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PubMed2005

Temperature-induced phase transitions of the ordered superlattice assembly of Au nanoclusters.

Superlattices of monolayer protected metallic and semiconducting nanoclusters have attracted significant attention due to their promising applications in nanotechnology. In this paper, we investigate the effect of temperature on the ordered superlattice structure of relatively larger sized Au nanoclusters passivated with dodecanethiol [ca. Au1415(RS)328] with the help of in situ temperature controlled X-ray diffraction (XRD) and infrared spectroscopy (IR) in conjunction with thermogravimetric (TG) and differential scanning calorimetric (DSC) analysis. In brief, monolayer protected Au nanoclusters (AuMPC) were prepared by a modified Brust synthesis technique, where dodecanethiol itself acts as both phase transfer and simultaneous capping agent during the reduction process, generating an average particle size of 3.72 +/- 0.4 nm after repeated solvent extraction and careful fractionation experiments. These particles are characterized with the help of UV-vis, transmission electron microscopic (TEM), IR, and NMR techniques, where effective capping as well as the superlattice formation on the TEM grid is evident from the combined analysis of these results. In situ low-angle XRD analysis shows that the particles undergo an irreversible phase transition in the temperature range of 100-115 degrees C, which is also reflected in the data from in situ IR analysis. However, the DSC analysis does not account for this phase transition, although the reversible phase transition due to the alkyl chain dynamics is in good agreement with the previously reported results. These results indicate the formation of temperature-induced, diffusion-limited phase transition involving nonequilibrium fractal structures, which is in good agreement with the previous available theoretical studies. The determination of the temperature window for the stability of these ordered assemblies would be used to understand the effect of thermal stress for device applications.

Journal Article↗
PubMed2006

Neuroparsin transcripts as molecular markers in the process of desert locust (Schistocerca gregaria) phase transition.

Desert locust swarms occasionally cause severe ecological and economic damage, particularly in countries of northwest Africa. However, the physiological mechanisms underlying locust phase transition, the switch of the solitarious to the gregarious phase, remain elusive. Therefore, identification of molecular changes linked to this phenomenon represents a primary requirement to start unraveling this enigma. The present paper provides novel information on phase-related molecular markers for locust phase transition. We present a detailed quantitative real-time RT-PCR analysis of two distinct neuroparsin precursor transcripts (Scg-NPP3 and Scg-NPP4) in the brain and in abdominal tissues of gregarious and solitarious desert locusts (Schistocerca gregaria). Our data reveal different temporal changes of these transcripts in the fat body during the adult stage of both phases. We, hereby, present novel scientific evidence for a phase-dependent regulation of these particular peptide hormone encoding transcripts and assign them as possible molecular markers in the process of locust phase transition.

Animals↗
PubMed2003

Hydration pressure and phase transitions of phospholipids. I. Piezotropic approach.

Dehydration reduces the main phase transition pressure of phospholipids. An analysis based on the Gibbs-Duhem equation shows how the shift of the transition pressure is correlated to the hydration pressure. By using Fourier transform infrared (FT-IR) spectroscopy we determined the hydration-dependent phase transition pressure. The application of our new approach gives hydration pressure values which agree with the values obtained with the osmotic stress method.

Hydrophobic and Hydrophilic Interactions↗
PubMed1984

Influence of oxygenated sterol compounds on phase transitions in model membranes. A study by differential scanning calorimetry.

A marked influence of oxygenated sterol compounds (OSC: 7 alpha-, 7 beta-, and 25-hydroxycholesterol and 7-ketocholestanol) on the reversible gel to liquid-crystalline phase transition behavior of cholesterol-free and cholesterol-containing model membranes is evidenced by high-sensitivity differential scanning calorimetry. Liposomes of dipalmitoylphosphatidylcholine (DPPC) were chosen as model membranes. Each of the investigated OSC exerts an individual influence on the phase transition of DPPC liposomes, which expresses itself in the temperature range, the corresponding enthalpy, and the peak shape of calorimetric curves. The onset temperature of the phase transition is lowered in the following range of effectiveness: 7 beta-hydroxycholesterol greater than 7 alpha-hydroxycholesterol greater than 7-ketocholestanol greater than cholesterol. The mutual presence of cholesterol and of OSC leads to the following order: 7 alpha-hydroxycholesterol approximately equal to 7 beta-hydroxycholesterol greater than 7-ketocholestanol greater than cholesterol (without OSC) greater than 25-hydroxycholesterol. The enthalpy of the phase transition is decreased with increasing content of cholesterol, 7 alpha- or 7 beta-hydroxycholesterol, or 7-ketocholestanol. At a concentration of about 10 mol % of the latter three OSC, the corresponding enthalpy value of the transition is lowered from 9.1 kcal/mol for pure DPPC to about 7.5 kcal/mol, whereas 10 mol % cholesterol lowers the enthalpy value to 7.0 kcal/mol.(ABSTRACT TRUNCATED AT 250 WORDS)

