Polymorphic transformation of 1,3-distearoyl-sn-2-linoleoyl-glycerol

Polymorphic transformation behavior of sub-α₁, sub-α₂, α, and γ in 1,3-distearoyl-sn-2-linoleoyl-glycerol (SLS) has been studied with X-ray diffraction, differential scanning caloremetry, and Fourier-transform infrared spectroscopy. Synchrotron radiation X-ray beam was employed to observe rapid transformation processes from the sub-α and α forms to the γ form. The chain length structures were double in sub-α₁, sub-α₂, and α, whereas γ was of triple chain-length structure. The subcell packing was pseudohexagonal for the two sub-α forms, hexagonal for the α form, and parallel type for the γ form. In comparison with 1,3-distearoyl-sn-2-oleoyl-glycerol (SOS), the occurrence behavior of sub-α, α, and γ of SLS was the same as that of SOS. However, the absence of β′ and β was unique for SLS. The chain-chain interactions between the linoleoyl moieties may stabilize the γ form, prohibiting the transformation into β′ and β forms. The presence of two cis double bonds may cause this stabilization, revealing the disordered chain conformation of the unsaturated chains.     

Polymorphic transformations in sn-1, 3-distearoyl-2-ricinoleyl-glycerol

Polymorphic transformations of sn-1,3-distearoyl-2-ricinoleyl-glycerol (SRS) have been studied with differential scanning calorimetry, X-ray powder diffraction (XRD), synchrotron radiation X-ray diffraction, and Fourier transform infrared spectroscopy (FTIR) techniques by using a 99.8% pure sample. Four polymorphs, α, γ, β′₂, and β′₁, were isolated. The thermal behavior of the four forms showed that the fusion of α at 25.8°C was followed by the crystallization of γ which melts at 40.6°C, and β′₂ and β′₁ revealed melting peaks at 44.3 and 48.0°C, respectively. No β form was observed, even when the two β′ forms were annealed around their melting points over one week. The XRD long spacing indicates that α packs into a double chain-length structure; however, γ and the two β′ phases pack into a triple chain-length structure. The polarized and nonpolarized FTIR spectra in methylene scissoring and methylene rocking regions indicated a parallel subcell packing in γ, and a mixture of orthorhombic perpendicular and parallel or hexagonal subcells in the β′₂ and β′₁ phases. Consequently, SRS exhibits quite a unique polymorphic behavior, compared to tristearoyl glycerol and sn-1,3-distearoyl-2-oleoyl-glycerol.     

Binary Phase Behavior of 1,3-Dipalmitoyl-2-oleoyl-<Emphasis Type="Italic">sn</Emphasis>-glycerol and 1,2-Dioleoyl-3-palmitoyl-<Emphasis Type="Italic">rac</Emphasis>-glycerol

1,3-Dipalmitoyl-2-oleoyl-sn-glycerol (POP) and 1,2-dioleoyl-3-palmitoyl-rac-glycerol (OOP) are two major molecular species that account for roughly half of the total triacylglycerols in palm oil. The binary phase behavior of a POP/OOP mixture plays an important role in the crystallization of palm oil. We conducted thermodynamic and kinetic studies of OOP and its mixtures with POP using differential scanning calorimetry and X-ray diffraction with a conventional generator and synchrotron radiation. We found that OOP has two polymorphs, α as a metastable form and β′ as the most stable form, and that the two forms are stacked in a triple-chain-length structure. The POP/OOP mixtures exhibited immiscible eutectic natures in both their metastable and their most stable states, in contrast to POP/1,2-dipalmitoyl-3-oleoyl-rac-glycerol and POP/1,3-dioleoyl-2-palmitoyl-sn-glycerol mixtures, in which molecular compounds of a double-chain-length structure were formed. A time-resolved synchrotron radiation X-ray diffraction study undertaken during the cooling and heating processes indicated that the α and β′ forms of the POP and OOP fractions crystallized and melted in separate manners, and that crystallization of the β′ form and the polymorphic transformation from α to β′ of POP and OOP are promoted in the presence of another component. The absence of molecular compound crystals in the binary mixtures of POP/OOP is explained by taking into account the molecular interactions of acyl chain packing, glycerol conformation, and methyl end stacking, among which glycerol conformation appeared to be most influential.     

