The glass transition temperature of mixtures of trehalose and hydroxyethyl starch

Although mixtures of HES and sugars are used to preserve cells during freezing or drying, little is known about the glass transition of HES, or how mixtures of HES and sugars vitrify. These difficulties may be due to the polydispersity between HES samples or differences in preparation techniques, as well as problems in measuring the glass transition temperature (T(g)) using differential scanning calorimetry (DSC). In this report, we examine the T(g) of mixtures of HES and trehalose sugar with <1% moisture content using DSC measurements. By extrapolating these measurements to pure HES using the Gordon-Taylor and Fox equations, we were able to estimate the T(g) of our HES sample at 44 degrees C. These results were additionally confirmed by using mixtures of glucose-HES which yielded a similar extrapolated T(g) value. Our approach to estimating the glass transition temperature of HES may be useful in other cases where glass transitions are not easily identified.     

Effects of thermal denaturation on the solid-state structure and molecular mobility of glycinin

The effects of moisture and thermal denaturation on the solid-state structure and molecular mobility of soy glycinin powder were investigated using multiple techniques that probe over a range of length and time scales. In native glycinin, increased moisture resulted in a decrease in both the glass transition temperature and the denaturation temperature. The sensitivity of the glass transition temperature to moisture is shown to follow the Gordon-Taylor equation, while the sensitivity of the denaturation temperature to moisture is modeled using Flory's melting point depression theory. While denaturation resulted in a loss of long-range order, the principal conformational structures as detected by infrared are maintained. The temperature range over which the glass to rubber transition occurred was extended on the high temperature side, leading to an increase in the midpoint glass transition temperature and suggesting that the amorphous regions of the newly disordered protein are less mobile. (13)C NMR results supported this hypothesis.     

Literature review: supplemented phase diagram of the trehalose-water binary mixture

Trehalose is of great interest in many fields, including freeze-drying, cryoprotection, and anhydrobiosis. Although data for the trehalose-water supplemented phase diagram have previously appeared in the literature, the data have been widely scattered and reported in several units. In this study, literature data for the binary trehalose-water system were collected and analyzed. The literature data were found to be reasonably consistent, with substantial agreement on the melting points for water, trehalose, and trehalose dihydrate and the glass transition temperature of water. There was also good agreement for the solubility, freezing, and glass transition curves. However, there was no general agreement on the glass transition temperature of pure trehalose. Additionally, the trehalose-water glass transition curve was modeled using the Gordon-Taylor equation, with a value for k of 5.2. The collected data in this report will be of much use in further studies of the protective abilities of trehalose.     

Thermal behavior of native and hydrophobized wheat gluten, gliadin and glutenin-rich fractions by modulated DSC

The glass transition temperature (T(g)) of hydrophobized and native wheat gluten and its protein fractions, with water mass fraction from 0 to 0.2, was studied using modulated differential scanning calorimetry. The T(g) values of unplasticized products were approximately 175 degrees C whatever the treatment (hydrophobization) or the fraction tested, except for the gliadin-rich fraction (162 degrees C). Experimental change in heat capacity at the glass transition (DeltaC(p)) ranged from 0.32 to 0. 50 J/g/ degrees C depending on the gluten fractions. The Gordon-Taylor fit of T(g) evolution as a function of water content showed that glutenin-rich fractions were more sensitive to water plasticization than the gliadin-rich fraction. The Kwei equation gave better fit to experimental data and demonstrated that the water plasticization of gluten and its fractions is influenced by secondary interactions. However, the application of the Couchman-Karasz equation without fitting predicts satisfactorily the plasticization of gluten proteins by water.     

