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At least 685 records · Page 38Linked to original sources
PubMed1995

Study of pH and temperature-induced transitions in urate oxidase (Uox-EC1.7.3.3) by microcalorimetry (DSC), size exclusion chromatography (SEC) and enzymatic activity experiments.

Purified recombinant urate oxidase (urate oxygen oxidoreductase EC 1.7.3.3. re-Uox) has been studied by means of differential scanning calorimetry (DSC) in correlation with enzymatic activity measurements and size exclusion chromatography. Differential scanning calorimetry curves versus pH show two endothermal effects in the pH range 6-10. The first endotherm reveals a maximum stability between pH 7.25 and pH 9.5 corresponding to a temperature of transition T(m1) of 49.0 degrees C and an enthalpy of transition of 326 kJ mol(-1). This value dramatically decreases below pH 7.25. The behavior of the second endotherm is more complex but the temperature of transition T(m2) is constant between pH 9 and 7.25 and a maximum for the corresponding enthalpy is obtained near pH 8 with DeltaH(2)=272 kJ mol(-1). An optimal pH of 8.0 for the stability of the enzymatic activity at elevated temperature was also found which was in good agreement with calorimetric results. Reversibility of the first endotherm is obtained from 20 to 51.5 degrees C. The calorimetric result is correlated to enzymatic activity, purity by size exclusion chromatography (SEC) and protein concentration measurements. In contrast, for the second endotherm, after heating up to 68.9 degrees C, no reversibility was found. Interaction with structural analogues of urate has been studied by DSC. 8-Azahyooxanthine has only a small effect and caffeine has no effect at all. With 8-azaxanthine, a rapid increase of the T(m1) function of the concentration is obtained. At high concentration T(m1) reached the T(m2) value which remained unaffected.

Journal Article↗
PubMed1999

Thermal stability of PNA/DNA and DNA/DNA duplexes by differential scanning calorimetry.

Thermodynamics of the thermal dissociation transitions of 10 bp PNA/DNA duplexes and their corresponding DNA/DNA duplexes in 10 mM sodium phosphate buffer (pH 7.0) were determined from differential scanning calorimetry (DSC) measurements. The PNA/DNA transition temperatures ranged from 329 to 343 K and the calorimetric transition enthalpies ranged from 209 +/- 6 to 283 +/- 37 kJ mol(-1). The corresponding DNA/DNA transition temperatures were 7-20 K lower and the transition enthalpies ranged from 72 +/- 29 to 236 +/- 24 kJ mol(-1). Agreement between the DSC and UV monitored melting (UVM) determined transition enthalpies validated analyzing the UVM transitions in terms of a two-state transition model. The transitions exhibited reversibility and were analyzed in terms of an AB = A + B two-state transition model which yielded van't Hoff enthalpies in agreement with the transition enthalpies. Extrapolation of the transition enthalpies and free energy changes to ambient temperatures yielded more negative values than those determined directly from isothermal titration calorimetry measurements on formation of the duplexes. This discrepancy was attributed to thermodynamic differences in the single-strand structures at ambient and at the transition temperatures, as indicated by UVM measurements on single DNA and PNA strands.

Base Sequence↗
PubMed1996

Tumor targeting in vivo by means of thermolabile fusogenic liposomes.

