ARTICLE pubs.acs.org/crystal
Crystallization, Polymorphism, and Binary Phase Behavior of Model Enantiopure and Racemic 1,3-Diacylglycerols R. John Craven and Robert W. Lencki* Department of Food Science, University of Guelph, Guelph, Ontario, Canada ABSTRACT: 1,3-Diacylglycerols (1,3-DAG) are components in many natural, commercial, and food systems. These compounds are always asymmetric, and diacid forms are chiral. To understand what effect this has on their crystallization behavior, model enantiopure (1-decanoyl-3-palmitoyl-sn-glycerol) and racemic (1,3-decanoyl-palmitoyl-rac-glycerol) 1,3-DAG were prepared and characterized. In addition, binary phase diagrams were prepared to investigate their phase behavior and the racemate’s crystalline tendency. The major finding for this work is eutectic phase behavior was seen for blends of opposite enantiomers indicating racemic mixtures form conglomerates (mechanical mixtures of enantiopure crystals) in the solid phase. Differential scanning calorimetry melting curves of the racemic mixture display marked polymorphism, whereas, the pure enantiomer did not. This can be understood from a structural perspective since chain-end matching and hydrogen-bond optimization (via orientation of glycerol) are simultaneous for enantiopure, but are multistage for racemic DAG. Thus, there are critical differences between the crystallization behavior of enantiopure and racemic 1,3-DAG, and future physical analysis of these compounds should reflect this finding.
’ INTRODUCTION Acylglycerols are the main components in fats, oils, and cell membranes. In nature, acylglycerols are often chiral due to the positional specificity of biosynthetic pathways. For instance, cell membranes typically only contain the (R)-stereoisomer of phospholipid (phosphate moiety in the sn-3 position of glycerol).1 Likewise, lipid components from natural sources incorporated into commercial products may also be chiral. For example, at least 15 commonly occurring triacylglycerols in palm oil have been identified as chiral.2 On the other hand, racemic mixtures (of chiral acylglycerols), monoacid acylglycerols and other nonchiral acylglycerols also occur in nature and are common in industry and research. It is well-known that there are significant differences between modes of crystallization for pure enantiomers and mixtures of opposite enantiomers,3 but these stereochemical aspects have been largely overlooked in most lipid crystallization studies to date. 1,3-Diacylglycerols (1,3-DAG) play a key role in a number of industrially important systems and show promise as food ingredients in a new class of weight-reducing products. When 1,3-DAG are substituted for dietary triacylglycerol (TAG), weight loss and improved blood cholesterol levels result.4 To develop 1,3-DAG-based products, however, a detailed understanding of their physical chemistry (in particular crystallization behavior) is required. For the most part, 1,3-DAG crystallize in one of two monotropic triclinic parallel β polymorphs which are, in order of increasing thermodynamic stability, β2 and β1. In broad terms, β1 is obtained by crystallizing 1,3-DAG from less polar solvents at higher temperatures and β2 is obtained by crystallizing 1,3-DAG from more polar solvents at lower temperatures or through melt r 2011 American Chemical Society
crystallization. In addition, it is also possible to obtain the β1 form by storing β2 crystals close to their melting point.5,6 β1-form 1,3-DAG adopt a V-shaped (herringbone) conformation in which the two hydrocarbon chains extend out from the polar center.7 This structure was determined using data from both single-crystal X-ray (for 1,3-di-(3)-thiadodecanoylglycerol) and infrared spectroscopy,8 and has since been confirmed for a different monoacid 1,3-DAG (viz. 1,3-di-(11)bromoundecanoyl-glycerol) and for a racemic diacid 1,3-DAG (viz. 1,3-stearoyl-oleoyl-rac-glycerol).9,10 Recently, it has been demonstrated that this same structure holds for 1,3-acyl-palmitoyl-rac-glycerols with acyl groups of varying chain length (from butyric through to oleic; 4:0 to 18:1).11 All diacid 1,3-DAG are chiral about the sn-2 position of glycerol. Nevertheless, there has been a tendency to examine the physical properties of 1,3-DAG without taking this into account. Consequently, the crystallization behavior of pure enantiomers has not been fully distinguished from that of mixed enantiomers or racemic mixtures in spite of critical differences. The role of stereochemistry in crystallization behavior is determined using phase diagrams derived from binary mixtures of pure enantiomer and the corresponding racemic mixture. In the resulting phase diagrams, a characteristic liquidus profile is associated with each particular crystalline tendency—racemic compound, conglomerate, or pseudoracemate.12
Received: November 19, 2010 Revised: March 29, 2011 Published: April 01, 2011 1566
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Figure 1. Stereochemistry of (R)-1-decanoyl-3-palmitoyl-sn-glycerol. Hydrogen atom displays stereochemistry at sn-2 position.
