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PubMed2000

On the Measurement of Phase Transition Temperatures in Physical Molecular Organogels.

Methods of measurement of the phase transition temperatures in physical molecular gels are analyzed. The "falling ball" technique, NMR, and rheology are compared. Temperature versus concentration phase diagrams can be easily obtained, and the reliability and workability of the methods are discussed. It appears that rheology is the more accurate and convenient technique while the falling ball method provides acceptable DeltaH, DeltaS values, but the melting temperature values are significantly altered. With NMR, kinetic information associated with the molecular aggregation process can also be extracted. Copyright 2000 Academic Press.

Journal Article↗
PubMed2004

Behavior of the layer compression modulus as a function of frequency near the nematic-smectic-A and re-entrant nematic-smectic-A phase transitions.

A progressive cross-over of the layer compression modulus B from a simple power law behavior to a saturation behavior when the frequency is increased from 1 Hz to 2 x 10(3) Hz is observed for the first time near the nematic-smectic-A and the re-entrant nematic-smectic-A phase transitions of a mixture of 4-cyano-4'-( n-hexyloxy)biphenyl and 4-cyano-4'-( n-octyloxy)biphenyl (6OCB-8OCB). This result shows that the saturation of B determined by second sound near the nematic-smectic-A phase transition of a similar mixture is dynamic in nature, and cannot be associated with the static saturation effect predicted by the dislocation loop model, as proposed previously.

Journal Article↗
PubMed1977

Conjugated polyene fatty acids as fluorescent probes: biosynthetic incorporation of parinaric acid by Escherichia coli and studies of phase transitions.

The use of the fluorescent fatty acid, parinaric acid (9, 11, 13, 15-octadecatetraenoic acid) (PnA), was studied in cells of an unsaturated fatty acid auxotroph of Escherichia coli. Growth conditions were found that permitted biosynthetic incorporation of PnA (up to 3%) into membrane phospholipids during growth on oleic or elaidic acid. Fluorescence measurements of incorporated PnA revealed phase transitions in cells, membranes, and phospholipids at temperatures that reflected the fatty acid composition of the sample. Transitions had a well-defined onset from high temperature, while the lower and end point was less well defined. cis- and trans-PnA (cis, trnas, trans, cis, and all trans, respectively) gave comparable results. Similar phase transitions were detected with PnA, which was not biosynthetically incorporated. Fluorescence of tryptophan was measured in E. coli membranes as a function of concentration of PnA. Significant quenching of tryptophan fluorescence by PnA was observed.

Cell Membrane↗
PubMed2005

Direct observation of geometrical phase transitions in mesoscopic superconductors by scanning tunneling microscopy.

Using scanning tunneling microscopy, we mapped the distribution of the local density of states in a single crystal superconductor heterostructure with an array of submicron normal metal islands. We observe the coexistence of strongly interacting multiquanta vortex lattice with interstitial Abrikosov vortices. The newly formed composite magnetic flux structure undergoes a series of phase transitions between different topological configuration states. The vortex configuration states are strongly dependent on the number of flux quanta and the nanoscale confinement architecture of the mesoscopic superconductor. Here, we present images of vortex phase transitions due to confinement effects when the number of magnetic flux quanta in the system changes. The vortex dynamics in these systems could serve as a model for behavior of confined many-body systems when the number of particles changes.

Journal Article↗
PubMed1980

Kinetic investigations on the phase transition of phospholipid bilayers.

Pressure-jump experiments were performed on vesicles and liposomes of dimyristoyl phosphatidylcholine and dipalmitoyl phosphatidylcholine following the time course of solution turbidity. For both lipids two relaxation effects were evaluated the time constants of which exhibit clear maxima at the mid-point of the phase transition. The time constants lie for vesicles in the 100 microseconds and 1 ms ranges and for liposomes in the 1 ms and 10 ms ranges. The processes are slightly faster for dimyristoyl phosphatidylcholine than for dipalmitoyl phosphatidylcholine. All relaxation times are concentration-independent. The time constant and amplitude behaviours indicate that all processes are cooperative in agreement with previous interpretations. It is demonstrated that cooperative units can be evaluated from the relaxation amplitudes. These are of the same order of magnitude as those obtained from static experiments. On the grounds of the present kinetic investigation we can state that the application of the linear Ising model to two-dimensional processes as attempted for the static lipid phase transition is inadequate.

