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PubMed2003

Orientational phase transition and the solvation force in a nematic liquid crystal confined between inhomogeneous substrates.

A nematic liquid crystal confined between two identical flat solid substrates, with an alternating stripe pattern of planar and homeotropic anchoring, is studied in the framework of the Frank-Oseen theory. By means of numerical minimization of the free energy functional we study the effect of the sample thickness D on the location of the phase transition between a uniform alignment, either planar or homeotropic, and a distorted nematic texture. The solvation force f due to distortions of the nematic director is also studied. It is found that f is always attractive, and for D small compared to the periodicity of the surface structure it exhibits two distinct asymptotic behaviors: f approximately -D(-1/2) or f approximately -D(-1), depending on the relation between D and the extrapolation lengths.

Journal Article↗

PubMed1987

Calcium-induced fusion of didodecylphosphate vesicles: the lamellar to hexagonal II (HII) phase transition.

Electron microscopic techniques have been employed to investigate the ability of didodecylphosphate vesicles (diameter approx. 900 A) to fuse in the presence of Ca2+. As revealed by negative staining, Ca2+ induces extensive fusion and large vesicles with diameters up to 7000 A are formed. In a process secondary to fusion, the fused vesicles display a tendency to flatten and are subsequently transformed into extended tubular structures. Freeze-fracture electron microscopy, in conjunction with 31P NMR and selected area electron diffraction measurements indicate that the tubes are packed in a hexagonal (HII) array and that the amphiphiles are converted from the lamellar to the hexagonal HII phase. The relationship between membrane fusion and the lamellar-to-hexagonal phase transition is discussed in terms of formation and abundance of transiently stable inverted micellar intermediates at contact regions between two interacting membranes. A model for the conversion of the (vesicular) lamellar into the (tubular) hexagonal HII phase is presented, taking into account the molecular shape of the amphiphile. The relevance of using simple synthetic amphiphiles as models for phospholipid bilayers and complex biomembrane behavior is briefly discussed.

Calcium↗

PubMed1991

Effects of small organic molecules on phospholipid phase transitions.

Small organic molecules are known to exhibit a wide spectrum of physiological or pharmacological effects and many of them are thought to be membrane associated. Therefore a great number of studies is devoted to the interaction between these molecules and phospholipid model membranes. Results obtained for molecular species of varying hydrophobic/hydrophilic balances will be described. It will be shown that, in general, these different molecules induce similar effects on phospholipid phase transitions, although they are located differently in the membrane. Detailed studies of these interactions will help to understand these processes on a molecular level.

Alcohols↗

PubMed1992

Optical and physical properties of wet-spun films of Na-hyaluronate. Evidence of a phase transition.

The refractive indices, water content, and volume of wet-spun films of Na-hyaluronate have been measured as a function of relative humidity (rh). These data are used with the Lorentz-Lorenz formula to determine the optical polarizabilities of Na-hyaluronate parallel and perpendicular to the helical axis. The analysis reveals a drop in the optical polarizabilities of approximately 20% between 80 and 88% rh, indicating a phase transition.

Carbohydrate Conformation↗

PubMed2002

Phase transition theory of many-mode ordering and pulse formation in lasers.

A novel theory for the ordering of many interacting modes in lasers is presented. By exactly solving a Fokker-Planck equation for the distribution of waveforms in the laser in steady state, equivalence of the system to a canonical ensemble is established, where the role of temperature is taken by amplifier noise. Passive mode locking is obtained as a phase transition of the first kind and threshold is calculated, employing mean field theory backed up by a numerical study. For zero noise, compliance with the existing noiseless theory is shown.

Journal Article↗

PubMed2005

Phase transitions in the liquid-vapor interface of dilute alloys of Bi in Ga: new experimental studies.

We report the results of measurements of x-ray reflectivity and grazing incidence x-ray diffraction from the liquid-vapor interfaces of four dilute alloys of Bi in Ga with mole fractions x(Bi)=0.0032, 0.0023, 0.00037, and 0.000037. The monolayer coverage of the alloys with x(Bi)=0.0023, and x(Bi)=0.00037 is about 0.85 and only very slightly temperature dependent. The monolayer coverage in the lowest-concentration alloy, with x(Bi)=0.000037, ranged from 0.82 at 29 degrees C to 0.58 at 110 degrees C. In none of these alloys, down to the lowest temperature used, 29 degrees C, can we find any evidence for crystallization of the Bi monolayer that segregates as the outermost stratum of the liquid-vapor interface. Drawing on theoretical arguments we propose that the transitions inferred from the second-harmonic generation and plasma generation studies of dilute Bi in Ga alloys are from the liquid state to the hexatic state of the Bi monolayer. The data for the alloy with x(Bi)=0.000037 suggest that near 80 degrees C there is a disordered phase-to-disordered phase transition.