Glass transition temperatures of cassava starch-whey protein concentrate systems at low and intermediate water content

Glass transition temperatures of cassava starch (CS)-whey protein concentrate (WPC) blends were determined by means of differential scanning calorimetry (DSC) in a water content range of 8-20% (dry basis, d.b.). Water equilibration in the samples was carried out by storing them at room temperature (25 °C) during four weeks. Physical aging and phase segregation were observed in some samples after this storage period depending on the water content. Both, first DSC heating scans and tan δ curves of CS-WPC blends with intermediate water content (10-18%), showed two endothermic thermal events. The first one appeared at around 60 °C and was independent of water content. The second one was detected at higher temperatures and moved towards the low-temperature peak as the water content increased. The results can be explained by a phase segregation process that can take place when the samples are conditioned below their glass transition temperatures. The Gordon-Taylor equation described well the plasticizing effect of water on the blends. WPC was also found to decrease the glass transition temperature, at constant water content, an effect attributed to additional water produced during browning reactions in the blends.     

Thermal properties of agave fructans (Agave tequilana Weber var. Azul)

Thermal properties of agave (A. tequilana Weber var. Azul) at different water contents were investigated. HP-TLC results showed a complex mixture of mono-, di-, oligo, and polysaccharides in agave fructans samples. The thermal decomposition temperatures were observed below to 200 °C. Modulated-differential scanning calorimetry studies showed a glass transition and a relaxation enthalpy processes in agave fructans. Samples with the highest moieties of monosaccharides showed the lower glass transition temperatures (Tg). The moisture sorption isotherm of agave fructans was determined at 20 °C and fitted to the GAB model. Gordon-Taylor equation was used to fit the Tg experimental data as a function of water content. Agave fructans was found to be an amorphous material. At low water activity (a_w) values (<0.4), agave fructans remained in a powdered amorphous state; and at intermediate a_w (0.4-0.75) collapsed and caked; and at high a_w (>0.75) changed in a highly viscous liquid-like solution.     

Effect of water as a diluent on the glass transition behaviour of malto-oligosaccharides, amylose and amylopectin

The glass transition behaviour of amorphous malto-oligomers from dimer to hexamer was investigated as a function of diluent (water) concentration using differential scanning calorimetry. The glass transition temperatures of the pure compounds ranged from 364 K for maltose to 448 K for maltohexaose. At low diluent concentrations the addition of water strongly depressed Tg. From the measurement of Tg and the heat capacity increment, delta Cp, of the transition for the pure compounds it was possible to predict the Tg of the malto-oligomer/water mixtures using a thermodynamic approach developed by Couchman. From the measurements on the malto-oligomers it was possible to obtain, by extrapolation, the high DP limits of delta Cp and Tg, which are appropriate to amylose and amylopectin. The predicted variation of Tg with diluent concentration for these materials was compared with the experimentally observed behaviour.     

Moisture sorption characteristics and thermophysical properties of trehalose-PBS mixtures

The goal of the study was to quantify the thermophysical properties and the moisture sorption characteristics of the trehalose-PBS (phosphate-buffered saline) from the desiccation preservation perspective. A moisture sorption study was undertaken to determine the desorption isotherms of the trehalose-PBS mixtures. The Brunauer, Emmett, and Teller (BET)-equation and the Guggenheim, Anderson, and de Boer equation were used to quantify the desorption data. The glass transition temperature of the mixtures of trehalose-PBS, equilibrated at different relative humidities was studied using a differential scanning calorimeter. Fourier transform infrared spectroscopy was used to study the molecular interaction between the trehalose and PBS mixtures. The results showed that the addition of PBS to the trehalose mixture causes a shift from the type II isotherm to a type III isotherm (characterized by BET equation) which may have detrimental effect on cell desiccation. The results showed that an increase in PBS mass fraction in the trehalose-PBS mixture causes a decrease in the glass transition temperature (Tg) of the mixture and also a decrease in the hydrogen bonding capacity of the trehalose glasses. The addition of PBS to trehalose posed some challenges and should be subject to further optimization to use it in desiccation preservation of biologics.     

