Melting, glass transition, and apparent heat capacity of α-D-glucose by thermal analysis

The thermal behaviors of α-D-glucose in the melting and glass transition regions were examined utilizing the calorimetric methods of standard differential scanning calorimetry (DSC), standard temperature-modulated differential scanning calorimetry (TMDSC), quasi-isothermal temperature-modulated differential scanning calorimetry (quasi-TMDSC), and thermogravimetric analysis (TGA). The quantitative thermal analyses of experimental data of crystalline and amorphous α-D-glucose were performed based on heat capacities. The total, apparent and reversingheat capacities, and phase transitions were evaluated on heating and cooling. The melting temperature (T(m)) of a crystalline carbohydrate such as α-D-glucose, shows a heating rate dependence, with the melting peak shifted to lower temperature for a lower heating rate, and with superheating of around 25K. The superheating of crystalline α-D-glucose is observed as shifting the melting peak for higher heating rates, above the equilibrium melting temperature due to of the slow melting process. The equilibrium melting temperature and heat of fusion of crystalline α-D-glucose were estimated. Changes of reversing heat capacity evaluated by TMDSC at glass transition (T(g)) of amorphous and melting process at T(m) of fully crystalline α-D-glucose are similar. In both, the amorphous and crystalline phases, the same origin of heat capacity changes, in the T(g) and T(m) area, are attributable to molecular rotational motion. Degradation occurs simultaneously with the melting process of the crystalline phase. The stability of crystalline α-D-glucose was examined by TGA and TMDSC in the melting region, with the degradation shown to be resulting from changes of mass with temperature and time. The experimental heat capacities of fully crystalline and amorphous α-D-glucose were analyzed in reference to the solid, vibrational, and liquid heat capacities, which were approximated based on the ATHAS scheme and Data Bank.     

An anhydrous polymorphic form of trehalose

An anhydrous polymorphic form of alpha,alpha-trehalose was prepared from trehalose dihydrate by two different drying methods: (1) heating under vacuum; and (2) heating in hot air. Preparation of this anhydrous form by vacuum heating showed good reproducibility. This form was characterized by X-ray powder diffraction analysis and differential scanning calorimetry. This anhydrous form was converted to an amorphous phase at 127 degrees C and was found to be hygroscopic. At 43% relative humidity at 25 degrees C, this form rapidly reverted to dihydrate, while the amorphous phase remained unchanged. When an amorphous phase coexisted with this form, the rate of water adsorption to the amorphous phase was slower than that to the amorphous phase alone. These properties of this anhydrous form of alpha,alpha-trehalose may explain the effects of trehalose in dehydration tolerance of plants and insects in the desert.     

Solid state amorphization kinetic of alpha lactose upon mechanical milling

It has been previously reported that α-lactose could be totally amorphized by ball milling. In this paper we report a detailed investigation of the structural and microstructural changes by which this solid state amorphization takes place. The investigations have been performed by Powder X-ray Diffraction, Solid State Nuclear Magnetic Resonance (¹³C CP-MAS) and Differential Scanning Calorimetry. The results reveal the structural complexity of the material in the course of its amorphization so that it cannot be considered as a simple mixture made of a decreasing crystalline fraction and an increasing amorphous fraction. Heating this complexity can give rise to a fully nano-crystalline material. The results also show that chemical degradations upon heating are strongly connected to the melting process.     

Effects of trehalose on the phase behavior of DPPC-cholesterol unilamellar vesicles

A systematic study is presented of the effects of trehalose on the physical properties of extruded DPPC-cholesterol unilamellar vesicles. Particular emphasis is placed on examining how the interactions present in the hydrated state translate into those in the dehydrated state. Observations from HSDSC and DSC are used to examine the phase behavior of hydrated and dehydrated vesicles, respectively. The concentration of trehalose inside and outside the vesicles is manipulated, and is shown to affect the relative stability of the membranes. Our results show for the first time that a combination of high inner and low outer trehalose concentration is able to decrease the gel-to-liquid crystalline phase temperature (T(m)), while any other combination will not. Upon dehydration, the T(m) of all lipid mixtures increases. The extent of the increase depends on the trehalose distribution across the bilayer. The T(m) changes in the same direction with trehalose concentration for both freeze-dried and fully hydrated samples, suggesting that the trehalose distribution across the vesicle membrane, as well as the trehalose-phospholipid interaction, is maintained upon lyophilization. The results presented in this work may aid in the formulation of systems to be used in the lyophilization of liposomes for drug delivery applications.     