Calorimetry, Differential Scanning↗
PubMed2004

Disorder-induced rounding of the phase transition in the large-q-state Potts model.

The phase transition in the q -state Potts model with homogeneous ferromagnetic couplings is strongly first order for large q, while it is rounded in the presence of quenched disorder. Here we study this phenomenon on different two-dimensional lattices by using the fact that the partition function of the model is dominated by a single diagram of the high-temperature expansion, which is calculated by an efficient combinatorial optimization algorithm. For a given finite sample with discrete randomness the free energy is a piecewise linear function of the temperature, which is rounded after averaging, however, the discontinuity of the internal energy at the transition point (i.e., the latent heat) stays finite even in the thermodynamic limit. For a continuous disorder, instead, the latent heat vanishes. At the phase transition point the dominant diagram percolates and the total magnetic moment is related to the size of the percolating cluster. Its fractal dimension is found d(f) = ( 5 + square root of 5)/4 and it is independent of the type of the lattice and the form of disorder. We argue that the critical behavior is exclusively determined by disorder and the corresponding fixed point is the isotropic version of the so-called infinite randomness fixed point, which is realized in random quantum spin chains. From this mapping we conjecture the values of the critical exponents as beta=2- d(f), beta(s) =1/2, and nu=1.

Journal Article↗
PubMed1986

Effect of ethylene glycol on the phase transition kinetics of gluco- and galactocerebrosides.

The effect of different concentrations of ethylene glycol in water on the phase transition (metastable----stable state) of Gaucher's glucocerebroside, of bovine brain cerebroside type II (non hydroxy acyl chains only) and of N-palmitoylgalactocerebroside has been investigated. The phase transition and its kinetics were inferred from the thermograms at different heating and cooling rates and confirmed by FTIR spectra of the cerebrosides in the different states. The significance of the conformational differences of the glucose and of the galactose residues with respect to their solvation, and the subsequent effect on the intermolecular interactions and the phase transition is discussed.

Calorimetry, Differential Scanning↗
PubMed1991

On the reversibility of the phase transitions in lipid-water systems.

Heating and cooling phase sequences observed in phospholipid and glycolipid dispersions in excess water have been summarized. Differences between heating and cooling sequences and also with respect to a reference phase sequence "subgel-gel-lamellar liquid crystalline-cubic-inverted hexagonal" have been pointed out. Together with kinetic data obtained by alternating current (AC) calorimetry, these data have been used for a discussion on the reversibility of the lipid phase transitions. Several typical symptoms of irreversible behavior are (i) undercooling of stable phases; (ii) formation of phases which are metastable over the whole range of their existence; (iii) slow formation of the nascent phase requiring isothermal annealing out of the transition region; (iv) different nonconvergent transition pathways in heating and cooling. These phenomena are related to the appearance of slow rearrangement modes during the phase transitions with characteristic times longer than experimental time scales. Similarly slow relaxations supporting the existence of long-lived non-equilibrium lipid states in the biomembranes may have also certain physiological significance.

Glycolipids↗
PubMed1977

Depression of phase-transition temperature in a model cell membrane by local anesthetics.

