen bonds C-Br.N and dihalogen interactions Br.Br play a crucial role in determining the crystal packing and compete successfully with other kinds of weak intermolecular interactions.

Bromine↗
PubMed2004

Atomistic and lattice model of a grain boundary defaceting phase transition.

Recent calculations have shown that grain boundary (GB) stress is too small to stabilize finite GB facets, suggesting that the existing theory of GB defaceting phase transitions is incomplete. We perform molecular dynamics calculations, which show a reversible phase transition at approximately 400 K with a concerted shuffle of two atoms at the facet junction as the elementary excitation. Based on this excitation we formulate an appropriate lattice model, perform Monte Carlo simulations, and establish an analytical relationship between the elementary excitation energy and the transition temperature.

Journal Article↗
PubMed2005

Dissipation-driven phase transition in two-dimensional Josephson arrays.

We analyze the interplay of dissipative and quantum effects in the proximity of a quantum phase transition. The prototypical system is a resistively shunted two-dimensional Josephson junction array, studied by means of an advanced Fourier path-integral Monte Carlo algorithm. The reentrant superconducting-to-normal phase transition driven by quantum fluctuations, recently discovered in the limit of infinite shunt resistance, persists for moderate dissipation strength but disappears in the limit of small resistance. For large quantum coupling our numerical results show that, beyond a critical dissipation strength, the superconducting phase is always stabilized at sufficiently low temperature. Our phase diagram explains recent experimental findings.

Journal Article↗
PubMed2001

Novel mechanism of photoinduced reversible phase transitions in molecule-based magnets.

A novel microscopic mechanism of bidirectional structural changes is proposed for the photoinduced magnetic phase transition in Co-Fe Prussian blue analogs on the basis of ab initio quantum chemical cluster calculations. It is shown that the local potential energies of various spin states of Co are sensitive to the number of nearest neighbor Fe vacancies. As a result, the forward and backward structural changes are most readily initiated by excitation of different local regions by different photons. This mechanism suggests an effective strategy to realize photoinduced reversible phase transitions in a general system consisting of two local components.

Journal Article↗
PubMed1995

Phase transition behavior and molecular structures of monounsaturated phosphatidylcholines. Calorimetric studies and molecular mechanics simulations.

High resolution differential scanning calorimetric studies were performed to investigate the thermotropic phase behavior of 26 molecular species of sn-1 saturated/sn-2 monounsaturated phosphatidylcholines. In parallel with calorimetric studies, the energy-minimized structures and steric energies of the diglyceride moieties of these monoenoic lipids were determined using a molecular mechanics approach. The combined calorimetric and computational studies led to the following results and conclusions. (i) When a single cis-carbon-carbon double bond (delta) is incorporated into a saturated diacylphosphatidylcholine molecule at any position within the central segment of the long sn-2 acyl chain, the resulting monoenoic lipid molecules will, in excess water, exhibit reduced phase transition temperature (Tm) and transition enthalpy (delta H) as they undergo the gel to liquid-crystalline phase transition. The Tm and delta H-lowering effects of the delta bond can be attributed to a decrease in the chain length of the sn-2 acyl chain, a change in the chain length difference between the sn-1 and sn-2 acyl chains, and a local perturbation of the chain-chain van der Waals interaction in the vicinity of the delta bond. (ii) For a series of positional isomers of 1-stearoyl-2-cis-octadecenoylphosphatidylcholine, C(18):C(18:1 delta n)PC, with a delta bond at different positions along the sn-2 acyl chain, the Tm value depends critically on the position of the delta bond. Specifically, the Tm value is minimal as the delta bond is located at the geometric center of the linear segment of the sn-2 acyl chain, and the Tm value is progressively increased as the delta bond migrates toward either end of the sn-2 acyl chain. (iii) The various monoenoic phosphatidylcholines under study can be divided into two groups. The Tm values of most lipids in each group can be correlated in an identical manner with their structural parameters, yielding a common Tm-structure relationship.

Calorimetry, Differential Scanning↗
PubMed1989

Infrared study of phospholipid hydration. New thermodynamic data about the main phase transition of saturated phosphatidylcholine water multidispersions.

Infrared spectra measurements are performed with fully hydrated phospholipid multilamellar dispersions. The temperature profiles of the delta (OH) bending vibration at 1645 cm-1, corresponding to the water molecules, show three marked transitions in such temperature ranges that they can be respectively associated with the fusion of free water molecules, with pretransition phenomena and with the main phase transition. From absorbance variations and calorimetric data, main phase transition enthalpies and entropies were calculated for both the acyl chains and the polar head-water regions. Negative contributions of the polar head-water region are consistent with previous results, indicating a more ordered water layer in the fluid phase. On the other hand, a decrease of the amount of bound water molecules with increasing chain length is pointed out.

