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1.
Differential scanning calorimetry and Raman spectroscopy have been used to examine the effects of ubiquinones (UQn) on the thermotropic phase behavior of dipalmitoylphosphatidylcholine (DPPC) in multilamellar vesicles, for UQ/DPPC molar ratios ranging from 0.01 to 0.1. The influence of the side chain length has been investigated by comparing the effect of a series of UQ with 2 (UQ2), 4 (UQ4), 6 (UQ6) and 10 isoprene units (UQ10).In the presence of increasing amount of UQ2 or UQ4, concomitant shift of the gel to liquid crystalline phase transition towards lower temperatures and vanishing of the pretransition are observed. Short-chain ubiquinones are thus inserted parallel to phospholipid chains, their benzoquinone ring being close to the DPPC polar headgroups. In addition, broadening and skewing of the main transition peak support the fact that UQ2 and UQ4 are laterally self-organized in highly concentrated regions located at the boundary of lipid domains. The lipid thermotropic behavior is not affected by the presence of other analogues of the series, UQ6 and UQ10. They remain homogeneously dispersed within the midplane of the phospholipid bilayer. Such a chain length dependence on the location and the organization of ubiquinones analogues may be correlated with their biological activities in biomembranes.  相似文献   

2.
The effect of 2,4-dichlorophenol (DCP) on the main transition and pretransition of fully hydrated (20 mass%) dipalmitoylphosphatidylcholine (DPPC) multilamellar liposomes has been studied by differential scanning calorimetry (DSC). It was observed that an increase in the molar ratio of DCP/DPPC (from 4·10-5 up to 2·10-2) causes progressive reductions in the temperature and enthalpy of the pretransition. The higher concentration of DCP eliminates the pretransition. The influence of DCP on the main transition in this molar ratio range is not drastic, but a decrease in temperature and in the enthalpy values was observed. In the molar ratio range (from 2·10-1 up to 4·10-1) the DSC scans show multiple main transition peaks instead of the characteristic single peak of the main transition. Above a DCP/DPPC molar ratio of 0.6 a new peak appears at 25°C having about the same transition enthalpy as the main transition of the pure system.This revised version was published online in November 2005 with corrections to the Cover Date.  相似文献   

3.
In this paper, the effect of increasing amounts of lysozyme (Lyso) and bovine serum albumin (BSA) on the behaviour of lecithin (DMPC) and cephalin (DMPE) liposomes was investigated by means of Raman and DSC techniques. The results showed that both proteins affected, but in a different way, both lecithin and cephalin liposomes. In the samples with lower Lyso concentrations (up to 2 % w/w), a small decrease on the main transition temperature (T m) was observed, whereas T m increased by further addition of Lyso (up to 15.0 % w/w). At the same time, an increase of about 20 % in the ΔH of the transition was observed. Pre-transition was also affected in a greater extent by protein presence. When BSA interacted with liposomes, a smaller increase in the T m values was observed with a contemporary increase of about 8 % in the associated ΔH. The data suggested that the BSA–liposomes interaction involves only the external surface of the bilayer, excluding thus any penetration into the liposomal hydrophobic core. On the contrary, a partial penetration into the bilayer is suggested when Lyso is added to liposomes. Both considered proteins strengthened the overall bilayer structure of DMPC liposomes, suggesting a decrease in the membrane permeability. Moreover, Lyso secondary structure changed by interaction with liposomes, as demonstrated by the Raman spectra behaviour, in particular in the case of DMPE.  相似文献   

4.
The monolayer behavior of three mixed systems of dipalmitoyl phosphatidyl choline (DPPC) with sterols; cholesterol (Ch), stigmasterol (Stig), and cholestanol (Chsta) formed at the interface of air/water (phosphate buffer solution at 7.4 with addition of NaCl) was investigated in terms of surface pressure (π) and molecular occupation surface area (A) relation. A series of πA curves at every 0.1 mol fraction of each sterol for the three combinations of mixed systems were obtained at 25.0 °C.

On the basis of the πA curves, the additivity rule in regard to A versus sterol mole fraction (Xst) was examined at discrete surface pressures such as 5, 10, 15, 20, 25, 30 mN m−1, and then from the obtained AXst curves the partial molecular areas (PMA) were determined. The AXst relation exhibited a marked negative deviation from ideal mixing in the pressure range below 10 mN m−1, i.e. in the expanded liquid film region (below the transition pressure of DPPC).