Disordering, fluidizing and dilating effects of anesthetics upon cell membranes are well recognized. The fluidization can be precisely measured with phospholipid membranes. When phospholipids are dispersed in water, they form globules of bilayer structure. These model membranes undergo transition between crystalline (ordered and less fluid) and liquid crystalline (less ordered and fluid) phases according to the temperature, the degree of packing of each molecule, and the chemical environment. The phase transition occurs in a cooperative fashion and the turbidity of the dispersion changes abruptly, clear in liquid crystalline phase and turbid in crystalline phase. The present study was undertaken to quantitate the fluidizing effects of local anesthetics on dipalmitoyl lecithin (DPL) bilayer dispersion by measuring the turbidity change. Tetracaine, bupivacaine, lidocaine, and procaine were studied. They all depressed the phase-transition temperature. The binding of the drugs to the model membrane followed unsaturable kinetics, and the pH titration curve showed that only uncharged molecules were active. The freezing point depression was analyzed according to the Van't Hoff model. From this model, the partition coefficients of the uncharged molecules between DPL and water were estimated: lidocaine 76, procaine 159, bupivacaine 812, and tetracaine 1,405. The concentration of local anesthetics in the DPL phase needed to decrease the phase-transition temperature 1 degree C showed a constant value of 0.132 M. The concentration of local anesthetics in the DPL phase is a function of pH, partition coefficient, and volume ratio between the DPL and aqueous phases. The normalized values of the fluidizing action of these drugs at physiologic conditions correlated well with their nerve-blocking potencies. The present results indicate that the uncharged molecules fluidize the lecithin membrane by unsaturable nonspecific binding. The possible effect of the charged molecules upon the fluidity of natural membranes remains to be established.

Anesthetics, Local↗
PubMed2002

Substrate-induced deformation and adhesion of phospholipid vesicles at the main phase transition.

The physiochemical properties of phospholipid vesicle, e.g. permeability, elasticity, etc., are directly modulated by the chain-melting transition of the lipid bilayer. Currently, there is a lack of understanding in the relationship between thermotropic transition, mechanical deformation and adhesion strength for an adherent vesicle at temperature close to main phase transition temperature T(m). In this study, the contact mechanics of dimyristoyl-phosphatidylcholine (DMPC) vesicle at the main phase transition are probed by confocal reflectance interference contrast microscopy in combination with phase contrast microscopy. It is shown that DMPC vesicles strongly adhere on pure fused silica substrate at T(m) and the degree of deformation as well as the adhesion energy is a decreasing function against the mid-plane diameter of the vesicles. Furthermore, an increase of osmotic pressure at the gel/liquid crystalline phase co-existence imposes insignificant changes in both the degree of deformation and adhesion energy of adherent vesicles when the lipid bilayer permeability is maximized. With the reverse of substrate charge, the mechanical deformation and adhesion strength for larger vesicles (mid-plane diameter >18 microm) are significantly reduced. By monitoring the parametric response of substrate-induced vesicle adhesion during main phase transition, it is shown that the degree of deformation and adhesion energy of adhering vesicle is increased and unchanged, respectively, against the increase of temperature.

Adhesiveness↗
PubMed1982

Changes in phase transition temperature of phospholipids induced by endotoxin.

The effects of endotoxin (lipopolysaccharide from Salmonella minnesota Re 595) on the phase transition temperature (Tm) of various phospholipids were studied. Endotoxin had no effect on the Tm and the width of the phase transition of dipalmitoyl-sn-3-phosphatidylcholine. Endotoxin at 100 micrograms/ml increased the Tm of dipalmitoyl-sn-3-phosphatidylethanolamine by 1.1 degrees C (P less than 0.01) and narrowed the range of transition from 4.5 to 2.6 degrees C; the endotoxin-induced changes in the Tm and the transition range were abolished by the presence of 0.25 mM CaCl2. Endotoxin increased the Tm of dipalmitoyl-sn-3-phosphatidic acid by 1.1 (P less than 0.01), 1.2 (P less than 0.01), and 3.1 (P less than 0.01) degrees C at 25, 50 and 100 micrograms/ml, respectively. Furthermore, the width of phase transition of phosphatidic acid was narrowed from 6.5 to 4.0 degrees C by endotoxin at 100 micrograms/ml. The endotoxin-induced changes in the Tm and the transition range of phosphatidic acid were not affected by the presence of EDTA (1 mM) or CaCl2 (0.05-0.1 mM). These results suggest that endotoxin decreases the fluidity of negatively charged phospholipids such as phosphatidic acid and phosphatidylethanolamine. A change in the physical properties of membrane lipid bilayers induced by endotoxin may have an adverse effect on the function of biological membranes.

Endotoxins↗
PubMed2006

Thermotropic and barotropic phase transitions in bilayer membranes of ether-linked phospholipids with varying alkyl chain lengths.