Polymorphic phases of galactocerebrosides: spectroscopic evidence of lamellar crystalline structures

Fourier transform infrared spectroscopy was applied to study the structural and thermal properties of bovine brain galactocerebroside (GalCer) containing amide linked non-hydroxylated or alpha-hydroxy fatty acids (NFA- and HFA-GalCer, respectively). Over the temperature range 0-90 degrees C, both GalCer displayed complex thermal transitions, characteristic of polymorphic phase behavior. Upon heating, aqueous dispersions of NFA- and HFA-GalCer exhibited high order-disorder transition temperatures near 80 and 72 degrees C, respectively. En route to the chain melting transition, the patterns of the amide I band of NFA-GalCer were indicative of two different lamellar crystalline phases, whereas those of HFA-GalCer were suggestive of lamellar gel and crystalline bilayers. Cooling from the liquid-crystalline phase resulted in the formation of another crystalline phase of NFA-GalCer and a gel phase of HFA-GalCer, with a phase transition near 62 and 66 degrees C, respectively. Prolonged incubation of GalCer bilayers at 38 degrees C revealed conversions among lamellar crystalline phases (NFA-GalCer) or between lamellar gel and crystalline bilayer structures (HFA-GalCer). Spectral changes indicated that the temperature and/or time induced formation of the lamellar crystalline structures of NFA- and HFA-GalCer was accompanied by partial dehydration and by rearrangements of the hydrogen bonding network and bilayer packing mode of GalCer.     

Melting, glass transition, and apparent heat capacity of α-d-glucose by thermal analysis

The thermal behaviors of α-d-glucose in the melting and glass transition regions were examined utilizing the calorimetric methods of standard differential scanning calorimetry (DSC), standard temperature-modulated differential scanning calorimetry (TMDSC), quasi-isothermal temperature-modulated differential scanning calorimetry (quasi-TMDSC), and thermogravimetric analysis (TGA). The quantitative thermal analyses of experimental data of crystalline and amorphous α-d-glucose were performed based on heat capacities. The total, apparent and reversing heat capacities, and phase transitions were evaluated on heating and cooling. The melting temperature (T_m) of a crystalline carbohydrate such as α-d-glucose, shows a heating rate dependence, with the melting peak shifted to lower temperature for a lower heating rate, and with superheating of around 25 K. The superheating of crystalline α-d-glucose is observed as shifting the melting peak for higher heating rates, above the equilibrium melting temperature due to of the slow melting process. The equilibrium melting temperature and heat of fusion of crystalline α-d-glucose were estimated. Changes of reversing heat capacity evaluated by TMDSC at glass transition (T_g) of amorphous and melting process at T_m of fully crystalline α-d-glucose are similar. In both, the amorphous and crystalline phases, the same origin of heat capacity changes, in the T_g and T_m area, are attributable to molecular rotational motion. Degradation occurs simultaneously with the melting process of the crystalline phase. The stability of crystalline α-d-glucose was examined by TGA and TMDSC in the melting region, with the degradation shown to be resulting from changes of mass with temperature and time. The experimental heat capacities of fully crystalline and amorphous α-d-glucose were analyzed in reference to the solid, vibrational, and liquid heat capacities, which were approximated based on the ATHAS scheme and Data Bank.     

Polymorphism, mesomorphism, and metastability of monoelaidin in excess water.