Liposomes↗
PubMed1986

Dose-dependent nonlinear response of the main phase-transition temperature of phospholipid membranes to alcohols.

The effect of 1-alkanols upon the main phase-transition temperature of phospholipid vesicle membranes between gel and liquid-crystalline phases was not a simple monotonic function of alkanol concentration. For instance, 1-decanol decreased the transition temperature at low concentrations, but increased it at high concentrations, displaying a minimal temperature. This concentration-induced biphasic effect cannot be explained by the van't Hoff model on the effect of impurities upon the freezing point. To explain this nonlinear response, a theory is presented which treats the effect of 1-alkanols (or any additives) on the transition temperature of phospholipid membranes in a three-component mixture. By fitting the experimental data to the theory, the enthalpy of the phase transition delta H* and the interaction energy, epsilon*AB between the additive and phospholipid molecules may be estimated. The theory predicts that when epsilon*AB greater than 2 (where epsilon*AB = epsilon AB/RT0, T0 being the transition temperature of phospholipid), both minimum and maximum transition temperatures should exist. When epsilon*AB = 2, only one inflection point exists. When epsilon*AB less than 2, neither maximum nor minimum exists. The alkanol concentration at which the transition temperature is minimum (Xmin) depends on the epsilon*AB value: the larger the epsilon*AB values, the smaller the Xmin. When epsilon*AB is large enough, Xmin values become so small that the plot delta T vs. X shows positive delta T in almost all alkanol concentrations. The interaction energy between 1-alkanols and phospholipid molecules increased with the increase in the carbon chain-length of 1-alkanols. In the case of the dipalmitoylphosphatidylcholine vesicle membrane, the carbon chain-length of 1-alkanols that caused predominantly positive delta T was about 12.

Alcohols↗
PubMed1992

Phase transitions of phospholipid vesicles under osmotic stress and in the presence of ethylene glycol.

The effects of poly(ethylene glycol) (PEG) on the phase transition of phospholipid multilamellar vesicles (MLVs) were investigated by using differential scanning calorimetry (DSC). Main transition temperature (Tm) and the pre-transition temperature (Tp) of neutral phospholipid-, DMPC-1, DPPC- and DSPC-MLVs increased with an increase in PEG concentration. The subtransition temperature of DPPC-MLV also increased with an increase in PEG concentration. These results could be qualitatively explained by enhancement of the lateral packing on the basis of the osmoelastic coupling theory. The pretransition temperature increased faster than the main transition temperature did with an increase in PEG concentration. The increment of Tm depended on the hydrocarbon chain length, the shorter the hydrocarbon chain length was, the larger the increment was. The transition width in the DSC peak was broadened with an increase in PEG concentration. These three above-mentioned effects are the main differences between the effects of the osmotic stress on the phase transition of MLVs and those of hydrostatic pressure. On the other hand, ethylene glycol (EG), which is the monomer of PEG, had a biphasic effect on transition temperature of DPPC-, DSPC-, and DMPC-MLV, reducing Tm and Tp at low concentrations, but increasing Tm and extinguishing pretransition at high concentrations. This is explained by the induction of an interdigitated gel phase at high concentrations of EG, which indicates that EG can easily penetrate into the head group region of the lipid, in contrast with PEG 6K, because EG is small. Temperature-EG concentration phase diagrams for the various PC-MLVs were determined.(ABSTRACT TRUNCATED AT 250 WORDS)

Calorimetry, Differential Scanning↗
PubMed2006

The D-type cyclin CYCD3;1 is limiting for the G1-to-S-phase transition in Arabidopsis.