Liquid-liquid phase transition temperatures increase when lipid bilayers are supported on glass

Membranes made from certain ternary mixtures of lipids can display coexisting liquid phases. In giant unilamellar vesicles, these phases appear as liquid domains which diffuse and coalesce after the vesicle is cooled below its miscibility transition temperature (T_m). Converting vesicles to supported lipid bilayers alters the mobility of the lipids and domains in the bilayer. At the same time, the miscibility transition temperature of the lipid mixture is altered. Here we compare T_m in vesicles and in supported bilayers formed by rupturing the same vesicles onto glass. We determine transition temperatures using fluorescence microscopy, and identify an increase in T_m when it is measured in identical membranes in solution and on a glass surface. We systematically alter the lipid composition of our membranes in order to observe the correlation between membrane composition and variation in T_m.     

Unexpected high pressure effects on the structural properties of condensed whey protein systems

We show that application of high hydrostatic pressure (600 MPa for 15 min) on condensed whey protein (WP) systems (e.g., 80% w/w solids content) results in unexpected structure–function behavior when compared with conventional thermal treatment. Unraveling the relaxation properties in first‐order thermodynamic transitions, the manifestation of glass transition phenomena and the preservation of native conformation in condensed preparations were recorded using small‐deformation dynamic oscillation in shear, modulated differential scanning calorimetry, and infrared spectroscopy. Informed temperature application results in the formation of continuous networks at the denaturation temperature, which undergo vitrification at subzero temperatures. In contrast, high‐pressure‐treated WPs resist physicochemical denaturation, hence preserving the native conformation of secondary and tertiary structures. This was rationalized on the basis of a critical concentration threshold where transfer of water molecules to nonpolar residues in the protein interior is minimized because of low moisture content and restricted molecular mobility. The physical state and morphology of these high‐solid preparations were further examined by the combined framework of reduced variables and Williams, Landel, and Ferry equation/free volume theory. Theoretical treatment of experimental observations unveils the dynamic range of the mechanical manifestation of the glass transition region in samples subjected to heat or pressure. In addition to preserving native conformation, WPs subjected to high pressure form glassy systems at parity with the structural functionality of the thermally treated counterparts. © 2012 Wiley Periodicals, Inc. Biopolymers 97:963–973, 2012.     

Calorimetric and spectroscopic studies of the phase behavior and organization of lipid bilayer model membranes composed of binary mixtures of dimyristoylphosphatidylcholine and dimyristoylphosphatidylglycerol

The thermotropic phase behavior of hydrated bilayers derived from binary mixtures of dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylglycerol (DMPG) was investigated by differential scanning calorimetry, Fourier-transform infrared spectroscopy and 31P-nuclear magnetic resonance spectroscopy. Binary mixtures of DMPC and DMPG that have not been annealed at low temperatures exhibit broad, weakly energetic pretransitions (approximately 11-15 degrees C) and highly cooperative, strongly energetic gel/liquid-crystalline phase transitions (approximately 23-25 degrees C). After low temperature incubation, these mixtures also exhibit a thermotropic transition form a lamellar-crystalline to a lamellar gel phase at temperatures below the onset of the gel/liquid-crystalline phase transition. The midpoint temperatures of the pretransitions and gel/liquid-crystalline phase transitions of these lipid mixtures are both maximal in mixtures containing approximately 30 mol% DMPG but the widths and enthalpies of the same thermotropic events exhibit no discernable composition dependence. In contrast, thermotropic transitions involving the Lc phase exhibit a very strong composition dependence, and the midpoint temperatures and transition enthalpies are both maximal with mixtures containing equimolar amounts of the two lipids. Our spectroscopic studies indicate that the Lc phases formed are structurally similar as regards their modes of hydrocarbon chain packing, interfacial hydration and hydrogen-bonding interactions, as well as the range and amplitudes of the reorientational motions of their phosphate headgroups. Our results indicate that although DMPC and DMPG are highly miscible, their mixtures do not exhibit ideal mixing. We attribute the non-ideality in their mixing behavior to the formation of preferential PC/PG contacts in the Lc phase due to the combined effects of steric crowding of the DMPC headgroups and charge repulsion between the negatively charged DMPG molecules.     