The polymorphic and metastable phase behavior of monoelaidin dry and in excess water was studied by using high-sensitivity differential scanning calorimetry and time-resolved x-ray diffraction in the temperature range of 4 degrees C to 60 degrees C. To overcome problems associated with a pronounced thermal history-dependent phase behavior, simultaneous calorimetry and time-resolved x-ray diffraction measurements were performed on individual samples. Monoelaidin/water samples were prepared at room temperature and stored at 4 degrees C for up to 1 week before measurement. The initial heating scan from 4 degrees C to 60 degrees C showed complex phase behavior with the sample in the lamellar crystalline (Lc0) and cubic (Im3m, Q229) phases at low and high temperatures, respectively. The Lc0 phase transforms to the lamellar liquid crystalline (L alpha) phase at 38 degrees C. At 45 degrees C, multiple unresolved lines appeared that coexisted with those from the L alpha phase in the low-angle region of the diffraction pattern that have been assigned previously to the so-called X phase (Caffrey, 1987, 1989). With further heating the X phase converts to the Im3m cubic phase. Regardless of previous thermal history, cooling calorimetric scans revealed a single exotherm at 22 degrees C, which was assigned to an L alpha+cubic (Im3m, Q229)-to-lamellar gel (L beta) phase transition. The response of the sample to a cooling followed by a reheating or isothermal protocol depended on the length of time the sample was incubated at 4 degrees C. A model is proposed that reconciles the complex polymorphic, mesomorphic, and metastability interrelationships observed with this lipid/water system. Dry monoelaidin exists in the lamellar crystalline (beta) phase in the 4 degrees C to 45 degrees C range. The beta phase transforms to a second lamellar crystalline polymorph identified as beta* at 45 degrees C that subsequently melts at 57 degrees C. The beta phase observed with dry monoelaidin is identical to the LcO phase formed by monoelaidin that was dispersed in excess water and that had not been previously heated.     

Lamellar gel-lamellar liquid crystal phase transition of dipalmitoylphosphatidylcholine multilayers freeze-dried from aqueous trehalose solutions. A real-time X-ray diffraction study

The mechanism of the phase transition of dipalmitoylphosphatidylcholine multilayers freeze-dried from fully hydrated gel phase (L beta') in the presence of trehalose has been investigated by real-time X-ray diffraction methods. Sequential diffraction patterns were recorded with an accumulation time of 3 s during heating and 1.2 s during cooling between about 20 and 80 degrees C. A transition is observed in the range 47-53 degrees C that involves structural events typical of a lamellar gel-lamellar liquid-crystal (L beta--L alpha) transformation. This transition is completely reversible with a temperature hysteresis of 2-3 degrees C and thereby resembles the main phase transition of fully hydrated dipalmitoylphosphatidylcholine multilayers. The mechanism of the transition from L beta to L alpha as seen in the wide-angle scattering profiles show that the sharp peak at about 0.41 nm, characteristic of the gel phase, broadens and shifts progressively to about 0.44 nm towards the end of the transition. A temperature jump of 6C degrees/s through the phase transition region of a freeze-dried dipalmitoylphosphatidylcholine: trehalose mixture (molar ratio 1:1) showed that the phase transition had a relaxation time of about 2 s which is similar to that of the main transition in the fully hydrated lipid. X-ray diffraction studies of the melting of dipalmitoylphosphatidylcholine freeze-dried from the lamellar-gel phase in the absence of trehalose showed a transition at above 70 degrees C. The low-angle diffraction data of phospholipid/trehalose mixtures are consistent with an arrangement of trehalose molecules in a loosely packed 'monolayer' separating bilayers of phospholipid. Trehalose appears to reduce the direct interbilayer hydrogen bond coupling thereby modifying the thermal stability and the phase transition mechanism of the bilayers.     

Dependence of trehalose protective action on the initial phase state of dipalmitoylphosphatidylcholine bilayers

Dipalmitoylphosphatidylcholine (DPPC) bilayers hydrated in the presence of trehalose were equilibrated at various temperatures (4, 20, and 60 degrees C) corresponding to the crystalline Lc, gel L beta', and liquid-crystalline L alpha phases, respectively, and then desiccated at these temperatures or freeze-dried at -80 degrees C to ca. DPPC dihydrate. The thermotropic behavior of the resulting DPPC/trehalose mixtures was investigated by differential scanning calorimetry and found to be dependent not only on the trehalose concentration but also on the phase state of the hydrated bilayers prior to their drying. Trehalose was most effective when the desiccation was carried out from the L alpha phase at 60 degrees C. In this case, one trehalose molecule per two DPPC molecules was sufficient to depress the melting temperature from values typical of DPPC dihydrate to 45 degrees C. Trehalose's influence decreased when dried from the L beta' phase and was significantly less pronounced when dried from the Lc phase. These data show that trehalose's protective influence depends on the initial phase state of the lipid bilayer and reaches its maximum in the liquid-crystalline state. The possible role of this effect in anhydrobiosis is pointed out.     