The main purpose of this work is to compare the physical chemistry of an enantiopure 1,3-DAG with its racemic counterpart. This will improve understanding of the role stereochemistry plays in crystallization and perhaps lend some insight into the polymorphism of 1,3-DAG. 1-Decanoyl-3-palmitoyl-sn-glycerol and 1,3-decanoyl-palmitoyl-rac-glycerol were chosen as models for study because the racemic form has relatively simple polymorphic behavior,11 and the difference in chain-length was regarded as sufficient for the potential effects of chirality to be detectable.
’ MATERIALS AND METHODS Unless noted otherwise materials, methods and procedures were identical to those described previously.12 Prepared samples were stored in a freezer (30 °C) to minimize acyl migration. Samples analyzed by infrared and X-ray spectroscopy were stored at 30 °C for at least one month prior to testing. Samples analyzed by differential scanning calorimetry (DSC) were stored at 30 °C for at least 3 days prior to testing. DSC analysis indicated that the high-melting form (β1) was obtained within this time frame. The purity of prepared compounds was determined by GC/FID analysis of trimethylsilyl- derivatives prepared using N-trimethylsilylimidazole and pyridine.13
Preparation of Enantiopure 1-Decanoyl-3-palmitoyl-snglycerol. Decanoyl chloride (2.84 g) dissolved in 30 mL of methylene chloride was added dropwise to a solution of 3-palmitoyl-sn-glycerol (5.0 g), triethylamine (2.73 g) and N,N-dimethylaminopyridine (0.18 g) in 70 mL of methylene chloride stirred in an ice bath.12 After addition, the solution was stirred at room temperature (∼22 °C) for 3 h. Solvent was removed under vacuum, the residue was taken back up in hexane and filtered, then the raw product was recrystallized from the filtrate. Enantiopure 1-decanoyl-3-palmitoyl-sn-glycerol (Figure 1) was further purified by flash chromatography with hexane/ethyl acetate (7:2, v/v) to yield 4.70 g of >99% (by GC) pure product.
Preparation of Racemic 1,3-Decanoyl-palmitoyl-rac-glycerol. This compound was produced as for the enantiopure 1-decanoyl3-palmitoyl-sn-glycerol but using racemic 1(3)-palmitoyl-rac-glycerol as a starting material. This produced 4.10 g of >99% (by GC) pure product.
’ CALCULATIONS The theoretical melting point depression was calculated using a form of the Hildebrand equation, ! RT 2E ΔT ln γx ð1Þ ΔH f where ΔT is the melting point depression, TE is assumed to be approximately equal to TR, and γ is assumed to be one.14 The molar entropy of mixing for the liquid was calculated using ΔSml ¼
ΔHE ΔHR ΔHE ΔHR TE ln TE TR TE TR TR
ð2Þ
Figure 2. Gas chromatography traces for (a) 1-decanoyl-3-palmitoylsn-glycerol and (b) 1,3-decanoyl-palmitoyl-rac-glycerol. Peaks prior to 5 min are solvent and derivatizing agent.
