Polymorphism of palmitoyl and stearoyl dioleins

Polymorphism has been explored for the pairs of homologs 1) 2-palmitoyl diolein (OPO) and 2-stearoyl diolein (OSO), and 2) 1-palmitoyl diolein (POO) and 1-stearoyl diolein (SOO). The symmetrical compounds show α, β′-2 and stable β-3 forms; the unsymmetrical compounds show α (presumed) and stable β′-3 forms.     

Polymorphism of stabilized and nonstabilized tristearin,pure and in the presence of food emulsifiers

The polymorphism of tristearin (SSS) was studied by means of differential scanning calorimetry and powder X-ray diffraction. The influence of 5% in weight of different food emulsifiers—i.e., 1-monostearin, sorbitan tristearate, and sugar monostearate—was also studied. Because polymorphism is sensitive to thermal treatment, two thermal conditionings were applied. According to the dynamic process (melting, quenching, and heating at 5°C/min), SSS showed three polymorphic forms: α, β′, and β₁. The presence of the emulsifiers hindered the β′→β transformation, and a destructured β₂ form was recorded. According to the stabilization process (stabilization at 57°C for various periods of time), SSS showed two β′ forms: β ₂ ^′ and β ₁ ^′ . Three hours of stabilization were necessary to recover the whole triglyceride under the β form. The emulsifiers slowed down the polymorphic transition rates. Indeed, after six hours of stabilization, mixtures of β′ and β were observed. Sugar monostearate seemed to have the most powerful effect on the transition kinetics because large amounts of α form were detected. Presented at the 86th AOCS Annual Meeting & Expo, in San Antonio, Texas, in May 1995.     

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.     

Non-Isothermal Crystallization of Bovine Milk Fat

The crystallization kinetics of milk fat were studied under non-isothermal and simulated adiabatic conditions using pulsed NMR spectroscopy. Isothermal experiments confirmed that when milk fat is shock cooled to below the α melting point it crystallizes in two steps due to the different crystallization kinetics of α and β′ modifications. In non-isothermal experiments, the fat samples were heated early during the plateau between steps to a temperature above the α melting point and β′ crystals formed more rapidly than in isothermal conditions. Fresh α crystals are believed to melt and form lamellar units containing triglyceride molecules with high degrees of isomorphism and these units can accelerate the nucleation and growth rates of β′ polymorph crystals. The crystallization behavior changed when the heating occurred late in the plateau and the α crystals are believed to have demixed, which allowed them to transform to β′ crystals directly in the solid state. Under simulated adiabatic conditions the rate of β′ crystallization was increased by a factor of 2–3 over the isothermal case. These findings were used to infer approaches to process difficult fat blends in scraped-surface heat exchanger plants.     

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.     

Synthesis of spin-labeled neutral lipids: Nitroxyl derivatives of triglycerides and sterol esters

Methods for the preparation of useful spin-labeled neutral lipids are described. A spin-labeled triglyceride has been prepared by acylation of 1,3-distearoylglycerol with stearic acid anhydride bearing the 4′,4′-dimethyloxazolidine-N-oxyl ring at carbon-12. The same fatty acid anhydride has been used to acylate the 3-hydroxy group of cholesterol to obtain a cholesteryl ester with the nitroxyl function in the fatty acyl chain. The 4′,4′-dimethyloxazolidinyl-1-oxyl derivative of 5α-an drostan-3-one-17β-ol has been esterified with stearic acid anhydride to obtain a steroid ester with the paramagnetic center in the steroid nucleus.     

Transitions of saturated monoacid triglycerides: Modeling conformational change at glycerol during α→β′→β transformation

The glycerol region geometry of modeled saturated monoacid triglycerides was altered by bond rotations and minor angle distortions to convert theoretical α-forms into bent β′- and β-forms. Direct α to β conversion involves lateral disruption of fatty chain packing to generate side-chain character typical of the β-form. Such disruption, which could contribute to fat bloom, allows additional molecular movement and shifts in molecular mechanics energy (MME) that may approximate thermal changes observed by differential scanning calorimetry during α to β transformations. Energy calculations at 100 points throughout each transformation identified plausible conversion routes. A two-stage conversion, α to either of two stereospecific β′-forms bent at glycerol followed by subsequent conversion to β, showed less chain movement and more favorable MME than direct α to β conversion. The preferred path, based on energy profiles of each conversion, involves a β′-D form and rotation of carbonyl to α-carbon bonds in chain #2 and a side chain (chain #3).     

