ARTICLE pubs.acs.org/JPCB
Solid-State Low Temperature f Middle Temperature Phase Transition of Linoleic Acid Studied by FTIR Spectroscopy Fuwei Pi,† Fumitoshi Kaneko,‡,* Makio Iwahashi,§ Masao Suzuki,^ and Yukihiro Ozaki*,† †
Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, Sanda, Hyogo 669-1337, Japan Department of Macromolecular Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan § Department of Chemistry, School of Science, Kitasato University, 1-15-1 Kitasato, Minami-ku, Sagamihara, Kanagawa-ken, 252-0373, Japan ^ Research Institute of Biological Materials, Keihanna Research Laboratory, Hikaridai 1-7, Seika, Soraku, Kyoto 619-0237, Japan ‡
bS Supporting Information ABSTRACT: Temperature-dependent FTIR spectra of linoleic acid were measured over a temperature range of 80 to 30 C to explore solid-state low-temperature (LT) f middle-temperature (MT) phase transition of linoleic acid. Band assignments were made mainly for the 900760 cm1 region, where the bands due to cis-olefin group’s dCH out-of-plane mode and the progression bands due to CH2 rockingCH2 twisting modes are expected to appear. The structural evolutions during the LT f MT phase transition of linoleic acid were investigated by analyzing the progression bands with the simple-coupled oscillator model to elucidate the features of acyl chain influenced with two cis-CdC bonds linked through one CH2 group. Structure changes in the LT f MT transition of linoleic acid were compared with those in the γ f R transition of oleic acid. It has been found that the former is rather localized around the cis-diene group, while the latter occurs in the larger portion between the cis-olefin group to the methyl end. It is also suggested that the conformational change in the cis-olefin group on the LT f MT transition is on a lesser scale than that on the γ f R transition.
1. INTRODUCTION cis-Polyunsaturated fatty acids, such as linoleic acid and arachidonic acid, are rather abundant in cerebral and retinal tissues for the special properties originated from the cis-olefin groups.1,2 These unsaturations in the acyl chain control physical features of biomembranes of these tissues,37 and play important roles in developing nervous and optical systems.810 Moreover, recent investigations indicate that the functions of G-protein coupled with membrane receptors such as rhodopsin are mainly dependent on the high concentration of polyunsaturated fatty acids.11 Even though the prominent functions of cis-polyunsaturated fatty acids have been elucidated, the dynamical properties and conformational changes at molecular level for their acyl chains with multiple cis-CdC bonds are still largely unclear. Thus, it is crucial to explore the structure and properties of hydrocarbon chains in cis-polyunsaturated lipids and related compounds for comprehensive understanding of their functions in biological systems. For this purpose, polymorphic investigations have been proved to be a very effective method,1222 which provides concrete information about the relationship between structure and physical properties. Most lipid compounds with cis-unsaturated acyl chain usually form several polymorphic phases; moreover, these phases usually carry out a variety of solid-state phase transitions. It has been clarified that the characteristics of the r 2011 American Chemical Society
solid-state transitions depend very sensitively on the chemical structures of cis-unsaturated lipid compounds, such as the position of cis-CdC bond.23 Vibrational spectroscopy has extensively been applied in investigations on structures and dynamics of cis-unsaturated lipids and related long-chain compounds.2430 With respect to cis-monounsaturated fatty acids such as palmitoleic, oleic and erucic acids, the structural changes on the γ f R polymorphic phase transitions have been explored by using Raman and IR spectroscopy, which has clarified that the higher temperature phase, R, contains conformational disorders in the vicinity of methyl terminal while the carboxyl-sided chain keeps the all-trans conformation.15,31 Furthermore, a recent IR study on oleic acid showed a possibility that the carboxyl group carries out a gradual structure change that progresses in a wide temperature range from 40 C to its melting point of 13 C.32 Biological functions of linoleic acid [CH3(CH2)4CHd CHCH2CHdCH(CH2)7COOH], one of essential fatty acids, has been investigated for elucidating its physiological and pathological roles.3336 However, the structures and dynamical properties of linoleic acid in solid states have not been fully clarified yet, though a single-crystal structure analysis was carried Received: January 24, 2011 Revised: April 8, 2011 Published: April 26, 2011 6289
