Structural analyses of polymorphic transitions of sn -1,3-distearoyl-2-oleoylglycerol (SOS) and sn -1,3-dioleoyl-2-stearoylglycerol (OSO): assessment on steric hindrance of unsaturated and saturated acyl chain interactions
1999; Elsevier BV; Volume: 40; Issue: 1 Linguagem: Inglês
10.1016/s0022-2275(20)33349-6
ISSN1539-7262
AutoresJunko Yano, Kiyotaka Sato, Fumitoshi Kaneko, Donald Small, Dharma R. Kodali,
Tópico(s)Fatty Acid Research and Health
ResumoPolymorphic transformations in two saturated-unsaturated mixed acid triacylglycerols, SOS (sn-1,3-distearoyl-2-oleoylglycerol) and OSO (sn-1,3-dioleoyl-2-stearoylglycerol), have been studied by FT-IR spectroscopy using deuterated specimens in which stearoyl chains are fully deuterated. A reversible phase transition between sub α and α and a series of irreversible transitions (α→γ→β′→β (β2, β1) for SOS and α→β′→β for OSO) were studied with an emphasis on the conformational ordering process of stearoyl and oleoyl chains. The α→sub α reversible transition was due to the orientational change of stearoyl chains in the lateral directions from the hexagonal subcell to a perpendicularly packed one. As the first stage of the series of irreversible transitions from α to β, the conformational ordering of saturated chains took place in the α→γ transition of SOS and in the α→β′ transition of OSO; one stearoyl chain in SOS and OSO takes the all-trans conformation and the second stearoyl chain in SOS takes the bent conformation like those observed in the most stable β-type. As the final stage, the ordering of unsaturated chains occurred in the β′→β transition both for SOS and OSO. A conversion in the layered structure from bilayer to trilayer was also accompanied by the conformational ordering in the α→γ transition of SOS and in the β′→β transition of OSO.—Yano, J., K. Sato, F. Kaneko, D. M. Small, and D. R. Kodali. Structural analyses of polymorphic transitions of sn-1,3-distearoyl-2-oleoylglycerol (SOS) and sn-1,3-dioleoyl-2-stearoylglycerol (OSO): assessment on steric hindrance of unsaturated and saturated acyl chain interactions. J. Lipid Res. 1999. 40: 140–151. Polymorphic transformations in two saturated-unsaturated mixed acid triacylglycerols, SOS (sn-1,3-distearoyl-2-oleoylglycerol) and OSO (sn-1,3-dioleoyl-2-stearoylglycerol), have been studied by FT-IR spectroscopy using deuterated specimens in which stearoyl chains are fully deuterated. A reversible phase transition between sub α and α and a series of irreversible transitions (α→γ→β′→β (β2, β1) for SOS and α→β′→β for OSO) were studied with an emphasis on the conformational ordering process of stearoyl and oleoyl chains. The α→sub α reversible transition was due to the orientational change of stearoyl chains in the lateral directions from the hexagonal subcell to a perpendicularly packed one. As the first stage of the series of irreversible transitions from α to β, the conformational ordering of saturated chains took place in the α→γ transition of SOS and in the α→β′ transition of OSO; one stearoyl chain in SOS and OSO takes the all-trans conformation and the second stearoyl chain in SOS takes the bent conformation like those observed in the most stable β-type. As the final stage, the ordering of unsaturated chains occurred in the β′→β transition both for SOS and OSO. A conversion in the layered structure from bilayer to trilayer was also accompanied by the conformational ordering in the α→γ transition of SOS and in the β′→β transition of OSO. —Yano, J., K. Sato, F. Kaneko, D. M. Small, and D. R. Kodali. Structural analyses of polymorphic transitions of sn-1,3-distearoyl-2-oleoylglycerol (SOS) and sn-1,3-dioleoyl-2-stearoylglycerol (OSO): assessment on steric hindrance of unsaturated and saturated acyl chain interactions. J. Lipid Res. 1999. 