Journal Article↗

PubMed1976

Electrostatic free energy and shift of the phase transition for charged lipid membranes.

For a charged membrane in an electrolyte solution the electrostatic free energy is derived treating the system as a diffuse double layer. The dependence of the energy on external parameters like surface charge density and temperature is obtained and the physical basis discussed. As an application the charges are shown to exert an electrostatic surface pressure on the lipid chain packing which leads to a shift in the phase transition of membranes. The results confirm the interpretation of experimental data as given by Träuble et al. in the accompanying paper.

Lipids↗

PubMed2002

Interplay between time-temperature transformation and the liquid-liquid phase transition in water.

We study the new water model proposed by Mahoney and Jorgensen [J. Chem. Phys. 112, 8910 (2000)], which is closer to real water than previously proposed classical pairwise additive potentials. We simulate the model in a wide range of deeply supercooled states and find (i) the existence of a nonmonotonic "nose-shaped" temperature of maximum density line and a nonreentrant spinodal, (ii) the presence of a low-temperature phase transition, (iii) the free evolution of bulk water to ice, and (iv) the time-temperature-transformation curves at different densities.

Journal Article↗

PubMed1988

A laser-induced europium (III) ion luminescence study of the interaction of this ion with phospholipid bilayer vesicles above and below the gel to liquid-crystalline phase transition temperature.

Laser-induced europium(III) luminescence spectroscopy was used to investigate the formation and integrity of phospholipid bilayer vesicles produced by ultrasonication and detergent dialysis. Eu(III) ion interactions with these model biological membrane systems were explored. 7F0----5D0 spectral and excited-state lifetime data reveal two distinct, temperature-dependent binding sites, one involving a weak, 'superficial' interaction with the phosphate moiety of the phosphatidylcholine head group, the other involving a more tightly bound ion in a relatively dehydrated region of the head group. This latter 'sequestered' species appeared only at temperatures equal to or below that of the gel----liquid-crystalline phase transition. Systems containing various amounts of cholesterol showed a decrease in the formation of the sequestered species, indicative of a decrease in ion permeability. The results of this study demonstrate that the Eu(III) luminescence technique is useful for detecting major phase alterations in phospholipid bilayer vesicles.

Europium↗

PubMed1994

On the main phase transition temperatures of highly asymmetric mixed-chain phosphatidylcholines.

The highly asymmetric mixed-chain phosphatidylcholines or C(X):C(Y)PC, which can self-assemble at T < Tm into the mixed interdigitated gel-state bilayer in excess water, can be divided into two groups. Group I consists of C(X):C(Y)PC with X > Y, and Group II consists of C(X):C(Y)PC with X < Y. The main phase transition temperatures (Tm) of these C(X):C(Y)PC have been analyzed in terms of two structural parameters (delta and Nf). Specifically, these structural parameters are related to the packing geometry of the lipid's acyl chains as the lipid molecules are packed either in a mixed (delta) or a hypothetically partially (Nf) interdigitated packing motif at T < Tm. Based on 28 and 29 Tm values of Group I and II C(X):C(Y)PC, two general equations are derived, respectively, by multiple regression analyses. These equations correlate systematically the Tm values of Group I and II phosphatidylcholines with their corresponding structural parameters. Using these two derived equations, the Tm values of a total of 92 molecular species of C(X):C(Y)PC are generated, and these calculated Tm values can be considered as the reliably predicted Tm values for highly asymmetric C(X):C(Y)PC which have delta C/CL values within the range of 0.42-0.66.

Calorimetry, Differential Scanning↗

PubMed2005

Relation between the Widom line and the dynamic crossover in systems with a liquid-liquid phase transition.

We investigate, for two water models displaying a liquid-liquid critical point, the relation between changes in dynamic and thermodynamic anomalies arising from the presence of the liquid-liquid critical point. We find a correlation between the dynamic crossover and the locus of specific heat maxima C(P)(max) ("Widom line") emanating from the critical point. Our findings are consistent with a possible relation between the previously hypothesized liquid-liquid phase transition and the transition in the dynamics recently observed in neutron scattering experiments on confined water. More generally, we argue that this connection between C(P)(max) and dynamic crossover is not limited to the case of water, a hydrogen bond network-forming liquid, but is a more general feature of crossing the Widom line. Specifically, we also study the Jagla potential, a spherically symmetric two-scale potential known to possess a liquid-liquid critical point, in which the competition between two liquid structures is generated by repulsive and attractive ramp interactions.

Journal Article↗

PubMed2000

Phase transitions above the Yrast line in 154Dy.

Spectra of the E2 quasicontinuum gamma rays feeding different spin regions of the 154Dy yrast line have been extracted. These are compared with corresponding theoretical spectra obtained by numerical simulations based on temperature-dependent Hartree-Fock theory, with thermal shape fluctuations. In this manner, different regions of the spin-energy plane can be examined. The results support the predictions of a smeared-out phase transition at high spin above the yrast line.