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Table 1. Enthalpy and Entropy of Solid-State Transitions for Oleic Acid and Linoleic Acid oleic acida Rtm
βtm
linoleic acidb
γ f R HTtmc LT f MT MT f HT
temperature (C)
13.3
16.2
2.2
5.8
51.3
33.5
ΔH (kJ mol1)
39.6
51.9
8.76
33.6
2.6
0.27
32.3
120.9
8.1
0.86
ΔS (J mol1 K1) 138.4 179.3
Data from refs 39and 40. b Data from ref 38. ΔS was recalculated with the temperature and ΔH data. c Abbreviations: tm, melting temperature; LT, low-temperature phase; MT, middle-temperature phase; HT, hightemperature phase. a
out on one of its solid phases about three decades ago.37 A recent investigation using differential scanning calorimetry (DSC) and powder X-ray diffraction (XRD) indicated that there are three kinds of polymorphic phases in linoleic acid, i.e., low-temperature (LT), middle-temperature (MT) and high-temperature (HT) phases, in the temperature region from 100 to 5 C.38 On heating process, the LT phase transforms to the MT phase at 51.3 C and the MT phase transforms to the HT phase at 33.5 C, reversibly. The thermodynamic parameters of both solid-state reversible phase transitions are summarized in Table 1, in comparison with those of oleic acid.3840 The transition enthalpy of the MT f HT phase transition is significantly smaller than that of the LT f MT transition, which suggests that the notable structural changes take place mainly on the LT f MT transition. The purpose of the present study is to investigate the structural changes in the LT f MT phase transition of linoleic acid by using FTIR spectroscopy for understanding the influence of two cis-CdC bonds linked through one CH2 group on the structure and dynamical properties of acyl chains. In our previous FTIR study on the γ f R phase transition of oleic acid,32 we elucidated the structural evolutions in detailed by analyzing the progression bands due to the CH2 rocking-CH2 twisting modes with the simple-coupled oscillator model. As for cis-monounsaturated fatty acids, the CH2 rocking-CH2 twisting region is simpler than the CH2 wagging region and thus easier to interpret. The study demonstrated that the conditions of the two hydrocarbon chains on both sides of cis-CdC bond can be monitored separately through the analysis of the progression bands assignable to each hydrocarbon chain. By applying the same methodology to linoleic acid, we are able to investigate the behavior of the carboxyl-sided and methyl-sided hydrocarbon chains of linoleic acid. Furthermore, we can probe the structural changes of cis-olefin group, carboxyl group and methyl terminal by observing the characteristic bands of these groups. The chemical structures of oleic acid and linoleic acid are plotted in Figure 1. On the other hand, according to the crystal structural analyses on oleic and linoleic acids, the LT phase of linoleic acid has structural features common to the γ phase of oleic acid with respect to the subcell and the conformation of cis-olefin group. Therefore, the comparisons of the phase transition behavior between the γ f R transition and the LT f MT transition would elucidate how the methylene interrupted diene structure (CHdCHCH2 CHdCH) affects the properties of the solid states of cispolyunsaturated fatty acids. This paper deals with the spectral features of the LT phase at first, mainly about the wavenumber region of 900670 cm1,
Figure 1. Schematic drawings of oleic acid and linoleic acid.
and then describes the spectral changes on the LT f MT phase transition and the characteristics of the transition derived from the spectral changes. It is shown that as compared with the γ f R transition, the structure changes on the LT f MT transition is on a small scale and localized around the cis-diene group. Finally, the influence of the methylene interrupted cis-diene structure on the properties of cis-polyunsaturated fatty acids is discussed on the basis of the comparison with oleic acid with respect to the transition behavior.