Combined effects of trehalose and cations on the thermal resistance of beta-galactosidase in freeze-dried systems

The purpose of this study was to investigate the combined effects of trehalose and cations on the preservation of beta-galactosidase in freeze-dried systems and their relationship to physical properties. Differential scanning calorimetry was employed to measure the glass transition temperature (T(g)) and the endothermal peak area, related to the amount of crystalline trehalose dihydrate present in the samples. In systems in which the trehalose matrix was humidified to conditions which allowed a high proportion of trehalose to crystallize, the enzyme was rapidly inactivated upon heating at 70 degrees C. In these conditions the addition of CsCl, NaCl and particularly KCl or MgCl(2), improved the enzyme stability with respect to that observed in matrices containing only trehalose. For a given moisture content, addition of salts produced very little change on the glass transition temperature; therefore the protective effect could not be attributed to a higher T(g) value. The crystallization of trehalose dihydrate in the humidified samples was delayed in the trehalose/salt systems (principally in the presence of Mg(2+)) and a parallel improvement of enzyme stability was observed.     

Solid-state NMR study of trehalose/1,2-dipalmitoyl-sn-phosphatidylcholine interactions

31P and 2H solid-state NMR studies of dry trehalose (TRE) and 1,2-dipalmitoyl-sn-phosphatidylcholine (DPPC) mixtures are reported. 31P spectra are consistent with a rigid head group above and below the calorimetric phase transition for both dry DPPC and a dry 2:1 TRE/DPPC mixture. In addition, 2H spectra of DPPC labeled at the 7-position of the sn-2 chain (2[7,7-2H2]DPPC) show exchange-narrowed line shapes with a width of 120 kHz over the temperature range 25-75 degrees C. These line shapes can be simulated with a model involving two-site jumps of the deuteron. In contrast, the 2H NMR spectrum of a dry 2:1 TRE/2[7,7-2H2]DPPC mixture above the phase transition (Tc = 46 degrees C) is narrowed by a factor of approximately 4 to a width of 29 kHz. Simulation of this spectrum requires a model involving four-site jumps of the deuteron and is indicative of highly disordered lipid acyl chains similar to those found in the L alpha-phases of hydrated lipids. Thus, TRE/DPPC mixtures above their transition temperatures exist in a new type of liquid crystalline like phase, which we term a lambda-phase. The observation of the dynamic properties of this new phase indicates the mechanism by which anhydrobiotic organisms maintain the integrity of their membranes upon dehydration.     

The phase behavior of hydrated cholesterol.

The thermotropic phase behavior of cholesterol monohydrate in water was investigated by differential scanning calorimetry, polarizing light microscopy, and x-ray diffraction. In contrast to anhydrous cholesterol which undergoes a polymorphic crystalline transition at 39 degrees C and a crystalline to liquid transition at 151 degrees C, the closed system of cholesterol monohydrate and water exhibited three reversible endothermic transitions at 86, 123, and 157 degrees C. At 86 degrees C, cholesterol monohydrate loses its water of hydration, forming the high temperature polymorph of anhydrous cholesterol. At least 24 hours were required for re-hydration of cholesterol and the rate of hydration was dependent on the polymorphic crystalline form of anhydrous cholesterol. At 123 degrees C, anhydrous crystalline cholesterol in the presence of excess water undergoes a sharp transition to a birefringent liquid crystalline phase of smectic texture. The x-ray diffraction pattern obtained from this phase contained two sharp low-angle reflections at 37.4 and 18.7 A and a diffuse wide-angle reflection centered at 5.7 A, indicating a layered smectic type of liquid crystalline structure with each layer being two cholesterol molecules thick. The liquid crystalline phase is stable over the temperature range of 123 to 157 degrees C before melting to a liquid dispersed in water. The observation of a smectic liquid crystalline phase for hydrated cholesterol correlates with its high surface activity and helps to explain its ability to exist in high concentrations in biological membranes.     