Modeling the kinetics of the pectin methylesterase catalyzed de-esterfication of pectin in frozen systems

The applicability of the William, Landel, and Ferry (WLF) equation with a modification to take into account the effect of melt-dilution and an empirical log-logistic equation were evaluated to model the kinetics of diffusion-controlled reactions in frozen systems. Kinetic data for the pectin methylesterase catalyzed hydrolysis of pectin in four model systems with different glass transition temperatures: sucrose, maltodextrin (DE = 16.5-19.5), carboxymethylcellulose (CMC) and fructose in a temperature range of -24 to 0 degrees C were used. The modified WLF equation was evaluated with a concentration-dependent glass transition temperature (T(g)) as well as the glass transition temperature of the maximally freeze-concentrated matrix (T(g)') as reference temperatures. The equation with temperature-dependent T(g) described the reaction kinetics reasonably well in all the model systems studied. However the kinetics was better described by a linear relationship between log(V(0)/V(0ref)) and (T - T(ref)) in all cases except CMC. The log-logistic equation also described the kinetics reasonably well. The effect of melt-dilution on reactant concentration was found to be minimal in all cases.     

Nonequilibrium behavior in supported lipid membranes containing cholesterol

We investigate lateral organization of lipid domains in vesicles versus supported membranes and monolayers. The lipid mixtures used are predominantly DOPC/DPPC/Chol and DOPC/BSM/Chol, which have been previously shown to produce coexisting liquid phases in vesicles and monolayers. In a monolayer at an air-water interface, these lipids have miscibility transition pressures of approximately 12-15 mN/m, which can rise to 32 mN/m if the monolayer is exposed to air. Lipid monolayers can be transferred by Langmuir-Sch?fer deposition onto either silanized glass or existing Langmuir-Blodgett supported monolayers. Micron-scale domains are present in the transferred lipids only if they were present in the original monolayer before deposition. This result is valid for transfers at 32 mN/m and also at lower pressures. Domains transferred to glass supports differ from liquid domains in vesicles because they are static, do not align in registration across leaflets, and do not reappear after temperature is cycled. Similar static domains are found for vesicles ruptured onto glass surfaces. Although supported membranes on glass capture some aspects of vesicles in equilibrium (e.g., gel-liquid transition temperatures and diffusion rates of individual lipids), the collective behavior of lipids in large liquid domains is poorly reproduced.     

Predicting glass transition temperatures of polyarylethersulphones using QSPR methods

The technique of Quantitative Structure Property Relationships has been applied to the glass transition temperatures of polyarylethersulphones. A general equation is reported that calculates the glass transition temperatures with acceptable accuracy (correlation coefficients of between 90-67%, indicating an error of 10-30% with regard to experimentally determined values) for a series of 42 reported polyarylethersulphones. This method is quite simple in assumption and relies on a relatively small number of parameters associated with the structural unit of the polymer: the number of rotatable bonds, the dipole moment, the heat of formation, the HOMO eigenvalue, the molar mass and molar volume. For smaller subsets of the main group (based on families of derivatives containing different substituents) the model can be simplified further to an equation that uses the volume of the substituents as the principal variable.     

Glassy State and Seed Storage Stability: The WLF Kinetics of Seed Viability Loss at T>Tgand the Plasticization Effect of Water on Storage Stability

