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PubMed2004

Studies of orthodontic elastomeric modules. Part 1: glass transition temperatures for representative pigmented products in the as-received condition and after orthodontic use.

The purpose of this study was to investigate the glass transition temperatures (T(g)) of representative elastomeric chain products (plastic modules) in the as-received condition and after orthodontic use to determine differences between brands and pigments. Values of T(g) were determined by differential scanning calorimetry. Products were obtained from 3 manufacturers: Rocky Mountain Orthodontics (RMO, Denver, Colo), Ormco (Glendora, Calif), and G&H (Greenwood, Ind). Three colors (gray, red, and purple) were selected for each brand-pigment combination to evaluate the as-received products, and test specimens for each brand-pigment combination from the same batches were placed in patients' mouths for 4 weeks to evaluate the products after clinical use (sample size of 7 specimens for both groups of experiments). Results were analyzed statistically by multiple nonparametric Mann-Whitney tests. A post hoc step-down Bonferroni analysis followed, to examine differences in T(g) due to pigmentation within brands and differences due to brands within each pigmentation category. For the as-received products, the RMO modules had mean T(g) ranging from -24 degrees C to -21 degrees C, whereas the Ormco and G&H modules had significantly lower mean T(g), ranging from -46 degrees C to -39 degrees C, indicating substantial compositional or polymer structural differences compared with the RMO modules. After clinical use, the RMO products had mean T(g) ranging from -31 degrees C to -25 degrees C, whereas the Ormco and G&H products had mean T(g) ranging from -46 degrees C to -30 degrees C. The in vivo specimens also exhibited a second, higher-temperature glass transition of unknown origin. Results suggest that the Ormco and G&H products should have greater flexibility than the RMO products and that there should be significant differences in clinical force-degradation behavior for the Ormco and G&H products compared with the RMO products.

Color↗
PubMed1978

The influence of branched-chain and omega-alicyclic fatty acids on the transition temperature of bacillus subtilis lipids.

The influence of branched-chain and omega-alicyclic fatty acids on the transition temperature of Bacillus subtilis lipids was studied by measuring the fluorescence depolarisation of the probe 1,6-diphenyl-1,3,5-hexatriene incorporated into lipid bilayers. Only anteiso-C15 and C17 fatty acid-enriched lipids showed no transition in the observed temperature range. Compared to the transition of normal lipids iso-fatty acid-enriched lipids have a slightly higher transition temperature. The incorporation of omega-alicyclic fatty acids with increasing size of the alicycle leads to a decrease in the transition temperature. A possible role of omega-cyclohexane fatty acids in Bacillus acidocaldarius is proposed.

Bacillus subtilis↗
PubMed1985

X-ray diffraction demonstrates that phosphatidyldiacylglycerol and phosphatidylcholesterol are not lamellar above the main transition temperature.

X-ray diffraction was used to investigate the lattice structure of aqueous dispersions of two phosphatidyldiacylglycerols and of a phosphatidylcholesterol above and below the chain melting transition temperature. Previously, Noggle et al. (Biochim. Biophys. Acta (1982) 691, 240-248) had investigated these lipids and had concluded on the basis of electron microscopy that the lipids were in a lamellar state above the transition temperature. However, they found the 31P-NMR signals were not characteristic of lamellar phases. It was, therefore, concluded that these lipids were yielding unexpected 31P-NMR spectra. The present X-ray results demonstrate that, in fact, the lipids are not in a lamellar state above the transition temperature and that the 31P-NMR and X-ray data are not necessarily in disagreement. Characteristics of the phases both above and below the chain melt temperature are discussed.

Cholesterol↗
PubMed1997

The relationship between protein aggregation and molecular mobility below the glass transition temperature of lyophilized formulations containing a monoclonal antibody.

PURPOSE: To find out if the physical instability of a lyophilized dosage form is related to molecular mobility below the glass transition temperature. Further, to explore if the stability data generated at temperatures below the glass transition temperature can be used to predict the stability of a lyophilized solid under recommended storage conditions. METHODS: The temperature dependence of relaxation time constant, tau, was obtained for sucrose and trehalose formulations of the monoclonal antibody (5 mg protein/vial) from enthalpy relaxation studies using differential scanning calorimetry. The non-exponentiality parameter, beta, in the relaxation behavior was also obtained using dielectric relaxation spectroscopy. RESULTS: For both sucrose and trehalose formulations, the variation in tau with temperature could be fitted Vogel-Tammann-Fulcher (VTF) equation. The two formulations exhibited difference sensitivities to temperature. Sucrose formulation was more fragile and exhibited a stronger non-Arrhenius behavior compared to trehalose formulation below glass transition. Both formulations exhibited < 2% aggregation at t/tau values < 10, where t is the time of storage. CONCLUSIONS: Since the relaxation times for sucrose and trehalose formulations at 5 degrees C are on the order of 10(8) and 10(6) hrs, it is likely that both formulations would undergo very little (< 2%) aggregation in a practical time scale under refrigerated conditions.

