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At least 451 records · Page 25Linked to original sources
PubMed2003

Reorientational optical nonlinearity of nematic liquid-crystal cells near the nematic-isotropic phase transition temperature.

We address the reorientational optical nonlinearity of homogeneously aligned neamtic liquid-crystal (NLC) cells. The propagation of light in NLC cells depend strongly on temperature. At a temperature approaching the clearing point, an undulating beam and multifocal points are observed in the NLC cell by use of a polarizing optical microscope. Using a conoscopic technique, we observed novel consecutive concentric and parabolic patterns projected onto a screen. Optical energy is considered to compete with thermal energy to affect NLC's orientation and to generate singularities in the steady state. A model of the configuration of the liquid crystal's orientation is proposed.

Journal Article↗
PubMed2004

Temperature- and glass transition temperature-dependence of bimolecular reaction rates in lyophilized formulations described by the Adam-Gibbs-Vogel equation.

Bimolecular reaction rates in lyophilized aspirin-sulfadiazine formulations containing poly(vinylpyrrolidone), dextran, and isomalto-oligomers of different molecular weights were determined in the presence of various water contents, and their temperature- and glass transition temperature (Tg)-dependence was compared with that of structural relaxation time (tau calculated according to the Adam-Gibbs-Vogel equation, in order to understand how chemical degradation rates of drugs in lyophilized formulations are affected by molecular mobility. The rate of acetyl transfer in poly(vinylpyrrolidone) K30 and dextran 40k formulations with a constant Tg, observed at various temperatures, exhibited a temperature dependence similar to that of tau at temperatures below Tg. Furthermore, the rates of acetyl transfer and the Maillard reaction in formulations containing alpha-glucose polymers and oligomers increased, as the Tg of formulations decreased, either associated with decreases in molecular weight of excipient or with increases in water content. The observed Tg dependence was similar to that of tau in the range of Tg higher than the experimental temperature. The results suggest a possibility that bimolecular reaction rate at temperatures below Tg can be predicted from that observed at the Tg on the basis of temperature dependence of structural relaxation time in amorphous systems, if the degradation rate is proportional to the diffusion rate of reacting compounds.

Algorithms↗
PubMed1977

Temperature induced analog reaction of adenylyl imidodiphosphate to an intermediate step of heavy meromyosin adenosine triphosphatase.

The UV absorption difference spectrum of heavy meromyosin induced by adenylyl imidodiphosphate (AMP-PNP) was found to be changed by temperature. At higher temperatures, the shape of the difference spectrum resembled the ATP-form of difference spectrum induced by ATP. At lower temperatures, a different shape was observed, resembling that induced by ADP. This temperature transition was found in the presence of both MgCl2 and MnCl2. The transition temperatures, were 21 degrees and 9 degrees in the presence of MnCl2 and MgCl2, respectively. A similar temperature dependence was observed with the difference spectrum induced by ATP at the steady state. The transition temperatures in this case were 11 degrees and 4.5 degrees in the presence of MnCl2 and MgCl2, respectively. The similarity of the effects of the two kinds of divalent cation on both transitions indicates that the temperature induced transition between two species of heavy meromyosin-AMP-PNP complex mimics the step in APTase [EC 3.6.1.3] reaction in which the intermediate complex showing the ATP-form of difference spectrum changes to that showing the ADP-form. The equilibrium constant of the decay step of the ATP-form of difference spectrum to the ADP-form in ATPase is, therefore, thought to be highly temperature dependent. Thermodynamic parameters were calculated for the transition between the two species of heavy meromyosin AMP-PNP complex. Large decreases in enthalpy and entropy were observed, while the standard free energy change was small. The results suggest that the intermediate showing the ATP-form of difference spectrum hardly changes to the forward direction in the ATPase reaction at higher temperature. The complex appears to be so stable in the steady state that almost all the myosin is present as this complex. The decay step in ATPase of the difference spectrum from the ATP-form to to the ADP-form may be coupled to muscular contraction. The temperature induced transition of heavy meromyosin AMP-PNP complex may, therefore, provide information concerning the state of myosin in active muscles.