where all enthalpy and temperature values are for fusion, temperature is in Kelvin, and the entropy of mixing for the solid is assumed to be zero.3 This calculated value was compared with the theoretical molar entropy of mixing calculated using ΔSml ¼ RðxE ln xE þ xE0 ln xE0 Þ
ð3Þ
’ RESULTS AND DISCUSSION For simplicity, enantiopure 1-decanoyl-3-palmitoyl-sn-glycerol (Figure 1) will occasionally be identified by the letter E or the short form E-DAG, and racemic 1,3-decanoyl-palmitoyl-racglycerol by the letter R or the short form R-DAG. It is worth noting that the racemic mixture (1,3-decanoyl-palmitoyl-racglycerol) is composed of 1-decanoyl-3-palmitoyl-sn-glycerol and 1-palmitoyl-3-decanoyl-sn-glycerol (E0 or E0 -DAG) in a 1:1 ratio. Consequently, R-DAG could also be identified as 50% E-DAG and 50% E0 -DAG in the accompanying figures, tables and text. For consistency, we will use terminology as defined in Jacques, Collet, and Wilen’s book: Enantiomers, Racemates and Resolutions.3 Chemical esterification with decanoyl chloride was used to produce both enantiopure and racemic DAG from the appropriate monopalmitin. Subsequent flash chromatography yielded product (E: 65% and R: 56% yield) in very high purity (>99% by GC) (Figure 2). As expected, NMR spectra were identical for both enantiomer and racemic mixture. δH = 0.88 (6H, t), 1,25 (36H, m), 1.63 (4H, m), 2.05 (1H, s), 2.35 (4H, t), 4.14 (5H, m) (Figure 3). Infrared spectra for the highest-melting forms of both the enantiopure compound and racemic mixture are similar, and any differences can be attributed to variations in the strength and orientation of hydrogen bonding between hydroxyl and carboxyl 1567
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Crystal Growth & Design groups (Figure 4, Table 1). As a result, absorbances around 3491, 1730, 1710, and 1194 cm1 are greater for the enantiopure compound. In addition, differences in location for the carbonyl stretching frequencies also occur (1707 and 1730 cm1 for E and 1712 and 1730 cm1 for R). All told, this indicates differences in hydrogen-bonding environment between E and R.15 Both the pure enantiomer and racemic mixture have a single absorbance (due to methylene rocking) at 716 cm1 indicating that methylene groups in adjacent acyl chains are packed in a triclinic parallel arrangement (indicating a β polymorph but not distinguishing between β1 and β2).16 Similarly, X-ray powder diffraction indicates that both compounds are in the β polymorph (Figure 5). Main peaks in the WAXS region for the enantiopure compound (3.69 vs, 3.81 s, and 4.56 s with a shoulder at 4.50 Å) and racemic mixture (3.72 vs, 3.80 s, and 4.60 s with a shoulder at 4.53 Å) are similar to those reported previously for the β1 polymorph of 1,3-lauroyl-palmitoyl-rac-glycerol (3.76, 3.80, and 4.58 Å).17 Some additional peaks occur at lower angles in the X-ray powder diffraction spectra (both E and R: 2θ = 2.22, 4.42, 6.6, 8.82° and R only: 2θ = 3.32, and 5.52°). Similarly, SAXS (1.2 to 4.5°) of both enantiomer and racemic mixture were identical (2θ = 2.20°) indicating their lamellae are the same size (39.75 Å). Several minor peaks were also observed in the SAXS measurement (E: 2θ = 4.43° and R: 2θ = 1.10, 3.31 and 4.44°).
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Substantial differences between 1-decanoyl-3-palmitoyl-snglycerol and 1,3-decanoyl-palmitoyl-rac-glycerol are apparent from an inspection of their respective DSC crystallization and melting data (Table 2). The initial melt of tempered solids (highmelting β1-form) yields a melting point (Te) of 60.69 °C for E Table 1. Main Infrared Absorbances for 1-Decanoyl-3-palmitoyl-sn-glycerol (E) and 1,3-Decanoyl-palmitoyl-rac-glycerol (R)16 wavenumber (cm1)
description
3491
OH stretch for OH H-bonded to
2849 and 2912
carbonyl O (E > R) CH stretch for acyl chain and
E: 1707 and 1730
CdO stretch (E > R); two peaks due to
glycerol backbone R: 1712 and 1730 1412
differences in H-bonding strongest CH deformation and wagging band
13001400
CH deformation and wagging bands
1194
COH bending and CO stretching (E > R)
1142 1186 8701150
COH bending and CO stretching bands CC stretch and CH rocking vibrations
716
methylene rocking vibrations single peak indicates triclinic parallel arrangement (β polymorph)
Figure 3. NMR spectrum of 1,3-decanoyl palmitoyl-rac-glycerol. Peak assignments are provided in the inset. Spectrum for 1-decanoyl-3palmitoyl-sn-glycerol was identical.