The PMA of Ch at π=5 mN m−1, for example, was found to be conspicuously negative in the range of XCh=0–0.2 (about −0.4 nm2 per molecule) and slightly positive (ca. 0.1 nm2 per molecule) in the range XCh=0.2 to 0.4. Above XCh=0.5, Ch’s PMA was almost the same as the surface area of pure Ch, while DPPC’s PMA was reduced to 60% of that of the pure system.

Excess Gibbs energy (ΔG(ex)) as a function of Xst was estimated at different pressures. Applying the regular solution theory to thermodynamic analysis of ΔG(ex), the activity coefficients (f1 and f2) of DPPC and the respective sterols as well as the interaction parameter (Ip) in the mixed film phase were evaluated; the results showed a marked dependence on Xst.

Compressibility Cs and elasticity Cs−1 were also examined. These physical parameters directly reflected the mechanical strength of formed monolayer film.

Phase diagrams plotting the collapse pressure (πc) against Xst were constructed, and the πc versus Xst curves were examined for the respective mixed systems in comparison with the simulated curves of ideal mixing based on the Joos equation.

Comparing the monolayer behavior of the three mixed systems, little remarkable difference was found in regard to various aspects. In common among the three combinations, the mole fraction dependence in monolayer properties was classified into three ranges: 0<Xst<0.2, 0.2<Xst<0.4 and 0.5<Xst<1. How the difference in the chemical structure of the sterols influenced the properties was examined in detail.  相似文献   


5.
Cationic liposomes are studied mainly as nonviral nucleic acid delivery systems and to a lesser extent as carriers/adjuvants of vaccines and as low-molecular-weight drug carriers. It is well established that the performance and the biological activity of liposomes in general are strongly related to their physicochemical properties. We investigated the thermotropic behavior and the size distribution of mixed cationic liposomes formulated with different percentages of 1,2 dimyristoyl-sn-glycero-3-phosphatidylcholine and one of four cationic amphiphiles characterized by a pyrrolidinium headgroup with the aim of achieving a better understanding of how the molecular structure of the cationic amphiphile and its mole percentage affect the physicochemical properties of the liposomes. Multilamellar vesicles and large unilamellar vesicles were studied by differential scanning calorimetry and turbidity, respectively, to characterize the thermotropic behavior and lipid phase, whereas dynamic light scattering was used to determine size distribution. This study shows that subtle modifications in the cationic amphiphile's molecular structure and in liposome composition may have dramatic effects on the organization of the liposome bilayer and hence on the morphological and physicochemical features of the liposomes, thus being highly relevant to the biological features investigated previously.  相似文献   

6.
A literature survey has been made of heat capacity and transition enthalpy values for ammonium nitrate. New DSC experiments lead to Δ H0 values for the IV å III, III å II and II å I transitions in good agreement with older data, and to a new value for the enthalpy of fusion. The effect of thermal history and moisture content is discussed.  相似文献   

7.
The new thermotropic polyester/polyaniline (PIn/PAni) blends have been prepared by solution blend of synthesized liquid crystalline poly[4,4′‐bis (ω‐alkoxy) biphenylisophthalate]s having four and six methylene units in spacer (PI4 and PI6) with PAni doped with camphorsolfonic acid (CSA). The percolation threshold electroactivity of prepared blend films has been determined by cyclic voltammetry. The effect of the PAni concentration, solvent nature and polyester structure on the electroactivity of the blends has been investigated. The extremely low percolation threshold of prepared PIn/PAni‐CSA blends from dimethylformamide (DMF) and m‐cresol solution was 3% weight of PAni‐CSA. The amount of conducting polymer necessary to retard the formation of the liquid crystalline (LC) phase is up to 45% by weight. Phase behavior studies by differential scanning calorimetry and polarizing microscopy show that blends with 45% of conducting polymer are both liquid crystal and conductive. The morphology of the blends has been investigated by scanning electron microscopy. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

