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PubMed2000

Density dependence of the transition temperature in a homogeneous bose-einstein condensate

Transition temperature data obtained as a function of particle density in the 4He-Vycor system are compared with recent theoretical calculations for 3D Bose-condensed systems. In the low density dilute Bose gas regime we find, in agreement with theory, a positive shift in the transition temperature of the form DeltaT/T0 = gamma(na(3))(1/3). At higher densities a maximum is found in the ratio of T(c)/T0 for a value of the interaction parameter, na(3), that is in agreement with path-integral Monte Carlo calculations.

Journal Article↗

PubMed1998

Effect of Crystallinity on the Glass Transition Temperature of Starch.

The glass transition temperature (T(g)) of potato and wheat starches, stored for several periods after gelatinization, was measured by differential scanning calorimetry (DSC), and the relative crystallinity of the starches was measured by X-ray diffractometry. T(g) of stored starches was higher than that of starches without storage, and the T(g) increment of starches gelatinized at 120 degrees C was higher than that of starches gelatinized at 60 degrees C. The water content at which the glass transition of a starch occurs at 25 degrees C was estimated from DSC data, and it increased linearly with relative crystallinity in two groups that differed in the gelatinization method. These results also showed the quantitative relationship between T(g) and retrogradation. In addition, these results suggested that the glass transition of starch could be interpreted in the same way as the glass transition of cross-linked synthetic polymers.

Journal Article↗

PubMed1991

A statistical mechanical analysis of the effect of long-chain alcohols and high pressure upon the phase transition temperature of lipid bilayer membranes.

Long-chain n-alcohols decrease the main phase-transition temperature of lipid vesicle membranes at low concentrations but increase it at high concentrations. The nonlinear phenomenon is unrelated to the interdigitation and is analyzed by assuming that alcohols form solid solutions with solid as well as liquid phases. The biphasic response originates from the balance of the free energy difference of alcohols in the liquid and solid membranes (delta gA) and the alcohol-lipid interaction free energy difference (delta u) between the two phases. When delta gA less than 0 and delta u greater than 0, or delta gA less than delta u less than 0, the transition temperature decreases monotonously according to the increase in the alcohol concentration. When delta gA greater than 0 and delta u less than 0, or delta gA greater than delta u greater than 0, it increases monotonously. Biphasic response occurs with a minimum temperature when delta u greater than delta gA greater than 0, and with a maximum temperature when delta u less than delta gA less than 0. When the alcohol carbon-chain length becomes closer to the lipid carbon-chain length, delta u is equalized by delta gA, and the temperature minimum of the main transition is shifted to extremely low alcohol concentrations. Hence, long-chain alcohols predominantly elevate the main transition temperature and lose their anesthetic potency. High pressure decreased both delta gA and delta u. Presumably, high pressure improves the packing efficiency of liquid membranes and decreases the difference between the solid and liquid membrane properties.

Alcohols↗

PubMed1990

Differential scanning calorimetry studies of the inverse temperature transition of the polypentapeptide of elastin and its analogues.

Differential scanning calorimetry studies have been carried out on the sequential polypeptide of elastin, (L-Val1-L-Pro2-Gly3-L-Val4-Gly5)n, abbreviated as PPP, and its more hydrophobic analogues (L-Leu1-L-Pro2-Gly3-L-Val4-Gly5)n, referred to as Leu1-PPP, and (L-Ile1-L-Pro2-Gly3-L-Val4-Gly5)n, referred to as Ile1-PPP Consistent with inverse temperature transitions, the temperatures of the transitions for which maximum heat absorption occurs are inversely proportional to the hydrophobicities of the polypentapeptides (31 degrees C for PPP, 16 degrees C for Leu1-PPP, and 12 degrees C for Ile1-PPP), and the endothermic heats of the transitions are small and increase with increasing hydrophobicity, i.e., 1.2, 2.9, and 3.0 kcal/mol pentamer for PPP, Leu1-PPP, and Ile1-PPP, respectively. Previous physical characterizations of the polypentapeptides have demonstrated the occurrence of an inverse temperature transition since increase in order, as the temperature is raised above that of the transition, has been repeatedly observed using different physical characterizations. Furthermore, the studies demonstrated identical conformations for PPP and Il21-PPP above and below the transition. Both heats and temperatures of the transitions vary with hydrophobicity, but not in simple proportionality.