The relationship between the glassy state in seeds and storage stability was examined, using the glass transition curve and a seed viability database from previous experiments. Storage data for seeds at various water contents were studied by Williams–Landel–Ferry (WLF) kinetics, whereas the glass transition curves of seeds with different storage stability were analysed by the Gordon–Taylor equation in terms of the plasticization effect of water on seed storage stability. It was found that the critical temperatures (T_c) for long-term storage of three orthodox seeds were near or below their glass transition temperatures (T_g), indicating the requirement for the presence of the glassy state for long-term seed storage. The rate of seed viability loss was a function of T-T_gat T>T_g, which fitted the WLF equation well, suggesting that storage stability was associated with the glass transition, and that the effect of water content on seed storage was correlated with the plasticization effect of water on intracellular glasses. A preliminary examination suggested a possible link between the glass transition curve and seed storage stability. According to the determined WLF constants, intracellular glasses in seeds fell into the second class of amorphous systems as defined by Slade and Levine (Critical Reviews in Food Science and Nutrition30: 115–360, 1991). These results support the interpretation that the glassy state plays an important role in storage stability and should be a major consideration in optimizing storage conditions.     

Forced and natural convective drying of trehalose/water thin films: implication in the desiccation preservation of Mammalian cells

Trehalose is believed to offer desiccation protection to mammalian cells by forming stable glassy matrices. The goal of the current study was to explore the desiccation kinetics of thin films of trehalose-water solution under forced and natural convective conditions and to investigate the thermophysical state of mammalian cells at the bottom of the thin film. We developed a finite difference model based on the mass and energy conservation equations coupled to the water transport model from the cells. The boundary conditions were obtained from correlations or experimental measurements and the Gordon-Taylor equation was used to predict the glass transition temperature at every location. Results indicated that there are three distinct regimes for drying for both forced and natural convection, characterized by the slope of the moisture content plot as a function of time. Our results also indicate that the surface of the solution reached the glassy state in less than 10 min for the Reynolds (forced) numbers explored and approximately 30 min for some Rayleigh (natural convective) numbers; however, significant water was trapped at this instant. Larger drying force hastened quicker glass formation but trapped more water. The numerical model was capable of predicting the drying kinetics for the dilute region accurately, but deviated while predicting the other regimes. Based on these experimental validations of the model, the osmotic response of different cells located at the bottom of the solution with orders of magnitude difference in their membrane permeability (Lp) was predicted. The results suggested that extracellular glass formed around cells at the bottom of a trehalose-water solution by the propagation of glass into the solution; however it takes more than an order of magnitude time (approximately 7 min to >100 min for forced convective drying) to remove sufficient water to form glass around cells from the time when the first surface glass is formed. This is attributed to low diffusivity of water through the glass. In addition, the water transport from the glassy matrix could be either diffusion or Lp limited. For diffusion-limited transport, lowering the film thickness at the beginning of drying by half almost lowers the drying time by an order of magnitude. In summary, the optimal design of convective desiccation protocols requires accounting for the size of the cell, their membrane permeability (Lp) and the starting thickness of the solution.     

Glass transition behavior of octyl β-D-glucoside and octyl β-D-thioglucoside/water binary mixtures

The lyotropic behavior and glass-forming properties of octyl β-d-glucoside (C8Glu) and octyl β-d-thioglucoside (C8SGlu)/water binary mixtures were evaluated using differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). The results clearly indicate that the mixture forms a glass in the supercooling state of liquid crystalline phases such as cubic, lamellar, and smectic. The glass transition temperature (T_g) of the mixture was strongly dependent on solute concentration, with a higher concentration correlating with a higher T_g. The experimental T_g was consistent with the predicted value calculated using the Couchman-Karasz equation in both the C8Glu and C8SGlu/water mixtures. The change of heat capacity at T_g showed the two bending points under variation of concentrations. And the highest temperature of phase transition from lamellar to isotropic solution was observed at around 50% molar concentration. It was expected that non-percolated state of water existed in extremely higher concentration ranges.     