Thermolabile fusogenic liposomes were devised based on the stoichiometric 1/2 mixtures of dipalmitoylphosphatidylcholine (DPPC) and elaidic acid (ELA) and from the similar stoichiometric mixtures of DPPC, dipalmitoylphosphatidylglycerol (DPPG) and elaidoyl alcohol (EL-OH) or palmitelaidoyl alcohol (PEL-OH). The resulting vesicle suspensions are fusogenic in the region of hyperthermia (> or = 42 degrees C) and can be targeted selectively to the heated tumor tissue. Incorporation of DPPG or fatty alcohols into the vesicle membranes also leads to a non-specific, temporary vesicle material accumulation in the lung, however, probably due to platelet activation. Vesicle material accumulation in A-431 tumors, xenotransplanted in nude mice, after 30 min of local hyperthermia (42 degrees C) is 4-fold higher for the DPPC/ELA (1/2), 2.8-fold higher for the DPPC/DPPG/EL-OH (0.8/0.2/2) and 3.7-fold higher for the DPPC/ELA/EL-OH (1/1/1) mixtures than for similar vesicles used at the physiological temperature. Extension of hyperthermia to 60 min induces a 7.8-fold relative material accumulation in the tumor tissue when the thermolabile, fusogenic DPPC/ELA/EL-OH (1/1/1) vesicles are used. Simple DPPC vesicles only reach concentrations in the heated tumor or muscle tissue that are 1.85-fold and 1.38-fold higher than in the normothermic control, respectively. This is probably a consequence of simple vasodilatation. In vitro experiments revealed that the adsorption of serum proteins to the vesicle membrane decreases the chain-melting phase transition temperature and the transition enthalpy of vesicle suspension. Adsorption is most prominent at the chain-melting phase transition temperature of the mixed lipid bilayers, which is also the critical temperature for the induction of liposome fusion. This hampers the practical use of the resulting vesicle suspension in vivo. The serum-induced decrease of the chain-melting phase transition temperature, which is likely to change as a function of time in vivo, depends on the lipid composition and on the local surface charge density of vesicles. Incorporation of ELA and DPPG concentrations above 15 mol-%, for example, reduce the extent of protein adsorption onto vesicles. This has to be borne in mind when devising vesicles for practical applications.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1995

Differential scanning calorimetric study of the effect of sterol side chain length and structure on dipalmitoylphosphatidylcholine thermotropic phase behavior.

We have investigated the thermotropic phase behavior of dipalmitoylphosphatidylcholine (DPPC) bilayers containing a series of cholesterol analogues varying in the length and structure of their alkyl side chains. We find that upon the incorporation of up to approximately 25 mol % of any of the side chain analogues, the DPPC main transition endotherm consists of superimposed sharp and broad components representing the hydrocarbon chain melting of sterol-poor and sterol-rich phospholipid domains, respectively. Moreover, the behavior of these components is dependent on sterol side chain length. Specifically, for all sterol/DPPC mixtures, the sharp component enthalpy decreases linearly to zero by 25 mol % sterol while the cooperativity is only moderately reduced from that observed in the pure phospholipid. In addition, the sharp component transition temperature decreases for all sterol/DPPC mixtures; however, the magnitude of the decrease is dependent on the sterol side chain length. With respect to the broad component, the enthalpy initially increases to a maximum around 25 mol % sterol, thereafter decreasing toward zero by 50 mol % sterol with the exception of the sterols with very short alkyl side chains. Both the transition temperature and cooperativity of the broad component clearly exhibit alkyl chain length-dependent effects, with both the transition temperature and cooperativity decreasing more dramatically for sterols with progressively shorter side chains. We ascribe the chain length-dependent effects on transition temperature and cooperativity to the hydrophobic mismatch between the sterol and the host DPPC bilayer (see McMullen, T. P. W., Lewis, R. N. A. H., and McElhaney, R. N. (1993) Biochemistry 32:516-522). Moreover, the effective stoichiometry of sterol/DPPC interactions is altered by a significantly large degree of hydrophobic mismatch between the sterol and the DPPC bilayer. Thus the short chain sterols appear to exhibit considerable immiscibility in gel state DPPC bilayers, effectively limiting their interaction with adjacent phospholipid molecules.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1980

Effect of n-butanol on Na+, K+-ATPase.

The effect of n-butanol on the activity of Na+, K+-ATPase has been found different at different temperatures. Below the phase-transition temperature, n-butanol has a stimulating effect provided the concentration is not higher than 0.3 M. But if the activity is determined above the phase-transition temperature, n-butanol has an inhibitory effect. The p-nitrophenyl phosphatase activity has been found to be inhibited by n-butanol even at comparatively low temperatures. This is in accord with the finding that the Arrhenius plot for p-nitrophenyl phosphatase did not show any break. It is suggested that at lower temperatures the conformational change involved in the transport of Na+, K+ ions may be the rate-limiting factor in the overall reaction, and n-butanol may be used as a probe for perturbation of the membrane to elucidate the mechanism of Na+, K+-ATPase.

