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At least 55 records · Page 3Linked to original sources
PubMed2002

Theory of bent-core liquid-crystal phases and phase transitions.

We study phases and phase transitions that can take place in the recently discovered bow-shaped or bent-core liquid-crystal molecules. We show that to completely characterize phases exhibited by such bent-core molecules a third-rank tensor T(ijk) order parameter is necessary in addition to the vector and the nematic (second-rank) tensor order parameters. We present an exhaustive list of possible liquid phases, characterizing them by their space-symmetry group and order parameters, and catalog the universality classes of the corresponding phase transitions that we expect to take place in such bent-core molecular liquid crystals. In addition to the conventional liquid-crystal phases such as the nematic phase, we predict the existence of other liquid phases, including the spontaneously chiral nematic (N(T)+2)(*) and chiral polar (V(T)+2)(*) phases, the orientationally ordered but optically isotropic tetrahedratic T phase, and a nematic N(T) phase with D(2d) symmetry that is neither uniaxial nor biaxial. Interestingly, the isotropic-tetrahedratic transition is continuous in mean-field theory, but is likely driven first order by thermal fluctuations. We conclude with a discussion of smectic analogs of these phases and their experimental signatures.

Journal Article↗
PubMed2003

Monte Carlo studies of three-dimensional O1 and O4 phi4 theory related to Bose-Einstein condensation phase transition temperatures.

The phase transition temperature for the Bose-Einstein condensation (BEC) of weakly interacting Bose gases in three dimensions is known to be related to certain nonuniversal properties of the phase transition of three-dimensional O(2) symmetric phi(4) theory. These properties have been measured previously in Monte Carlo lattice simulations. They have also been approximated analytically, with moderate success, by large N approximations to O(N) symmetric phi(4) theory. To begin investigating the region of validity of the large N approximation in this application, the same Monte Carlo technique developed for the O(2) model [P. Arnold and G. Moore, Phys. Rev. E 64, 066113 (2001)] to O(1) and O(4) theories has been applied. The results indicate that there might exist some theoretically unanticipated systematic errors in the extrapolation of the continuum value from lattice Monte Carlo results. The final results show that the difference between simulations and next-to-leading order large N calculations does not improve significantly from N=2 to N=4. This suggests that one would need to simulate yet larger N's to see true large N scaling of the difference. Quite unexpectedly (and presumably accidentally), the Monte Carlo result for N=1 seems to give the best agreement with the large N approximation among the three cases.

Journal Article↗
PubMed1981

On kinetics of phase transitions in cell membranes.

Phase transitions in a bicomponent lipid membrane are considered. It is shown that in this case metastable states practically do not arise and phase transitions are smooth and hysteresisless. An elastic frame on the surface of the membrane changes the character of phase transitions: they become sharp and hysteretic. The role of membrane phase transitions for regulation of cell processes is considered.

Cell Membrane↗
PubMed1982

An assessment of phase transitions in soybean membranes.

Phase transitions were measured in vesicles of phospholipids, alone and in various combinations, and in pelleted mitochondrial membranes, using thermal (DSC) and optical methods. The objective was to consider their possible involvement in chilling injury of soybeans (Glycine max [L.] Merr. cv Wayne 1977). Saturated phospholipids showed clear transitions in the temperature range of 50 degrees C to near 0 degrees C. When mixtures of two phospholipids were examined, there was a marked lowering and broadening of the transition peaks, and a shift in the transition temperatures to intermediate temperatures. The unsaturated phospholipids that occur naturally in soybeans showed no detectable phase transitions in this temperature range, alone or in combinations. Examination of the polar lipids from soybean asolectin revealed no transitions in the biological temperature range; the additions of cations such as Ca(2+) and La(3+) did not evoke a detectable phase transition in them. Mitochondrial membrane pellets likewise showed no transitions. The application of these two direct methods of examination of membrane components without the addition of foreign agents did not support the suggested occurrence of a bulk phase transition which could be related to chilling injury in soybeans.

Journal Article↗
PubMed1986

Dynamic approach to thermotropic and inotropic phase transitions.

Thermotropic and inotropic phase transitions have been analysed with a dynamic theory on a self-organization. An equation of motion of a molecular assembly with strong interactions may be approximately described as: dQ/dt' congruent to -K1Q-K3Q3, where Q is a displacement from the equilibrium point Q0(identical to 0) in a vibrational state, K1 is a transition parameter. When the parameter K1 concerned with an internal driving force (partial system) changes from positive to negative through the potential bifurcation, the system transfers to a new stable state breaking down the symmetry. Such a sign change of K1 serves as a trigger to a phase transition. Using Weiss' approximation, we have evaluated the change of K1 by a function of temperature, kappa (T-TC), and have obtained the critical temperature TC of thermotropic phase transition. We have furthermore treated inotropic phase transition caused by the binding of divalent cations like Ca2+ using the function kappa (T-beta TC), where beta is a shift parameter of the critical temperature.

Calcium↗
PubMed1991

The study of lipid-protein interactions: effect of melittin on phase transition of phosphatidylethanolamine and sensitivity of phospholipases to phase state.