The bilayer phase transitions of a series of ether-linked phospholipids, 1,2-dialkylphosphatidylcholines containing linear saturated alkyl chain (C(n)=12, 14, 16 and 18), were observed by differential scanning calorimetry (DSC) under ambient pressure and light-transmittance measurements under high pressure. The thermodynamic quantities of the pre- and main-transitions for the ether-linked PC bilayer membranes were calculated and compared with those of a series of ester-linked PCs, 1,2-diacylphosphatidylcholines. The thermodynamic quantities of the main transition for the ether-linked PC bilayers showed distinct dependence on alkyl-chain length and were slightly different from those of the ester-linked PC bilayers. From the comparison of thermodynamic quantities for the main transition between both PC bilayers, we revealed that the attractive interaction in the gel phase for the ether-linked PC bilayers is weaker than that for the ester-linked PC bilayers. Regarding the pretransition, although changes in enthalpy and entropy for both PC bilayers were comparable to each other, the volume changes of the ether-linked PC bilayers roughly doubled those of the ester-linked PC bilayers. The larger volume change results from the smallest partial molar volume of the ether-linked PC molecule in the interdigitated gel phase. Further, we constructed the temperature-pressure phase diagrams for the ether-linked PC bilayers by using the phase-transition data. The region of the interdigitated gel phase in the phase diagrams was extended by applying pressure and by increasing the alkyl-chain length of the molecule. Comparing the phase diagrams with those for the ester-linked PC bilayers, it was proved that the phase behavior of the ester-linked PC bilayers under high temperature and pressure is almost equivalent to that of the ether-linked PC bilayers in the vicinity of ambient pressure.

Calorimetry, Differential Scanning↗
PubMed2004

Poly[(2,2'-bipyridine)tetrakis(imidazolato)diiron(II)]: structural and spin-state phase transitions and low-temperature magnetic ordering in a unique 2-dimensional material.

Poly[(2,2'-bipyridine)tetrakis(imidazolato)diiron(II)] was synthesized by the reaction of ferrocene with imidazole in an excess of 2,2'- bipyridine in a Carius tube at 130 degrees C. Dc magnetic susceptibility studies at an applied field of 1000 G reveal that on cooling from room temperature, the material undergoes two structural phase transitions designated alpha --> beta and beta --> gamma at 151 and 133 K, respectively. On warming, the gamma --> beta and beta --> alpha transitions are observed at 137 and 151 K, respectively; a 4 K thermal hysteresis clearly detectable in the lower temperature beta <--> gamma transition. These structural phase transitions have also been studied by detailed, variable-temperature, ac susceptibility and Mössbauer spectroscopy techniques. Single-crystal X-ray diffraction studies done at 294, 143, and 113 K reveal 2, 12, and 6 unique iron centers in the alpha-, beta-, and gamma-forms, respectively. All three forms have the same basic structure involving 2D extended double layer sheets (bilayers) of alternating tetrahedral and octahedral irons singly bridged by imidazolate ligands, with the octahedral centers additionally coordinated by 2,2'-bipyridine ligands that occupy positions between the sheets. Magnetic susceptibility and bond length data reveal that in the gamma-phase one in three six-coordinate sites corresponds to spin singlet ground-state iron(II); i.e., the overall alpha --> gamma process involves a spin-crossover transition such that at least 1/6 of the iron sites in the gamma-phase correspond to S = 0. This is supported by the low-temperature Mössbauer spectra of the gamma-phase, which reveal the simultaneous presence of both S = 2 and S = 0 iron(II) centers. The compound magnetically orders, with a resultant small remnant magnetization, at low temperatures. The magnetic phase transition, studied by a combination of dc and ac susceptibility and Mössbauer techniques, occurs at T(c) approximately 6.5 K.

Journal Article↗
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↗
PubMed1988

Depression of phase-transition temperature by anesthetics: nonzero solid membrane binding.

The anesthetic-induced depression of the main phase-transition temperature of phospholipid membranes is often analyzed according to the van't Hoff model on the freezing point depression. In this procedure, zero interaction between anesthetics and solid-gel membranes is assumed. Nevertheless, anesthetics bind to solid-gel membranes to a significant degree. It is necessary to analyze the difference in the anesthetic binding between the liquid-crystal and solid-gel membranes to probe the anesthetic action on the lipid membranes. This article describes a theory to estimate the anesthetic binding to each state at the phase-transition temperature. The equations derived here reveal the relation between the partition coefficients of anesthetics and the anesthetic effects on the transition characters: the change in the transition temperature, and the broadening of transition. The theory revealed that the width of transition temperature is determined primarily by the membrane/buffer partition coefficients of anesthetics. Our previous data on the local anesthetic action on the transition temperature of the dipalmitoylphosphatidylcholine vesicle membrane (Ueda, I., Tashiro, C. and Arakawa, K. (1977) Anesthesiology 46, 327-332) are analyzed by this method. The numerical values for the partition of local anesthetics into the liquid-crystal and solid-gel dipalmitoyl-phosphatidylcholine vesicle membranes at the phase-transition temperature are: procaine 8.0 x 10(3) and 4.7 x 10(3), lidocaine, 3.7 x 10(3) and 2.3 x 10(3), bupivacaine 4.1 x 10(4), and 2.6 x 10(4), and tetracaine 7.3 x 10(4) and 4.7 x 10(4), respectively.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed2003