4-Nitrophenylphosphatase↗
PubMedDate not supplied

Mechanisms controlling the temperature-dependent binding of proteins to poly(N-isopropylacrylamide) microgels.

Poly(N-isopropylacrylamide) (PNIPA) microgels may offer several advantages over PNIPA-modified surfaces when used as sorbents in temperature-sensitive chromatography. Yet, a full exploitation of these advantages requires a better understanding of the mechanisms controlling the separation process. As a model system, we have studied the binding of three proteins (bovine serum albumin (BSA), ovalbumin, and lysozyme) to PNIPA microgels. Binding experiments were conducted both below (25 degrees C) and above (37 degrees C) the volume phase transition temperature of the gel, T(c). The analysis of the binding isotherms has shown that although an average gel particle contained a larger amount of protein below the phase transition temperature, the concentration of the protein within the particle was higher above this temperature. These findings were attributed to changes in the binding loci due to temperature swings around T(c): whereas a sorption mechanism is dominant below this temperature, surface-adsorption was more important above it. A comparison between the three studied proteins has shown that below T(c) the binding increases with a decrease in the molecular weight. On the other hand, no significant difference in the bound protein amounts was observed above the phase transition temperature. Our results imply that, despite the increase in the gel's hydrophobicity above the phase transition temperature, the resolution in bioseparations based on PNIPA gels is not necessarily better above T(c).

Acrylic Resins↗
PubMed1991

Effect of sugars and polyols on water in agarose gels.

Effects of ribose, glucose, sucrose, ethylene glycol, glycerin, propylene glycol, and sorbitol on water in concentrated agarose gels were studied by differential scanning calorimetry at low temperatures. Changes in the phase transition temperatures of 40% agarose gels, induced by the addition of these chemical reagents, are discussed, together with rheological and thermal data for the same systems at ambient and higher temperatures. Both sugars and polyols are believed to reduce the amount of freezable water and to promote plasticization and molecular mobility of agarose chains in gels, thus shifting the glass transition temperatures to lower temperatures. However, the effects of decreasing freezable water, and the direct effect on the junction zones, produced by sugars seem to be different from the effects produced by polyols.

Calorimetry, Differential Scanning↗
PubMed1982

Structure and thermotropic behavior of phosphatidylserine bilayer membranes.

The structure and thermotropic properties of a homologous series of diacylphosphatidylserines (PS) in the anhydrous and hydrated states have been examined with low-angle X-ray diffraction and differential scanning calorimetry. In the anhydrous state at low temperatures both acidic PS and its NH4+ salts exhibit lamellar bilayer crystal forms that transform to liquid-crystalline hexagonal (type II) structures at higher temperatures. The crystal leads to liquid-crystal transition temperature increases with increasing chain length, the transition temperature of an NH4+-PS being higher than that of its corresponding acidic form. In contrast, the transition enthalpies of the acidic PS are higher than those of the NH4+ salt forms. Hydrated cyclic PS and NH4+-PS exhibit reversible lamellar gel leads to liquid-crystal transitions. In this case the acidic form undergoes this chain length dependent transition at a higher temperature, but with a lower enthalpy change, than the NH4+-PS. Both below and above the hydrocarbon chain melting transition, hydrated lamellar bilayer structures are present. The temperature-composition phase diagram of the NH4+-dimyristoyl-PS/H2O system has been studied in detail. The chain melting transition decreases with increasing hydration, reaching a limiting value of 39 degrees C. X-ray diffraction shows that both the bilayer gel structure and the bilayer liquid-crystal form take up water continuously (i.e., no hydration limit), a characteristic of lipid bilayers with a net charge. Electron-density profiles a NH4+-dimyristoyl-PS at different hydration levels permit detailed analysis of the structural parameters of the PS bilayer.

Calorimetry↗
PubMed1982

Calorimetric investigation of polymyxin binding to phosphatidic acid bilayers.