2. EXPERIMENTAL SECTION 2.1. Materials. Samples of oleic and linoleic acids with very high purity were supplied by Research Institute of Biological Materials (Kyoto, Japan), and were used without further purification. Their purities were guaranteed to be more than 99.9%. 2.2. Fourier-Transform Infrared Spectroscopy. Temperaturedependent FTIR spectra of oleic and linoleic acids were measured with normal transmission method by a Thermo Nicolet Nexus 470 Fourier-transform spectrometer equipped with a liquid nitrogen cooled mercurycadmium-telluride (MCT) detector. The same sample preparation and IR measurement procedures were utilized in the investigations of both acids for obtaining the determined polymorphic phases, i.e., the γ and R phases for oleic acid, and the LT and MT phases for linoleic acid. For obtaining the determined polymorphs of oleic and linoleic acids, one drop of liquid sample (specimens of oleic and linoleic acids were first kept at room temperature for 1 h to melt all crystals before experimental preparation) was sandwiched between a pair of KBr plates. The plates were then fixed on the coldfinger of a cryostat in the sample compartment of the FTIR spectrometer, and cooled to 40 and 80 C at a rate of 2 C/min for oleic acid and linoleic acid, respectively, by pumping liquid nitrogen to the cryostat. After 1 h, the cooled samples were heated at a rate of 2 C/min to predetermined temperatures for IR measurement. Prior to the spectral collection, the sample temperature was maintained for 10 min after each temperature change to ensure the thermal equilibrium between the sample and the cryostate. The sample temperature was recorded with an accuracy of (0.1 C by using a thermoelectric device whose sensor was attached on the KBr plate. Original FTIR spectra obtained with coadded 128 scans at a 1 cm1 resolution for the LT and MT phases of linoleic acid from 80 to 33 C are reproduced in Figure 2.
3. BACKGROUND: STRUCTURES OF OLEIC AND LINOLEIC ACIDS AT LOW TEMPERATURE Concerning the low-temperature phase (γ phase) of oleic acid and that (LT phase) of linoleic acid, the cell parameters are summarized in Table 2. The structure of the γ phase was 6290
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Figure 2. Temperature dependent FTIR spectra at selected temperatures.
Table 2. Cell Parametersa of Oleic Acid and Linoleic Acid at Low Temperature37,42 temperature/ space acid oleic acid42 linoleic acid37
C
group
a/Å
b/Å
c/Å
20
P21/a 9.51
4.74
40.6
110
β/deg V/ Å3 ∼90
P21/a 9.377 4.632 42.98 109.38
∼1830 1761
a
In this table, the a and c axes of the original paper are exchanged, following the convention that the longest axis is taken as the c axis.41
determined by Abrahamsson and Rydersteht-Nahringbauer.42 The unit cell consisting of four molecules belongs to a pseudoorthorhombic system of space group P21/a, and the internal rotation angles of the two CC bonds linked to cis-CdC bond are þ133 and 133, respectively, which can be recognized as skew and skew0 . With these conformations, two all-trans hydrocarbon chains on both sides of cis-CdC bond form the O0 || subcell in the γ phase. The structure of the LT phase of linoleic acid was determined by Ernst et al.37 The unit cell consists of four molecules and belongs to a monoclinic system of space group P21/a. The internal rotation angles of four CC bonds linked to the two cis-CdC bonds are 119, þ123, þ124, and 121, respectively, from the carboxyl side toward the methyl side, which can be recognized as skew0 , skew, skew, and skew0 . In other words, the two olefin groups in the LT phase of linoleic acid take skew, cis, skew0 conformations as that of oleic acid in the γ phase. With these conformations, the skeletal planes of the methyl-sided and carboxyl-sided hydrocarbon chains are parallel to each other, and the two hydrocarbon chains are packed in the O0 || subcell (Figure S1 in the Supporting Information).
4. RESULTS 4.1. Bands Due to cis-Olefin Groups. In the range of 900670 cm1, the LT phase of linoleic acid shows different spectral features from those in the γ phase of oleic acid, as shown in Figure 3a. The most remarkable difference is the two intense bands appearing at 720 and 711 cm1 in the LT phase of linoleic acid. These two bands at 720 and 711 cm1 have a common property, i.e., their peak height intensities significantly decrease with an increase in temperature as described later. Lipid compounds having cis-monounsaturated acyl chains generally generate an
Figure 3. Enlargements of the γ phase oleic acid and the LT phase linoleic acid: (a) the progression bands due to CH2 rocking-CH2 twisting modes, (b) the CdC stretching mode from 1670 to 1640 cm1.