Neutron diffraction study of the size-induced tetragonal to monoclinic phase transition in zirconia nanocrystals.

Accurate neutron powder diffraction experiments at several temperatures allow one to monitor the reconstructive tetragonal to monoclinic phase transition as a function of the size of zirconia nanoparticles. The structure of the tetragonal phase observed in the nanocrystals is identical to that observed in micrometric zirconia above 1400 K. A uniaxial strain depending on grain size is observed. The phase transition occurs above a threshold crystal size. These results are analyzed within the Landau theory and can be understood as a mechanism of size-dependent phase transition where the primary order parameter is altered by the nanoparticle size.

Journal Article↗
PubMed1997

X-ray diffraction study of bilayer to non-bilayer phase transitions in aqueous dispersions of di-polyenoic phosphatidylethanolamines.

The low temperature phase properties of aqueous dispersions of di-18:2 and di-18:3 phosphatidylethanolamine are strongly influenced by the presence of ice. In the presence of cryoprotectants to inhibit ice formation, these lipids persist in the H(II) phase down to at least -50 degrees C. Ice formation, however, leads to a drastic reduction in the amount of available free water and a rapid reduction in the diameter of the inverted cylindrical micelles of the H(II) phase. The resulting increase in surface curvature of the micelles induces an imbalance in the forces acting in the lipid surface and the hydrophobic core which is relieved by formation of the L(alpha) phase. On reheating the lipid samples undergo an abrupt L(alpha) --> H(II) phase transition at about -20 degrees C. The radius of the water core of the inverted micelles at their point of formation is estimated to be 0.9 nm. This increases with temperature as more unfrozen water becomes available until the normal equilibrium radius of about 2.3 nm is reached at 0 degrees C when the bulk water in the sample finally melts. A small proportion of the H(II) phase lipid enters an as yet unidentified cubic phase on freezing. The spacings of the (10) planes of the H(II) phase, the (111) planes of the cubic phase and the d-spacing of the L(alpha) phase were found to be almost identical at the phase transition temperature. The cubic phase appears to disappear at low temperature but to reform on heating. Freeze-fracture studies revealed no unequivocal evidence for cubic phase lipid but the presence of residual non-bilayer lipid structures was observed even at temperatures as low as -80 degrees C. The presence of intersecting stacks of lamellar sheets in the replicas strongly suggest the existence of an epitaxial relationship between the L(alpha) and H(II) phases in these systems.

Freeze Fracturing↗
PubMed2001

Pseudosymmetry and phase transition in dimethyl 2,3-bis(tricyclo.

The crystal structure of the title compound, C(26)H(34)O(4), shows a reversible phase transition at about 178 K. The structure of the high-temperature phase contains two independent molecules related by pseudosymmetry elements. Cooling through the phase-transition temperature results in a doubling of the c axis. The low-temperature structure contains four independent molecules related by pseudosymmetry elements. The phase transition results in a rearrangement of some weak intermolecular C-H.O interactions. The number of very weak C-H.O interactions, with H.O distances between 2.8 and 2.9 A, is increased in the low-temperature structure.

Journal Article↗
PubMedDate not supplied

[Solubility and phase transitions in the water-protein-salt system].

The previously derived formulas for the curves corresponding to sol-gel, liquid-liquid, and liquid-solid phase transitions, which correlate the critical molar composition of the water-protein-salt system with individual characteristic features of its component (protein charge z, the number of ions adsorbed v, the function of electrolyte activity A) are presented as curves in ordinary coordinates of protein solubility logS against salt concentration m3. Tendencies in changes in phase transition lines versus the v, z, and v/z ratio have been determined. Correlations of the salting-out curve and the salting-out coefficient with phase transitions are discussed.

Algorithms↗