Amino Acid Sequence↗

PubMed1992

Hydrophobicity scale for proteins based on inverse temperature transitions.

In general, proteins fold with hydrophobic residues buried, away from water. Reversible protein folding due to hydrophobic interactions results from inverse temperature transitions where folding occurs on raising the temperature. Because homoiothermic animals constitute an infinite heat reservoir, it is the transition temperature, Tt, not the endothermic heat of the transition, that determines the hydrophobically folded state of polypeptides at body temperature. Reported here is a new hydrophobicity scale based on the values of Tt for each amino acid residue as a guest in a natural repeating peptide sequence, the high polymers of which exhibit reversible inverse temperature transitions. Significantly, a number of ways have been demonstrated for changing Tt such that reversibly lowering Tt from above to below physiological temperature becomes a means of isothermally and reversibly driving hydrophobic folding. Accordingly, controlling Tt becomes a mechanism whereby proteins can be induced to carry out isothermal free energy transduction.

Amino Acid Sequence↗

PubMed2004

Does the transition temperature of Cu-NiTi archwires affect the amount of tooth movement during alignment?

OBJECTIVES: To examine whether the transition temperature of Cu-NiTi archwires has an effect on the tooth movement during the alignment phase of orthodontic treatment. DESIGN: 'Split mouth' design in randomly selected patients. SETTING AND SAMPLE POPULATION: The Department of Orthodontics, School of Dentistry, University of Aarhus. Fifteen randomly selected patients with identical level of irregularity in the alignment phase of their treatment. Experiment Variable - Specially manufactured Cu-NiTi archwires for the upper arch were inserted. These consisted of two separate halves, each with its own transition temperature, respectively 27 degrees and 40 degrees C, and clamped together in the middle. OUTCOME MEASURE: The tooth movement, expressed as two translations and a rotation, in the occlusal plane was measured from the patients' intraoral photographs taken upon insertion of the archwires and again after 1 month. RESULTS: Tooth movements tended to be larger on the 40 degrees C-side, however only in case of the total translation of the premolars was this difference significant. In general, patients had not noticed any difference between the two sides of the archwire, although one patient stated the 27 degrees C-side to be more comfortable as the 40 degrees C-side had bothered her when drinking hot beverages. CONCLUSION: The transition temperature of Cu-NiTi archwires has indeed an effect on the amount of tooth movement during alignment. However, the differences are so small though that it is the question whether they can be noticed clinically. The study corroborates the trend towards the use of lower forces within orthodontics.

Copper↗

PubMed1990

Electric field increases the phase transition temperature in the bilayer membrane of phosphatidic acid.

The effect of the electric field on the phase transition temperature (Tc) of acidic 1,2-dipalmitoyl-sn-glycero-3-phosphate (DPPA) and 1,2-dipalmitoyl-sn-glycero-3-thionphosphate (thion-DPPA) and zwitterion, i.e. 1,2-dipalmitoyl-rac-3-phosphocholine and 1,2-distearoyl-rac-glycero-3-phosphocholine (DPPC and DSPC), lipids has been investigated. The phase transition was detected using the jump-like increase effect in the conductance of the planar bilayer membrane. A voltage increase to 150 mV has been shown to increase the phase transition temperature in a bilayer lipid membrane (BLM) of phosphatidic acids (DPPA and thion-DPPA) by 8-12 degrees C while the transition temperature in the bilayer of zwitterion lipids (DPPC and DSPC) increases insignificantly. The increasing of Tt in BLM of acidic lipids is attributed to the voltage-induced changes in the molecule packing density.