Antibodies, Monoclonal↗
PubMed1991

How membrane chain-melting phase-transition temperature is affected by the lipid chain asymmetry and degree of unsaturation: an effective chain-length model.

Hydrocarbon effects on the lipid chain-melting phase-transition temperature are analyzed. The membrane fluidization temperature is shown to increase with the effective chain length, which is proportional to the thickness of the well-packed hydrocarbon region. The latter, as a rule, increases with the length of the longest ordered and aligned segment on each chain. This conclusion is independent of the cause for the reduced chain packing in membrane interior: chain unsaturation (which effectively decouples the two hydrocarbon segments disjoined by a double bond) or chain asymmetry (which causes the terminal hydrocarbon segments to lose close contact) both affect the bilayer chain-melting phase-transition temperature comparably on the effective chain-length scale. Thermodynamic consequences of the trans unsaturation are approximately 50% smaller than the effects of the double bonds in the cis conformation, owing to the smaller membrane perturbation by the former double bonds. A simple quantitative model is introduced for the analysis of the phospholipid chain-melting phase behavior. This new model permits quantitative predictions of the chain-melting transition temperature solely on the basis of the known lipid chemical composition. It also explains lipid sensitivity to the hydrocarbon type and attachment. The model agreement with the experimental data is usually better than to within 99% and thus comparable to experimental scatter, even when only a few or no adjustable parameters are used. The membrane fluidization temperature is calculated for a number of potentially interesting, also as yet unexplored, phospholipids, and the biological significance of the effective chain-length concept is discussed.

Hydrocarbons↗
PubMed2004

A molecular dynamics simulation of the melting points and glass transition temperatures of myo- and neo-inositol.

The heat of sublimation, density, melting point, and glass transition temperature are calculated for myo- and neo-inositol, using the condensed-phase optimized molecular potentials for atomistic simulation studies (COMPASS) force field and molecular dynamics techniques. Our results show that the calculated heats of sublimation and density are very close to the experimental values for both compounds. Furthermore, our simulated melting temperatures for myo- and neo-inositol also compare very well to the experimentally obtained data. The glass transition temperatures for myo- and neo-inositol have been calculated to be ca. 494 K and ca. 518 K, respectively, and the shape of the volume versus temperature plots produced are typical for a glass transition. As a result, it is our view that the COMPASS force field suitably describes these two compounds in molecular simulations and that molecular dynamics techniques, combined with this force field, can be used to simulate the melt and glass transitions for such molecules.

Journal Article↗
PubMed1999

Thermodynamic analysis of chain-melting transition temperatures for monounsaturated phospholipid membranes: dependence on cis-monoenoic double bond position.

Unsaturated phospholipid is the membrane component that is essential to the dynamic environment needed for biomembrane function. The dependence of the chain-melting transition temperature, T(t), of phospholipid bilayer membranes on the position, n(u), of the cis double bond in the glycerophospholipid sn-2 chain can be described by an expression of the form T(t) = T(t)(c)(1 + h'(c)|n(u) - n(c)|)/(1 + s'(c)|n(u) - n(c)|), where n(c) is the chain position of the double bond corresponding to the minimum transition temperature, T(t)(c), for constant diacyl lipid chain lengths. This implies that the incremental transition enthalpy (and entropy) contributed by the sn-2 chain is greater for whichever of the chain segments, above or below the double-bond position, is the longer. The critical position, n(c), of the double bond is offset from the center of the sn-2 chain by an approximately constant amount, deltan(c) approximately 1. 5 C-atom units. The dependence of the parameters T(t)(c), h'(c), and s'(c) on sn-1 and sn-2 chain lengths can be interpreted consistently when allowance is made for the chain packing mismatch between the sn-1 and sn-2 chains. The length of the sn-2 chain is reduced by approximately 0.8 C-atom units by the cis double bond, in addition to a shortening by approximately 1.3 C-atom units by the bent configuration at the C-2 position. Based on this analysis, a general thermodynamic expression is proposed for the dependence of the chain-melting transition temperature on the position of the cis double bond and on the sn-1 and sn-2 chain lengths. The above treatment is restricted mostly to double-bond positions close to the center of the sn-2 chain. For double bonds positioned closer to the carboxyl or terminal methyl ends of the sn-2 chain, the effects on transition enthalpy can be considerably larger. They may be interpreted by the same formalism, but with different characteristic parameters, h'(c) and s'(c), such that the shorter of the chain segments makes a considerably smaller contribution to the calorimetric properties of the chain-melting transition.