Phase Equilibrium and Optimization Tools: Application for Enhanced Structured Lipids for Foods

Solid–liquid phase equilibrium modeling of triacylglycerol mixtures is essential for lipids design. Considering the α polymorphism and liquid phase as ideal, the Margules 2-suffix excess Gibbs energy model with predictive binary parameter correlations describes the non ideal β and β′ solid polymorphs. Solving by direct optimization of the Gibbs free energy enables one to predict from a bulk mixture composition the phases composition at a given temperature and thus the SFC curve, the melting profile and the Differential Scanning Calorimetry (DSC) curve that are related to end-user lipid properties. Phase diagram, SFC and DSC curve experimental data are qualitatively and quantitatively well predicted for the binary mixture 1,3-dipalmitoyl-2-oleoyl-sn-glycerol (POP) and 1,2,3-tripalmitoyl-sn-glycerol (PPP), the ternary mixture 1,3-dimyristoyl-2-palmitoyl-sn-glycerol (MPM), 1,2-distearoyl-3-oleoyl-sn-glycerol (SSO) and 1,2,3-trioleoyl-sn-glycerol (OOO), for palm oil and cocoa butter. Then, addition to palm oil of Medium-Long-Medium type structured lipids is evaluated, using caprylic acid as medium chain and long chain fatty acids (EPA-eicosapentaenoic acid, DHA-docosahexaenoic acid, γ-linolenic-octadecatrienoic acid and AA-arachidonic acid), as sn-2 substitutes. EPA, DHA and AA increase the melting range on both the fusion and crystallization side. γ-linolenic shifts the melting range upwards. This predictive tool is useful for the pre-screening of lipids matching desired properties set a priori.     

Thermal and structural properties of sn-1,3-dipalmitoyl-2-oleoylglycerol and sn-1,3-dioleoyl-2-palmitoylglycerol binary mixtures examined with synchrotron radiation X-ray diffraction

Thermodynamic and polymorphic behavior of POP (sn-1,3-dipalmitoyl-2-oleoylglycerol) and OPO (sn-1,3-dioleoyl-2-palmitoylglycerol) binary mixtures was examined using differential scanning calorimetry and conventional and synchrotron radiation X-ray diffraction. A molecular compound, β_C, was formed at the 1:1 (w/w) concentration ratio of POP and OPO, giving rise to two monotectic phases of POP/compound and compound/OPO in juxtaposition. β_C has a long-spacing value of 4.2 nm with a double chainlength structure and the melting point of 31.9°C. A structural model of the POP-OPO compound is proposed, involving the separation of palmitoyl and oleoyl chain leaflets in the double chainlength structure. In the polymorphic occurrence of the POP-OPO mixtures, the POP fraction transformed from α to β′ with no passage through γ, then transformed to β. The presence of OPO in POP promoted the β′-β transformation of POP during the melt-mediated crystallization.     

13C cross-polarization and magic-angle spinning nuclear magnetic resonance of polymorphic forms of three triacylglycerols

The cross-polarization and magic-angle spinning nuclear magnetic resonance (CP/MAS-NMR) technique has been used to analyze the polymorphic forms of three triacylglycerols, 1,3-dipalmitoyl-2-oleoyl glycerol (POP), 1, 3-racpalmitoyl-stearoyl-2-oleoyl glycerol (POS), and 1,3-distearoyl-2-oleoyl glycerol (SOS). Specific attention has been paid to glycerol, carbonyl, olefinic, and methyl end carbon resonances. Many distinct differences were observed in each polymorphic form of SOS. In the α form, the saturated and unsaturated acyl chains exhibit liquid state-like conformations. However, olefinic conformations of the γ and β′ forms were asymmetric with respect to thecis double bond. Spectral difference between β₂ and β₁ was observed only for the methylene carbon, and not in the other regions. Spectra of corresponding polymorphic forms of POP and POS were almost identical to those of SOS. However, some spectral differences were observed in the glycerol and methyl regions of γ and β′. From the chemical shifts of the methylene carbons, the crystal structures of the polymorphic form have been discussed, particularly in terms of the subcell structures.     