Kinetics↗
PubMed2006

Effects of phospholipid hydrolysis on the aggregate structure in DPPC/DSPE-PEG2000 liposome preparations after gel to liquid crystalline phase transition.

Upon storage of phospholipid liposome samples, lysolipids, fatty acids, and glycerol-3-phosphatidylcholine are generated as a result of acid- or base-catalyzed hydrolysis. Accumulation of hydrolysis products in the liposome membrane can induce fusion, leakage, and structural transformations of the liposomes, which may be detrimental or beneficial to their performance depending on their applications as, e.g., drug delivery devices. We investigated in the present study the influence of phospholipid hydrolysis on the aggregate morphology of DPPC/DSPE-PEG2000 liposomes after transition of the phospholipid membrane from the gel phase to liquid crystalline phase using high performance liquid chromatography (HPLC) in combination with static light scattering, dynamic light scattering, and cryo-transmission electron microscopy (cryo-TEM). The rates of DPPC hydrolysis in DPPC/DSPE-PEG2000 liposomes were investigated at a pH of 2, 4, or 6.5 and temperatures of 22 degrees C or 4 degrees C. Results indicate that following phase transition, severe structural reorganizations occurred in liposome samples that were partially hydrolyzed in the gel phase. The most prominent effect was an increasing tendency of liposomes to disintegrate into membrane discs in accordance with an increasing degree of phospholipid hydrolysis. Complete disintegration occurred when DPPC concentrations had decreased by, in some cases, as little as 3.6%. After extensive phospholipid hydrolysis, liposomes and discs fused to form large bilayer sheets as well as other more complex bilayer structures apparently due to a decreased ratio of lysolipid to palmitic acid levels in the liposome membrane.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed2003

Second order phase transition in neural rate coding: binary encoding is optimal for rapid signal transmission.

Here, we derive optimal tuning functions for minimum mean square reconstruction from neural rate responses subjected to Poisson noise. The shape of these tuning functions strongly depends on the length T of the time window within which action potentials (spikes) are counted in order to estimate the underlying firing rate. A phase transition towards pure binary encoding occurs if the maximum mean spike count becomes smaller than approximately three. For a particular function class, we prove the existence of a second-order phase transition. The analytically derived critical decoding time window length is in precise agreement with numerical results. Our analysis reveals that binary rate encoding should dominate in the brain wherever time is the critical constraint.

Action Potentials↗
PubMed1981

Function of phospholipids in Escherichia coli. Influence of changes in polar head group composition on the lipid phase transition and characterization of a mutant containing only saturated phospholipid acyl chains.

The cls mutation conferring a defect in cardiolipin synthesis (Pluschke, G., Hirota, Y., and Overath, P. (1978) J. Biol. Chem. 253, 5048-5055) has been introduced into an Escherichia coli strain defective in unsaturated fatty acid synthesis in order to study the effect of changes in polar head group composition on the ordered in equilibrium fluid phase transition of the membrane phospholipids. The defect in cardiolipin formation is compensated by an increase in phosphatidylglycerol content, resulting in a decrease of the midpoint of the phase transition by 6 degrees C. Starvation of the cls mutant strain for the unsaturated fatty acid supplement leads to the incorporation of saturated acyl chains of reduced average length into the phospholipids, and growth is inhibited although the membrane remains in a fluid state. A revertant of this strain is described which retains the parental fabB-, fadE-, and cls markers and grows in the absence of an unsaturated fatty acid supplement. A cls+ derivative of the revertant can multiply in a restricted temperature range (35-43 degrees C). It contains only saturated phospholipid acyl chains of an anomalously short average length of 14 carbon atoms but has the same polar head group composition as wild type E. coli. The results demonstrate that, under defined conditions, saturated acyl chains of reduced length are functionally equivalent to unsaturated chains.