Biophysical Phenomena↗
PubMed1993

Do low-power lasers change phase transition temperature of dipalmitoyl phosphatidylcholine (DPPC) membrane?

This study was designed to investigate the effects of the He-Ne laser (632.8 nm of wavelength, 8.5 mW in power, Senko Med. Co. Ltd., Tokyo, Japan) on the phase transition temperature of dipalmitoyl phosphatidylcholine (DPPC) membranes. Liposomal bilayers of DPPC (12.5 mM) were obtained with ultrasonification for 45 min at 40 degrees C and refrigerated for 2-3 days at 5 degrees C. The bilayers vesicle solution was divided into 3-ml working aliquots, which were assigned to three groups. The aliquots in Group 1 were used as controls without irradiation and the aliquots in Groups 2 and 3 were irradiated in 37 degrees C baths for 15 to 30 min, respectively. To determine phase transition temperature, optical density (%T; percent of permittance) of each aliquot was measured spectrophotomechanically at a wavelength of 440 nm while increasing its temperature at a rate of 0.5 degrees C every minute. Main phase transition temperatures in Groups 1, 2, and 3 were 41.54 +/- 0.23 degrees C (n = 10), 41.50 +/- 0.27 degrees C (n = 10), and 41.30 +/- 0.36 degrees C (n = 10), respectively. No significant difference between the nonirradiation and irradiation groups was established. These results suggest that irradiations of low-power He-Ne lasers up to 30 min do not change important physical characteristics of artificial DPPC membranes.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1991

Differential scanning calorimetry studies of NaCl effect on the inverse temperature transition of some elastin-based polytetra-, polypenta-, and polynonapeptides.

Differential scanning calorimetry studies of the effect of NaCl on protein-based polymer self-assembly has been carried out on six elastin-based synthetic sequential polypeptides--i.e., the polypentapeptide (L-Val1-L-Pro2-Gly3-L-Val4-Gly5)n and its more hydrophobic analogues (L-Leu1-L-Pro2-Gly3-L-Val4-Gly5)n and (L-Val1-L-Pro2-L-Ala3-L-Val4-Gly5)n; the polytetrapeptide (L-Val1-L-Pro2-Gly3-Gly4)n and its more hydrophobic analogue (L-Ile1-L-Pro2-Gly3-Gly4)n; and the polynonapeptide (a pentatetra hybrid), (L-Val1-L-Pro2-Gly3-L-Val4-Gly5-L-Val6-L-Pro7-Gly8-Gly9++ +)n. Previous physical characterizations of the polypentapeptides have demonstrated the occurrence of an inverse temperature transition since increase in order of the polypentapeptide, as the temperature is raised from below to above that of the transition, has been repeatedly observed using different physical characterizations. In the present experiments, it is observed that the transition temperatures of the polypeptides studied are linearly dependent on NaCl concentration. The molar effectiveness of NaCl in shifting the transition temperature delta Tm/[N], is about 14 degrees C/[N], with the dependence on peptide hydrophobicity being fairly small. Interestingly, however, the delta delta Q/[N] does depend on the hydrophobicity of a polypeptide.

Amino Acid Sequence↗
PubMed1996

Water-solid interactions. III. Effect of glass transition temperature, Tg, and processing on tensile strength of compacts of lactose and lactose/polyvinyl pyrrolidone.