intense and highly characteristic band assignable to dCH out-of-plane bending, γ(dCH), in this region;43 for example, an intense band appears at 703 cm1 for the γ phase of oleic acid.12,32 Since linoleic acid has two cis-CdC bonds in the acyl chain at the ninth and 12th positions (Figure 1), it is reasonable to tentatively assign both bands to the γ(dCH) mode of each olefin group. As can be seen from the spectrum of the γ phase of oleic acid, the γ(dCH) band is the most intense band in this region. Furthermore, according to the spectral analysis on the γ phase of oleic acid, the bands due to the hydrocarbon chains would appear at higher frequencies than these two bands in the LT phase.32,44 These facts strongly back up the assignments that the bands at 720 and 711 cm1 are due to the two cis-olefin groups and the bands appearing at higher frequencies are associated with the hydrocarbon segments. Although there is a possibility that the γ(dCH) modes of the two olefin groups contribute equally to both the 720 and 711 cm1 bands through the strong coupling between them, we consider that each olefin group makes a larger contribution to either of these two bands, since the two bands exhibit somewhat different characteristics as described later. Polymorphic studies on cis-monounsaturated fatty acids have clarified that the methylsided hydrocarbon chain is more mobile than the carboxyl-sided hydrocarbon chain with a dimerized carboxyl group through hydrogen bonding.14,32 Therefore, it should be considered that the bands due to the methyl-sided cis-olefin group would exhibit a larger temperature dependence and a larger spectral change than those due to the carboxyl-sided cis-olefin group on the LT f MT phase transition. On the basis of this criterion and the observed spectral changes described in a later section, we may assign the 711 cm1 band to the γ(dCH) mode of the methyl-sided olefin group (at the 12th position), and the 720 cm1 band to that of the carboxylsided olefin group (at the ninth position). Similarly, two weak 6291
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Table 3. Frequencies and Assignments of the ν8 Progression Bands of Oleic and Linoleic Acids Carboxyl-Sided Chain linoleic acid
oleic acid
LT phase
MT phase
727
727
γ phase
R phase
assignmentsa
727
727
R1
730
729
R2
737
738
737
737
R3
765
765
765
768
R4
817
818
820
821
R5
Methyl-Sided Chain linoleic acid LT
MT
caproic acid K
phase phase salt
oleic acid
Na salt
assignments γ phase R phase assignments
723 724
R1
724
722
R1
757
759
765 766
R2
730
729
R2
837
839
843 843
R3
745 788
794
R4
854
859
R5
R3
“R” means “ν8 mode”, and the digit is allotted in the order in which the phase angle increases. a
bands appearing at 1665 and 1650 cm1 in the LT phase of linoleic acid (Figure 3b) are assignable to the stretching modes, ν(CdC), of the methyl-sided and carboxyl-sided olefin groups, respectively. In contrast, the γ phase of oleic acid gives only one band at 1661 cm1.12,32 4.2. Progression Bands due to CH2 RockingCH2 Twisting Modes. Similarly to other unsaturated fatty acids and esters and n-alkanes,43,44 a series of progression bands due to CH2 rocking CH2 twisting modes (ν8 branch modes) due to hydrocarbon segments appear in the 900720 cm1 region in the LT phase,45,46 as shown in Figure 3a. Since linoleic acid has two hydrocarbon segments at both sides of the diene group, two series of progression bands should appear and overlap with each other in this region. In our previous study, we analyzed the progression bands in this region for oleic acid by using a simple-coupled oscillator model and found that not only the number of methylene groups but also the terminal conditions determine the frequencies of the progression bands. The fixedfixed boundary model is suitable for the carboxyl-sided chain sandwiched with cis-olefin and carboxyl groups, and the free-fixed boundary model is suitable for the methyl-sided chain.47 The allowed phase angles for an N-oscillator system with the fixedfixed boundary are given by δk ¼ kπ=ðN þ 1Þ
k ¼ 1, 2, :::, N,
ð1Þ
and those with fixed-free boundary are given by δk ¼ ð2k 1Þπ=2N
k ¼ 1, 2, :::, N,
ð2Þ
(details are given in the Supporting Informaiton). Therefore, there are systematic frequency differences for the progression bands between the carboxyl-sided chain and the methyl-sided chain even both having the same number of methylene groups. On the basis of the chemical and crystal structures of oleic and linoleic acids,37,42 it is expected that the progression bands due to
Figure 4. Enlargements of the spectra: (a) the region from 755 to 675 cm1 in the LT f MT transition (upper part) and the γ f R transition (bottom part); (b) the region from 1670 to 1640 cm1 in the LT f MT phase transition (upper part) and the γ f R phase transition (bottom part) (V, an intensity decrease with a temperature increase; v, an intensity increase with a temperature increase). In part a, the shoulder at 734 cm1 appears obviously in a narrow temperature region around 50 C.