Cardiolipins↗
PubMed2003

Continuous quantum phase transition in a Kndo lattice model.

We study the magnetic quantum phase transition in an anisotropic Kondo lattice model. The dynamical competition between the RKKY and Kondo interactions is treated using an extended dynamic mean field theory appropriate for both the antiferromagnetic and paramagnetic phases. A quantum Monte Carlo approach is used, which is able to reach very low temperatures, of the order of 1% of the bare Kondo scale. We find that the finite-temperature magnetic transition, which occurs for sufficiently large RKKY interactions, is first order. The extrapolated zero-temperature magnetic transition, on the other hand, is continuous and locally critical.

Journal Article↗
PubMed2004

Quantum phase transition in an atomic bose gas with a feshbach resonance.

We show that in an atomic Bose gas near a Feshbach resonance a quantum phase transition occurs between a phase with only a molecular Bose-Einstein condensate and a phase with both an atomic and a molecular Bose-Einstein condensate. We show that the transition is characterized by an Ising order parameter. We also determine the phase diagram of the gas as a function of magnetic field and temperature: the quantum critical point extends into a line of finite temperature Ising transitions.

Comment↗
PubMed2005

Phase transition in the link weight structure of networks.

When transport in networks follows the shortest paths, the link weights are shown to play a crucial role. If the underlying topology with nodes N is not changed and if the link weights are independent from each other, then we show that, by tuning the link weights, a phase transition occurs around a critical extreme value index alphac of the link weight distribution alpha<alphac. If the extreme value index of the link weight distribution , transport in the network traverses many links whereas for , all transport flows over a critical backbone consisting of N-1 links. For connected Erdös-Rényi random graphs Gp(N) and square lattices, we have characterised the phase transition and found that alphac approximately =bN(-beta) with betaGp(N) and betalattice approximately = 0.62.

Journal Article↗
PubMed2001

Phase transitions in n-alkylammonium dihydrogenphosphates and -arsenates and ferroelastic n-hexyl- and n-octylammonium dihydrogenarsenate.

n-Hexylammonium dihydrogenarsenate, (C6H16N)[AsO2-(OH)2], and n-octylammonium dihydrogenarsenate, (C8H20-N)[AsO2OH2], are both ferroelastic at room temperature. The samples used in this study were not subjected to a phase transition after they had been crystallized. The structures are monoclinic (P2(1)/n) and isostructural with the corresponding dihydrogenphosphates. Each sample contained two domains and each structure was refined as a twin. There are strong hydrogen bonds between dihydrogenarsenates and moderate hydrogen bonds between dihydrogenarsenates and n-alkylammonium groups. The hydrogen-bond distances correspond well to those observed in the dihydrogenphosphates. All the atoms except two H atoms exist in pairs linked by the lost symmetry operations derived from the prototypic space group P2/b2(1)/n2(1)/a. Each of these two different H atoms is involved in an asymmetric hydrogen bond between an oxygen pair. These oxygens are supposed to change their roles as hydrogen-bond donors and acceptors during the ferroelastic switching. The phase-transition sequences are affected by interactions between the neighbouring organic chains in the structure.

Journal Article↗
PubMed1997

Differential scanning calorimetry of chain-melting phase transitions of N-acylphosphatidylethanolamines.