Figure 5. X-ray powder diffraction patterns for (a) 1-decanoyl-3palmitoyl-sn-glycerol and (b) 1,3-decanoyl palmitoyl-rac-glycerol.
Figure 4. Infrared spectra for (a) 1-decanoyl-3-palmitoyl-sn-glycerol with inset emphasizing two intense absorbance bands at 1707 and 1730 cm1 and (b) 1,3-decanoyl palmitoyl-rac-glycerol with inset emphasizing two absorbance bands at 1712 and 1730 cm1. 1568
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Table 2. Crystallization and Melting Data for Pure Enantiomer and Racemic Mixture 1-decanoyl-3-palmitoyl-sn-glycerol crystallization
melting ΔHc
ΔHf
Te (°C) Tp (°C) (kJ/mol) Te (°C) Tp (°C) (kJ/mol) 5 °C/mina 2 °C/min
49.18
49.52
96.95
60.69 58.57
61.76 60.86
95.69 94.77
5 °C/min
48.79
48.04
96.80
58.33
61.26
95.69
10 °C/min
47.78
47.65
97.24
58.37
61.98
96.17
1,3-decanoyl-palmitoyl-rac-glycerol crystallization
melting ΔHc
ΔHf
Te (°C) Tp (°C) (kJ/mol) Te (°C) Tp (°C) (kJ/mol) 5 °C/mina 2 °C/min 5 °C/min
45.18 44.03
10 °C/min 42.7 a
44.47
97.53
54.60
56.79
100.62
50.87b
52.7b
47.92b
52.55b
54.49b
49.56b
43.51
97.28
51.04
53.43
97.14
41.91
96.85
50.76
54.03
95.54
High-melting form. b First and second peak; total ΔHf = 97.48 kJ/mol.
and 54.60 °C for R with very similar enthalpies of fusion (95.69 and 100.62 kJ/mol, respectively). In cooling/heating cycles conducted at 2, 5, and 10 °C/min, lower-melting solids (β2-form) are obtained, and the difference (ΔTe) between the higher-melting (β1) and lower-melting (β2) forms is less for E than it is for R (on average 2.27 and 3.71°, respectively). There are marked differences between the DSC crystallization and melting curves for 1-decanoyl-3-palmitoyl-sn-glycerol and 1,3decanoyl-palmitoyl-rac-glycerol (Figure 6). Most notably, the crystallization of metastable forms is visible in distorted exothermic peaks (5 and 2 °C/min) and the endothermic double peak (2 °C/min) for R, whereas the polymorphism of E appears to be limited to the occurrence of β1 and β2 forms with slightly different melting points and, perhaps, to the crystallization of a metastable form leading to the distorted exothermic peak in the sample cooled at 2 °C/min. The DSC melting curves for binary mixtures of 1-decanoyl3-palmitoyl-sn-glycerol and 1,3-decanoyl palmitoyl-rac-glycerol in 10% increments were plotted together to facilitate comparison (Figure 7). These melting curves were analyzed, and Tp for each binary mixture was used to display liquidus data (Figure 8). The observed trend is also evident from casual observation of the compiled melting curves (Figure 7). The line in Figure 8 represents a linear regression on data obtained from complete cooling/heating cycles (see below). The enantiopure compound has the highest melting point (Tp). The melting point decreases
Figure 6. DSC curves for the (a) crystallization and (b) melting of enantiopure 1-decanoyl-3-palmitoyl-sn-glycerol, and the (c) crystallization and (d) melting of 1,3-decanoyl-palmitoyl-rac-glycerol. Dashed line distinguishes the high-melting form and solid lines delineate cooling/heating cycles starting with completely melted samples. 1569
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Figure 7. DSC melting curves for high-melting form of binary blends of 1-decanoyl-3-palmitoyl-sn-glycerol (E) and 1,3-decanoyl palmitoyl-racglycerol (R). Labels refer to relative proportion of enantiopure compound and racemic mixture.
