Thermal behavior of amylose-lipid complexes

Melting and crystallization phenomena of amylose-lipid complexes, crystallized either from dilute solution or concentrated amylose ‘melts’ under various conditions, were studied using differential scanning calorimetry (DSC). The melting enthalpies (complex/H₂O: 0·1–0·5 w/w) of the solution-grown crystalline complexes were 20·4±0·8 J g^?1 for amylosemonopalmitin (AM-1-C16), 26·5±1·5 J g^?1 for amylose-lysolecithin (AM-lys/in) and 26·6±1·6 J g^?1 for amylose-lauric acid (AM-C12). While melting of the AM-lys/in complex showed a single transition for all concentrations studied, the melting behavior of the AM-1-C16 and the AM-C12 was rather complex at low or intermediate water contents. At a heating rate of 10°C min^?1 two endothermic transitions with an intermediate exothermic peak were observed, indicative of non-equilibrium melting. A process of partial melting, followed by recrystallization and final melting, is suggested to explain such multiple-melting characteristics. These phenomena become less prominent with increasing water content; presumably due to the depression of the glass transition (T_g) and the melting temperatures (T_m). The annealing behavior of AM-1-C16 further suggested that the development of new structural order upon heating takes place primarily after partial melting of the initial crystalline structure. Overall, the DSC data are typical of those for semicrystalline polymers and consistent with a lamellar type of molecular organization in the crystalline regions of these materials.     

Ceramide-1-Phosphate, in Contrast to Ceramide, Is Not Segregated into Lateral Lipid Domains in Phosphatidylcholine Bilayers

Sphingolipids are key lipid regulators of cell viability: ceramide is one of the key molecules in inducing programmed cell death (apoptosis), whereas other sphingolipids, such as ceramide 1-phosphate, are mitogenic. The thermotropic and structural behavior of binary systems of N-hexadecanoyl-D-erythro-ceramide (C₁₆-ceramide) or N-hexadecanoyl-D-erythro-ceramide-1-phosphate (C₁₆-ceramide-1-phosphate; C₁₆-C1P) with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) was studied with DSC and deuterium nuclear magnetic resonance (²H-NMR). Partial-phase diagrams (up to a mole fraction of sphingolipids X = 0.40) for both mixtures were constructed based on DSC and ²H-NMR observations. For C₁₆-ceramide-containing bilayers DSC heating scans showed already at X_cer = 0.025 a complex structure of the main-phase transition peak suggestive of lateral-phase separation. The transition width increased significantly upon increasing X_cer, and the upper-phase boundary temperature of the mixture shifted to ∼65°C at X_cer = 0.40. The temperature range over which ²H-NMR spectra of C₁₆-ceramide/DPPC-d₆₂ mixtures displayed coexistence of gel and liquid crystalline domains increased from ∼10° for X_cer = 0.1 to ∼21° for X_cer = 0.4. For C16-C1P/DPPC mixtures, DSC and ²H-NMR observations indicated that two-phase coexistence was limited to significantly narrower temperature ranges for corresponding C1P concentrations. To complement these findings, C₁₆-ceramide/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and C16-C1P/POPC mixtures were also studied by ²H-NMR and fluorescence techniques. These observations indicate that DPPC and POPC bilayers are significantly less perturbed by C₁₆-C1P than by C₁₆-ceramide and that C₁₆-C1P is miscible within DPPC bilayers at least up to X_C1P = 0.30.     

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.     

Theoretical Aspects of Differential Scanning Calorimetry as a Tool for the Studies of Equilibrium Thermodynamics in Pharmaceutical Solid Phase Transitions

Although differential scanning calorimetry (DSC) is a non-equilibrium technique, it has been used to gain energetic information that involves phase equilibria. DSC has been widely used to characterize the equilibrium melting parameters of small organic pharmaceutical compounds. An understanding of how DSC measures an equilibrium event could make for a better interpretation of the results. The aim of this mini-review was to provide a theoretical insight into the DSC measurement to obtain the equilibrium thermodynamics of a phase transition especially the melting process. It was demonstrated that the heat quantity obtained from the DSC thermogram (ΔH) was related to the thermodynamic enthalpy of the phase transition (ΔH ^P ) via: ΔH?=?ΔH ^P /(1?+?K ^??1) where K was the equilibrium constant. In melting, the solid and liquefied phases presumably coexist resulting in a null Gibbs free energy that produces an infinitely larger K. Thus, ΔH could be interpreted as ΔH ^P. Issues of DSC investigations on melting behavior of crystalline solids including polymorphism, degradation impurity due to heating in situ, and eutectic melting were discussed. In addition, DSC has been a tool for determination of the impurity based on an ideal solution of the melt that is one of the official methods used to establish the reference standard.     