8.
Transmission electron micrographs (TEM) showed that liposome vesicles prepared from DL-α-phosphatidylcholine dimyristoyl (1,2-ditetradecanoyl-rac-glycerol-3-phosphocholine) (DMPC) by the modified reverse-phase evaporation method (mREV) were spherical in shape and in majority of them were less than 100 nm in diameter. Differential scanning calorimetry (DSC) method was used to determine the influence of cholesterol content and pH of Tris-HCl buffer used for the preparation of liposomes on the temperature of phase transition T C of phospholipids which form the investigated liposome vesicles. The use of DSC method made it possible to determine not only the temperature of the main phase transition of phospholipids but also the temperature of the phospholipid phase transition from the tilted gel phase(L β′) to the ripple gel phase(P β′). The results were compared with those obtained with EPR study. EPR study was carried out in the temperature range from 284 to 310 K i.e. below and above the phase transition temperature T C of DMPC. On the basis of EPR spectra of spin marker 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) incorporated into the liposome, the values of parameters f were determined. Hence TEMPO can be used to observe the change in partition between aqueous and fluid lipid regions. The change in the relative values of f determined for DMPC as a function of temperature shows that this phospholipid undergoes a transition from a ‘gel phase’ to a lamellar smectic liquid crystalline phase in the presence of excess water. The EPR study of TEMPO allowed us to determine the transition temperature T C. The results were compared with those obtained with DSC method.  相似文献   

9.
The temperature (T)—pressure (P) phase diagrams of aqueous solutions of a homologous series of cationic surfactants, tetradecyl- (C14TAB), hexadecyl- (C16TAB), and octadecyltrimethylammonium bromide (C18TAB), have been determined by observing the sudden change of the transmittance accompanying the phase transition under high pressure up to 160 MPa. Regarding three kinds of phase transitions which have been previously assigned by the differential scanning calorimetry (DSC) (S. Kaneshina and M. Yamanaka, J. Colloid Interface Sci.131, 493, 1989), all the transition temperatures were linearly elevated by applying pressure. The volume changes associated with the transitions were estimated from the Clapeyron—Clausius equation by using the values of the T—P slopes on the phase diagrams and of the transition entropies taken from the DSC study. A chemical potential vs pressure profile, of which slope reflects the partial molar volume, among the states of surfactant assemblies, i.e., micelle, gel, and coagel, was drawn schematically on the basis of the transition volumes. The phase boundary between the coagel phase and the micellar solution should be the critical solution line of the surfactant, representing the pressure dependence on the Krafft temperature. In the C18TAB-water system, the phase boundary line between the metastable gel and the supercooled micelle had a break point at 45 MPa, suggesting the existence of a new pressure-induced mesophase above 45 MPa. The metastable gel phase of C14TAB disappeared in the pressure range up to 160 MPa.  相似文献   

10.
The phase behavior of mixtures formed by palmitic acid (PA), cholesterol (Chol), and sodium cholesteryl sulfate (Schol) has been characterized by differential scanning calorimetry and infrared and 2H NMR spectroscopy. It is reported that it is possible to form, with PA/sterol mixtures, fluid lamellar phases where the sterol content is very high (a sterol mole fraction of 0.7). As a consequence of the rigidifying ability of the sterols, the PA acyl chains are very ordered. The stability of these self-assembled bilayers is found to be pH-dependent. This property can be controlled by the Chol/Schol molar ratio, and it is proposed that this parameter modulates the balance between the intermolecular interactions between the constituting species. A phase-composition diagram summarizing the behavior of these mixtures as a function of pH, at room temperature, is presented. It is also shown that it is possible to produce large unilamellar vesicles (LUVs) from these mixtures, using standard extrusion techniques. The resulting LUVs display a very limited passive release of the entrapped material. In addition, these LUVs constitute a versatile vector for pH-triggered release.  相似文献   

11.
The effects of diphenyltin dichloride (DPhT), triphenyltin chloride (TPhT) and tetraphenyltin (TTPhT) on the thermotropic phase behaviour of phosphatidylcholine bilayers were studied. All the phenyltin compounds investigated affected phase transitions differently. TTPhT broadened the main phase transition but it left the transition temperatures and enthalpy unchanged. TPhT reduced the transition temperatures and the enthalpies while DPhT showed a dual effect on the pretransition and the main transition. At low concentrations DPhT reduced the temperatures of the transitions slightly and at higher concentrations it increased them. Based on differential scanning calorimetry (DSC) and also 1H NMR and 31P NMR measurements, it is suggested that DPhT induces interdigitated gel phase formation and TPhT induces hexagonal phase formation. TTPhT seems to affect the structure only a little. The toxic activity of DPhT and TPhT seems to be connected with their ability to induce changes in the membrane structure. Copyright © 2000 John Wiley & Sons, Ltd.  相似文献   