Polymorphism of pos. I. occurrence and polymorphic transformation

Polymorphic behavior of 1,3-rac-palmitoyl-stearoy 1-2-oleoylglycerol, 99.9% purity (POS) was examined by X-ray diffraction (XRD), differential scanning calorimetry (DSC), solubility measurements and optical microscopy in comparison with 1,3-dipalmitoyl-2-oleoylglycerol (POP) and 1,3-distearoyl-2-oleoylglycerol (SOS). Melt crystallization and solvent crystallization were examined for the occurrence of metastable and stable polymorphs. The number of independent polymorphs was four; α,δ, pseudoβ′ andβ. The lowest melting form, α, was identical to that commonly observed in POP and SOS lowest melting forms. As to the highest melting form,β, the XRD shortspacing pattern was identical toβ ₁ of POP and SOS. This is consistent with crystal habit:β single crystals of POS showed the same shape as those of β₁ of POP and SOS. However, the melting point ofβ (POS), 35.9°C, was lower than those ofβ ₁ of POP, 36.7°C, and of SOS, 43.0°C. Correspondingly, solubility ofβ of POS was lower than that of β₁ of POP below about 13°C, but higher above 13°C. POS did not possessβ ₂ , which is the second stable form in POP and SOS. Two forms of6 and pseudoβ′ occurred, the latter being more stable. The structural properties ofδ showed thatδ is not identical toγ previously observed in POP and SOS. Transformation behavior from the metastable to stable polymorphs of POS showed some differences from those of POP and SOS. Presented at the AOCS annual meeting in Cincinnati, Ohio, in May 1989.     

Polymorphism of POP and SOS. I. occurrence and polymorphic transformation

The polymorphic modifications of POP and SOS were identified with X-ray diffraction (XRD), DSC and Raman spectroscopy by using pure samples (99.9%). In POP, six polymorphs, α,γ, pseudo-β′₂, pseudo-β′₁, β′₂ and β′₁, were obtained, whereas five polymorphs, α, γ, pseudo-β′, β₂ and β₁, were isolated in SOS. Thermodynamic stability increased from α to β₁ straightforwardly both in POP and SOS, because the polymorphic transformation went monotropically in the order described above. Additionally, the 99.2% sample of POP crystallized another form, δ, but the 99.9% sample did not, implying subtle influences of the impurity. The four forms, α, γ, β₂ and β₁, of POP, revealed XRD and DSC patterns identical to the four forms of SOS designated by the same symbols. The chain length structure was double inα and triple in the other three forms in both POP and SOS. Peculiarity of POP was revealed partly in the chain length structure of pseudo-β′₂ and pseudo-β′₁ which were double, whereas pseudo-β′ of SOS was triple. This apparently showed contrast to the facts that the three forms revealed rather similar XRD short spacing patterns. Another peculiarity of POP was revealed in enthalpy value of the melt crystallization of α: ΔH_c (α) = 68.1 kJ/mol which was much larger than that of SOS (47.7 kJ/mol), and also than AOA and BOB. These peculiarities mean that the double chain length structures of POP are more stabilized than the others. Raman bands of CH₂ scissoring mode of SOS indicated parallel packing in γ, β₂ and β₁, and orthorhombic perpendicular packing in pseudo-β′. The polymorphic transformation mechanisms were discussed based on the proposed polymorphic structure models. Presented at the AOCS annual meetings in New Orleans, Louisiana in May 1987 and Phoenix, Arizona in May 1988.     

Polymorphism and solidification kinetics of the binary system POS-SOS

As a first approach to modeling the crystallization behavior of cocoa butter, the binary system of its two major components, 1-palmitoyl-2-oleoyl-3-stearoyl-s n-glycerol (POS) and 1,3-distearoyl-2-oleoyl-s n-glycerol (SOS), was studied. Differential scanning calorimetry, coupled with polarized light microscopy, was used to determine the phase diagrams of the various polymorphic forms (the most metastable sub-α and α, intermediate δ and β′, and stable β). Associated theoretical phase diagrams were also built. The presence of a solid solution for β and the ideal behavior of α were confirmed. Kinetics of isothermal solidification as a function of temperature were studied for three compositions of the POS-SOS system. Results were displayed as time-temperature-transformation (TTT) diagrams and crystal morphology maps. Dependence of the crystallization kinetics on composition of the binary system was interpreted in terms of nucleation and growth mechanisms. The asymmetry of the POS molecule induces a slower growth rate of the mixture when the concentration of this triacylglycerol is increased. Solidification kinetics during continuous cooling were studied at various cooling rates for the system POS-SOS 25:75. Experimental results were compared with numerical predictions of a solidification model based upon TTT data and an additivity principle. The calculated and measured volume fractions of the different phases formed and times of onset and finish of the solidification were in good agreement.     