The cooperative binding process between the antibiotic peptide polymyxin-B and negatively-charged phosphatidic acid bilayers was investigated by differential thermal analysis and completed by fluorescence polarization measurements. The sigmoidal binding curves were analyzed in terms of the interaction energy within a domain formed by polymyxin and phosphatidic acid molecules. The formation of such a heterogeneous domain structure was favoured by high concentration of external monovalent ions. The cooperativity of the binding increased while a charge-induced decrease in the phase transition temperature of the pure lipid phase was observed with increasing ion concentration at a given pH. The reduced lateral coupling within the lipid bilayer in the presence of salt ions, as demonstrated by an increase in the lipid phase transition enthalpy, was considered to facilitate the cooperative domain formation. Moreover, an increase in the cooperativity of the polymyxin binding could be observed if phosphatidic acids of smaller chain length and thus of a lowered phase transition temperature were used. By the use of chemically-modified polymyxin we were able to demonstrate the effect of electrostatic and hydrophobic interaction. Acetylated polymyxin with a reduced positive charge was used to demonstrate the pure hydrophobic effect of polymyxin binding leading to a decrease in the phosphatidic acid phase transition temperature by about 20 degrees C. The cooperativity of the binding was strongly reduced. Cleavage of the hydrophobic polymyxin tail yielded a colistinnonapeptide which caused an electrostatically-induced increase in the phosphatidic acid phase transition temperature. With unmodified polymyxin we observed the combined effects of electrostatic as well as hydrophobic interaction making this model system interesting for the understanding of lipid-protein interactions. Evidence is presented that the formation of the polymyxin-phosphatidic acid complex is a lateral phase separation phenomenon.

Calorimetry, Differential Scanning↗
PubMed1995

Structure and toxicity of amphotericin B/triglyceride emulsion formulations.

This paper investigates the relationship between the in-vitro toxicity of amphotericin B/triglyceride emulsion formulations, and their phospholipid composition and method of manufacture. Emulsion toxicity, as measured by the erythrocyte potassium leakage assay, was found to be dependent on the transition temperature of the phospholipid emulsifier, and the thermal history of the emulsion. Emulsions made using unsaturated phospholipids showed a low toxicity irrespective of the method of manufacture. Emulsions made using phospholipids with phase transition temperatures above the maximum process temperature displayed a toxicity comparable to the deoxycholate-solubilized formulation. This toxicity was significantly reduced when such formulations were heated to a temperature above the phase transition temperature for the phospholipid. We interpret these data as demonstrating that the amphotericin molecule must be inserted fully into the phospholipid emulsifier in order to produce a low toxicity emulsion, and that this insertion is critically dependent on the composition of the emulsifier and its phase transition properties.

Amphotericin B↗
PubMed1980

A theory of the effects of head-group structure and chain unsaturation on the chain melting transition of phospholipid dispersions.

We have developed statistical mechanical descriptions of the effects of head-group structure and acyl chain unsaturation on the chain melting phase transition of aqueous dispersions of bilayers containing glycerophosphocholines and glycerophosphoethanolamines. The theoretical framework is an extension of the model of Jacobs et al. [Jacobs, R. E., Hudson, B. S., & Andersen, H. C. (1975) Proc. Natl. Acad. Sci. U.S.A. 72, 3993]. There are several systematic trends in the experimental transition data for various types of phospholipids. Assumptions about the physical origins of these trends were incorporated into statistical mechanical models, which were used to calculate transition temperatures and enthalpies. The extent to which the calculated results of a model reproduce the experimental trends is taken as a measure of the validity of the assumptions on which the model is based. We found that the gross differences among the transition temperatures of phospholipids with two saturated chains, two trans-unsaturated chains, two cis-unsaturated chains, and one cis-unsaturated and one saturated chain can all be explained in terms of the effect of the double bonds on molecular shape and the subsequent effect of shape on the ability of molecules to pack together into a low-energy state at high density. The dependence of transition temperature on the location of the double bond in cis-unsaturated molecules can be understood on the same basis. The differences between the transition temperatures of glycerophosphocholines and glycerophosphoethanolamines with the same hydrocarbon chains can be explained in terms of a larger intermolecular attraction (or smaller repulsion) for the latter than for the former. These differences depend on the presence or absence of unsaturation in the hydrocarbon chains in a way that is consistent with the postulate that hydrogen bonding between glycerophosphoethanolamines is responsible for the differences.