the carboxyl-sided chain appear almost at the same frequencies in the LT phase of linoleic acid as in the γ phase of oleic acid by the following reasons: the carboxyl-sided chain consists of seven CH2 groups and is accommodated in the O0 || subcell for both unsaturated fatty acids. As listed in Table 3, the frequencies of the progression bands associated with the carboxyl-sided chain are actually almost the same between the LT phase of linoleic acid and the γ phase of oleic acid. On the other hand, the methyl-sided chain of linoleic acid consisting of only four CH2 groups is expected to give a few peaks in this region. With respect to the boundary conditions, it can be considered that the free-fixed boundary model is applicable also to the acyl chain in n-saturated fatty acids and derivatives. Therefore, the derivatives of caproic acid which has the same number (four) of CH2 groups in its acyl chain could be used as references for assignments. Referring the IR spectra of several caproic acid salts,48 the remaining bands at 837 and 757 cm1 can be assigned to the methyl-sided chain (Table 3). The band due to R1 mode of the methyl-sided chain, which can not be observed directly in the raw spectra, may be buried under the R1 and R2 modes of the carboxyl-sided chain for the strong peak overlapping. 4.3. Phase Transition Behavior in the LT f MT Phase Transition. Linoleic acid shows marked spectral variances in the heating process to the MT phase even below the transition point. 6292
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band at 757 cm1 shifts to 759 cm1, and the R3 band at 837 cm1 shifts to 839 cm1. The region of 737727 cm1 region sandwiched by the carboxyl-sided R1 and R3 bands is clearly elevated, and a shoulder appears at 734 cm1 at 51 and 50 C. The shoulder flattens out at 45 C. There are no drastic changes for the characteristic bands due to the methyl group in the LT f MT phase transition: the methyl rocking r(CH3) band at 889 cm1 (Figure 5a), the symmetric CH3 bending around 1384 cm1, and the asymmetric bending at 1456 cm1. Similarly, the following bands due to the carboxyl group exhibit no remarkable discontinuous changes at the phase transition point: OCdO deformation at 689 cm1 (Figure 5a), CdO stretching at 1687 cm1 and OH outof-plane deformation at 923 cm1 (Figure 2 and Figure S2 in the Supporting Information). There are no noticeable changes also in the CH2 wagging region of 13801180 cm1 (Figure 2), where the progression bands due to the carboxyl-sided CH2 wagging modes coupled with the CO stretching of the carboxyl group dominate.
5. DISCUSSION Figure 5. The spectral changes of the progression bands due to CH2 rocking-CH2 twisting modes: (a) on the LT f MT phase transition; (b) on the γ f R phase transition.
As shown in Figure 4a, the two γ(dCH) bands at 720 and 711 cm1 largely decrease in intensity as the temperature increases from 80 to 52 C; in particular, above 60 C the intensity changes are very conspicuous, whereas the other bands such as the CH2 rocking bands at 737 and 727 cm1 and the OCdO deformation band at 689 cm1 do not show so large intensity changes at least up to 55 C. The intensity of the 711 cm1 band decreases at a higher rate than that of the 720 cm1 band, from which we regard the bands at 711 and 720 cm1 as the γ(dCH) modes due to the methyl-sided and carboxyl-sided olefin groups, respectively. The both bands show a striking intensity decrease in the temperature interval of 55 to 52 C, and the higher frequency sides of these bands around 715 and 724 cm1 are elevated. Finally, the 711 and 720 cm1 bands shift to 715 and 723 cm1, respectively. Discontinuous spectral variations are observed also in the CdC stretching band region (Figure 4b). The ν(CdC) bands at 1665 and 1650 cm1 show gradual changes in intensity and frequency in the temperature range of the LT phase, nevertheless, the former band disappears and the latter band shifts to 1649 cm1 accompanying with a slight intensity increase at the transition point. On the basis of these spectral changes, we reasonably assign the 1665 cm1 band to the ν(CdC) mode of the methyl-sided olefin group and the 1649 cm1 band to the ν(CdC) mode of the carboxyl-sided olefin group, respectively. The ν(dCH) band also shows a drastic change at the transition point, i.e., it remarkably shifts from 3002 to 3006 cm1 with a significant decrease of the peak height intensity at the phase transition point (Figure 2). In comparison with the conspicuous variances associated with the cis-olefin groups, the spectral changes of the progression bands of the ν8 branch modes are not so large. As plotted in Figure 5a, the bands due to the carboxyl-sided chain do not show any remarkable frequency shift in the LT f MT phase transition, whereas a small but discontinuous frequency shift is observed on the bands assigned to the methyl-sided chain; the R2
5.1. Structural Changes of Acyl Chain in the LT f MT Transition. As described in the results, the bands due to the two