Polymorphic transitions of mixed triglycerides,SOS, in the presence of sorbitan monostearate

The polymorphic behavior of 1,3-distearoyl-2-oleo glycerol (SOS) has been investigated in the presence of a selected food emulsifier, sorbitan monostearate. Five polymorphs, named α, γ, pseudo-β′, and β₂, and β₁ were crystallized and identified by both x-rays and DSC. Each polymorph was treated using two selected protocols—the “screen cycle” and the “aging cycle.” It has been demonstrated that sorbitan monostearate, when present in the molten fat, will significantly retard the α to γ transition during both the screen cycle and the aging cycle. A retardation effect was not detected for the γ to pseudo-β′ transition (during the screen or the aging cycle). The emulsifier that is inserted into the fat behaves as a conventional impurity, causing significant reduction in the melting temperatures of each polymorph. As a result, the fat tends to melt prior to its transition. The behavior of SOS in the presence of the sorbitan monostearate is therefore different than that of SSS (tristearin), and may explain, in part, the absence of the strong blooming transition to high melting polymorph that occurs when the fat is rich in SOS. This behavior can also shed some light on the behavior of cocoa butter in the presence of sorbitan esters.     

Synchrotron radiation X-ray diffraction study on phase behavior of PPP-POP binary mixtures

The phase behavior of thesn-1,3-dipalmitoyl-2-oleoylglycerol (PPP-POP) binary mixture system was studied by powder X-ray diffraction with synchrotron radiation and by differential scanning calorimetry. The results showed that the immiscible phases were observed in metastable and in the most stable forms. In particular, synchrotron X-ray diffraction enabled us to reveal the monotectic nature of α as a kinetic phase behavior. The equilibrium phase diagram of the PPP-POP mixture is divided into two regions. In POP concentration ratios below 40%, solid-state transformation from α to β was observed, indicating that the α-β transition of PPP was promoted in the presence of POP. By contrast, the polymorphic transition proceeds from α to β through the occurrence of the intermediate β′ form at POP concentration ratios above 50%.     

α-Melt-mediated crystallization of 1-palmitoyl-2-oleoyl-3-stearoyl-sn-glycerol

The α-melt-mediated crystallization of 1-palmitoyl-2-oleoyl-3-stearoyl-sn-glycerol (POS) has been investigated by differential scanning calorimetry (DSC), combined with polarized-light microscopy. Starting from a completely liquid state, the melt was first cooled down and maintained at a temperature, T ₁, during a time, t ₁, where the α-phase formed. Then it was heated to a temperature, T ₂, above the melting point of α for isothermal solidification into a solid phase, which was identified as δ. Based upon DSC solidification peaks, the time-temperature-transformation (TTT) diagram of POS was constructed for these solidification conditions and was compared with the TTT diagram of direct crystallization from the melt. The α-melt-mediated solidification showed accelerated kinetics of the δ-phase. The effects of T ₁ and t ₁ were also studied: at short t ₁, crystallization was faster with a decreasing value of T ₁, whereas the opposite trend was observed for a longer plateau at T ₁. These tendencies were interpreted in terms of three competing phenomena: the density of δ-nuclei that can form during the plateau at T ₁, α-δ solid-state transformation, and memory effects of molecule arrangements in the α-remelted phase.     

Separation of γ- and α-linolenic acid containing triacylglycerols by capillary supercritical fluid chromatography

The separation of γ- and α-linolenic acid containing triacylglycerols with an identical acyl carbon number and degree of unsaturation was obtained on capillary supercritical fluid chromatography using a 25% cyanopropyl−75% methylpolysiloxane stationary phase. The resolution of 1,3-dioleoyl-2-γ-linolenoyl-sn-glycerol and 1,3-dioleoyl-2α-linolenoyl-sn-glycerol was 1.35 on a 10 m×50 μm i.d. column, whereas the resolution was enhanced to 1.66 by combining two 10-meter columns in series. The difference in the position of double bonds in one linolenic acid residue of triacylglycerols resulted in two series of peaks in the separation of alpine currant (Ribes alpinum) and black currant (R. nigrum) seed oils. The use of the 10-meter column was found to be appropriate for the screening of the triacylglycerol profile in both seed oils studied.     