Chemical Phenomena↗
PubMed1976

The thermal transitions and structural properties of 5alpha-cholestan-3beta-ol esters of aliphatic acids.

5 alpha-Cholestan-3 beta-ol esters of aliphatic acids undergo both enantiotropic and monotropic changes of state. Ten saturated and three unsaturated esters have been examined by differential scanning calorimetry and polarizing microscopy to determine transition temperatures, enthalpies, and entropies. The results are compared with an analogous series of cholesterol esters. All esters of even-numbered n-alkanoic acids from C2 to C20 melt from a crystalline state to an isotropic liquid. The crystalline state has been studied by X-ray powder diffraction. The C8 to C20 esters have progressively increasing crystalline melting transition temperatures from 76 to 99 degrees C and possess similar X-ray powder diffraction patterns, suggesting that these compounds form an isostructural series. Esters of C2, C4, and C6 acids exhibit polymorphism. Crystalline cholestanol oleate melts to an isotropic liquid, whereas cholestanol linoleate and linolenate fail to crystallize, even after several months at -20 degrees C. Esters of the even-numbered saturated acids from C4 to C14 form monotropic cholesteric liquid crystalline phases. Esters C10, C12, and C14 form smectic liquid crystalline phases. Cholestanol oleate, linoleate, and linolenate form both cholesteric and smectic mesophases. The lower smectic to cholesteric and cholesteric to isotropic transition temperatures of the cholestanol esters compared to the corresponding transition temperatures of the analogous cholesterol esters suggest that the delta 5 double bond in cholesterol increases the thermal stability of the mesophases of cholesterol esters.

Binding Sites↗
PubMed2000

Optimization of storage stability of lyophilized actin using combinations of disaccharides and dextran.

The storage stability of a dry protein depends on the structure of the dried protein, as well as on the storage temperature relative to the glass transition temperature of the dried preparation. Disaccharides are known to preserve the native conformation of a dried protein; however, the resulting T(g) of the sample may be too low ensure adequate storage stability. On the other hand, formulations dried with high molecular weight carbohydrates, such as dextran, have higher glass transition temperatures, but fail to preserve native protein conformation. We tested the hypothesis that optimizing both protein structure and T(g) by freeze-drying actin with mixtures of disaccharides and dextran would result in increased storage stability compared to actin dried with either disaccharide or dextran alone. Protein structure in the dried solid was analyzed immediately after lyophilization and after storage at elevated temperatures with infrared spectroscopy, and after rehydration by infrared and circular dichroism spectroscopy. Structural results were related to the polymerization activity recovered after rehydration. Degradation was noted with storage for formulations containing either sucrose, trehalose, or dextran alone. Slight increases in T(g) observed in trehalose formulations compared to sucrose formulations did not result in appreciable increases in storage stability. Addition of dextran to sucrose or trehalose increased formulation T(g) without affecting the capacity of the sugar to inhibit protein unfolding during lyophilization and resulted in improved storage stability. Also, dextran provides an excellent amorphous bulking agent, which can be lyophilized rapidly with formation of strong, elegant cake structure. These results suggest that the strategy of using a mixture of disaccharide and polymeric carbohydrates can optimize protein storage stability.

Actins↗
PubMed2005

Dynamical zero-temperature phase transitions and cosmic inflation or deflation.

For a rather general class of scenarios, sweeping through a zero-temperature phase transition by means of a time-dependent external parameter entails universal behavior: In the vicinity of the critical point, excitations behave as quantum fields in an expanding or contracting universe. The resulting effects such as the amplification or suppression of quantum fluctuations (due to horizon crossing, freezing, and squeezing) including the induced spectrum can be derived using the curved space-time analogy. The observed similarity entices the question of whether cosmic inflation itself might perhaps have been such a phase transition.

Journal Article↗
PubMed2000

Preformulation studies and characterization of the physicochemical properties of amorphous polymers using artificial neural networks.