Polymorphism of cocoa butter

Largely by x-ray diffraction six crystalline states, I–VI, in order of increasing melting point, have been identified for cocoa butter. Of these states II, IV, V and VI are pure and identifiable with previously (or presently) identified polymorphs of 2-oleoylpalmitoyl stearin (POS), namelyα-2,β′-2,β-3 (“V”) andβ-3 (“VI”); V and VI representing distinct but very closely related crystalline structures. State I is a definite but fleeting and not readily characterized subα state and may be a phase mixture, as state III may be also. Melting points, heats of fusion and dilatometric data are reported for all states to the extent that their stability permits. The normal state of cocoa butter in chocolate is apparently V, certainlyβ-3. While it is true that “bloom” has not been observed for pure V nor observed to exist in the absence of VI, it is premature to say that VI is specifically the phase of chocolate “bloom”.     

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.     

Polymorphic stability of some shortenings as influenced by the fatty acid and glyceride composition of the solid phase

A number of North American vegetable and animal fat shortenings, which had been analyzed previously for their physical and textural characteristics, were analyzed also for their chemical composition. The fatty acid and triglyceride composition of the solids were calculated by analyzing the composition of the original product and the liquid phase, and by determination of the solid fat content (SFC) of the fat. The solids were also isolated by isopropanol (IP) separation, and the high melting glycerides (HMG) by acetone crystallization at 15°C. There was not much difference in total saturates andtrans content between vegetable and animal fat shortenings. Changing formulations from soy-palm to soy-cottonseed does not change the total saturates plustrans content. The solids of the vegetable shortenings in the β form contained about 20% of 16:0, those in the β′ form 30% or more. The animal fat shortenings were mainly in the β form, their solids contained 30% or more of 16:0. C54 triglyceride content of the solids of β vegetable shortenings (calculated and IP-separated) was >45%, that of all animal fats was <25%. Solids of animal fat shortenings contain high levels of C52. The C54 triglycerides are β-tending and should be kept low in vegetable shortening. In the HMG the C54 should not exceed 30%. This can only be achieved by incorporation of a β′ hard fat, preferably palm hard fat. Animal fat, especially lard, crystallizes in the β form because the palmitic acid in the glyceride molecule is located in the 2-position, whereas those of vegetable fats are in the 1- and 3-position.     

Binary phase behavior of 1,3-distearoyl-2-oleoyl-sn-glycerol (SOS) and 1,3-distearoyl-2-linoleoyl-sn-glycerol (SLS)

The binary phase behavior of SOS (1,3-distearoyl-2-oleoyl-sn-glycerol) and SLS (1,3-distearoyl-2-linoleoyl-sn-glycerol) was examined by using DSC and conventional and synchrotron radiation X-ray diffraction. The solid-solution phases were observed in the metastable α and γ forms in all concentration ranges. Results indicated that the miscible γ form did not transform to the β′ form when the mixtures were subjected to simple cooling from a high-temperature liquid to a low-temperature solid phase. However, and α-melt-mediated transformation into β′ and β₂ resulted in the formation of immiscible phases in concentration ranges of SLS below 30%. By contrast, at SLS concentration ranges above 30%, the α-melt-mediated transformation caused crystallization of only the γ form, and β′ and β₂ crystals did not appear. These results show that the specific interactions between SOS and SLS are operative in the phase behavior of the mixture states of SOS and SLS.     

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.     

The influence of polymorphic form on oxygen and water vapor transmission through lipid films

Fully-hydrogenated soybean and rapeseed oil was blended 2:1, layered on a filter paper support, adjusted to the desired polymorphic form and tested for resistance to transmission to oxygen and water vapor. Resistance to oxygen transport decreased upon conversion from α to the β′ form and then increased substantially upon conversion to the β polymorph. This was attributed to the greater solid-state density of the β form, which likely affects resistance to gas flux by lowering the oxygen diffusion constant through the film. Resistance to water vapor transmission also decreased following the α to β′ transition and then increased somewhat upon conversion to the β form. However, resistance of the β form did not exceed that of the α form at any of the temperatures tested. Moisture sorption characteristics of the various polymorphic forms apparently caused relative resistance values for water vapor flux to differ, somewhat from those for oxygen flux.     

The rele of chain length and an emulsifier on the polymorphism of mixtures of triglycerides

The appearance of the β′ form in the α-β transformation in tristearin is hardly detectable. On the other hand, in mixtures of tristearin and tripalmitin at different ratios, β′ formation has been observed to be favored. This observation confirms the statement in the literature that in mixtures of different chain lengths orthorhombic packing is stabilized. When an emulsifier was added to the mixtures, both the β′ and β formation were inhibited. The effect caused by the addition of the emulsifier as an impurity to tristearin is compared to that caused by the addition of tripalmitin: their effects, although both kinetic, were very different. In spite of this difference, they both have implications in the confectionery and fats industries.     

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%.     