Monitoring the crystallization of amylose–lipid complexes during maize starch melting by synchrotron x‐ray diffraction

Most starch granules exhibit a natural crystallinity, with different diffraction patterns according to their botanical origin: A‐type from cereals and B‐type from tubers. The V polymorph results essentially from the complexing of amylose with compounds such as iodine, alcohols, or lipids. The intensity and nature of phase transitions (annealing, melting, polymorphic transitions, recrystallization, etc.) induced by hydrothermal treatments in crystalline structures are related to temperature and water content. Despite its small concentration, the lipid phase present mainly in cereal starches has a large influence on starch properties, particularly in complexing amylose. The formation of Vh crystalline structures was observed by synchrotron x‐ray diffraction in native maize starch heated at intermediate and high moisture contents (between 19 and 80%). For the first time, the crystallization of amylose–lipid complexes was evidenced in situ by x‐ray diffraction without any preliminary cooling, at heating rates corresponding to the usual conditions for differential scanning calorimetry experiments. For higher water contents, the crystallization of Vh complexes clearly occurred at 110–115°C. For intermediate water contents, mixed A + Vh (or B + Vh for high amylose starch) diffraction diagrams were recorded. Two mechanisms can be involved in amylose complexing: the first relating to crystallization of the amylose and lipid released during starch gelatinization, and the second to crystalline packing of separate complexed amylose chains (amorphous complexes) present in native cereal starches. © 1999 John Wiley & Sons, Inc. Biopoly 50: 99–110, 1999     

Structural and thermodynamic properties of rice starches with different genetic background Part 1. Differentiation of amylopectin and amylose defects

A combined DSC - HPAEC-PAD approach, gel permeation chromatography and mild long-term acidic hydrolysis were employed to study the effects of amylopectin chain-length distributional and amylose defects on the assembly structures of amylopectin (crystalline lamellae, amylopectin clusters) in A-type polymorphic starches extracted from 11 Thai cultivars of rice with different amylose level. Joint analysis of the data allowed determining the contributions of different populations of amylopectin chains to the thermodynamic melting parameters of crystalline lamellae. It was shown that amylopectin chains with DP 6-12 and 25or=37 could be related to chains stabilizing these structures. The total effect of amylose and amylopectin defects can be described by means of Thomson-Gibbs' equation. The increase of defects in the assembly structures is accompanied by rise of the rates of acidic hydrolysis of both amorphous and crystalline parts in starches.     

X-Ray diffraction of ethylenediamine–amylose complex

Solutions of amylose in ethylenediamine yield a crystalline film complex upon evaporation of solvent. The x-ray analysis indicates the presence of a tetragonal-shaped cell with a symmetry approximating that of space group P2₁2₁2₁. The amylose sixfold helix has a diameter of 13.3 Å and a translation period of 8.0Å. Chemical and physical analyses support a complexing ratio of one ethylenediamine molecule to every two glucose units. The structure is nearly identical to any amylose–dimethyl sulfoxide complex previously examined. The square mode of packing arrangement appears to result from complexation between amylose chains. Such complexing indicates a much greater degree of amylose interaction than is observed in amylose complex structures having a hexagonal close-packing arrangement.     