Chemical Phenomena↗

PubMed2006

Glass transition temperature of glucose, sucrose, and trehalose: an experimental and in silico study.

Isothermal-isobaric molecular dynamics simulations are used to calculate the specific volume of models of different amorphous carbohydrates (glucose, sucrose, and trehalose) as a function of temperature. Plots of specific volume vs temperature exhibit a characteristic change in slope when the amorphous systems change from the glassy to the rubbery state. The intersection of the regression lines of data below (glassy state) and above (rubbery state) the change in slope provides the glass transition temperature (T(g)). These predicted glass transition temperatures are compared to experimental T(g) values as obtained from differential scanning calorimetry measurements. As expected, the predicted values are systematically higher than the experimental ones (about 12-34 K) as the cooling rates of the modeling methods are about a factor of 10(12) faster. Nevertheless, the calculated trend of T(g) values agrees exactly with the experimental trend: T(g)(glucose) < T(g)(sucrose) < T(g)(trehalose). Furthermore, the relative differences between the glass transition temperatures were also computed precisely, implying that atomistic molecular dynamics simulations can reproduce trends of T(g) values in amorphous carbohydrates with high quality.

Biophysics↗

PubMed1985

Fluorescence decay of pyrene in small and large unilamellar L, alpha-dipalmitoylphosphatidylcholine vesicles above and below the phase transition temperature.

The fluorescence decays of pyrene in small and large unilamellar L, alpha-dipalmitoylphosphatidylcholine vesicles have been investigated as a function of probe concentration and temperature. When the molar ratio of pyrene to phospholipid equals 1:3000, no excimer emission is observed and the fluorescence decays are mono-exponential. When this ratio is equal to or higher than 1:120, excimer formation is observed. Above the phase transition temperature the observed fluorescence decays of monomer and excimer can be adequately described by a bi-exponential function. The monomer decays can be equally well fitted to a decay law which takes into account a time-dependence in the probe diffusion rate constant. The fluorescence decay kinetics are compatible with the excimer formation scheme which is valid in an isotropic medium. The excimer lifetime and the (apparent) rate constant of excimer formation have been determined as a function of probe concentration at different temperatures above the phase transition temperature. The activation energy of excimer formation is found to be 29.4 +/- 1.3 kJ/mol. In small unilamellar vesicles the diffusion constant associated with the pyrene excimer formation process varies from 8.0 X 10(-7) cm2/s at 40 degrees C to 2.2 X 10(-6) cm2/s at 70 degrees C. Below the phase transition temperature the monomer decays can be described by a decay law which takes into account a time dependence of the rate constant of excimer formation. The lateral diffusion coefficient of pyrene calculated from the decay fitting parameters of the monomer region varies from 4.0 X 10(-9) cm2/s at 20 degrees C to 7.9 X 10(-8) cm2/s at 35 degrees C. No significant difference could be observed between the pyrene fluorescence decay kinetics in small and large unilamellar vesicles.

Diffusion↗

PubMed1988

Mechanochemical coupling in synthetic polypeptides by modulation of an inverse temperature transition.