12.
Three samples of semiflexible thermotropic liquid-crystalline polyesters based on mesogenic aromatic triads and a decamethylene spacer have been investigated by IR spectroscopy at various temperatures between room temperature and 290°C. The crystal-to-liquid crystal (either nematic or smectic) phase transition was accompanied by fairly strong spectral variations, whereas slight, but significant, changes in the IR profiles were detected at the liquid crystal-to-isotropic transition. By comparing the results obtained with the spectral behavior recorded for corresponding structural analogs of low molar mass, it was possible to attribute the spectral variations observed in the polymer samples to a decrease in intermolecular interactions rather than to conformational changes. The thermal transitions indicated by IR spectra were in good agreement with the analogous data obtained by calorimetric or optical microscopy techniques.  相似文献   

13.
The urethane forming cure reactions of hydroxyl terminated polybutadiene (HTPB) binder with three different isocyanate curatives, viz., toluene diisocyanate (TDI), isophorone diisocyanate (IPDI) and 4,4-methylene bis(cyclohexyl isocyanate) (MCHI), were investigated by differential scanning calorimetry (DSC). The effect of two cure catalysts, viz., dibutyl tin dilaurate (DBTDL) and ferrric tris-acetylacetonate (FeAA) on the cure reactions was also studied. Cure kinetics was evaluated using the multiple heating rate Ozawa method. The reactivities of the three isocyanates and catalytic efficiencies were compared based on the DSC reaction temperatures, activation energies and rate constants. Viscosity build-up in these systems at isothermal temperature was also studied and compared with the results from DSC.  相似文献   

14.
Upon slow cooling, a small fraction of the thermotropic liquid crystal S1496 exhibits a new phase which shows a high-resolution carbon-13 NMR spectrum with isotropic chemical shifts. This new phase is probably either cubic phase or a plastic phase. Its formation is facilitated by the addition of small amounts of solutes. Some other liquid crystals containing alkylcyclohexanes behave similarly.  相似文献   

15.
The thermotropic and lyotropic phase behavior of 1- and 5-decyl urea, and 1-, 2-, 4-, and 6-dodecyl urea have been studied. This allowed the effect of positional isomerism to be examined. Intermolecular hydrogen bonding by the urea moiety is the dominant factor in determining the solid-state thermal behavior and crystal solubility boundary of these linear nonionic surfactants. The positional isomers where the urea moiety was not situated at the terminus of the hydrocarbon chain exhibited higher melting points than the 1-alkyl ureas. This has been rationalized by postulating interdigitated chains in the solid state. In the urea surfactant-water systems, three phases are observed, viz. crystalline solid, a dilute aqueous solution of the alkyl urea, and an isotropic liquid. The last two phases coexist in the low-surfactant, high-temperature region of the binary phase diagram. An overview of structure-property correlations for linear nonionic urea surfactants is presented in light of the new physicochemical data obtained for the decyl urea and dodecyl urea positional isomers.  相似文献   

16.
The effect of γ-radiation on the cis-1,4-polyisoprene in the presence of oxygen is investigated by ATR-FTIR technique and non-isothermal DSC measurements. FTIR measurements have shown that the formation of hydroperoxides, ketones, alcohols and/or ethers is apparent already at lower, 20–50 kGy, doses of γ-radiation and it increases significantly with the exposure time. Besides, lactones, anhydrides, peresters, carboxylic acids, and esters are formed, too. Spectral changes in the region of C=C conjugated double bonds indicate a formation of shorter polyene structures and aromatic rings. Kinetic parameters describing the temperature dependence of the induction period have been obtained from DSC measurements using the isoconversional method. Residual stabilities have been calculated in order to characterize the gamma radiation effect on polyisoprene thermooxidative stability. Both methods proved that doses lower than 50 kGy do not cause severe changes in polymer properties.  相似文献   