Effect of molecular structure on the conductivity of amorphous carbohydrate-water-KCl mixtures in the supercooled liquid state

The effect of carbohydrate structure on the conductivity of low water content amorphous carbohydrate-water, and carbohydrate-water-KCl mixtures, has been measured using both direct current and alternating current techniques at temperatures in the supercooled liquid and glassy range, ranging from -40 to 80 degrees C. The structures included homologous mono-, di- and trisaccharides (glucose, maltose and maltotriose), a monosaccharide with no exocyclic hydroxymethyl group (xylose) and a second trisaccharide (raffinose). The KCl-mixtures contained 9.3% w/w water and 0.74% w/w KCl which resulted in calorimetric glass transition temperatures, T(g), in the range -29-19 degrees C. At this concentration conduction due to KCl dominated that due to intrinsic conductors originating from the carbohydrates and water. In the supercooled liquid region, as temperature, T, is reduced to T(g), the activation energy of the molar conductivity of KCl, Lambda(m), increased as described by a Vogel-Tamman-Fulcher-type equation, Lambda(m)=Lambda(m0)exp[B/(T-T(0))], where Lambda(m0), B and T(0) are constants. Comparison of the molar conductivity of KCl in the carbohydrate mixtures at T(g) with that in aqueous solutions showed that conductivity is, to varying extents, uncoupled from viscosity. The uncoupling increased in the order D-xylose相似文献   

Calorimetric and spectroscopic studies of the phase behavior and organization of lipid bilayer model membranes composed of binary mixtures of dimyristoylphosphatidylcholine and dimyristoylphosphatidylglycerol

The thermotropic phase behavior of hydrated bilayers derived from binary mixtures of dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylglycerol (DMPG) was investigated by differential scanning calorimetry, Fourier-transform infrared spectroscopy and ³¹P-nuclear magnetic resonance spectroscopy. Binary mixtures of DMPC and DMPG that have not been annealed at low temperatures exhibit broad, weakly energetic pretransitions (∼11-15 °C) and highly cooperative, strongly energetic gel/liquid-crystalline phase transitions (∼23-25 °C). After low temperature incubation, these mixtures also exhibit a thermotropic transition form a lamellar-crystalline to a lamellar gel phase at temperatures below the onset of the gel/liquid-crystalline phase transition. The midpoint temperatures of the pretransitions and gel/liquid-crystalline phase transitions of these lipid mixtures are both maximal in mixtures containing ∼30 mol% DMPG but the widths and enthalpies of the same thermotropic events exhibit no discernable composition dependence. In contrast, thermotropic transitions involving the L_c phase exhibit a very strong composition dependence, and the midpoint temperatures and transition enthalpies are both maximal with mixtures containing equimolar amounts of the two lipids. Our spectroscopic studies indicate that the L_c phases formed are structurally similar as regards their modes of hydrocarbon chain packing, interfacial hydration and hydrogen-bonding interactions, as well as the range and amplitudes of the reorientational motions of their phosphate headgroups. Our results indicate that although DMPC and DMPG are highly miscible, their mixtures do not exhibit ideal mixing. We attribute the non-ideality in their mixing behavior to the formation of preferential PC/PG contacts in the L_c phase due to the combined effects of steric crowding of the DMPC headgroups and charge repulsion between the negatively charged DMPG molecules.     

Effect of sugar-phosphate mixtures on the stability of DPPC membranes in dehydrated systems

The stabilizing role of sugars on dehydrated membranes is well established. The formation of a glassy matrix and the direct interaction between the sugars and the lipids are some of the mechanisms proposed to be involved in this stabilizing effect. Phospholipidic systems have been studied extensively as models for biological membranes and also due to the practical applications of liposomes as vehicles for drug delivery. In this work, we evaluate the effect of sugar-phosphate mixtures on the transition temperature of dehydrated 1,2-dipalmitoylphosphatidylcholine, and also examine some physical characteristics of these mixtures, such as the glass transition temperature and water sorption properties. The addition of phosphate salts to sugar systems has several interesting features that merit its consideration in formulations to protect dehydrated labile biomaterials. In particular, sucrose-phosphate mixtures provide an interesting alternative to pure saccharide formulations due to their high glass transition temperatures and their increased ability to maintain a low melting transition temperature in the presence of small amounts of water.