40: 140–151. The aliphatic chain–chain interactions are critical factors for the determination of physical properties such as softness and flexibility of the fats and lipids present in bio-tissues. Their packing conditions depend on chemical natures of fatty acid moieties: chain length, parity (odd or even), unsaturation (cis or trans), etc. Most of the biologically important lipid specimens are composed of saturated and unsaturated acyl chains as revealed in diacylglycerols and phospholipids. Diacylglycerols act as an activator of protein kinase C and a signal transductor in metabolism (1Berridge M.J. Irvine R.F. Inositol triphosphate, a novel second messenger in cellular signal transduction.Nature. 1984; 312: 315-321Google Scholar, 2Nishizuka Y. The role of protein kinase C in cell surface signal transduction and tumor promotion.Nature. 1984; 308: 693-698Google Scholar, 3Rando R.R. Yong N. The stereospecific activation of protein kinase C.Biochem. Biophys. Res. Commun. 1984; 122: 818-823Google Scholar, 4Seker M.C. Hokin L.E. The role of phosphoinositides in signal transduction.J. Membr. Biol. 1986; 89: 193-210Google Scholar, 5Ganong B.R. Loomis C.R. Hannun Y.A. Bell R.M. Specificity and mechanism of protein kinase C activation by sn-1,2-diacylglycerols.Proc. Natl. Acad. Sci. USA. 1986; 83: 1184-1188Google Scholar). Phospholipids produce fluidity of membrane lipids, often containing a saturated chain in the sn-glycerol 1 position and an unsaturated chain in the sn-glycerol 2 position (6Small D.M. Lateral chain packing in lipids and membranes.J. Lipid Res. 1984; 25: 1490-1500Google Scholar, 7Wang G. Lin H. Lin S. Huang C. Phosphatidylcho-lines with sn-1 saturated and sn-2 cis-monounsaturated acyl chains. Their melting behavior and structures.J. Biol. Chem. 1995; 270: 22738-22746Google Scholar). Triacylglycerols (TAGs), which are a major metabolic fuel, also consist of saturated and unsaturated acyl chains. Unsaturated acyl moieties have a critical role to prevent the solidification of TAGs; solidification causes lipase activity to be remarkably reduced in poikilothermal animals (8Sugiura M. Isobe M. Studies on the lipase of Chromobacterium viscosum. IV. Substrate specificity of a low molecular weight lipase.Chem. Pharm. Bull. 1975; 23: 1226-1230Google Scholar, 9Ohtsu T. Katagiri C. Kumura M.T. Hori S.H. Cold adaptations in drosophila. Qualitative changes of triacylglycerols with relation to overwintering.J. Biol. Chem. 1993; 268: 1830-1834Google Scholar). The presence of these moieties affects melting behavior more effectively than the modification of aliphatic chain length (10Kodali D.R. Atkinson D. Small D.M. Molecular packing in triacyl-sn-glycerols: influences of acyl chain length and unsaturation.J. Dispersion Sci. Technol. 1989; 10: 393-440Google Scholar). The aliphatic compounds containing cis-unsaturated chains reveal quite diversified polymorphism due to the structural flexibility of olefinic groups. Concerning the cis-unsaturated fatty acids, their molecular structures have been unveiled by X-ray diffraction crystal structure analyses and vibrational spectroscopy, IR and Raman (11Abrahamsson S. Ryderstadt-Nahringbauer I. The crystal structure of the low-melting form of oleic acid.Acta Crystallogr. 