Comparison of the effects of NaCl on the thermotropic behaviour of sn-1' and sn-3' stereoisomers of 1,2-dimyristoyl-sn-glycero-3-phosphatidylglycerol

The phase behaviour of liposomes of 1,2-dimyristoyl-sn-glycero-3-phosphatidyl-sn-1'-glycerol (1'-DMPG) and the corresponding sn-3' stereoisomer (3'-DMPG) were studied by DSC as a function of NaCl concentration. The melting of the metastable gel phase to the liquid-crystalline phase was similar for both lipids. However, in the presence of salt and at 6 degrees C (T less than Tp) the gel phase of both stereoisomers of DMPG was shown to be metastable and a new phase nominated here as the highly crystalline phase was formed as the stable state. However, significant differences in the formation and melting of the highly crystalline phase were evident between the two polar headgroup stereoisomers. For 3'-DMPG in the presence of 300 mM NaCl the melting enthalpy of this phase is approx. 82 kJ/mol and the transition temperature about 11 degrees higher (at 33.6 degrees C) than for the gel to liquid-crystalline phase transition (25 kJ/mol at 23.0 degrees C). In the presence of 0.15-1.2 M NaCl at 6 to 10 degrees C the formation of the highly crystalline phase of 3'-DMPG is complete within 2 to 5 days, increasing [NaCl] facilitates the rate. For a 1:1 mixture of 1'- and 3'-DMPG the formation of the highly crystalline phase requires several weeks and melts at about 20 degrees higher than the gel phase (at approx. 40 degrees C). For 1'-DMPG partial conversion into the highly crystalline phase requires several months. For 3'-DMPG several intermediate phases appeared as endothermic peaks between the main phase transition temperature and the melting temperature of the highly crystalline phase. In contrast, for 1'-DMPG and the 1:1 mixture the subgel phase appears to be the only metastable intermediate phase. Different monovalent cations differ in their effect on the metastable behaviour.     

De- and rehydration behavior of alpha,alpha-trehalose dihydrate under humidity-controlled atmospheres

Effects of humidity were investigated on de- and rehydration behavior of alpha,alpha-trehalose dihydrate (T(h)) throughout simultaneous measurements of differential scanning calorimetry and X-ray diffractometry (DSC-XRD) and simultaneous thermogravimetry and differential thermal analysis (TG-DTA). When T(h) was heated from room temperature under dry nitrogen atmosphere, a metastable anhydrous crystal (T(alpha)) was formed at 105 degrees C after dehydration of T(h). The resulting T(alpha) melted at 125 degrees C and became amorphous, followed by cold crystallization from 150 degrees C giving rise to a stable anhydrous crystal T(beta). Under a highly humid atmosphere, on the other hand, T(beta) was formed at 90 degrees C directly as a result of T(h) dehydration. T(alpha) was readily rehydrated and turned back to T(h) when nitrogen gas with low water vapor pressure of 2.1kPa was admitted, whereas high water vapor pressure up to 7.4kPa was required for rehydration of T(beta) into T(h). This study provided a picture of pathways that link various solid forms of trehalose, taking into account the effects of a humid environment.     

In-plane miscibility and mixed bilayer microstructure in mixtures of catanionic glycolipids and zwitterionic phospholipids

SAXS/WAXS studies were performed in combination with freeze fracture electron microscopy using mixtures of a new Gemini catanionic surfactant (Gem16-12, formed by two sugar groups bound by a hydrocarbon spacer with 12 carbons and two 16-carbon chains) and the zwitterionic phospholipid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) to establish the phase diagram. Gem16-12 in water forms bilayers with the same amount of hydration water as DPPC. A frozen interdigitated phase with a low hydration number is observed below room temperature. The kinetics of the formation of this crystalline phase is very slow. Above the chain melting temperature, multilayered vesicles are formed. Mixing with DPPC produces mixed bilayers above the corresponding chain melting temperature. At room temperature, partially lamellar aggregates with local nematic order are observed. Splitting of infinite lamellae into discs is linked to immiscibility in frozen state. The ordering process is always accompanied by dehydration of the system. As a consequence, an unusual order-disorder phase transition upon cooling is observed.     

Acoustic emission during cooling and heating of aqueous solutions of polyethylene glycols of molecular masses from 300 to 3000

The object of this research is to study acoustic emissions (AE) in aqueous solutions of polyethylene glycols (PEGs) of molecular masses from 300 to 3000 during cooling at 100 degrees C/min and heating at 70 degrees C/min in the temperature range from 0 to -196 degrees C. The dependence of AE intensity on PEG concentration and molecular mass is analysed. The AE intensity correlated with the crystalline to amorphous phase ratio in the frozen system.     