The effects of melittin on the bilayer-to-inverted hexagonal (HII) phase transition of egg phosphatidylethanolamine (EPE) and the influence of the phase state of membrane matrix on hydrolysis of EPE by phospholipases have been studied. The phase transitions were measured using the fluorescent probe N-(7-nitro-2,1,3-benzoxadiazol-4-yl)phosphatidylethanolamine (N-NBD-PE) and differential scanning calorimetry. In the presence of melittin at a lipid-to-melittin molar ratio (R1) of 200, 100, and 20, the phase transition of EPE disappeared, indicating that melittin stabilizes the bilayer structure. In the presence of 10 mol% of cholesterol, the phase transition temperature (TH) decreased and TH was observed even in the presence of melittin at R1 of 200 and 100. The fluorescence intensity of the tryptophan residue of melittin is sensitive to the phase transition and the wavelength of emission maxima shift from 352 to 337 nm upon addition of EPE and EPE-cholesterol (10 mol%) at R1 of 200. Kinetic parameters for phospholipase-catalyzed hydrolysis of EPE in bilayer and HII phases showed that HII phase of EPE is a poorer substrate for phospholipases and that cholesterol decreases the susceptibility of EPE to phospholipases.

Enzyme Stability↗
PubMed1999

Experimental observation of a torus-doubling transition to chaos near the ferroelectric phase transition of a KH2PO4 crystal.

A ferroelectric KH2PO4 crystal is implemented in a simple series connection of an RLC circuit and the transition to chaos near the phase-transition temperature is investigated. The torus-doubling scenario to chaos, the theory of which was expounded by Kaneko for high-dimensional dynamical systems, has been found in the crystal. These experimental results suggest that observation of the nonlinear dynamical behavior in condensed matter can give much information about the correlation between the generation of nonlinearity and the order-parameter dynamics of the crystal near the phase-transition temperature.

Journal Article↗
PubMed1991

Isothermal lipid phase transitions.

In liotropic lipid systems phase transitions can be induced isothermally by changing the solvent concentration or composition; alternatively, lipid composition can be modified by (bio)chemical means. The probability for isothermal phase transitions increases with the decreasing transition entropy; it is proportional to the magnitude of the transition temperature shift caused by transformation-inducing system variation. Manipulations causing large thermodynamic effects, such as lipid (de)hydration, binding of protons or divalent ions and macromolecular adsorption, but also close bilayer approach are, therefore, likely to cause structural lipid change(s) at a constant temperature. Net lipid charges enhance the membrane susceptibility to salt-induced isothermal phase transitions; a large proportion of this effect is due to the bilayer dehydration, however, rather than being a consequence of the decreased Coulombic electrostatic interactions. Membrane propensity for isothermal phase transitions, consequently, always increases with the hydrophilicity of the lipid heads, as well as with the desaturation and shortening of the lipid chains. Upon a phase change at a constant temperature, some of the interfacially bound solutes (e.g. protons or calcium) are released in the solution. Membrane permeability and fusogenicity simultaneously increase. In mixed systems, isothermal phase transitions, moreover, may result in lateral phase separation. All this opens up ways for the involvement of isothermal phase transitions in the regulation of biological processes.

Lipid Bilayers↗
PubMed2006

Barotropic phase transition between the lamellar liquid crystal phase and the inverted hexagonal phase of dioleoylphosphatidylethanolamine.

The phase transition between the lamellar liquid crystal (Lalpha) phase and the inverted hexagonal (H(II)) phase of dioleoylphosphatidylethanolamine (DOPE) in aqueous NaCl solutions was observed by means of differential scanning calorimetry (DSC) under ambient pressure and light-transmittance technique under high pressure. The pressure dependence of the transition temperature (dT/dp) and the thermodynamic quantities for the Lalpha/H(II) transition were compared with those of another phase transition found in the DOPE bilayer membrane, which is the transition from the lamellar crystal (Lc) phase to the Lalpha phase. The dT/dp value of the Lalpha/H(II) transition was about 3.5 times as large as that of the Lc/Lalpha transition while the thermodynamic quantities were significantly smaller than those of the latter to the contrary. Comparing the enthalpy and volume behavior of the Lalpha/H(II) transition with that of the Lc/Lalpha transition, we concluded that the Lalpha/H(II) transition can be regarded as the volume-controlled transition for the reconstruction of molecular packing.

Calorimetry, Differential Scanning↗
PubMed1997

Phase transition between hexagonal II (H[II]) and liquid-crystalline phase induced by interaction between solvents and segments of the membrane surface of dioleoylphosphatidylethanolamine.

We have investigated effects of several water-soluble organic solvents such as acetone, acetonitrile, and ethanol, which also have high solubility in alkane, on the structure and phase behavior of dioleoylphosphatidylethanolamine (DOPE) dispersion. X-ray diffraction data indicated that a phase transition from hexagonal II (H[II]) to liquid-crystalline (L alpha) phase in DOPE dispersion, occurred at 13% (v/v) acetone in H2O at 20 degrees C. The temperature of the L alpha-H(II) phase transition of DOPE dispersion increased with an increase in acetone concentration, and it was 37 degrees C at 20% (v/v) acetone. These results indicated that acetone stabilized L alpha phase relative to H(II) phase. Similar results were obtained in interactions of DOPE dispersions in H2O with acetonitrile or ethanol. X-ray diffraction data indicated that the H(II)-L alpha phase transition occurred at 9.0% (v/v) acetonitrile or at 9.8% (v/v) ethanol in water at 20 degrees C. The L alpha-H(II) phase transition temperature of DOPE dispersion increased with an increase in acetonitrile or ethanol concentration, and it was 66 degrees C at 20% (v/v) acetonitrile. Substitution of H2O by D2O (deuterium oxide) increased their threshold concentrations of the H(II)-L alpha phase transition induced by these organic solvents. A mechanism of these phase transitions and the effect of the substitution of H2O by D2O is proposed and discussed; an interaction free energy between solvents and the hydrophobic segments of the alkyl chains in the membrane surface, and also a packing parameter of the phospholipid may be main factors to explain these phenomena reasonably.

Acetone↗