Structural investigation of amylose complexes with small ligands: helical conformation, crystalline structure and thermostability

Crystalline amylose complexes were prepared with decanal, 1-butanol, menthone and alpha-naphtol. Their crystalline structure and the related helical conformation, determined by wide angle X-ray diffraction (WAXD) and 13C CPMAS solid state NMR, were assigned to V6I, V6II, V6III and V8 types, respectively. It was possible to propose some hypotheses on the possible nature of interactions and especially intra-/inter-helical inclusion. Some shifts in the NMR C1 carbon signals were attributed to the presence of ligand in specific sites inside the structure for a same type of V6 helical conformation. Moreover, the crystallinity and polymorphic changes induced by desorption/rehydration were studied. A general increase of the carbon resonances sharpness upon rehydration has been observed, but also a V6II-V6I transition when decreasing the water content. Differential scanning calorimetry (DSC) experiments were also performed to approach the thermostability of the four types of complex and also the way they form again after melting/cooling sequences.     

Amylose–dye complexes in cationic micelles: an optical spectroscopy study

The formation of amylose complexes with rose bengal (RB), erythrosine B (ER), and phenolphthalein (PP) in the presence of the cationic detergent tetradecyltrimethylammonium bromide (TTABr) was studied using optical spectroscopy methods. Absorption spectroscopy, steady-state fluorescence spectroscopy and picosecond time-resolved fluorescence spectroscopy were used to derive association constants k_s of the dyes, critical micelle concentration (CMC) values and structural information on the complexes formed. It seems that PP fits very well into amylose sites, where it forms an efficient inclusion complex with k_s=44,500 M⁻¹. The molecular diameter of RB is too big to fit the amylose cavity. Only part of the xanthene unit may be adopted in the helical cavity of amylose, whereas most of the interaction occurs through electrostatic and/or dipole–dipole interactions with the amylose chain. The ER molecule is an intermediate case, because it may fit the amylose cavity or adsorb on the amylose surface to form a complex. The presence of a surfactant in the amylose–ligand system increases the association constant for all dyes. In the presence of amylose, a decrease of the detergent CMC value of about one order of magnitude is observed. It is probable that the increased number of micelles incorporate more dyes into the amylose vicinity, which finally changes the structure of the amylose chain. On a macro scale, it was noted that the samples with dyes and detergent have a lower tendency to precipitate and the gelation process is delayed compared to that in water.     

Interaction between dry starch and plasticisers glycerol or ethylene glycol, measured by differential scanning calorimetry and solid state NMR spectroscopy

The interaction of crystalline amylose and of crystalline and amorphous amylopectin with the plasticisers glycerol or ethylene glycol in the absence of water was studied, by using differential scanning calorimetry (DSC) and solid state nuclear magnetic resonance (NMR) spectroscopy. Upon heating starch freshly mixed with plasticisers, a strong exothermal interaction enthalpy of ΔH−35 J/g was detected by DSC. At room temperature glycerol interacts mainly with the amorphous starch regions, the interaction taking 8 days to reach equilibrium. For ethylene glycol the interaction is faster, taking four days to reach equilibrium, and the rate is not affected by crystallinity. Ethylene glycol interacts in a more ordered manner with amorphous than with crystalline material, resulting in a narrower ethylene glycol cross-polarisation magic angle spinning (CP/MAS) signal when equilibrium is reached at room temperature. Upon heating, more glycerol or ethylene glycol is immobilised, but in a less ordered manner than upon storage at room temperature. This results in a more intense, but broader plasticiser CP/MAS signal upon heating. Interaction in a more ordered manner probably implies interaction with more of the hydroxy groups of the plasticiser. The polysaccharide mobility is increased more when the plasticiser interacts in a more ordered manner, as observed by small starch signals in HP/DEC spectra.     

Effects of annealing on the polymorphic structure of starches from sweet potatoes (Ayamurasaki and Sunnyred cultivars) grown at various soil temperatures