Figure 8. Liquidus data (Tp) for high-melting form of binary blends of 1-decanoyl-3-palmitoyl-sn-glycerol (E) and 1,3-decanoyl palmitoyl-racglycerol (R). Line represents linear regression on data collected in complete cooling/heating cycles (Figure 12).
with each successive addition of racemic mixture, finally reaching the lowest melting point, that of the racemic mixture. In this case, the racemic mixture of 1-decanoyl-3-palmitoyl-sn-glycerol and 1-palmitoyl-3-decanoyl-sn-glycerol has a lower melting point than either of the enantiopure compounds. This eutectic phase behavior, where the pure enantiomers’ melting points are mutually depressed, is typical for systems crystallizing as conglomerates.3 The actual melting point depression (6.1°) is close to the predicted value (6.7°) determined using the Hildebrand equation (eq 1) and DSC data for pure E. Thus, reduction in the melting point of 1-decanoyl-3-palmitoyl-sn-glycerol by addition of 1-palmitoyl-3-decanoyl-sn-glycerol (in the form of a racemic mixture) is a colligative property and similar melting point reduction would be expected for addition of a completely dissimilar compound (in place of the opposite enantiomer) in
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Figure 9. DSC melting curves for binary blends of 1-decanoyl-3palmitoyl-sn-glycerol (E) and 1,3-decanoyl-palmitoyl-rac-glycerol (R). Samples were cooled from 100 °C to 20 at 2 °C/min and then melted at 2 °C/min. Labels refer to relative proportion of enantiopure compound and racemic mixture.
Figure 10. DSC melting curves for binary blends of 1-decanoyl-3palmitoyl-sn-glycerol (E) and 1,3-decanoyl palmitoyl-rac-glycerol (R). Samples were cooled from 100 °C to 20 at 5 °C/min and then melted at 5 °C/min. Labels refer to relative proportion of enantiopure compound and racemic mixture.
the binary mixture. Likewise, the calculated entropy of mixing for liquid E- and E0 -DAG (5.5 J/mol by eq 2) is close to the theoretical value (5.76 J/mol) predicted using eq 3. In addition to those for the high melting (β1) form (Figure 7), DSC melting curves for cooling/heating cycles of binary mixtures of 1-decanoyl-3-palmitoyl-sn-glycerol and 1,3-decanoylpalmitoyl-rac-glycerol crystallized and melted at 2, 5, and 10 °C are presented in Figures 9, 10, and 11, respectively. These cooling/heating curves provide information on the low-melting form’s (β2) phase behavior including any additional polymorphism. It is interesting to note that samples rich in the enantiopure compound (E7R3 to pure E) tend to produce one main peak whereas samples rich in racemic compound (pure R to E6R4) often display distorted or double peaks. This echoes the differing extent of polymorphism found for pure enantiomer and racemic mixture above (Figure 6). From a structural perspective this makes sense, since chain-end matching and hydrogen-bond optimization (orientation of glycerol) would occur 1570
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Table 4. Melting Data (Te) for Binary Mixtures of 1-Decanoyl-3-palmitoyl-sn-glycerol (E) and 1,3-Decanoyl-palmitoylrac-glycerol (R) at Three Different Cooling/Heating Rates Te (°C)
Figure 11. DSC melting curves for binary blends of 1-decanoyl-3palmitoyl-sn-glycerol (E) and 1,3-decanoyl palmitoyl-rac-glycerol (R). Samples were cooled from 100 °C to 20 at 10 °C/min and then melted at 10 °C/min. Labels refer to relative proportion of enantiopure compound and racemic mixture.