The G1-to-S-phase transition is a key regulatory point in the cell cycle, but the rate-limiting component in plants is unknown. Overexpression of CYCLIN D3;1 (CYCD3;1) in transgenic plants increases mitotic cycles and reduces endocycles, but its effects on cell cycle progression cannot be unambiguously determined. To analyze the cell cycle roles of plant D-type cyclins, we overexpressed CYCD3;1 in Arabidopsis thaliana cell suspension cultures. Changes in cell number and doubling time were insignificant, but cultures exhibited an increased proportion of G2- over G1-phase cells, as well as increased G2 arrest in response to stationary phase and sucrose starvation. Synchronized cultures confirm that CYCD3;1-expressing (but not CYCD2;1-expressing) cells show increased G2-phase length and delayed activation of mitotic genes such as B-type cyclins, suggesting that CYCD3;1 has a specific G1/S role. Analysis of putative cyclin-dependent kinase phosphorylation sites within CYCD3;1 shows that mutating Ser-343 to Ala enhances CYCD3;1 potency without affecting its rate of turnover and results in a fivefold increase in the level of cell death in response to sucrose removal. We conclude that CYCD3;1 dominantly drives the G1/S transition, and in sucrose-depleted cells the decline in CYCD3;1 levels leads to G1 arrest, which is overcome by ectopic CYCD3;1 expression. Ser-343 is likely a key residue in modulating CYCD3;1 activity in response to sucrose depletion.

Aphidicolin↗
PubMed2000

Nonequilibrium phase transition in a model for social influence

We present extensive numerical simulations of the Axelrod's model for social influence, aimed at understanding the formation of cultural domains. This is a nonequilibrium model with short range interactions and a remarkably rich dynamical behavior. We study the phase diagram of the model and uncover a nonequilibrium phase transition separating an ordered (culturally polarized) phase from a disordered (culturally fragmented) one. The nature of the phase transition can be continuous or discontinuous depending on the model parameters. At the transition, the size of cultural regions is power-law distributed.

Journal Article↗
PubMed2005

Real-space observation of nanoscale inhomogeneities and fluctuations in a phase transition of a surface quasi-one-dimensional system: In/Si(111).

We report direct visualizations of the fluctuation and condensation phenomena in a phase transition of a one-dimensional (1D) In/Si(111) system using scanning tunneling microscopy. The high-temperature (HT) and low-temperature (LT) phases are found to coexist on the nanometer scale near Tc. Above Tc, 1D LT-phase stripes fluctuate in the HT phase and coalesce into 2D islands with decreasing temperature. They condense to make the LT phase below Tc. Small areas of the HT phase also exist below Tc. The observed temperature-dependent evolution of the nanoscale inhomogeneities is consistent with the theoretical predictions for a second-order phase transition.

Journal Article↗
PubMed1994

Modulation by gangliosides of the lamellar-inverted micelle (hexagonal II) phase transition in mixtures containing phosphatidylethanolamine and dioleoylglycerol.

We studied the effect of gangliosides GD1a and GM1 on the lamellar-to-hexagonal II phase transition of mixtures of dioleoylphosphatidylethanolamine/dioleoylphosphatidyl choline, 3:1, and of transphosphatidylated phosphatidylethanolamine with dioleoylglycerol by high-sensitivity differential scanning calorimetry, 31P-NMR, and pyrene fluorescence of a phosphatidylcholine probe. Gangliosides had a dual effect. Below 1 mol % ganglioside the hexagonal II phase transition was affected but still occurred at lower temperature than in the absence of gangliosides. The presence of between 1 and 2 mol % gangliosides increased the temperature for formation of the hexagonal II phase and progressively decreased its cooperativity. Above 3 mol % gangliosides totally inhibited the formation of both the temperature-induced and composition-induced hexagonal phase, probably by opposing the geometric distortions necessary for the inverted micellar structures.

Diglycerides↗
PubMed1996

Membrane phase transition of intact human platelets: correlation with cold-induced activation.

Using Fourier transform infrared spectroscopy (FTIR), we have determined the phase transition temperature (Tm) of lipids in intact human platelets and have shown that it occurs between 15 and 18 degrees C, the temperature at which cold activation of platelets has previously been reported (Zucker and Borrelli, 1954, Blood, 28:602-608; White and Krivit, 1967, Blood, 30:625-635). The temperature at which the platelets pass through Tm is highly correlated with initial platelet shape change. However, shape change continues after the cells have passed through the phase transition. Cold-induced activation has previously prevented long-term storage of platelets at 4 degrees C. Antifreeze glycoproteins (AFGPs) isolated from polar fishes previously have been used to prevent ice crystal growth during freezing of tissues as well as leakage of solutes from liposomes as they were chilled through their Tm. We sought to determine if these AFGPs were able to stabilize platelets for long-term storage at 4 degrees C. Incubating platelets with antifreeze glycoproteins during long-term storage and rapid rewarming to 37 degrees C abrogated granule secretion associated with cold activation in a dose-dependent manner. This work suggests that AFGPs may be a possible solute for use in long-term low temperature storage of platelets.