Adenosine Triphosphatases↗
PubMed1989

Hairpin formation of d(CGCG-TA-CGCG), d(CGCG-TG-CGCG) and their cytosine methylated analogs.

Hairpin formations of decamers d(CGCG-TA-CGCG), d(CGCG-TG-CGCG), and their m5dC analogs are evidenced by the existence of biphasic absorbance melting profiles in which the lower transition temperature increases with increasing oligomer concentration, whereas the higher melting temperature is concentration independent. The corresponding temperature dependent CD intensity at 285 nm exhibits a maximum around 55 degrees C. These observations are consistent with the interpretation that the lower temperature transition corresponds to the duplex to hairpin transformation while the melting of hairpins into single strands constitutes the higher temperature transition. The CD spectrum of the hairpin conformation appears to be characterized by a couplet with nearly equal positive and negative intensities at 285 and 255 nm, respectively, while a significantly smaller intensity at 285 nm is apparent for the duplex form. The hairpin conformation is suspected to contain a two-nucleotide loop. Titrations with NaCl further suggest that, in contrast to the TA sequence, the TG sequence with wobble base pairing favors Z formation under high salt conditions.

Base Sequence↗
PubMed1983

Solubilization of phospholipids by chaotropic ion solutions.

Phosphatidylcholine (PC) dissolves in solutions of concentrated neutral trichloroacetate and tribromoacetate, anions known to be extremely chaotropic. Solubilization of egg yolk PC occurs in Na-trichloroacetate between 2 and 3 M and in Na-tribromoacetate between 1.5 and 2 M. The resulting optically clear solutions were found by gel exclusion chromatography to consist of micelles containing of the order of 10(2) lipid molecules which, according to 31P NMR, are undergoing rapid, isotropic motion. Dipalmitoyl-PC is not solubilized below its normal phase transition temperature, although it remains in solution if first dissolved above that temperature; transition temperature and enthalpy change are both drastically depressed in 3 M Na-trichloroacetate. Na-trichloroacetate (3 M) also dissolves sphingomyelin but not phosphatidylethanolamine, acidic phospholipids, or egg PC-cholesterol mixtures of more than 33% cholesterol. Destabilization of the lamellar phase of egg PC relative to the micellar phase is promoted by trichloroacetate binding to and intercalating between the lipid molecules. Trichloroacetate was found to bind to egg PC liposomes with an affinity constant of about 1 M-1, so that nearly 1:1 association between egg PC and trichloroacetate is predicted at the concentration at which solubilization occurs. A surface area dilation by an amount (50+%) consistent with formation of micelles was confirmed by surface tension isotherms of egg PC on 3 M Na-trichloroacetate. The chaotropic effect may synergize binding by reducing the energy cost of the exposure of hydrophobic portions of lipid that is necessitated by the small radius of curvature of micelles. Several chaotropic agents less potent than trichloroacetate and tribromoacetate did not solubilize egg PC.

Calorimetry, Differential Scanning↗
PubMedDate not supplied

Biodegradable polymeric nanospheres formed by temperature-induced phase transition in a mixture of poly(lactide-co-glycolide) and poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer.

The mixture of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer(F-127) and PLGA (poly(lactide-co-gycolide)) forms a liquid state above their phase transition temperatures, and the phase-separated state is induced by decreasing the temperature below the phase transition temperature. On the basis of the temperature-induced phase transition behavior in the mixture of F-127 and PLGA, a novel method for the preparation of drug-loaded PLGA nanospheres was designed and characterized by measuring the loading amount, the encapsulation efficiency, and the drug release pattern. Paclitaxel, used as a potent anticancer drug, was selected as a model drug.

Antineoplastic Agents↗
PubMed1999

Calorimetric studies on the stability of the ribosome-inactivating protein abrin II: effects of pH and ligand binding.