The effect of moisture sorption at different relative humidities on the tensile strength and the physical stability of compacts of crystalline and partly amorphous lactose, alone and in binary mixtures with PVP, has been studied. Furthermore, the role of moisture as a plasticizer and its effect on the glass transition temperature, Tg, are related to the compactibiltiy. Samples were conditioned for 2 hr using a climate test chamber at different relative humidities. Moisture sorption was determined, the radial crushing strength for compacts was measured immediately and after storage, and the tensile strength was calculated. The glass transition temperature, Tg, was determined using DSC. The tensile strength of the compacts was found to depend on both the conditioning humidity and the humidity during storage. An increase in humidity to a level at which the glass transition temperature, Tg, fell below the operating temperature, T, resulted in transition from a rigid glassy state to a mobile rubbery state. For compacts of partly amorphous lactose, an increase in the tensile strength was observed during storage of tablets, due to recrystallization of the amorphous regions above Tg. Tablets of mixtures of lactose and PVP exhibit a sharp decrease in tensile strength at humidities above 70% RH, due to the glass-to-rubber transition of PVP.

Calorimetry, Differential Scanning↗
PubMed1989

Minimal surface tension, squeeze-out and transition temperatures of binary mixtures of dipalmitoylphosphatidylcholine and unsaturated phospholipids.

Fluorescence polarization (FP) measurements and surface tension (ST) experiments were performed to determine the gel-to-liquid-crystal transition or melting temperature of phospholipid mixtures. The FP-temperature diagrams showed main transition temperatures of 41 degrees C for dipalmitoylphosphatidylcholine (DPPC). The 7:3 and 9:1 binary mixtures of DPPC and phosphatidylinositol (PI), phosphatidylglycerol (PG) and phosphatidylcholine (PC) had main transition temperatures of, respectively, 32-36 degrees C and 37-39 degrees C. The minimal surface tension of DPPC monolayers increased rapidly at 40 degrees C, suggesting that this was the transition temperature for the melting of these monolayers. This value was in close accordance with the main transition temperature of DPPC, observed with the fluorescence polarization measurements. Melting temperatures of monolayers were higher for almost all mixtures than the temperatures at which the transition started, indicating preferential squeeze out of the unsaturated component and enrichment of the monolayer with DPPC. However, neither the 7:3 DPPC/PC nor the DPPC/PG mixtures could withstand high surface pressures at temperatures above 30 degrees C, whereas monolayers of DPPC/PG (9:1) became fluid at temperatures above 35 degrees C. Preferential squeeze-out of the unsaturated phospholipid was especially effective in both the 7:3 and 9:1 DPPC/PI mixtures. These monolayers started to melt at 39-40 degrees C, which is above their main transition temperatures of, respectively, 32 and 37 degrees C, and which approximate the melting temperature of DPPC. Preferential squeeze-out is essential for an artificial lung surfactant. The estimation of this phenomenon by determining the monolayer melting temperatures is therefore useful for distinguishing between mixtures which are effective surfactants at body temperature and those which are less effective.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1983

[Diagnostic significance of phase transition temperatures of blood serum cholesterol in atherosclerosis].

Lipid parameters and temperatures of serum cholesterol; ethers phasic transition were investigated in 35 patients with atherosclerosis of lower-limb vessels, 93 patients with atherosclerosis of coronary arteries and 40 normal subjects. The correlation between transition temperatures and the fatty-acid spectrum of cholesterol ether fraction was examined. Temperatures of cholesterol ethers phasic transition are suggested as diagnostic criteria to supplement conventional biochemical tests.

Adult↗
PubMed2002

Depression of the nematic-isotropic phase transition temperature at nanopatterned surfaces.

A spatially varying herringbone pattern with an easy axis alternating in angle between 0 and psi and having a period of 200 nm was scribed into a polyimide-coated substrate. The depression of the nematic-isotropic transition temperature for a nematic layer at the patterned surface relative to its value at a uniformly rubbed surface was investigated as a function of psi for 15 < psi < 88 degrees. It was found that the depression of the transition temperature increases with psi, up to approximately 7 mK at psi=88 degrees. A simple model was developed that includes not only elasticity, but also anchoring effects at the polyimide. The model, which is used to calculate the thickness of the nematic layer, indicates that anchoring-rather than elastic-effects play the dominant role in the depression of the layer's transition temperature.

Journal Article↗
PubMed2004

Effect of powder to liquid ratio on tensile strength and glass transition temperature of alumina filled poly methyl methacrylate (PMMA) denture base material.