Starches extracted from the sweet potato cultivars Sunnyred and Ayamurasaki grown at 15 or 33 degrees C (soil temperature) were annealed in excess water (3 mg starch/mL water) for different times (1, 4, 8 or 10h) at the temperatures 2-3 degrees K below the onset melting temperature. The structures of annealed starches, as well as their gelatinisation (melting) properties, were studied using high-sensitivity differential scanning calorimetry (HSDSC). In excess water, the single endothermic peak shifted to higher temperatures, while the melting (gelatinisation) enthalpy changed only very slightly, if any. The elevation of gelatinisation temperature was associated with increasing order/thickness of the crystalline lamellae. The only DSC endotherm identified in 0.6 M KCl for Sunnyred starch grown at 33 degrees C was attributed to A-type polymorphic structure. The multiple endothermic forms observed by DSC performed in 0.6M KCl for annealed starches from both cultivars grown at 15 degrees C provided evidence of a complex C-type (A- plus B-type) polymorphic structure of crystalline lamellae. The A:B-ratio of two polymorphic forms increased upon annealing due to partial transformation of B- to A-polymorph, which was time dependent. Long heating periods facilitated the maximal transformation of B- to A-polymorph associated with limited A:B ratio.     

Mobility of lipid in complexes of amylose–fatty acids by deuterium and C solid state NMR

Palmitic and lauric acid complexes with amylose were studied by solid state methods: ¹³C CP/MAS NMR, deuterium NMR, X-ray powder diffraction and differential scanning calorimetry (DSC). The crystalline amylose complexes were found to be in a V-type sixfold single chain helix. The melting points of the complexes were over 100°C, at least 40–50°C higher than the melting points of the free fatty acids. CP/MAS ¹³C NMR spectra revealed two resonance peaks at 33.6 and 32.4 ppm for the palmitic acid, which were assigned as free and complexed fatty acid, respectively. A single resonance peak at 32.4 ppm was found for the lauric acid and assigned to the complex. The chemical shift of 32.4 ppm for the complexed fatty acids suggests a combined trans and gauche conformation for the fatty acid chain in the complex. T₁ relaxation measurements on the two palmitic acid resonances show different behavior: a very slow relaxation for the 33.6 ppm and a much faster relaxation (1.2 s) for the 32.4 ppm resonances. The latter was similar to the relaxation of the single resonance of the lauric acid (1.1 s). Temperature dependent deuterium spectra of the amylose–lauric acid and amylose–palmitic acid complexes suggest a complete complexation for the amylose–lauric acid, whereas the amylose–palmitic acid complex is partially disassociated by the thermal treatment. Based on the overall data, a partially disordered model is proposed: an imperfect helix with the fatty acid partly inside and partly out, depending on crystallization conditions and the necessity of placing the carboxyl head outside the V-helix.     

Formation and crystallization of amylose–monoglyceride complex in a starch matrix

The formation of amylose–lipid complexes in a gelatinized potato starch matrix was investigated using potato starch and glycerol monopalmitin. These complexes exist in two forms, with the amounts of each of the forms being dependent on the temperatures and durations of the pre-treatments.

Differential scanning calorimetry (DSC) was used to analyze transition temperatures and melting enthalpies, and thereby determine the amount of the complexes in the samples. X-ray diffraction analysis was used to investigate their crystallinity.

In measurements with DSC, form I started to melt at 88.5°C, and form II at 112.9°C. When complex form II was preheated at 100 or 110°C, its melting point rose to 116.3 and 119.7°C, respectively, because of an annealing effect. The same phenomenon occurred with complex form I: when preheated at 90°C, its melting point rose to 96.8°C. The crystal formation of form II appeared to be slower when treated at 110°C than at 100°C. Their maximum melting enthalpies were reached after about 24 h and 4 h of preheating, respectively. In X-ray diffraction analyses, form II showed a V-pattern, but form I did not. This indicates that form II is more crystalline than form I. It was possible to transform form I into form II when it was heat treated, because form I was then partially or totally melted.

As a comparison, the charged substance cetyltrimethylammonium bromide created complex form I with amylose in the starch matrix, but not form II.     