The effects of pH and ligand binding on the stability of abrin II, a heterodimeric ribosome-inactivating protein, and its subunits have been studied using high-sensitivity differential scanning calorimetry. At pH7.2, the calorimetric scan consists of two transitions, which correspond to the B-subunit [transition temperature (Tm) 319.2K] and the A-subunit (Tm 324.6K) of abrin II, as also confirmed by studies on the isolated A-subunit. The calorimetric enthalpy of the isolated A-subunit of abrin II is similar to that of the higher-temperature transition. However, its Tm is 2.4K lower than that of the higher-temperature peak of intact abrin II. This indicates that there is some interaction between the two subunits. Abrin II displays increased stability as the pH is decreased to 4.5. Lactose increases the Tm values as well as the enthalpies of both transitions. This effect is more pronounced at pH7.2 than at pH4.5. This suggests that ligand binding stabilizes the native conformation of abrin II. Analysis of the B-subunit transition temperature as a function of lactose concentration suggests that two lactose molecules bind to one molecule of abrin II at pH7.2. The presence of two binding sites for lactose on the abrin II molecule is also indicated by isothermal titration calorimetry. Plotting DeltaHm (the molar transition enthalpy at Tm) against Tm yielded values for DeltaCp (change in excess heat capacity) of 27+/-2 kJ.mol-1.K-1 for the B-subunit and 20+/-1 kJ.mol-1.K-1 for the A-subunit. These values have been used to calculate the thermal stability of abrin II and to surmise the mechanism of its transmembrane translocation.

Abrin↗
PubMed2006

Predictions of onset of crystallization from experimental relaxation times I-correlation of molecular mobility from temperatures above the glass transition to temperatures below the glass transition.

PURPOSE: Predicting onsets of crystallization at temperatures below T (g), from data above T (g), would require that the correlation between crystallization onset and mobility is same above and below T (g), and the techniques being used to measure mobility above and below T (g) are measuring essentially the same kind of mobility. The aim of this work is to determine if the relaxation times obtained using different techniques (DSC, TAM) below T (g) correlate with relaxation time obtained above T (g) using dielectric spectroscopy. METHODS: Model compounds for this work were chosen based on their varied DeltaH (f), DeltaC (p)(T (g)) and H-bonding in crystalline state vs. amorphous state. Relaxation times above T (g) were determined by the simultaneous fit of real and imaginary permittivity to the Cole-Davidson model. Tau and beta below T (g) were determined using isothermal microcalorimetry (TAM) or MDSC. MDSC was used to calculate Kauzmann temperature and strength of the glass using established relationships. RESULTS: Indomethacin, nifedipine and flopropione showed Arrhenius temperature dependence throughout the entire temperature range and extrapolation of tau ( beta ) measured above T (g) by dielectric relaxation agreed with tau ( beta ) measured below T (g) by TAM/MDSC. Ketoconazole, however, showed the expected VTF behavior. For at least two compounds compared (indomethacin and ketoconazole), relaxation times measured by TAM and MDSC did not agree, with TAM giving significantly lower values of tau ( beta ), but TAM and MDSC relaxation times appeared to extrapolate to a common value at T (g). CONCLUSIONS: It was found that, for all cases studied, relaxation time constants determined above and below T (g) did appear to extrapolate to the same value around T (g) indicating that molecular mobility measured above and below T (g) using different techniques is highly correlated.

Calorimetry↗
PubMed1991

Thermotropic phase properties of the hydroxyceramide/cholesterol system.