The aim of this study was to evaluate the mechanical properties and glass transition temperature (Tg) of a denture base material prepared from high molecular weight poly methyl methacrylate (PMMA) and alumina (Al2O3). The glass transition temperature was studied by using differential scanning calorimetry (DSC). The effect of powder-to-liquid ratio was investigated. The result showed that the tensile properties and the Tg were slightly effected by the powder-to-liquid ratio. The ratio of 2.2:1 by weight of powder to liquid was found to be the best ratio for mixing the material to give the best result in this formulation.

Aluminum Oxide↗
PubMed2006

Crystallization rate of amorphous nifedipine analogues unrelated to the glass transition temperature.

To examine the relative contributions of molecular mobility and thermodynamic factor, the relationship between glass transition temperature (T(g)) and the crystallization rate was examined using amorphous dihydropyridines (nifedipine (NFD), m-nifedipine (m-NFD), nitrendipine (NTR) and nilvadipine (NLV)) with differing T(g) values. The time required for 10% crystallization, t(90), was calculated from the time course of decreases in the heat capacity change at T(g). The t(90) of NLV and NTR decreased with decreases in T(g) associated with water sorption. The t(90) versus T(g)/T plots almost overlapped for samples of differing water contents, indicating that the crystallization rate is determined by molecular mobility as indicated by T(g). In contrast, differences in the crystallization rate between these four drugs cannot be explained only by molecular mobility, since the t(90) values at a given T(g)/T were in the order: NLV>NTR>NFD approximately m-NFD. A lower rate was obtained for amorphous drugs with lower structural symmetry and more bulky functional groups, suggesting that these factors are also important. Furthermore, the crystallization rate of NTR in solid dispersions with poly(vinylpyrrolidone) (PVP) and hydroxypropyl methylcellulose (HPMC) decreased to a greater extent than expected from the increased T(g). This also suggests that factors other than molecular mobility affect the crystallization rate.

Calorimetry, Differential Scanning↗
PubMed1986

Destabilization of phosphatidylethanolamine liposomes at the hexagonal phase transition temperature.

We have examined whether there is a relationship between the lamellar-hexagonal phase transition temperature, TH, and the initial kinetics of H+- and Ca2+-induced destabilization of phosphatidylethanolamine (PE) liposomes. The liposomes were composed of dioleoylphosphatidylethanolamine, egg phosphatidylethanolamine (EPE), or phosphatidylethanolamine prepared from egg phosphatidylcholine by transesterification (TPE). These lipids have well-spaced lamellar-hexagonal phase transition temperatures (approximately 12, approximately 45, and approximately 57 degrees C) in a temperature range that allows us to measure the initial kinetics of bilayer destabilization, both below and above TH. The liposomes were prepared at pH 9.5. The TH of EPE and TPE was measured by using differential scanning calorimetry, and it was found that the TH was essentially the same at low pH or at high pH in the presence of 20 mM Ca2+. At temperatures well below TH, either at pH 4.5 or at pH 9.5 in the presence of Ca2+, the liposomes aggregate, leak, and undergo lipid mixing and mixing of contents. We show that liposome/liposome contact is involved in the destabilization of the PE liposomes. The temperature dependence of leakage, lipid mixing, and mixing of contents shows that there is a massive enhancement in the rate of leakage when the temperature approaches the TH of the particular PE and that lipid mixing appears to be enhanced. However, the fusion (mixing of aqueous contents) is diminished or even abolished at temperatures above TH. At and above the TH, a new mechanism of liposome destabilization arises, evidently dependent upon the ability of the PE molecules to adapt new morphological structures at these temperatures. We propose that this destabilization demarks the first step in the pathway to the eventual formation of the HII phase. Thus, the polymorphism accessible to PE is a powerful agent for membrane destabilization, but additional factors are required for fusion.

Calcium↗
PubMed1980

Liposome filtration. Dependence on transition temperature.

Liposomes formed by vortexing and passed through polycarbonate surface retention membranes showed appreciable differences in filtration behavior depending on the temperature of filtration relation relative to the liposome gel-liquid crystal transition temperature. Below transition, liposomes were filterable and size distributions could be determined; the cumulative volume distributions were log-normal. Above transition, liposomes were not filterable: smaller liposomes were formed until a limiting size was reached. These results suggest that liquid crystal liposome size distributions cannot be determined by filtration. This filtration behavior is a physical property of liposomes, related to the gel-liquid crystal transition, not previously reported. This property could be exploited as a new method for controlling liposome size distributions, but the implications for lipid membranes, including biological membranes, are general.

Filtration↗