The utility of artificial neural networks (ANNs) as a preformulation tool to determine the physicochemical properties of amorphous polymers such as the hydration characteristics, glass transition temperatures and rheological properties was investigated. The neural network simulator, CAD/Chem, based on the delta back-propagation paradigm was used for this study. The ANNs software was trained with sets of experimental data consisting of different polymer blends with known water-uptake profiles, glass transition temperatures and viscosity values. A set of similar data, not initially exposed to the ANNs was used to validate the ability of the ANNs to recognize patterns. The results of this investigation indicate that the ANNs accurately predicted the water-uptake, glass transition temperatures and viscosities of different amorphous polymers and their physical blends with a low % error (0-8%) of prediction. The ANNs also showed good correlation between the water-uptake and changes in the glass transition temperatures of the polymers. This study demonstrated the potential of the ANNs as a preformulation tool to evaluate the characteristics of amorphous polymers. This is particularly relevant when designing sustained release formulations that require the use of a fast hydrating polymer matrix.

Alginates↗
PubMed1988

Differential scanning calorimetry and 2H NMR studies of the phase behavior of gramicidin-phosphatidylcholine mixtures.

The extents of two-phase coexistence in the phase diagrams of mixtures of gramicidin with 1,2-bis(perdeuteriopalmitoyl)-sn-glycero-3-phosphocholine (DPPC-d62) and with 1,2-bis(perdeuteriomyristoyl)-sn-glycero-3-phosphocholine (DMPC-d54) mixtures have been explored with differential scanning calorimetry (DSC) and deuterium nuclear magnetic resonance (2H NMR). For both systems, increased gramicidin content causes a decrease in transition enthalpy and a broadening of the peak in excess heat capacity at the transition. In DMPC-d54-based mixtures, the broadening is roughly symmetric about the pure lipid transition temperature. Addition of gramicidin to DPPC-d62 extends the excess heat capacity peak on the low-temperature side, resulting in a slightly asymmetric scan. Deuterium NMR spectra showing a superposition of gel and liquid-crystalline components, observed for both mixtures, indicate the presence of two-phase coexistence. For the DPPC-d62-based mixtures, two-phase coexistence is restricted to an approximately 2 degrees C temperature range below the pure transition temperature. For DMPC-d54-based mixtures, the region of two-phase coexistence is even narrower. For both mixtures, beyond a gramicidin mole fraction of 2%, distinct gel and liquid-crystal contributions to the spectra cannot be distinguished. Along with the broad featureless nature of the DSC scan in this region, this is taken to indicate that the transition has been replaced by a continuous phase change. These results are consistent with the existence of a closed two-phase region having a critical concentration of gramicidin below 2 mol%.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1992

Physicochemical behavior of cytotoxic ether lipids.

The physicochemical properties of four biologically active alkyllysophosphatidylcholine analogs have been investigated using differential scanning calorimetry (DSC), 31P NMR, and fluorescence spectroscopy. Each of the alkyl ether lipids, suspended in a buffer/ethylene glycol matrix, undergo a thermotropic structural rearrangement at temperatures near 0 degree C. The axially asymmetric 31P NMR spectra obtained for all four lipids, after prolonged incubation at -20 degrees C, are consistent with the notion that these amphiphiles are able to form extended lamellar structures at temperatures below the main phase transition temperature (Tm). We have attributed the sharp, isotropic 31P resonance, observed for each derivative at temperatures above the Tm, to the presence of a micellar state. This has been confirmed via 31P NMR experiments employing the lanthanide shift reagent PrCl3. The critical micelle concentrations (cmc) of these lysolipids have been determined via the fluorescence titration of [6-palmitoyl-2-[[(2-trimethylammonium)ethyl]methyl]amino] naphthalene (PATMAN). The low values obtained for both the lamellar to micellar phase transition temperature (-7.88 +/- 0.19 degree C) and the cmc (0.15 +/- 0.02 microM) for the 2-benzyloxylysolipid (1) indicate that this analog exhibits a marked preference for the micellar state relative to the 2-methoxy (2), 2-deoxy (3), and 2-hydroxy (4) substituted compounds.

Calorimetry, Differential Scanning↗