cis-olefin groups show characteristic spectral variations in the LT f MT transition process. In particular, the bands at 720 and 711 cm1 exhibit noticeable changes; the intensities start to decrease from several degrees below the transition point, and then striking frequency shifts to 715 and 723 cm1 are followed at the transition point. Compared with the spectral changes in the bands associated with cis-olefin groups, those observed in the bands due to other portions are practically indistinctive. On the γfR phase transition of oleic acid, a significant conformational change around cis-CdC group brings about a large discontinuous spectral change in the γ(dCH) band. Therefore, these characteristics indicate that the structural changes in the LT f MT phase transition may mainly take place around the two successive cis-CdC bonds linked through one CH2 group. The influence of the conformational change of the cis-olefin groups is seen in the v8 progression bands of the methyl-sided chain as frequency shift, while the frequencies of the carboxylsided bands remain almost unchanged (Figure 5a). It can be inferred that although a certain structural change is induced in the methyl-sided chain, no distinctive structural changes occur in the carboxyl-sided chain, which is consistent with the experimental data that the carboxyl bands as well as the CH2 wagging bands do not show any noticeable discontinuous changes at the transition point. In contrast with the small but definite spectral changes of the methyl-sided ν8 bands, the changes of the r(CH3) band at 889 cm1 are not so evident as shown in Figure 5a, which suggests that the LT f MT phase transition does not induce any large structural modification around the methyl terminal. The intensity increase in the region between the carboxyl-sided R1 (727 cm1) and R3 (737 cm1) bands is probably caused by the broadening and higher frequency shift of the two γ(dCH) bands. At the moment, there is no definite explanation for the cause of the shoulder at 734 cm1 that appears in a narrow temperature region around 50 C. 5.2. Comparison of Linoleic Acid LT f MT Transition with Oleic Acid γ f r Transition. The γ f R phase transition is one of the important characteristics of cis-monounsaturated fatty acids. The comparison between the LT f MT transition of 6293
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The Journal of Physical Chemistry B linoleic acid and the γ f R transition of oleic acid that has the same number of carbon atoms as linoleic acid would make the characteristics of linoleic acid clear and comprehensible. The spectral changes in the range of 900670 cm1 are significant differences between these two transitions. Compared with linoleic acid, oleic acid shows striking frequency shifts and intensity changes for the v8 progression bands in the phase transition. The bands due to the methyl-sided chain shift largely and significantly decrease in intensity. Even the bands due to the carboxyl-sided chain exhibit smaller but clear spectral changes (Figure 5b). The most remarkable difference is found in the r(CH3) band at 893 cm1; it is smeared out on the γ f R transition (Figure 5b),32 whereas it is kept almost unchanged on the LT f MT transition (Figure 5a). These differences imply that the structural variances in the hydrocarbon chains accompanying with the conformational change of cis-olefin group are much larger on the γ f R transition than on the LT f MT transition. In other words, the structural changes on the LT f MT transition are localized around the cis-diene group, whereas those on the γ f R transition take place in the larger portion (from the cis-olefin group to the methyl terminal). Another crucial difference between these two transitions is the changes of the γ(dCH) band. The γ(dCH) band of oleic acid appearing at 703 cm1 collapses into the background on the γ f R transition. Nevertheless, although the two γ(dCH) bands of linoleic acid at 720 and 711 cm1 carry out large frequency shifts and intensity changes at the phase transition point, the corresponding γ(dCH) bands still can be clearly confirmed in the MT phase. On the basis of these differences, it can be inferred that the conformational change of cis-olefin group on the LT f MT transition is on a lesser scale compared with that on the γ f R transition, i.e., no large structural changes of the hydrocarbon chains are accompanied by the conformational change at the cis-diene group on the LT f MT transition. The difference in the magnitude of the spectral change is fully consistent with the thermodynamic data in Table 1. As can be seen in Table 1, the transition enthalpy ΔH and the transition entropy ΔS of the LT f MT transition are about one-third of those of the γ f R transition, which also suggests the smaller magnitude of structural changes on the LT f MT transition. 