1962; 15: 1261-1268Google Scholar, 12Kobayashi M. Kaneko F. Sato K. Suzuki M. Vibrational spectroscopic study on polymorphism and order–disorder phase transition in oleic acid.J. Phys. Chem. 1986; 90: 6371-6378Google Scholar, 13Kaneko F. Yamazaki K. Kitagawa K. Kikyo T. Kobayashi M. Kitagawa Y. Matsuura Y. Sato K. Suzuki M. Structure and crystallization behavior of the β phase of oleic acid.J. Phys. Chem. B. 1997; 101: 1803-1809Google Scholar, 14Kaneko F. Yamazaki K. Kobayashi M. Kitagawa Y. Matsuura Y. Sato K. Suzuki M. Mechanism of the γ→α and γ1→α1 reversible solid-state phase transitions of erucic acid.J. Phys. Chem. 1996; 100: 9138-9148Google Scholar, 15Kaneko F. Kobayashi M. Sato K. Suzuki M. Martensitic phase transition of petroselinic acid: Influence of polytypic structure.J. Phys. Chem. 1997; 101: 285-292Google Scholar, 16Sato K. Yano J. Kawada I. Kawano M. Kaneko F. Suzuki M. Polymorphic behavior of gondoic acid and phase behavior of its binary mixtures with asclepic acid and oleic acid.J. Am. Oil Chem. Soc. 1997; 74: 1153-1160Google Scholar). The molecular structures of cis-unsaturated fatty acid esters of cholesterol were also clarified by the use of X-ray diffraction methods (17Craven B.M. Guerina N.G. The crystal structure of cholesteryl oleate.Chem. Phys. Lipids. 1979; 29: 91-98Google Scholar, 18Craven B.M. Sawzik P. Conformation and packing of unsaturated chains in cholesteryl linolelaidate at 123 K.J. Lipid Res. 1983; 24: 784-789Google Scholar, 19Gao Q. Craven B.M. Conformation of the oleate chains in crystals of cholesteryl oleate at 123 K.J. Lipid Res. 1986; 27: 1214-1221Google Scholar). Through those studies, it has become apparent that the cis-olefin group can take various internal-rotation angles in solid states to compensate for the conformational disadvantage, depending on the balance between the acyl chain length and the double bond position. The mixed-acid TAGs (20Sato K. Arishima T. Wang Z.H. Ojima K. Sagi N. Mori H. Polymorphism of POP and SOS. I. Occurrence and polymorphic transformation.J. Am. Oil Chem. Soc. 1989; 66: 664-674Google Scholar, 21Arishima T. Sagi N. Mori H. Sato K. Polymorphism of POS. I. Occurrence and polymorphic transformation.J. Am. Oil Chem. Soc. 1991; 68: 710-715Google Scholar, 22Kodali D.R. Atkinson D. Redgrave T.G. Small D.M. Structure and polymorphism of 18-carbon fatty acyl triacylglycerols: effect of unsaturation and substitution in the 2-position.J. Lipid Res. 1987; 28: 403-413Google Scholar, 23Engstrom L. Triglyceride systems forming molecular compounds.J. Fat Sci. Technol. 1992; 94: 173-181Google Scholar) and diacylglycerols (24Goto M. Honda K. Di L. Small D.M. Crystal structure of a mixed chain diacylglycerol, 1-stearoyl-3-oleyl-glycerol.J. Lipid Res. 1995; 36: 2185-2190Google Scholar, 25Di L. Small D.M. Physical behavior of the mixed chain diacylglycerol, 1-stearoyl-2-oleyl-sn-glycerol: difficulties in chain packing produce marked polymorphism.J. Lipid Res. 1993; 34: 1611-1623Google Scholar) also exhibited diversified polymorphic behavior, all indicating complicated molecular interactions. The mixing behavior of aliphatic chains is important for understanding the interactions between acyl chains. The miscibility between saturated and unsaturated constituents has been studied in the binary systems of fatty acids and those of TAGs. It was clarified that a saturated constituent was hard to mix with an unsaturated one (26Yoshimoto N. Nakamura T. Suzuki M. Sato K. Phase properties of binary mixtures of petroselinic acid/oleic acid and asclepic acid/oleic acid.J. Phys. Chem. 1991; 95: 3384-3390Google Scholar, 27Inoue T. Motoda I. Hiramatsu N. Suzuki M. Sato K. Phase behavior of binary mixture of palmitoleic acid (cis-9-hexadecenoic acid) and asclepic acid (cis-11-octadecenoic acid).Chem. Phys. Lipids. 1993; 66: 209-214Google Scholar, 28Koyano T. Hachiya I. Sato K. Phase behavior of mixed systems of SOS and OSO.J. Phys. Chem. 1992; 96: 10514-10520Google Scholar, 29Minato A. Ueno S. Yano J. Wang Z.H. Seto H. Amemiya Y. Sato K. Synchrotron radiation X-ray diffraction study on phase behavior of PPP–POP binary mixtures.J. Am. Oil Chem. Soc. 1996; 73: 1567-1572Google Scholar, 30Minato A. Ueno S. Smith K. Amemiya Y. Sato K. Thermodynamic and kinetic study on phase behavior of binary mixtures of POP and PPO forming molecular compound systems.J. Phys. Chem. B. 1997; 101: 3498-3505Google Scholar, 31Minato A. Ueno S. Yano J. Smith K. Seto H. Amemiya Y. Sato K. 