The interaction of cholesterol with ceramides containing alpha-hydroxy fatty acyl chains (hydroxyceramides) has been studied as a foundation for characterizing the lipid bilayers of the stratum corneum. A relatively large quantity of cerebrosides was obtained from bovine brain and converted to ceramides through removal of the carbohydrate side chain. The ceramides were separated based on the absence or presence of hydroxy fatty acyl chains. The lyophilized hydroxyceramides showed a broad melting region at 92 degrees C. Hydroxyceramides dispersed in water produced a relatively narrow, thermotropic transition at 75 degrees C. The effect of cholesterol on this thermotropic phase transition of hydroxyceramides was determined by differential scanning calorimetry. With respect to the main transition, cholesterol caused a broadening of the phase transition at relatively low levels as well as a decrease in the peak transition temperature. The presence of cholesterol at levels in excess of 7 wt% gave rise to an additional low-temperature transition at 55 degrees C. Upon immediate rescanning, this transition was exothermic, but with increasing incubation time the area under the excess heat capacity curve as a function of temperature became smaller. After two days or more, the transition observed was endothermic. At cholesterol levels between 40 and 50 wt%, multiple peaks were observed. From comparisons with related systems, the cooperative thermal transitions of hydroxyceramides with cholesterol are suggested to result from changes in hydrogen bonding or be due to phase separation. The composition of isolated brain ceramides is being compared with that reported for the stratum corneum.

Animals↗
PubMed2001

Isoprene-styrene copolymer elastomer and tetrahydrofurfuryl methacrylate mixtures for soft prosthetic applications.

Novel elastomer/methacrylate systems have been developed for potential soft prosthetic applications. Mixtures of varying compositions of an isoprene-styrene copolymer elastomer and tetrahydrofurfuryl methacrylate (SIS/THFMA) formed one-gel systems and were heat cured with a peroxide initiator. The blends were characterised in terms of sorption in deionised water and simulated body fluids (SBF), tensile properties and viscoelastic parameters of storage modulus and tan delta, as well as glass transition temperatures using dynamic mechanical analysis (DMA). DMA data gave two distinct peaks in tan delta, a lower temperature transition due to the isoprene phase in SIS and one at high temperature thought to be a combination of THFMA and the styrene phase in SIS. The tensile data showed a clear phase inversion within the mid range compositions changing from plastic to elastomeric behaviour. The sorption studies in deionised water showed a two stage uptake with an initial Fickian region that was linear to t 1/2 followed by a droplet growth/clustering system. The slope of the linear region was dependent on the composition ratio. The extent of overall uptake was osmotically dependent as all materials equilibrated at a much lower uptake in SBF. The diffusion coefficients were found to be concentration dependent.

Absorption↗
PubMed2005

Fatty acid and hydroxy acid adaptation in three gram-negative hydrocarbon-degrading bacteria in relation to carbon source.

The lipids of three gram-negative bacteria, Acinetobacter calcoaceticus, Marinobacter aquaeolei, and Pseudomonas oleovorans grown on mineral media supplemented with ammonium acetate or hydrocarbons, were isolated, purified, and their structures determined. Three pools of lipids were isolated according to a sequential procedure: unbound lipids extracted with organic solvents, comprising metabolic lipids and the main part of membrane lipids, OH--labile lipids (mainly ester-bound in the lipopolysaccharides, LPS) and H+-labile lipids (mainly amide-bound in the LPS). Unsaturated FA composition gave evidence for an aerobic desaturation pathway for the synthesis of these acids in A. calcoaceticus and M. aquaeolei, a nonclassic route in gram-negative bacteria. Surprisingly, both aerobic and anaerobic pathways are operating in the studied strain of P. oleovorans. The increase of the proportion of saturated FA observed for the strain of P. oleovorans grown on light hydrocarbons would increase the temperature transition of the lipids for maintaining the inner membrane fluidity. An opposite phenomenon occurs in A. calcoaceticus and M. aquaeolei grown on solid or highly viscous C19 hydrocarbons. The increases of FA < C18 when the bacteria were grown on n-nonadecane, or of iso-FA in cultures on isononadecane would decrease the transition temperature of the lipids, to maintain the fluidity of the inner membranes. Moreover, P. oleovorans grown on hydrocarbons greatly decreases the proportion of P-hydroxy acids of LPS, thus likely maintaining the physical properties of the outer membrane. By contrast, no dramatic change in hydroxy acid composition occurred in the other two bacteria.

Acinetobacter calcoaceticus↗