Polymorphism of POP and SOS. II. kinetics of melt crystallization

Melt crystallization of the polymorphs of SOS, α,γ, pseudo-β′, β₂ and β₁. and of POP, α,γ, pseudo-β′₂ pseudo-β ₁, β₂andβ ₁was examined using pure samples (99.9%). Induction time τ for newly occurring crystals in the melt phase was measured with a polarizing microscope equipped with a temperature-controlled growth cell. Rate of crystallization, 1/τ, was obtained for each polymorph of POP and SOS whose identification was done with X-ray diffraction and differential scanning calorimetry (DSC). Two modes of crystallization, melt-cooling and melt-mediation, yielded approximately the same results for POP and SOS: (a) The rates of crystallization were always higher in less stable than in more stable forms,β ₂only crystallized via a γ-melt mediation, butβ ₁did not occur by the melt crystallization; (b) the rate of meltmediated crystallization was always higher than the simple melt-cooling as examined at the same crystallization temperature; (c) the occurrence behavior of the polymorphs differed between the simple-cooling and meltmediation. The results were related to the solidification behavior of the polymorphs of cocoa butter. Presented at the AOCS Annual Meeting in Phoenix, Arizona in May 1988.     

Polymorphism of POS. II. kinetics of melt crystallization

Melt crystallization of four polymorphs of POS, α,δ, pseudo-β′ andβ, was examined with pure samples (>99.9%). Induction time, τ, for newly occurring crystals was measured with a polarizing microscope equipped with a temperature-controlled growth cell. Rate of crystallization, 1/τ, was obtained for each polymorph, whose identification was done with x-ray diffraction (XRD) and differential scanning calorimetry (DSC). Two modes of crystallization, melt cooling and melt mediation, were applied. From these experiments, the following conclusions were obtained: (i) The rate of melt-mediated crystallization was always higher than of simple melt cooling; (ii) the pseudo-β′ form was crystallized in a wider range of temperature than the less stable δ form; (iii) the occurrence behavior of the polymorphs differed between simple melt cooling and melt mediation; (iv) the δ form was crystallized only by simple melt cooling in a narrow range of temperature, 25.5°C∼28.3°C. This means that there is a possibility that δ may result from racemic compounds that are crystallized in a specific manner. The experimental results are discussed in comparison to 1,3-dipalmitoyl2-oleoylglycerol (POP), 1,3-distearoyl-2-oleoylglycerol (SOS) and cocoa butter.     

Lipolytic synthesis of optically active 1,2-dibutyryl-sn-glycerol. Identification of diglyceride by solvent-dependent specific rotation

The objective was to determine whether the initial pregastric lipase catalyzed hydrolysis of a triacylglycerol to 1,2(2,3)-diacylglycerol was a consequence of sn-specific hydrolysis. The identity of the reaction products for the enzyme-assisted hydrolysis and uncatalyzed acyl-transfer reaction sequence of tributyrylglycerol was assigned by ¹³C nuclear magnetic resonance. The optical activity of the product 1,2-dibutyryl-sn-glycerol (yield >50%, pH 6.5, 35°C, 13 min) was solvent dependent, being −2.92° (c ∼1.3, CHCl₃) and +3.32° (c ∼1.2, pyridine), and confirmation of sn-3 specificity by pregastric lipase was obtained.     

Hydrothermal synthesis of crystalline α-/β-MnO2 nanorods via γ-MnOOH nanorod precursors

The crystalline α-MnO₂ and β-MnO₂ nanorods have been successfully prepared via a facile hydrothermal method from γ-MnOOH nanorods precursor, respectively. The samples were characterized by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), field emission scanning electron microscope (FESEM) and Fourier transformed infrared spectra (FTIR). The morphology and structure of γ-MnOOH nanorods precursors have a great influence on the crystal structure of the obtained products. The α-MnO₂ nanorods are prepared from the 100°C γ-MnOOH precursor, while the β-MnO₂ nanorods are obtained from the 150°C γ-MnOOH precursor, respectively. Besides, the catalytic activity of the prepared α-MnO₂ and β-MnO₂ nanorods for the H₂O₂ decomposition has been investigated comparatively, and the latter shows better catalytic activity.     