Effect of Milling on DSC Thermogram of Excipient Adipic Acid

The purpose of this research was to investigate why and how mechanical milling results in an unexpected shift in differential scanning calorimetry (DSC) measured fusion enthalpy (∆_fus H) and melting point (T _m) of adipic acid, a pharmaceutical excipient. Hyper differential scanning calorimetry (hyper-DSC) was used to characterize adipic acid before and after ball-milling. An experimental study was conducted to evaluate previous postulations such as electrostatic charging using the Faraday cage method, crystallinity loss using powder X-ray diffraction (PXRD), thermal annealing using DSC, impurities removal using thermal gravimetric analysis (TGA) and Karl Fischer titration. DSC thermograms showed that after milling, the values of ∆_fus H and T _m were increased by approximately 9% and 5 K, respectively. Previous suggestions of increased electrostatic attraction, change in particle size distribution, and thermal annealing during measurements did not explain the differences. Instead, theoretical analysis and experimental findings suggested that the residual solvent (water) plays a key role. Water entrapped as inclusions inside adipic acid during solution crystallization was partially evaporated by localized heating at the cleaved surfaces during milling. The correlation between the removal of water and melting properties measured was shown via drying and crystallization experiments. These findings show that milling can reduce residual solvent content and causes a shift in DSC results.     

X-ray diffraction of oriented amylose fibers. III. The structure of amylose–n-butanol complexes

Complexes of amylose with n-butanol were prepared both as crystalline precipitates and as oriented fibers. These complexes were subjected to x-ray analysis, their unit cells were calculated, and the space group of P2₁2₁2₁ was confirmed. n-Butanol complexes exist in both hydrated and anhydrous forms. There is no evidence for methanol, ethanol, or n-propanol structures similar to those shown by the n-butanol complex. The Complexes are unstable in the open air and revert to V-amylose hydrate on standing.     

Synthesis and spectroscopic studies of platinum(II) complexes of N,N′-dicyclohexylethylenediamine

The platinum(II) complexes of the formula [Pt(DCHEDA)X₂], where DCHEDA is N,N′-dicyclohexylethylenediamine and X⁻ is CL⁻, Br⁻, I⁻, 0.5C₂O₄²⁻ (oxalate), 0.5C₃H₂O₄²⁻ (malonate), 0.5C₉H₄O₆²⁻ (4-carboxyphthalate), 0.5S₂O₃²⁻ or 0.5SO₄²⁻, have been synthesized and characterized by UVVis, IR, and ¹H NMR spectral techniques. All the above complexes are non-electrolytes in DMF/H₂O, except the sulphate complex which becomes a 1:1 electrolyte after incubation for 24 h at 28 °C. The halide complexes were also studied by X-ray photoelectron spectroscopy and these data suggest that there is π-bonding from platinum to halide in these complexes. The oxalate, malonate and sulphate bind in their complexes as bidentate ligands to platinum through two oxygen atoms whereas the thiosulphate in its complex binds as a bidentate ligand to platinum through one oxygen atom and one sulphur atom.     

Synthesis, structure and thermochemical behavior of bis-(1,1,1,5,5,5-hexafluoro-2,4-pentanedionato)-(1,4,7,10,13,16-hexaoxa-cyclooctadecane)-strontium in comparison with its structural and thermochemical analogous

The trans-[M(18-crown-6)(C₅HO₂F₆)₂] (M = Ca, Sr, Ba, Pb) complexes have been synthesized and identified. The crystal structure of the Sr complex has been determined by X-ray diffraction. The basic structural unit is the mononuclear complex trans-[Sr(18-crown-6)(C₅HO₂F₆)₂]. The thermal properties of the complexes have been correlated to their structure. The melting and decomposition ranges of the Ca, Sr and Ba complexes and their sublimation temperatures at ∼10⁻² mm Hg have been determined. Experimental evidence is presented that the complexes are similar in volatility.     

Computer simulation of the cyclodextrin–phenylalanine complex

The results of the molecular dynamics simulations of the complexes of α-cyclodextrin-l-phenylalanine and β-cyclodextrine- L- phenylalanine in vacuo and in aqueous solution are presented. The trajectories of the insertion angle, rotation of the aromatic ring of the phenylalanine inside the macrocycle and the dihedral angle χ² (Cα–Cβ–Cγ–C_D2) describing the relative movement of the aromatic ring with respect to the polar region give detailed information of the dynamics of the complexes. It is found that the complex with α-cyclodextrin in water is not stable, in agreement with experimental data, while in all other situations studied the complex is stable within the computational limits. Comparing the different cases and the experimental evidence it comes out that a simulation of the complexes without an explicit treatment of the solvent gives unreliable results.