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.J. Am. Oil Chem. Soc. 1997; 74: 1213-1220Google Scholar). We were interested also in the lateral packing of a TAG containing both saturated and unsaturated acyl chains, which would relate to a significant question of how the saturated and unsaturated chains are accommodated together in biological tissues and industrial products. In the previous studies on the mixed-acid TAGs of this type, it has been clarified that the steric hindrance between the saturated and unsaturated acyl chains resulted in various metastable phases and a series of polymorphic transformation to the stable phase. From the studies on three kinds of TAGs, Sat-O-Sat (1,3-disaturated-acyl-2-oleoylglycerol) (20Sato K. Arishima T. Wang Z.H. Ojima K. Sagi N. Mori H. Polymorphism of POP and SOS. I. Occurrence and polymorphic transformation.J. Am. Oil Chem. Soc. 1989; 66: 664-674Google Scholar, 21Arishima T. Sagi N. Mori H. Sato K. Polymorphism of POS. I. Occurrence and polymorphic transformation.J. Am. Oil Chem. Soc. 1991; 68: 710-715Google Scholar), O-Sat-O (1,3-dioleoyl-2-saturated-acylglycerol) (22Kodali D.R. Atkinson D. Redgrave T.G. Small D.M. Structure and polymorphism of 18-carbon fatty acyl triacylglycerols: effect of unsaturation and substitution in the 2-position.J. Lipid Res. 1987; 28: 403-413Google Scholar), and Sat-Sat-O (1,2-disaturated-acyl-3-oleoylglycerol) (23Engstrom L. Triglyceride systems forming molecular compounds.J. Fat Sci. Technol. 1992; 94: 173-181Google Scholar), the following conclusions were obtained. First, the position and quantity of oleoyl groups drastically change polymorphism. For example, SOS (sn-1,3-distearoyl-2-oleoylglycerol) shows five polymorphs, α, γ, β′, β2, and β1, while OSO (sn-1,3-dioleoyl-2-stearoylglycerol) shows three polymorphs, α, β′, and β (Figs. 1a, b) (20Sato K. Arishima T. Wang Z.H. Ojima K. Sagi N. Mori H. Polymorphism of POP and SOS. I. Occurrence and polymorphic transformation.J. Am. Oil Chem. Soc. 1989; 66: 664-674Google Scholar, 22Kodali D.R. Atkinson D. Redgrave T.G. Small D.M. Structure and polymorphism of 18-carbon fatty acyl triacylglycerols: effect of unsaturation and substitution in the 2-position.J. Lipid Res. 1987; 28: 403-413Google Scholar, 32de Jong S. van Soest T.C. van Schaick M.A. Crystal structures and melting points of unsaturated triacylglycerols in the β phase.J. Am. Oil Chem. Soc. 1991; 68: 371-378Google Scholar, 33Larsson K. Molecular arrangement in glycerides.Fette Seifen Anstrichm. 1972; 74: 136-142Google Scholar). Second, there are specific composition ratios for binary systems to form molecular compounds. Concerning this point, the binary systems of SOS/rac-1,2-distearoyl-3-oleoylglycerol (SSO) (23Engstrom L. Triglyceride systems forming molecular compounds.J. Fat Sci. Technol. 1992; 94: 173-181Google Scholar), SOS/OSO (28Koyano T. Hachiya I. Sato K. Phase behavior of mixed systems of SOS and OSO.J. Phys. Chem. 1992; 96: 10514-10520Google Scholar), sn-1, 3-dipalmitoyl-2-oleoylglycerol (POP)/sn-1,3-dioleoyl-2-palmitoylglycerol (OPO) (31Minato A. Ueno S. Yano J. Smith K. Seto H. Amemiya Y. Sato K. 