A Fourier transform infrared spectroscopic study of the interaction of alkaline earth cations with the negatively charged phospholipid 1, 2-dimyristoyl-sn-glycero-3-phosphoglycerol

The interaction of aqueous phospholipid dispersions of negatively charged 1,2-dimyristoyl-sn-glycero-3-phosphoglycerol, sodium salt (DMPG) with the divalent cations Mg(2+), Ca(2+) and Sr(2+) at equimolar ratios in 100 mM NaCl at pH 7 was investigated by Fourier transform infrared spectroscopy. The binding of the three cations induces a crystalline-like gel phase with highly ordered and rigid all-trans acyl chains. These features are observed after storage below room temperature for 24 h. When the gel phase is heated after prolonged incubation at low temperature phase transitions into the liquid crystalline phase are observed at 58 degrees C for the DMPG:Sr(2+), 65 degrees C for the DMPG:Mg(2+), and 80 degrees C for the DMPG:Ca(2+) complex. By subsequent cooling from temperatures above T(m) these complexes retain the features of a liquid crystalline phase with disordered acyl chains until a metastable gel phase is formed at temperatures between 38 and 32 degrees C. This phase is characterized by predominantly all-trans acyl chains, arranged in a loosely packed hexagonal or distorted hexagonal subcell lattice. Reheating the DMPG:Sr(2+) samples after a storage time of 2 h at 4 degrees C results in the transition of the metastable gel to the liquid crystalline phase at 35 degrees C. This phase transition into the liquid crystalline state at 35 degrees C is also observed for the Mg(2+) complex. However, for DMPG:Mg(2+) at higher temperatures, a partial recrystallization of the acyl chains occurs and the high temperature phase transition at 65 degrees C is also detected. In contrast, DMPG:Ca(2+) exhibits only the phase transition at 80 degrees C from the crystalline gel into the fluid state upon reheating. Below 20 degrees C, the rate of conversion from the metastable gel to a thermodynamically stable, crystalline-like gel phase decreases in the order Ca(2+)&z. Gt;Mg(2+)>Sr(2+). This conversion into the crystalline gel phase is accompanied by a complete dehydration of the phosphate groups in DMPG:Mg(2+) and by a reorientation of the polar lipid head groups in DMPG:Ca(2+) and in DMPG:Sr(2+). The primary binding sites of the cations are the PO(2)(-) groups of the phosphodiester moiety. Our infrared spectroscopic results suggest a deep penetration of the divalent cations into the polar head group region of DMPG bilayers, whereby the ester carbonyl groups, located in the interfacial region of the bilayers, are indirectly affected by strong hydrogen bonding of immobilized water molecules. In the liquid crystalline phase, the interaction of all three cations with DMPG is weak, but still observable in the infrared spectra of the DMPG:Ca(2+) complex by a slight ordering effect induced in the acyl chains, when compared to pure DMPG liposomes.     

Structure and thermotropic properties of 1-stearoyl-2-acetyl-phosphatidylcholine bilayer membranes.

The structural and thermotropic properties of 1-stearoyl-2-acetyl-phosphatidylcholine (C(18):C(2)-PC) were studied as a function of hydration. A combination of differential scanning calorimetry and x-ray diffraction techniques have been used to investigate the phase behavior of C(18):C(2)-PC. At low hydration (e.g., 20% H2O), the differential scanning calorimetry heating curve shows a single reversible endothermic transition at 44.6 degrees C with transition enthalpy delta H = 6.4 kcal/mol. The x-ray diffraction pattern at -8 degrees C shows a lamellar structure with a small bilayer periodicity d = 46.3 A and two wide angle reflections at 4.3 and 3.95 A, characteristic of a tilted chain, L beta' bilayer gel structure. Above the main transition temperature, a liquid crystalline L alpha phase is observed with d = 53.3 A. Electron density profiles at 20% hydration suggest that C(18):C(2)-PC forms a fully interdigitated bilayer at -8 degrees C and a noninterdigitated, liquid crystalline phase above its transition temperature (T > Tm). Between 30 and 50% hydration, on heating C(18):C(2)-PC converts from a highly ordered, fully interdigitated gel phase (L beta') to a less ordered, interdigitated gel phase (L beta), which on further heating converts to a noninterdigitated liquid crystalline L alpha phase. However, the fully hydrated (> 60% H2O) C(18):C(2)-PC, after incubation at 0 degrees C, displays three endothermic transitions at 8.9 degrees C (transition I, delta H = 1.6 kcal/mol), 18.0 degrees C (transition II), and 20.1 degrees C (transition III, delta HII+III = 4.8 kcal/mol). X-ray diffraction at -8 degrees C again showed a lamellar gel phase (L beta') with a small periodicity d = 52.3 A. At 14 degrees C a less ordered, lamellar gel phase (L beta) is observed with d = 60.5 A. However, above the transition III, a broad, diffuse reflection is observed at approximately 39 A, consistent with the presence of a micellar phase. The following scheme is proposed for structural changes of fully hydrated C(18):C(2)-PC, occurring with temperature: L beta' (interdigitated)-->L beta (interdigitated)-->L alpha(noninterdigitated)-->Micelles. Thus, at low temperature C(18):C(2)-PC forms a bilayer gel phase (L beta') at all hydrations, whereas above the main transition temperature it forms a bilayer liquid crystalline phase L alpha at low hydrations and a micellar phase at high hydrations (> 60 wt% water).     