Antifreeze Proteins↗
PubMed2006

Holographic phase transitions with fundamental matter.

The holographic dual of a finite-temperature gauge theory with a small number of flavors typically contains D-brane probes in a black hole background. At low temperature, the branes sit outside the black hole and the meson spectrum is discrete and possesses a mass gap. As the temperature increases, the branes approach a critical solution. Eventually, they fall into the horizon and a phase transition occurs. In the new phase, the meson spectrum is continuous and gapless. At large Nc and large 't Hooft coupling, we show that this phase transition is always first order. In confining theories with heavy quarks, it occurs above the deconfinement transition for the glue.

Journal Article↗
PubMed2000

Inversion in the change of the refractive index and memory effect near the nematic-isotropic phase transition in a lyotropic liquid crystal.

This work demonstrates the occurrence of dn/dT inversion from negative to positive near the nematic-isotropic phase transition in a lyotropic liquid crystal. It is suggested that this effect can be attributed to a sudden increase of the electronic polarizability due to a change in the micelle shape near this phase transition. Formation of a long lasting lenslike element within the sample when it is irradiated at moderately high laser powers is also reported. This permanent lens is erasable by increasing the temperature above the nematic-isotropic transition temperature.

Crystallization↗
PubMed1980

Asymmetric antagonistic effects of an inhalation anesthetic and high pressure on the phase transition temperature of dipalmitoyl phosphatidic acid bilayers.

The phase transition temperature (Tt) of dipalmitoyl phosphatidic acid multilamellar liposomes is depressed 10 degrees C by the inhalation anesthetic methoxyflurane at a concentration of 100 mmol/mol lipid. Application of 100 atm of helium pressure to pure phosphatidic acid liposomes increased Tt only 1.5 degrees C. However, application of 100 atm helium pressure to dipalmitoyl phosphatidic acid lipsomes containing 100 mmol methoxyflurane/mol lipid almost completely antagonized the effect of the anesthetic. A non-linear pressure effect is observed. In a previous study, a concentration of 60 mmol methoxyflurane/mol dipalmitoyl phosphatidylcholine depressed Tt only 1.5 degrees C, exhibiting a linear pressure effect. The completely different behavior in the charged membrane is best explained by extrusion of the anesthetic from the lipid phase.

Atmospheric Pressure↗
PubMed1984

The effects of water-soluble solutes on the phase transitions of phospholipids.

A previous treatment [Sturtevant, J.M. (1982) Proc. Natl. Acad. Sci. USA 79, 3963-3967], based on ideal solution theory, of the influence of small molecules on the main phase transitions of phospholipid bilayers is extended to include the effect of water solubility of the added solute. This effect is manifested primarily in a sharpening of the phase transition, resulting from a buffering by the aqueous phase of the concentration of the solute in the lipid phases.

Lipid Bilayers↗
PubMed1998

Radicals as probes of ferroelectric phase transitions in EPR investigations of pyridinium tetrafluoroborate.

This paper reports CW-EPR studies of pyridinium tetrafluoroborate undergoing solid-solid phase transitions at 240 and 205 K, of which the one occurring at the higher temperature was found to be paraelectric-ferroelectric. EPR spectra of gamma-irradiated polycrystalline samples were taken at room temperature, at the liquid nitrogen temperature, and at temperatures ranging from 163 to 273 K in the cycles of cooling and heating. The radical appearing under the effect of gamma irradiation was established to be formed by attachment of a hydrogen atom to the carbon atom C(2) in the pyridine ring. The unpaired electron was found to delocalize onto the pyridine ring. The g factor was established as 2.0021(4) and the values of the hyperfine splitting constants testify to the occurrence of an alpha-type interaction, Aalpha = 0.98(5) mT, of the unpaired electron with protons at the C(3), C(5), and N(1) positions, two beta1- and beta2-type interactions with the hydrogen nuclei at the C(2) position, Abeta1 = 6.68(5) mT and Abeta2 = 4.85(5) mT, and the interaction with the nitrogen nucleus at N(1), AN = 0.86(5) mT. The complex character of the spectra obtained prompted us to employ computer analysis of the signals to enhance their resolution. Analysis of the temperature dependence of the linewidth parameter of individual components of the pyridine radical spectrum shows that EPR together with the computer resolution enhancement method can be a useful method for studying the nature of phase transitions in pyridinium tetrafluoroborate.

Electron Spin Resonance Spectroscopy↗