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.J. Am. Oil Chem. Soc. 1997; 74: 1213-1220Google Scholar), and POP/rac-1,2-dipalmitoyl-3-oleoylglycerol (PPO) (30Minato A. Ueno S. Smith K. Amemiya Y. Sato K. Thermodynamic and kinetic study on phase behavior of binary mixtures of POP and PPO forming molecular compound systems.J. Phys. Chem. B. 1997; 101: 3498-3505Google Scholar) were investigated. Although each TAG component has different polymorphism, a specific chain–chain interaction caused the formation of the molecular compounds at the 1:1 ratio for each combination. In this study we aimed to clarify the conformation and lateral packing of the saturated and unsaturated acyl chains in each polymorphic form of SOS and OSO. For this purpose we used vibrational spectroscopy, which is a useful method to investigate the complicated polymorphism and intermolecular interactions on a molecular level. The information about subcell structure and molecular conformation of aliphatic chain can be derived. We have already applied infrared absorption spectroscopy for the polymorphic studies on mixed-acid TAGs such as SOS, POP, POS, and on binary systems of POP/PPO and POP/OPO (34Yano J. Ueno S. Sato K. Arishima T. Sagi N. Kaneko F. Kobayashi M. FT–IR study of polymorphic transformations in SOS, POP, and POS.J. Phys. Chem. 1993; 97: 12967-12973Google Scholar, 35Minato A. Yano J. Ueno S. Smith K. Sato K. FT-IR study on microscopic structures and conformations of POP-PPO and POP-OPO molecular compounds.Chem. Phys. Lipids. 1997; 88: 63-71Google Scholar). In the present study, we adopted the following tactics. For molecular conformation, the progression bands due to the ν3 branch (CH2 wagging) modes of polymethylene segments were used as a probe (36Jones R.N. Mckay A.F. Sinclair R.G. Band progressions in the infrared spectra of fatty acids and related compounds.J. Am. Chem. Soc. 1952; 74: 2575-2578Google Scholar, 37Primas H. Gunthard Hs.H. Theorie der intensitaten der schwingungsspektren von kettenmolekeln. 1. Allgemeine theorie der berechnung von intensitaten der infrarotspecktren von grossen molekeln.Helv. Chem. Acta. 1955; 38: 1254-1262Google Scholar, 38Primas H. Gunthard Hs.H. Die infrarotspektren von kettenmolekeln der formel R′CO(CH2CH2)nCOR″. II. Die normalschwingungen des symmetrietypus Bu.Helv. Chem. Acta. 1953; 36: 1791-1803Google Scholar, 39Tasumi M. Shimanouchi T. Watanabe A. Goto R. Infrared spectra of normal higher alcohols—I.Spectrochimica Acta. 1964; 20: 629-666Google Scholar, 40Fichmeister I. Infrared absorption spectroscopy of normal and substituted long-chain fatty acids and esters in the solid state.Prog. Chem. Fats Other Lipids. 1974; 24: 91-162Google Scholar, 41Ruig W.G. Infrared spectra of diacid and triacid triglycerides.Appl. Spectrosc. 1977; 31: 122-131Google Scholar, 42Amey R.L. Chapman D. Infrared spectroscopic studies of model and natural biomembranes.in: Chapman D. Biomembrane Structure and Function, Topics in Molecular and Structural Biology. 4. Basel, Weinheim1984: 199-256Google Scholar). The frequencies of these progression bands are so sensitive to the length of all-trans hydrocarbon segments that we can derive the information about which acyl chain of a TAG takes a bend conformation (43Yano J. Kaneko F. Kobayashi M. Sato K. Structural analyses of triacylglycerol polymorphs with FT–IR techniques: I. Assignments of CH2 progression bands of saturated monoacid triacylglycerols.J. Phys. Chem. B. 1997; 101: 8112-8119Google Scholar, 44Yano J. Kaneko F. Kobayashi M. Kodali D.R. Small D.M. Sato K. Structural analyses of triacylglycerol polymorphs with FT–IR techniques: II. β′1-form of 1,2-dipalmitoyl-3-myristoyl-sn-glycerol.J. Phys. Chem. B. 1997; 101: 8120-8128Google Scholar). To get the information about the lateral packing of stearoyl and oleoyl groups separately, we synthesized partially deuterated specimens of SOS and OSO where stearoyl chains are fully deuterated. Using these deuterated specimens, we could avoid the problem of the ordinary hydrogenated specimens (45Heibert G.L. Hornig D.F. A new tool for the study of crystalline spectra.J. Chem. Phys. 1952; 20: 918-919Google Scholar, 46Tasumi M. Krimm S. Vibrational analysis of chain folding in polyethylene crystals.J. Polym. Sci. 1968; 6: 995-1010Google Scholar, 47Spell S.J. Sadler D.M. Keller A. Chain trajectory in solution grown polyethylene crystals: correction between infra-red spectroscopy and small-angle neutron scattering.Polymer. 