For the polypentapeptide of elastin, (L-Val-L-Pro-Gly-L-Val-Gly)n, and appropriate analogs when suitably cross-linked, it has been previously demonstrated that development of elastomeric force at fixed length and length changes at fixed load occur as the result of an inverse temperature transition, with the temperature of the transition being inversely dependent on the hydrophobicity of the polypeptide. This suggests that at fixed temperature a chemical means of reversibly changing the hydrophobicity could be used for mechanochemical coupling. Evidence for this mechanism of mechanochemical coupling is given here with a 4%-Glu-polypentapeptide, in which the valine in position 4 is replaced in 1 out of 5 pentamers by a glutamic acid residue. Before cross-linking, the temperature for aggregation of 4%-Glu-polypentapeptide is remarkably sensitive to pH, shifting from 25 degrees C at pH 2 to 70 degrees C at pH 7.4 in phosphate-buffered saline (PBS). At 37 degrees C, the cross-linked 4%-Glu-polypentapeptide matrix in PBS undergoes a pH-modulated contraction and relaxation with a change from pH 4.3 to 3.3 and back. The mean distance between carboxylates at pH 4.3 in the elastomeric matrix is greater than 40 A, twice the mean distance between negatively charged species in PBS. Accordingly, charge-charge repulsion is expected to make little or no contribution to the coupling. Mechanochemical coupling is demonstrated at fixed load by monitoring pH dependence of length and at constant length by monitoring pH dependence of force. To our knowledge, this is the first demonstration of mechanochemical coupling in a synthetic polypeptide and the first system to provide a test of the recent proposal that chemical modulation of an inverse temperature transition can be a mechanism for mechanochemical coupling. It is suggested that phosphorylation and dephosphorylation may modulate structure and forces in proteins by locally shifting the temperatures of inverse temperature transitions.

Amino Acid Sequence↗

PubMed2001

Pseudoelasticity and thermoelasticity of nickel-titanium alloys: a clinically oriented review. Part I: Temperature transitional ranges.

The purpose of this review was to organize a systematic reference to help orthodontists evaluate commonly used orthodontic nickel-titanium alloys. Part I of the article reviews the data available in the literature regarding the temperature transitional ranges of the alloys. The thermomechanical behavior of these compounds is, in fact, strictly dependent on the correlation between the temperature transitional range and the oral temperature range. Part II of the article will focus on the mechanical characteristics of the alloys, such as the magnitude of the forces delivered and its correlations with temperature transitional range and oral temperature.

Body Temperature↗

PubMed1999

Impact of melting conditions of sucrose on its glass transition temperature.

The impact of the melting conditions of sucrose crystals on the glass transition temperature (T(g)) of the sucrose melt was studied. Final temperature, heating rate, and the residence time at the final temperature were the experimental conditions considered. The glass transition temperature of the different glasses was measured by differential scanning calorimetry, and the degradation of sucrose during the thermal treatments was studied by high-performance liquid chromatography. The results showed that the T(g) is sensitive to the degradation of sucrose: T(g) decreases with the appearance of small molecules and then increases with the appearance of polymerization products. Thus, the choice of thermal treatment is of the utmost importance for the determination of the T(g) of pure sucrose.

Calorimetry, Differential Scanning↗

PubMed2006

Early-stage roughening of the polymer-polymer interface approaching the glass transition temperature by real-time neutron reflection.

The early-stage roughening of the interface between thin deuterated poly(methyl methacrylate) (d-PMMA) layers on thick polystyrene (PS) films was studied as a function of the temperature using real-time specular neutron reflectivity. By measuring the growth of the interface roughness as a precursor of the dewetting, the characteristic time constant of the early stages of the process was studied as a function of the temperature approaching the glass transition temperature (T(g)) of the two polymers from above and compared with the prediction of the growth of the interface by the spinodal process. Both solid and liquid regimes were probed, in which the viscosity of the thin film or the substrate dominates respectively. The characteristic growth time of the process also depends on the upper film thickness to a power of 5 or 6 in the solid or liquid regimes, respectively, as predicted by the theory of spinodal dewetting.

Journal Article↗

PubMed1997

Glass transition temperatures of dental porcelains determined by DSC measurement.

The differential scanning calorimetry (DSC) curves for three commercial dentin and incisal porcelains fused-to metal were measured using high-temperature DSC. The glass transition temperatures (Tg) were determined from the DSC curves at heating rates of 7-20 degrees C/min, and the activation energy was derived from an Arrhenius plot of negative reciprocal Tg vs. logarithm of heating rate. The Tg of the dental porcelains depended on the content of aluminum oxide, whereas the activation energy depended on the content of sodium oxide. The ultra-low fusing type porcelains had a low activation energy due to the higher content of sodium oxide than the other porcelains.