17.
Lipid bilayers consisting of lipids with terminally perfluoroalkylated chains have remarkable properties. They exhibit increased stability and phase-separated nanoscale patterns in mixtures with nonfluorinated lipids. In order to understand the bilayer properties that are responsible for this behavior, we have analyzed the structure of solid-supported bilayers composed of 1,2-dipalmitoyl- sn-glycero-3-phosphocholine (DPPC) and of a DPPC analogue with 6 terminal perfluorinated methylene units (F6-DPPC). Polarized attenuated total reflection Fourier-transform infrared spectroscopy indicates that for F6-DPPC, the tilt of the lipid acyl chains to the bilayer normal is increased to 39 degrees as compared to 21 degrees for native DPPC, for both lipids in the gel phase. This substantial increase of the tilt angle is responsible for a decrease of the bilayer thickness from 5.4 nm for DPPC to 4.5 nm for F6-DPPC, as revealed by temperature-controlled imaging ellipsometry on microstructured lipid bilayers and solution atomic force microscopy. During the main phase transition from the gel to the fluid phase, both the relative bilayer thickness change and the relative area change are substantially smaller for F6-DPPC than for DPPC. In light of these structural and thermotropic data, we propose a model in which the higher acyl-chain tilt angle in F6-DPPC is the result of a conformational rearrangement to minimize unfavorable fluorocarbon-hydrocarbon interactions in the center of the bilayer due to chain staggering.  相似文献   

18.
The correlation between morphological changes and DSC recordings gives valuable information on the mechanism of phase transformations. The present paper describes a new instrument for simultaneous DSC and thermomicroscopy in transmitted light, where a DSC device is placed in a commercially available hot-stage. The application of this DSC/thermomicroscopy is exemplified by study of the phase diagrams for KNO3-NaNO3, diphenylamine-benzophenone and a liquid crystal system.
Zusammenfassung Die Korrelation zwischen morphologischen Veränderungen und DSC-Kurven ergibt wertvolle Informationen über den Mechanismus von Phasenumwandlungen. Es wird ein neues Gerät zur simultanen DSC und Thermomikroskopie im durchfallenden Licht beschrieben, in dem die DSC-Einheit in einem im Handel erhältlichen hot-stage untergebracht ist. Die Anwendbarkeit dieses DSC/Thermomikroskops wird durch Untersuchung der Phasendiagramme KNO3-NaNO3, Dyphenylamin-Benzophenon und eines Flüssigkeitskristallsystems demonstriert.

. , . KNO3-NaNO3, - .
  相似文献   

19.
Cold pressed linseed oil and paints prepared using the inorganic pigments; lead white and red lead, were characterized using non-isothermal differential scanning calorimetry (DSC) in an air atmosphere to determine the effect of the pigment on the oxidative polymerisation of the drying oil medium. For each paint sample, the onset temperature for oxidation was reduced from 166°C to the range 50 to 60°C when a heating rate of 5 K min-1 was used. In order to determine the rate of drying, the non-isothermal experiments were carried out using a range of heating rates. A change in the mechanism oxidative polymerization was observed as the heating rate was increased.  相似文献   

20.
An intensive study has been conducted to compare the effects of malei hydrazine (MH) and hydroquinone (HQ) on the liquid crystallinity and phase transition behavior in the ABA/HQ/TFTA and ABA/MH/TFTA copolyesters (p‐acetoxybenzoic acid (ABA) and tetrafluoroterephthalic acid (TFTA)). These two copolyesters were prepared by thin‐film polymerization and characterized by differential scanning calorimetry (DSC), polarizing light microscope (PLM), wide‐angle X‐ray diffraction (WAXD), as well as Cerius2 computational simulation. Characterization and comparison of the liquid crystalline (LC) evolution and morphology changes of HQ moiety with corresponding MH moiety suggest that ABA/MH/TFTA system is energetically favorable to mesophase formation than ABA/HQ/TFTA system. When the films are quenched, a surface microcrack decoration is observed in both systems. Both systems, which have the persistence ratio larger than 6.42, satisfy the minimum requirement for the LC formation by molecular science software. The ABA/MH/TFTA film exhibits only one single peak transition. However, two distinct transitions have been observed in the ABA/HQ/TFTA system. The average Avrami exponent, n, is ~1.2, and PLM and WAXD results suggest mesophase transition in ABA/MH/TFTA film. As reflected by the results obtained from PLM, WAXD, and DSC studies, the phase transition is confirmed as crystal → nematic → isotropic in ABA/HQ/TFTA copolyester. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2230–2242, 2005  相似文献   

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