1980; 21: 1121-1128Google Scholar): the overlapping of IR bands due to oleoyl and stearoyl groups. Each group showed IR bands in its own frequency regions due to the mass difference between hydrogen and deuterium. In this paper, we will demonstrate the utilities of these spectral techniques and show that the conformational ordering of each acyl group is accompanied by an ordering in the lateral packing and/or a structural change in the layer structure. Hydrogenated-SOS (purity >99%) was provided by Fuji Oil Co. Ltd. Hydrogenated-OSO (purity >99%) was purchased from Sigma Co. Ltd. sn-1,3-Distearoyl-d70-2-oleoylglycerol and sn-1,3-dioleoyl-2-stearoyl-d35-glycerol were synthesized by adopting the methods described in the literature (22Kodali D.R. Atkinson D. Redgrave T.G. Small D.M. Structure and polymorphism of 18-carbon fatty acyl triacylglycerols: effect of unsaturation and substitution in the 2-position.J. Lipid Res. 1987; 28: 403-413Google Scholar, 48Bentley P.H. McCrae W. An efficient synthesis of symmetrical 1,3-diglycerides.J. Org. Chem. 1970; 35: 2082-2083Google Scholar) by using perdeuterated stearic-d35 acid (obtained from Aldrich Chemical Company) having >98 atom % deuterium. In this paper we have used the words "d-SOS" and "d-OSO" for the deuterated specimens and "h-SOS" and "h-OSO" for the usual hydrogenated specimens. The following procedures were applied to obtain each polymorphic form both for hydrogenated and deuterated specimens. SOS. Specimens of the α and γ phases were prepared by holding samples at 80°C for 10 min and then quenching to 10°C and 30°C, respectively (20Sato K. Arishima T. Wang Z.H. Ojima K. Sagi N. Mori H. Polymorphism of POP and SOS. I. Occurrence and polymorphic transformation.J. Am. Oil Chem. Soc. 1989; 66: 664-674Google Scholar). A film of the β′ phase was made from a chloroform solution on KBr by evaporation method. By holding the β′ specimen at 35°C for 8 h, we obtained a film of β (20Sato K. Arishima T. Wang Z.H. Ojima K. Sagi N. Mori H. Polymorphism of POP and SOS. I. Occurrence and polymorphic transformation.J. Am. Oil Chem. Soc. 1989; 66: 664-674Google Scholar). As for the two β phases of SOS, we previously concluded that the molecular structures of β2 and β1 are almost the same, but the crystal symmetry is different between them (34Yano J. Ueno S. Sato K. Arishima T. Sagi N. Kaneko F. Kobayashi M. FT–IR study of polymorphic transformations in SOS, POP, and POS.J. Phys. Chem. 1993; 97: 12967-12973Google Scholar). As the β2-type crystal was not obtained from the deuterated specimen in this study, we focused on the most stable β1 form of SOS, which is described as β in this paper. Single crystals of the β form were grown from an acetonitrile solution by slow cooling. OSO. A specimen of the α phase was prepared by quenching a melt to −10°C. By heating the α phases to −5° and 10°C, we obtained specimens of the β′ and β phases, respectively (22Kodali D.R. Atkinson D. Redgrave T.G. Small D.M. Structure and polymorphism of 18-carbon fatty acyl triacylglycerols: effect of unsaturation and substitution in the 2-position.J. Lipid Res. 1987; 28: 403-413Google Scholar). Single crystals of the β form were grown from an acetonitrile solution by cooling. There were a few differences in polymorphic behavior between hydrogenated and deuterated specimens. Each polymorph melted at a slightly lower temperature in a deuterated specimen than in a hydrogenated one, and