Enzymatic Synthesis of Structured Triacylglycerols Containing CLA Isomers Starting from <Emphasis Type="Italic">sn</Emphasis>-1,3-Diacylglycerols

The synthesis of structured triacylglycerols (TAG) by the enzymatic reaction between sn-1,3-diacylglycerols (sn-1,3-DAG) and conjugated linoleic acid (CLA) isomers was studied. Both the substrates of the reaction were produced from vegetable oils, the sn-1,3-DAG from extra virgin olive oil and the CLA isomers from sunflower oil. The enzymatic reactions between these substrates were catalyzed for 96 h by an immobilized lipase from Rhizomucor miehei (Lipozyme IM) and the reactions carried out in solvent were monitored every 24 h by using high-performance liquid chromatography-evaporative light scattering detector (HPLC-ELSD). The enzymatic reactions were carried out in different reaction media (hexane, isooctane and solvent free) and with different CLA/sn-1,3-DAG ratios. Total % acidic composition and structural analysis data were evaluated to verify the presence of CLA isomers in sn-2- position of synthesized TAG. The results showed good levels of CLA incorporation in sn-1,3-DAG, from 19.2% of TAG synthesized in solvent free conditions with a 0.5:1 substrate ratio, to 47.5% of TAG synthesized in isooctane with a 2:1 substrate ratio. It was observed that for all the reaction media, the best sn-2- acylic specificity was obtained with a 0.5:1 substrate ratio.     

Binary Phase Behavior of Diacid 1,3-Diacylglycerols

Fifteen phase diagrams were prepared using data from differential scanning calorimetry analysis of binary blends of representative diacid 1,3-DAG. The behavior observed in binary phase diagrams is related to the difference in T _m (ΔT _m) between system components—eutectic for ΔT _m < 26 °C and monotectic for ΔT _m > 30 °C. Binary blends were prepared using six diacid 1,3-DAG: 1,3-hexanoyl-lauroyl-rac-glycerol, 1,3-hexanoyl-palmitoyl-rac-glycerol, 1,3-hexanoyl-oleoyl-rac-glycerol, 1,3-lauroyl-palmitoyl-rac-glycerol, 1,3-lauroyl-oleoyl-rac-glycerol and 1,3-palmitoyl-oleoyl-rac-glycerol. Diacid 1,3-DAG were synthesized using representative FA: hexanoic (6:0)—short-chain FA; lauric (12:0)—medium-chain FA; palmitic (16:0)—long-chain FA; and oleic (18:1)—mono-unsaturated FA. In addition to the aforementioned phase diagrams, the physical chemistry of 1,3-hexanoyl-lauroyl-rac-glycerol, 1,3-hexanoyl-oleoyl-rac-glycerol and 1,3-lauroyl-oleoyl-rac-glycerol is reported.     

Synthesis and surface properties of chemodegradable anionic surfactants: Diastereomeric (2-n-alkyl-1,3-dioxan-5-yl) sulfates with monovalent counter-ions

Sodium, potassium and ammonium cis- and trans-(2-n-alkyl-1,3-dioxan-5-yl) sulfates 6–8 (alkyl: n-C₉H₁₉, 6a–8a, and n-C₁₁H₂₃, 6b–8b) were synthesized in a reaction of aliphatic aldehydes 1a,b with glycerol 2 followed by separation in high yields of individual geometric isomers of cis-and trans-2-n-alkyl-5-hydroxy-1,3-dioxanes, cis-3a,b and trans-3a,b, followed by sulfation with sulfur trioxide-pyridine complex, and finally neutralization with NaOH, KOH, and NH₄OH, respectively. Physical data of the compounds and some surface properties of 2-n-nonyl derivatives, such as critical micelle concentration (CMC), effectiveness of aqueous surface tension reduction (Π_CMC), surface excess concentration Γ_CMC, and the surface area demand per molecule (A_CMC), were determined. It was shown that the surface activity of these compounds is influenced both by their geometric structure and by the monovalent counter-ion.