Aluminum Oxide↗

PubMed1986

Changes of transition temperatures of phosphatidylcholine and phosphatidylglycerol in presence of amphiphilic drugs.

The transition temperature of phosphatidylglycerol (PG) was reduced to lower temperatures in presence of propranolol, imipramine, amitriptyline and chlorpromazine. This effect was dependent on drug concentration and was smallest with propranolol. The fluidizing effect, however, increased from propranolol to chlorpromazine according to the octanol/H2O partition coefficients. When the two phospholipids PG and phosphatidylcholine (PC) were compared, the presence of drug lead to a more pronounced reduction of the transition temperature in the case of the acidic phospholipid than in the case of the neutral one.

Amitriptyline↗

PubMed2006

First-principles combinatorial design of transition temperatures in multicomponent systems: the case of Mn in GaAs.

The transition temperature TC of multicomponent systems--ferromagnetic, superconducting, or ferroelectric--depends strongly on the atomic arrangement, but an exhaustive search of all configurations for those that optimize TC is difficult, due to the astronomically large number of possibilities. Here we address this problem by parametrizing the TC of a set of approximately 50 input configurations, calculated from first principles, in terms of configuration variables ("cluster expansion"). Once established, this expansion allows us to search almost effortlessly the transition temperature of arbitrary configurations. We apply this approach to search for the configuration of Mn dopants in GaAs having the highest ferromagnetic Curie temperature. Our general approach of cluster expanding physical properties opens the way to design based on exploring a large space of configurations.

Journal Article↗

PubMed2005

A fundamental approach for the estimation of the mechanical glass transition temperature in gelatin.

The paper constitutes an attempt to overcome the empiricism prevalent in the estimation of the glass transition temperature (Tg) of gelatin networks using rheological techniques. In doing so, it presents a study of the viscoelastic properties of a well-characterised gelatin sample covering the structural properties from the rubbery region to the glassy state. The pattern of oscillatory behaviour on shear is given by a master curve produced by shifting data obtained at different temperatures along the logarithmic time scale. Data reduction does not hold for all temperatures thus giving rise to thermorheological complexity. Within the temperature range at which molecular processes are represented by a simple distribution of relaxation times, a fundamental argument is developed to pinpoint the mechanical Tg. This should improve confidence in measured glassy properties over the empirical indicators found in the literature. As a demonstration, the glass transition temperature of gelatin at "zero moisture" obtained using the proposed framework of analysis is contrasted with earlier attempts to identify the mechanical Tg of gelatin solids.

Elasticity↗

PubMed2002

A novel method for estimation of transition temperature for polymorphic pairs in pharmaceuticals using heat of solution and solubility data.

A novel method for thermodynamic stability studies of polymorphic drug substances has been developed. In order to estimate the transition temperature for an enantiotropic polymorphic pair, a formula for calculating the temperature at which the solubilities of each polymorph become equal has been derived with heat of solution and solubility as the variables. This formula is based on the assumption that van't Hoff plots (logarithmic solubility versus reciprocal of absolute temperature plots) of each polymorph show a straight line (heat of solution is independent of temperature) whose slope can be expressed as a function of heat of solution. The transition temperatures for seratrodast, acetazolamide and carbamazepine polymorphic pairs calculated by the formula were in good agreement with the results of previous studies. Furthermore, the calculated transition temperature for the indomethacin polymorphic pair was above the melting point, an unrealistic temperature range, suggesting that these polymorphs are monotropically related. Since this formula requires solubility data at only one arbitrary temperature other than heat of solution data for both polymorphs in a polymorphic pair, the proposed method is much faster than the conventional method requiring solubility data at five or more different temperatures for the preparation of van't Hoff plots.

Hot Temperature↗