there was a new metastable phase in d-SOS that melted at a temperature between the melting points of β′ (36.5°C) and β2 (41.0°C). In spite of these differences, there were no significant structural differences between hydrogenated and deuterated specimens. Each polymorph of the deuterated specimens exhibited almost the same X-ray diffraction pattern as that of the hydrogenated specimens. The X-ray diffraction pattern of each polymorphic form was measured using a Rigaku X-ray diffractometer (40 kV, 10 mA) with Cu-Kα radiation. For the measurement at a low temperature, a Rigaku cryostat was used. The IR spectra were taken with a Perkin-Elmer Spectrum 2000 spectrometer attached to a Perkin-Elmer i-Series FT-IR microscope. An Oxford flow type cryostat CF1104 and an Oxford temperature controller ITC-4 were used for the measurements at low temperatures. The polarized IR spectra were measured with a MCT detector and a wire-grid polarizer (Perkin-Elmer PR500). The spectra without mention of the conditions were taken at room temperature. To determine the directions of transition moments three-dimensionally, we used the following three methods of IR measurement. Polarized IR transmission method. Using single crystal specimens we measured the polarized IR spectra of the β phases with the experimental conditions depicted in Fig. 2a. The incident IR rays impinge normally to the flat face of the crystal. The long crystal edge is set parallel to the direction of φ = 90°. As the other metastable phases such as γ and β′ had difficulty in growing single crystals for the polarized IR measurement, we made a well-oriented film between two KBr windows. The (001) plane of this film is parallel to the KBr window's faces. The polarization dependence of the IR spectra gives the information about the projections of transition moments onto the (001) plane, i.e., the information is two-dimensional. Therefore, the RAS (reflection absorption spectroscopy) and ATR (attenuated total reflection) methods were necessary in this study. RAS. For the RAS measurement, a variable angle reflection accessory (Perkin-Elmer) was used. A thin film of TAG was built on a flat aluminum plate by developing a chloroform solution. Bycontrolling the temperature of the substrate, we made a film of each polymorph where the lamellar interfaces are parallel to the metal surface (Fig. 2b). The IR bands whose transition moments are parallel to the chain axis are emphasized with a strong electric field normal to the metal surface (49Allara D.L. Swalen J.D. An infrared reflection spectroscopy study of oriented cadmium arachidate monolayer films on evaporated silver.J. Phys. Chem. 1982; 86: 2700-2704Google Scholar, 50Rabolt J.F. Burns F.C. Schlotter N.E. Swalen J.D. Anisotropic orientation in molecular monolayers by infrared spectroscopy.J. Chem. Phys. 1983; 78: 946-952Google Scholar). ATR. Polarized ATR spectra were measured using an ATR accessory (PIKE technologies) and a wire-grid polarizer. A Ge plate (wedge angle: 45°) was used as an internal reflection element (IRE). Single crystals or films built on the Ge plate were used as specimens. The x- and z-components of a transition moment are observed with p-polarization, while the y-component is taken with the s-polarization (Fig. 2c) (51Kaneko F. Miyamoto H. Kobayashi M. Polarized infrared attenuated total reflection spectroscopy for three-dimensional structural analysis on long-chain compounds.J. Chem. Phys. 1996; 105: 4812-4822Google Scholar). To study the conformation state of stearoyl chains in each polymorph of SOS and OSO, the information about the IR bands of tristearoylglycerol (SSS) is important as a reference, in particular the progression bands due to hydrocarbon chains (40Fichmeister I. Infrared absorption spectroscopy of normal and substituted long-chain fatty acids and esters in the solid state.Prog. Chem. Fats Other Lipids. 