samples
2 °C/min
5 °C/min
10 °C/min
mean
SD
pure E
58.56
58.33
58.37
58.42
0.12
E9R1
57.24
56.98
57.00
57.07
0.14
E8R2
55.62
55.44
55.63
55.56
0.11
E7R3
53.97
53.45
53.64
53.69
0.26
E6R4 E5R5
51.42 51.07
51.38 50.45
51.31 50.21
51.37 50.58
0.06 0.44
E4R6
51.87
51.56
51.55
51.66
0.18
E3R7
52.10
51.82
49.60
51.17
1.37
E2R8
52.17
49.43
50.46
50.69
1.38
50.62
50.79
50.71
0.12
51.04
50.76
50.89
0.14
E1R9 pure R
50.86
Table 3. Melting Data (Tp) for Binary Mixtures of 1-Decanoyl-3-palmitoyl-sn-glycerol (E) and 1,3-Decanoyl-palmitoylrac-glycerol (R) at Three Different Cooling/Heating Rates Tp (°C) samples
2 °C/min
5 °C/min
10 °C/min
mean
SD
pure E
60.86
61.26
61.98
61.37
0.57
E9R1
60.28
60.82
61.00
60.70
0.37
E8R2 E7R3
59.77 58.80
60.29 59.43
60.72 60.26
60.26 59.50
0.48 0.73
E6R4
58.37
58.75
59.37
58.83
0.50
E5R5
57.44
57.79
58.33
57.85
0.45
E4R6
54.76
57.80
59.16
57.24
2.25
E3R7
54.63
55.70
57.01
55.78
1.19
E2R8
55.15
56.38
56.70
56.08
0.82
E1R9
54.76
54.78
55.23
54.92
0.27
pure R
54.49
54.43
54.03
53.98
0.53
simultaneously for the enantiopure compound. In contrast, these two processes would be separate and independent for racemic mixtures and, ultimately, both processes would be necessary for the formation of the most-stable crystal form. At present, however, it is difficult to know the extent to which polymorphism and phase behavior (i.e., formation of addition compounds) contribute to the observed melting behavior of binary mixtures. Melting data for cooling/heating cycles illustrated in Figures 9, 10, and 11 is summarized in Tables 3 and 4 and plotted in Figure 12. In spite of differences in cooling/heating rate the data (especially for XR e 0.5) shows good reproducibility (Tables 3 and 4). Linear regression for the mean values of Tp (peak maximum) yields an equation (Tp (°C) = 7.418x þ 61.57) with R2 = 0.988. This line is shown in Figures 8 and 12. Linear regression for the mean values of Te (extrapolated onset) for XR e 0.5 yields an equation (Te (°C) = 16.63x þ 58.61) with R2 = 0.989. This line is shown in Figures 12. Phase behavior on the basis of Tp values is eutectic indicating the solid is a conglomerate for 1:1 mixtures of E- and E0 -DAG. In addition, solid solution
Figure 12. Melting data for cooling/heating cycles on binary blends of 1-decanoyl-3-palmitoyl-sn-glycerol (E) and 1,3-decanoyl-palmitoyl-racglycerol (R). Triangles are for data collected at 2 °C/min, diamonds for 5 °C/min and squares for 10 °C/min. Extrapolated onset of melting (Te) values are represented by open symbols, maximum peak temperature (Tp) values are represented by filled symbols. Solid lines were derived by linear regression on mean data and the dashed line is the suggested eutectic solidus.