5.3. Influence of Methylene Interrupted Diene Structure. From the spectral changes described above, it can be inferred that the fluctuation of the cis-diene group starts to be activated around 60 C prior to the LT f MT transition point and the large conformational change occurs at the transition point of 51.3 C. These temperatures are quite low compared with the transition temperature of 2.2 C for the γ f R transition of oleic acid. Furthermore, the conformational changes in the transition induce no significant structural changes both in the methyl-sided and carboxyl-sided hydrocarbon chains, which are striking differences from the γ f R phase transition of oleic acid where a significant change of the subcell structure and a conformational disordering take place in the methyl-sided chain. According to the molecular structures of oleic and linoleic acids (Figure 1), we infer that these specific features of linoleic acid in the LT f MT transition are ascribable to the methylene interrupted diene structure, CHdCHCH2CHdCH.14 The CC bonds adjacent to cis-CdC bond are able to adopt various rotation angles different from the standard skew and skew0 .14 Therefore, the methylene interrupted diene part with four CC bonds of this type should have a highly flexible nature. In
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particular, the central CH2 group can be regarded as highly mobile. The four CC bonds may also provide the acyl chain a high adjustability to form a stable packing of hydrocarbon segments depending on circumstances. The evenodd effect of carbon atoms is another factor that can be assumed to affect the properties of the methyl-sided chains.41,49 This effect has a significant influence on the polymorphism of both n-fatty acids and cis-unsaturated fatty acids. For example, an odd-number fatty acid melts at a lower temperature than the even-number fatty acid consisting of only one less carbon atom. cis-Monounsaturated fatty acids that show the γ f R transition have odd-number carbon atoms in the methylsided chain. In contrast, linoleic acid has even-number (six) carbon atoms in the methyl-sided chain. With respect to the lamellar interface consisting of methyl terminals, the LT phase and the γ phase are quite similar to each other. However, when the rotation angle of the bottom CC bond linked to the CdC bond deviates from skew toward trans, the orientation of the methyl group with respect to the lamellar interface becomes different between linoleic acid and oleic acid; the angle that the terminal CC bond makes with the interface decreases in the former and increases in the latter. In this situation, linoleic acid resembles even-number n-fatty acid in the arrangement of the methyl group, and oleic acid resembles odd-number n-fatty acid. The difference would contribute to make the stability of the lamellar interface consisting of methyl terminals larger in linoleic acid. In order to confirm this evenodd effect of the methylsided chain, systematic studies on cis-polyunsaturated fatty acids with cis-olefin groups at different positions are necessary.
6. CONCLUSION Structural variations in the LT f MT phase transition of linoleic acid were investigated by using FTIR spectroscopy with particular emphasis on the elucidation of the features of acyl chain influenced with two cis-CdC bonds linked through one CH2 group. Band assignments were made mainly for the 900670 cm1 region, where the bands due to dCH outof-plane bending of cis-olefin and the progression bands due to CH2 rockingCH2 twisting modes are observed, and then the spectral variations during the LT f MT phase transition were investigated, from which the characteristics of the transition were derived. Finally, the transition behavior was compared between the LT f MT transition of linoleic acid and the γ f R transition of oleic acid to explore the influence of the methylene interrupted cis-diene structure on the properties of cis-polyunsaturated fatty acids. The following important conclusions can be reached from the present study: 1 Structural changes on the LT f MT transition are localized around the cis-diene group, whereas those on the γ f R transition of oleic acid take place in the larger portion (from the cis-olefin group to the methyl terminal). 2 The conformational change in the cis-olefin group on the LT f MT transition is on a lesser scale compared with that on the γ f R transition. 3 The methylene interrupted diene part with four CC single bonds of linoleic acid should have a highly flexible nature. The four CC single bonds may also provide the acyl chain with a high adjustability to form a stable packing of hydrocarbon segments depending on circumstances. 6294
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’ ASSOCIATED CONTENT
bS
Supporting Information. Schematic representation of crystal structures of oleic acid and linoleic acid and O0 // subcell, explanation for simple coupled oscillator models with fixedfixed and freefixed boundary conditions, and the spectra regarding the bands of 1687 and 923 cm1 from the LT phase to MT phase. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*(Y.O.) Telephone: þ81-79-565-8349. Fax: þ81-79-565-9077. E-mail:
[email protected]. (F.K.) Telephone: þ81-6-6850-5453. Fax: þ81-6-6850-5455. E-mail:
[email protected].
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