Phosphatidylethanolamines in which the polar headgroup is N-acylated by a long-chain fatty acid (N-acyl PEs) are present in many plasma membranes under normal conditions, and their content increases dramatically in response to membrane stress in a variety of organisms. The thermotropic phase behavior of a homologous series of saturated N-acyl PEs, in which the length of the N-acyl chain is equal to that of the O-acyl chains attached at the glycerol backbone, has been investigated by differential scanning calorimetry (DSC). All fully hydrated N-acyl PEs with even chain lengths from C-12 to C-18 exhibit sharp endothermic chain-melting phase transitions in the absence of salt and in 1 M NaCl. Cooperative chain-melting is demonstrated directly by the temperature dependence of the electron spin resonance spectra from probe phospholipids bearing a spin label group in the acyl chain. The calorimetric transition enthalpy and the transition entropy obtained from DSC depend approximately linearly on the chain length with incremental values per CH2 group that exceed those of normal diacyl phosphatidylethanolamines, but to an extent that underrepresents the additional N-acyl chain. A thermodynamic model is constructed for the chain-length dependences and end effects of the calorimetric quantities, which includes a deficit proportional to the difference in O-acyl and N-acyl chain lengths for nonmatched chains, as is found and justified structurally for mixed-chain diacyl phospholipids. From data on the chain-length dependence of N-acyl diC16PEs, it is then deduced that the N-acyl chains are less well packed than the O-acyl chains and, from the data on the matched-chain N-acyl PEs, that the O-acyl chain packing is similar to that in normal diacyl PEs. The gel-to-fluid phase transition temperatures of the N-acyl PEs in the absence of salt are practically the same as those of the normal diacyl PEs of the corresponding chain lengths, although the transition enthalpies and entropies are appreciably greater, indicating entropy-enthalpy compensation. In 1 M NaCl, the transition temperatures are 3-4.5 degrees higher than in the absence of salt, representing the contribution of the electrostatic surface potential of the N-acyl PEs.

Acylation↗
PubMed2001

Ultrafast laser-induced phase transitions in amorphous GeSb films.

Time-resolved measurements of the spectral dielectric function reveal new information about ultrafast phase transitions induced by femtosecond laser pulses in Sb-rich amorphous GeSb films. The excitation generates a nonthermal phase within 200 fs. The dielectric function of this phase differs from that of the crystalline phase, contrary to previous suggestions of a disorder-to-order transition. The observed dielectric function is close to that of the liquid phase, indicating an ultrafast transition from the amorphous phase to a different disordered state.

Journal Article↗
PubMed2002

Unusual magnetic properties of one-dimensional molecule-based magnets associated with a structural phase transition.

Three ion-pair complexes, [RbzPy](+)[Ni(mnt)(2)](-) (mnt(2)(-) = maleonitriledithiolate; [RbzPy](+) = 4-R-benzylpyridinium; R = Br (1), Cl (2), and NO(2) (3)), with unusual magnetic properties have been synthesized and characterized. The crystal structures of 1 and 2 have been solved. The two complexes belong to the P2(1)/c space group with Z = 4 and C(20)H(11)BrN(5)NiS(4), a = 12.0744(17) A, b = 26.369(4) A, c = 7.440(3) A, and beta = 102.63(3) degrees for 1 and C(20)H(11)ClN(5)NiS(4), a = 12.105(2) A, b = 26.218(4) A, c = 7.374(2) A, and beta = 102.55(2) degrees for 2, respectively. The [Ni(mnt)(2)](-) anions in 1-3 form uniformly spaced one-dimensional (1-D) magnetic chains of s = 1/2 at room temperature. The temperature dependences of the susceptibility for 1-3 show that they undergo phase transitions. All three complexes are paramagnetic in their high-temperature (abbreviation HT) phase and diamagnetic in the low-temperature (abbreviation LT) phase because of strong dimerization along the stacking direction. The results of thermal analysis (DSC) further confirm that the phase transition for 1 and 2 is first-order but maybe second-order for 3. The phenomena observed in this study are similar to those of the 1-D radical systems.

Journal Article↗
PubMed2005

Modeling phase transitions during the crystallization of a multicomponent fat under shear.