behavior is evident for mixtures rich in enantiopure compound (XR e 0.5). Thus, the phase behavior can be summarized as eutectic (between E- and E0 -DAG) having limited solid solution formation (for enantiomer-rich mixtures), and the racemic mixture crystallizes as a mechanical mixture (conglomerate). Values for Tp and Te are far less regular for racemic and near racemic mixtures (XR > 0.5) than for enantiomer-rich mixtures (XR e 0.5) (Figure 12). The melting data suggests that either polymorphs or addition compounds are formed for E- and E0 -DAG at a number of ratios in particular for E4R6 cooled 1571
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Crystal Growth & Design and heated at 5 and 10 °C/min (XR = 0.6). To a lesser degree, similar behavior is seen for E3R7 and E2R8 (XR = 0.7 and 0.8). As a consequence, the current data implies, but does not explicitly delineate, the eutectic solidus location. A dashed line represents the likely location for the eutectic solidus (Figure 12). In conclusion, investigation of phase behavior through binary phase diagrams indicates 1,3-decanoyl-palmitoyl-rac-glycerol crystallizes as a conglomerate. In the solid phase, opposite enantiomers of 1,3-DAG are mutually immiscible and thus form a mechanical mixture. Future crystal structure determinations for 1,3-DAG should reflect this finding. Furthermore, more complex melting behavior was observed for the racemic mixture and systems rich in racemic mixture. This may be due to greater polymorphism, the formation of addition compounds, or both. In sum, this demonstrates the importance of stereochemistry in determinations of crystal structure and understanding the crystallization behavior of 1,3-DAG.
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(16) Chapman, D. The Structure of Lipids by Spectroscopic and X-ray Techniques: With a Chapter on Separation Techniques including thin layer and gas liquid chromatography; Methuen and Co Ltd: London, 1965. (17) Sidhu, S. S.; Daubert, B. F. J. Am. Chem. Soc. 1946, 68 (12), 2603–2605.
’ AUTHOR INFORMATION Corresponding Author
*Phone: 519-824-4120, x54327. E-mail:
[email protected].
’ ACKNOWLEDGMENT Financial support for this project was provided by National Sciences and Engineering Research Council of Canada; we are grateful for their generosity. Thanks to ACD (Toronto) for their free chemical structure drawing and NMR analysis software (Chemsketch, 3D Viewer and 1D NMR Processor). Thanks also to Japan VAM & POVAL Co., Ltd. for their donation of vinyl esters of fatty acids. ’ REFERENCES (1) Weiss, R. M.; McConnell, H. M. Nature 1984, 310, 47–49. (2) Loh, S. K.; Choo, Y.-M. J. Oil Palm Res. 2003, 15 (1), 6–11. (3) Jacques, J.; Collet, A.; Wilen, S. H. Enantiomers, Racemates and Resolutions; John Wiley and Sons: New York, 1981. (4) Christophe, A. B. Structural Effects on Adsorption, Metabolism, and Health Effects of Lipids. In Bailey’s Industrial Oil and Fat Products, 6th ed. ed.; Shahidi, F., Ed.; John Wiley & Sons, Inc.: Hoboken, NJ, 2005; Vol. 3, pp 535553. (5) Larsson, K. Physical Properties - Structural and Physical Characteristics. In The Lipid Handbook; Gunstone, F. D., Harwood, J. L., Padley, F. B., Eds.; Chapman and Hall Ltd: London, 1986; pp 321384. (6) Shannon, R. J.; Fenerty, J.; Hamilton, R. J.; Padley, F. B. J. Sci. Food Agric. 1992, 60, 405–417. (7) Larsson, K. Acta Crystallogr. 1963, 16, 741–748. (8) Chapman, D. J. Chem. Soc. 1956, 2522–2528. (9) Hybl, A.; Dorset, D. Acta Crystallogr. 1971, B27, 977–986. (10) Goto, M.; Honda, K.; Di, L.; Small, D. M. J. Lipid Res. 1995, 36, 2185–2190. (11) Craven, R. J.; Lencki, R. W., J. Am. Oil Chem. Soc. 2011, DOI: 10.1007/s11746-011-1769-0. (12) Craven, R. J. Lencki, R. W. Cryst. Growth Des., 2011, DOI: 10.1021/cg101654c. (13) Craven, R. J.; Lencki, R. W. J. Am. Oil Chem. Soc. 2010, 87 (11), 1281–1291. (14) Atkins, P. W. Physical Chemistry, 2nd ed.; Oxford University Press: Oxford, 1982. (15) Chapman, D. Chem. Rev. 1962, 62, 433–456. 1572
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