Journal Article↗

PubMed2002

Properties of a "phase transition" induced by antiangiogenetic therapeutical protocols.

Inhibiting angiogenesis has been found to be an interesting therapeutical strategy against cancer. In fact, the success of tumor growth is subordinated to the corresponding increase of the vascular system feeding the neoplasm. However, optimization and design of proper antiangiogenetic therapeutical strategies is still an open problem. We apply a recently developed angiogenesis model to study how variations in the relevant parameters, e.g., induced by chemicals, may cause a "phase transition" to a region in the parameter space in which angiogenesis is not succesful. To demonstrate the reliability of our approach and its usefulness, we will study some specific drugs and use our model to investigate the influence of the main variables involved in a clinical treatment: the administration time, the duration of the drug effect, and the drug dose.

Angiogenesis Inhibitors↗

PubMedDate not supplied

[Salt-induced critical type phase transition in a water-protein matrix of serum albumin molecules].

Effective thermodynamic parameters of activation of label transition between two microsurroundings in water-protein matrix (WPM) of spin-labelled molecules of serum albumin (HSA-SL) in 0.001 M phosphate buffer, pH 7.3 within concentration range of NaCl (m3) (10(-3) M to 2 M and concentration range of protein (m2) 50 to 200 mg/ml were determined. The phase transition (PT) between two structures of water in WPM is revealed within the m3 range 0.01 M to 0.1 M. It is close to the type I PT and more expressed at high protein concentrations. As m3 increases (to 0.2 M when m2 = 50 mg/ml and to 0.7 when m2 = 200 mg/ml) PT-I is transformed into PT of the critical type. This is indicated to zero values of the higher order derivatives of Gibbs energy of activation with respect to salt concentration, as well as by the maximum values of positive disjoining pressure in these points. The biological significance of this phenomenon is discussed.

Humans↗

PubMed1996

Phase transitions and the molecular mechanism of contraction.

In this paper, the rotating cross-bridge mechanism for muscle contraction is discussed and much contradictory evidence is put forward. As an alternative, a model is given in which the motor of muscle contraction is placed in the myosin-rod hinge and/or in the actin filament. No definite choice for one of the proposed models can be made yet, although it is clear that some kind of phase transition plays an important role in the mechanism.

Amino Acid Sequence↗

PubMed1980

Statistical mechanical analysis of Raman spectroscopic order parameter changes in pressure-induced lipid bilayer phase transitions.

The statistical mechanical cluster theory of Fisher as applied by Kanehisa and Tsong to phospholipid bilayers is modified to describe the effects of hydrostatic pressure on the state of an aqueous dispersion of the phospholipid dipalmitoyl phosphatidylcholine. A high pressure Raman scattering cell has been built to obtain the Raman spectra of aqueous dispersions of phospholipids as a function of the applied hydrostatic pressure from 0 to 100 atmospheres. Predicted thermal and pressure-induced phase transitions are compared with an experimentally obtained Raman order parameter derived from the ratio of two bands in the C-H stretching region of the Raman spectrum of the sample. The parameters of the theory are adjusted to obtain a satisfactory fit of the Raman order parameter versus temperature. The theory is then found to give an excellent prediction of the observed pressure dependence of the Raman order parameter with no changes in the adjustable parameters. The implications of the success of the theoretical fit is discussed. Particularly of interest is the rather high value of the critical temperature, Tc, for lipid bilayers which is predicted by the model.

Hydrostatic Pressure↗

PubMed2005

Concomitant dimorphs of tri-O-[p-halobenzoyl]-myo-inositol 1,3,5-orthoformates with different halogen bonding contacts: first order crystal-to-crystal thermal phase transition of kinetic form to the thermodynamic form.

Crystallization of tri-O-[p-halobenzoyl]-myo-inositol 1,3,5-orthoformates from ethyl acetate-petroleum ether solution produced concomitant dimorphs that have different halogen bonding contacts; the kinetic form with C-Br...O-C contacts upon heating to 185 degrees C, converts completely to the thermodynamic form with C-Br...O=C contacts via crystal-to-crystal first order phase transition.

Journal Article↗

PubMed2004

Evidence for a quantum phase transition in Pr2-xCe(x)CuO4-delta from transport measurements.

The doping and temperature dependences of the Hall coefficient, R(H), and ab-plane resistivity in the normal state down to 350 mK is reported for oriented films of the electron-doped high-T(c) superconductor Pr(2-x)Ce(x)CuO(4-delta). The doping dependences of beta (rho=rho(0)+ATbeta) and R(H) (at 350 mK) suggest a quantum phase transition at a critical doping near x=0.165.

Journal Article↗