The crystallization of multicomponent systems involves several competing physicochemical processes that depend on composition, temperature profiles, and shear rates applied. Research on these mechanisms is necessary in order to understand how natural materials form crystalline structures. Palm oil was crystallized in a Couette cell at 17 and 22 degrees C under shear rates ranging from 0 to 2880 s(-1) at a synchrotron beamline. Two-dimensional x-ray diffraction patterns were captured at short time intervals during the crystallization process. Radial analysis of these patterns showed shear-induced acceleration of the phase transition from alpha to beta(') . This effect can be explained by a simple model where the alpha phase nucleates from the melt, a process which occurs independently of shear rate. The alpha phase grows according to an Avrami growth model. The beta(') phase nucleates on the alpha crystallites, with the amount of beta(') crystal formation dependent on the rate of transformation of alpha to beta(') as well as the growth rate of the beta(') phase from the melt. The shear induced alpha- beta(') phase transition acceleration occurs because under shear, the alpha nuclei form many distinct small crystallites which can easily transform to the beta(') form, while at lower shear rates, the alpha nuclei tend to aggregate, thus retarding the nucleation of the beta(') crystals. The displacement of the diffraction peak positions revealed that increased shear rate promotes the crystallization of the higher melting fraction, affecting the composition of the crystallites. Crystalline orientation was observed only at shear rates above 180 s(-1) at 17 degrees C and 720 s(-1) at 22 degrees C .

Journal Article↗
PubMed2004

The effect of bacteriorhodopsin, detergent and hydration on the cubic-to-lamellar phase transition in the monoolein-distearoyl phosphatidyl glycerol-water system.

The cubic phase of monoolein (MO) has successfully been used for crystallization of membrane proteins. It is likely that the transition to a lamellar phase upon dehydration is important for the crystallization process, and that the internal dimensions of the lipid phases (i.e., water pore diameter) are crucial for the inclusion and the diffusion of membrane proteins. In the present study, we investigated the cubic-to-lamellar phase transitions in the MO-water and the MO-distearoyl phosphatidyl glycerol (DSPG) systems. The MO-water system was investigated by means of isothermal sorption and desorption microcalorimetry. We show that the transition from cubic to lamellar phase induced by desorption is driven by entropy. At 25 degrees C, this occurs at a water activity of 0.98 with a transition enthalpy of 860 J/mol (MO). The phase behavior was also investigated in the presence of a small amount of the transmembrane protein bacteriorhodopsin (bR), and a detergent, octyl glucoside (OG), and it was shown that both bR and OG stabilize the lamellar phase. Analogous results were obtained for the MO-DSPG-water system. The latter system resembles the MO-water system in that a cubic-to-lamellar phase transition is induced by dehydration, although the structural properties of these phases are slightly different. Finally, we demonstrate that bR can be crystallized from a cubic phase of MO-DSPG-buffer.

Bacteriorhodopsins↗
PubMed1995

[Effect of changes in the lipid composition on the plasma membrane of Saccharomyces cerevisiae through mutation of the phase transition and mixing behavior of the lipid fraction].

Mutation of Saccharomyces cerevisiae leads to an alteration of the size and surface structure of the mutant cell. These phenomena are correlated with change in the lipid composition and hence membrane fluidity of the plasma membranes. Such alterations are in the fatty acyl constituents of phospholipids and glycolipids and include changes in the saturation or length of fatty acyl chains. Simultaneously, the "melting point" of phospholipid fractions, i.e. the temperature of the ordered-disordered phase transition and mixing behavior, changed. Consequently calorimetry on isolated phospholipid fractions extracted from plasma membranes is a useful analytical method to demonstrate directly such phase transitions and change in the mixing behavior of the cellular lipid fractions in connection with mutation. A correlation between the alteration of the lipid composition in the plasma membrane of strains of S. cerevisiae S 288 and its mutant VY 1160 is discussed.

Calorimetry↗