Shape-memory polymer networks from oligo[(epsilon-hydroxycaproate)-co-glycolate]dimethacrylates and butyl acrylate with adjustable hydrolytic degradation rate

Degradable shape-memory polymer networks intended for biomedical applications were synthesized from oligo[(epsilon-hydroxycaproate)-co-glycolate]dimethacrylates with glycolate contents between 0 and 30 mol % using a photopolymerization process. In addition AB copolymer networks were prepared by adding 60 wt % n-butyl acrylate as comonomer. All synthesized polymer networks are semicrystalline at room temperature. A melting transition T(m) between 18 and 53 degrees C which can be used as switching transition for the shape-memory effect can be attributed to the crystalline poly(epsilon-hydroxycaproate) phase. At temperatures below T(m) the elastic properties are dominated by these physical cross-links. At temperatures higher than T(m) the E modulus of the amorphous polymer networks is lowered by up to 2 orders of magnitude, depending on the chemical cross-link density. Copolymer networks based on macrodimethacrylates with a M(n) of up to 13,500 g x mol(-1) and a maximum glycolate content of 21 mol % show quantitative strain recovery rates in stress-controlled cyclic thermomechanical experiments. Hydrolytic degradation experiments of polymer networks performed in phosphate buffer solution at 37 degrees C show that the degradation rate can be accelerated by increasing the glycolate content and decelerated by the incorporation of n-butyl acrylate.     

Energetic basis of molecular recognition in a DNA aptamer

The thermal stability and ligand binding properties of the L-argininamide-binding DNA aptamer (5'-GATCGAAACGTAGCGCCTTCGATC-3') were studied by spectroscopic and calorimetric methods. Differential calorimetric studies showed that the uncomplexed aptamer melted in a two-state reaction with a melting temperature T(m)=50.2+/-0.2 degrees C and a folding enthalpy DeltaH(0)(fold)=-49.0+/-2.1 kcal mol(-1). These values agree with values of T(m)=49.6 degrees C and DeltaH(0)(fold)=-51.2 kcal mol(-1) predicted for a simple hairpin structure. Melting of the uncomplexed aptamer was dependent upon salt concentration, but independent of strand concentration. The T(m) of aptamer melting was found to increase as L-argininamide concentrations increased. Analysis of circular dichroism titration data using a single-site binding model resulted in the determination of a binding free energy DeltaG(0)(bind)=-5.1 kcal mol(-1). Isothermal titration calorimetry studies revealed an exothermic binding reaction with DeltaH(0)(bind)=-8.7 kcal mol(-1). Combination of enthalpy and free energy produce an unfavorable entropy of -TDeltaS(0)=+3.6 kcal mol(-1). A molar heat capacity change of -116 cal mol(-1) K(-1) was determined from calorimetric measurements at four temperatures over the range of 15-40 degrees C. Molecular dynamics simulations were used to explore the structures of the unligated and ligated aptamer structures. From the calculated changes in solvent accessible surface areas of these structures a molar heat capacity change of -125 cal mol(-1) K(-1) was calculated, a value in excellent agreement with the experimental value. The thermodynamic signature, along with the coupled CD spectral changes, suggest that the binding of L-argininamide to its DNA aptamer is an induced-fit process in which the binding of the ligand is thermodynamically coupled to a conformational ordering of the nucleic acid.