Computer modeling ofα- toβ′- form phase transitions using theoretical triglyceride structures

Intermolecular energy calculations were performed on theoretical triarachidinα-form structures as selected bond rotations converted them intoβ′-forms with a chain-tilt change in the glycerol region. Interactions across the methyl gaps amounted to only 2-3% of the total energy in initialα- and finalβ′-forms, but computer generated energy profiles duringα- toβ′-phase transitions revealed highly repulsive regions due to the close approach of methyl groups. This methyl gap interaction, plus additional repulsive interactions in the lateral packing of molecules during rigid chain rotations, necessitated modification of certain chain movements during phase transition to reduce excessive repulsive energy. These results suggest that phase transitions proceed in a particular sequence of events that either distribute energy to promote further phase excitation or that lead to collapse into the stable polymorphic form. Phase transition energy curves also reveal that secondaryα- andβ′-forms are possible and are dependent on the startingα-forms, the direction of chain rotation and the subcell arrangement.     

Polymorphism of glyceryl ethers and ether esters

Phase behavior has been studied by thermal and diffraction methods for 1-and 2-palmityl and stearyl ethers of glycerol and for 14 trialkyl glyceryl ethers, dialkyl monoacyl glyceryl ethers and monoalkyl diacyl glyceryl ethers, all of which were saturated trichain substituted glycerol compounds containing one or more of the following chains: palmityl (Py), palmitoyl (P), stearyl (Sy) and stearoyl (S). The monoalkyl glyceryl ethers resemble monoglycerides in crystallization behavior but with significant differences. Isomeric 1- and 2-ethers are very close in melting point. The 1-ethers show, besides a stable form, two other forms which transform reversibly to each other. The 2-ethers are polymorphic but with only one clearly established melting level. All trichain compounds were polymorphic also, most being dimorphic, each exhibiting a metastable α form, typically more stable than that of related triglycerides. Forms other than α were labeled I, II, etc., in order of decreasing melting point and were typically obtained from solvent. Polymorphic behavior showed some rather large departures from that of related triglycerides and appeared generally more sensitive to impurities. The two triethers, PyPyPy and SySySy were dimorphic each with a stable form much resembling metastable α in diffraction pattern, hence presumed to be of a new (more dense) hexagonal type of cross sectional structure. Three dialkyl monoacyl compounds PyPyP, SySyS and SyPyS and also three monoalkyl diacyl compounds PPyP, SSyS and SPyS were dimorphic, with Form I a stable, nontilted, somewhat β′-like form. PySyS and PSyP, which were trimorphic, showed such a β′-like form as a Form II, i.e., a second highest melting point. The stable phase of four compounds, namely PyPP, SySS, PySyS (all trimorphic) and dimorphic PySS (and possibly that of SyPP) could be called a β phase. Presence of an alkyl group on the 2 position of glycerol, in all but the PySyS, prevented β structure. PSyP and SyPP exhibited triple chain length structures, not encountered in saturated mixed triglycerides with less than four carbons difference in the acyl chains. SyPP was exceptional in showing four forms.     

Specific heats of the solid-state phases of trimargarin and tristearin

Differential scanning calorimetry is used to obtain specific heats of the α, β′₂, β′₁ and β phases of trimargarin and tristearin in the temperature range from 190–350 K. Unequal specific heats are observed for β′ phases of the 2 lipids in contrast to nearly coincident values for their respective α and β phases. These results are discussed on the basis of odd vs even chain length triglycerides.     

A new ceramide from the basidiomycete Russula cyanoxantha

A new phytosphingosine-type ceramide (1) was isolated along with nine other compounds—5α,8α-epidioxy-(22E,24R)-ergosta-6,22-dien-3β-ol, 5α,8α-epidioxy-(24S)-ergosta-6-en-3β-ol, (24S)-ergosta-7-ene-3β,5α,6β-triol,(22E,24R)-ergosta-7, 22-dien-3β,5α,6β-triol, inosine, adenine, l-pyroglutamic acid, fumaric acid, and d-allitol from the ethanol and chloroform/methanol extract of the fruiting bodies of the basidiomycete Russula cyanoxanotha (Schaeff.) Fr. The structure of (1) was established as (2S,3S,4R,2′R)-2-(2′-hydroxytetracosanoylamino) octadecane-1,3,4-triol by means of spectroscopic and chemical methods.     

A Feeding Stimulant for Manduca sexta from Solanum surattenses

The acceptance of Solanum surattenses as a host plant for the larvae of Manduca sexta was explained by the presence of feeding stimulants in foliage. Bioassay-guided fractionation of plant extracts resulted in the isolation of a highly active compound (1), which was identified as a furostan derivative {26-O-β-d-glucopyranosyl-(25R)-furosta-5-ene-3-β-yl-O-α-l-rhamnopyranosyl-(1″-2′)-O-α-l-rhamnopyranosyl-(1′″-3″)-O-β-d-glucopyranoside}. This compound has the same steroidal core substructure as that in a stimulant (indioside D) previously identified from potato foliage. However, the sugar substituents attached to the core are different.