1974; 24: 91-162Google Scholar, 41Ruig W.G. Infrared spectra of diacid and triacid triglycerides.Appl. Spectrosc. 1977; 31: 122-131Google Scholar, 43Yano J. Kaneko F. Kobayashi M. Sato K. Structural analyses of triacylglycerol polymorphs with FT–IR techniques: I. Assignments of CH2 progression bands of saturated monoacid triacylglycerols.J. Phys. Chem. B. 1997; 101: 8112-8119Google Scholar). The spectral pattern in the region of 1380–1150 cm−1 reflects sensitively the stem length of the trans zigzag chain. Two series of progression bands overlap in this region, one is due to the ν3 branch modes (CH2 wagging: 1360–1150 cm−1) and the other is due to the ν7 branch modes (CH2 twisting-rocking: 1300–1150 cm−1) (52Snyder R.G. Vibrational spectra of crystalline n-paraffins part I. Methylene rocking and wagging modes.J. Mol. Spectrosc. 1960; 4: 411-434Google Scholar, 53Snyder R.G. A revised assignment of the B2g methylene wagging fundamental of the planar polyethylene chain.J. Mol. Spectrosc. 1967; 23: 224-228Google Scholar). The ν3 progression bands appear clearly and can be used as a convenient tool. Figure 3 shows the IR spectra of the β phase of SSS with the band assignments determined in the previous study (43Yano J. Kaneko F. Kobayashi M. Sato K. Structural analyses of triacylglycerol polymorphs with FT–IR techniques: I. Assignments of CH2 progression bands of saturated monoacid triacylglycerols.J. Phys. Chem. B. 1997; 101: 8112-8119Google Scholar). The bands marked with open circles are of the all-trans stearoyl chains at the sn-1 and sn-2 positions, and those with wedges are of the bent stearoyl chain at the sn-3 position whose C(2)–C(3) bond takes a gauche conformation (see top left of Fig. 3) (54Knoop E. Samhammer E. Rontgenographische untersuchungen uber die kristallstruktur einiger triglyceride.Milchwissenschaft. 1961; 16: 201-209Google Scholar, 55de Jong S. van Soest T.C. Crystal structures and melting points of saturated triglycerides in the β-2 phase.Acta Crystallogr. 1978; B34: 1570-1583Google Scholar, 56Larsson K. The crystal structure of the β-form of trilaurin.Ark. Kemi. 1964; 23: 1-15Google Scholar, 57Jensen L.H. Mabis A.J. Refinement of the structure of β-tricaprin.Acta Crystallogr. 1966; 21: 770-781Google Scholar). The conformational difference has two effects on the ν3 progression bands. First, the polarization of the ν3 bands is changed. Because the CH2 wagging modes of all-trans hydrocarbon chains have transition moments parallel to the chain axis, the ν3 bands of the straight sn-1,2 chains clearly appear in RAS. On the other hand, the ν3 bands of the sn-3 chain appear both in the transmission spectra and in the RAS spectra, because the ν3 modes of the sn-3 chain couple with the C–O stretching mode and have both the perpendicular and parallel components to the molecular axis. Second, the gauche bond makes these bands shift to higher frequencies as shown in Fig. 3, due to the shortened stem length of the all-trans moiety. The α phase of monoacid TAGs exhibits a reversible transition to the sub α phase (58Jackson F.L. Lutton E.S. The polymorphism of certain behenyl mixed triglycerides. A new metastable crystalline form of triglycerides.J. Am. Oil Chem. Soc. 1950; 27: 4519-4521Google Scholar), but we did not confirm the existence of the sub α phase in the previous studies on SOS and OSO. Figure 4 shows X-ray diffraction patterns of SOS α on cooling. The α form exhibited a single peak of 0.42 nm due to the hexagonal subcell. Below −50°C, this hexagonal subcell seemed to approach gradually to a pseudohexagonal packing having a main peak of 0.42 nm and a shoul
Referência(s)