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LETTER TO THE EDITOR
Triacylglycerol Polymorphism: What Can We Learnfrom Space Groups and Crystalline Tendency?
R. John Craven • Robert W. Lencki
Received: 20 July 2011 / Revised: 16 September 2011 / Accepted: 6 October 2011
� AOCS 2011
Polymorphism strongly affects the functionality of fat and
fat-containing foods. For instance, margarine in the desir-
able b0 form is smooth and creamy, whereas, the more
stable b polymorph is associated with a grainy texture.
Similarly, cocoa butter in form V produces chocolate that
is glossy, snaps nicely, and ‘‘melts in your mouth,’’
whereas, the more stable form VI is a dull white or grey
film that does not melt as readily [1]. Many organic com-
pounds are polymorphic and this polymorphism occurs via
numerous mechanisms—some of which are ill-defined [2].
Similarly, the current understanding of triacylglycerol
(TAG) polymorphism is, in many ways, incomplete.
A number of factors are thought to contribute to TAG
polymorphism. In the solid phase, TAG molecules adopt one
of several possible chain-length (double or triple chain
length aka 2L or 3L) and glycerol conformation (chair or
tuning fork) structures. These structures are determined to a
large part by the TAG’s substituent fatty acids and their
relative affinity for each other, with acyl chains congregating
due to similarities in length and degree of saturation [3, 4]. In
addition, variations in acyl-chain tilt as well as acyl-chain-
and methyl-end-packing are also thought to contribute to, or
be indicative of, polymorphic behavior [5, 6]. While this
descriptive mechanism has been useful, particularly for
molecules with minimal stereochemistry (i.e. n-paraffins and
waxes) [7] it is incapable of predicting or explaining some
common polymorphic behavior of TAG. For example the
current model cannot explain why, while simple (monoacid)
TAG are b-tending, enantiopure mixed (di- and triacid) TAG
(e.g. milk fat, sn-10:0-10:0-16:0 and sn-16:0-16:0-14:0) are
b0-stable [8–10]. Moreover, it does not explain why enan-
tiopure TAG (sn-10:0-10:0-16:0 and sn-16:0-16:0-14:0)
are b0-stable while the corresponding racemic mixtures
(rac-10:0-10:0-16:0 and rac-16:0-16:0-14:0) are b-stable
[9, 10]. Perhaps the current descriptive mechanism for TAG
polymorphism could be improved by including some con-
sideration for the stereochemical conformation of constitu-
ent molecules.
Crystalline Tendency
The relative stereochemistry of molecules within the unit cell
can be determined by spectroscopic means (single-crystal-
and powder diffraction X-ray) or from the phase behavior of
enantiomeric mixtures (crystalline tendency) [11–13]. In our
lab we employed the latter technique to understand the
relationship between polymorphism and stereochemis-
try for a chiral TAG system (sn-10:0-10:0-16:0 and
sn-16:0-10:0-10:0). Samples of enantiopure 1,2-bisdeca-
noyl-3-palmitoyl-sn-glycerol (sn-10:0-10:0-16:0; E-TAG)
and racemic bisdecanoyl-1(3)-palmitoyl-rac-glycerol
(rac-10:0-10:0-16:0 : 50% sn-10:0-10:0-16:0 ? 50%
sn-16:0-10:0-10:0; R-TAG) were prepared in [99%
purity (by GC) [9]. While their melting points were simi-
lar (sn-10:0-10:0-16:0: Tp = 32.9 �C; rac-10:0-10:0-16:0:
Tp = 33.38 �C) their polymorphism (b0-tending E-TAG vs
b-tending R-TAG) and consequently their crystallization and
melting behavior was remarkably different [9]. A similar
polymorphic tendency has been reported for sn-16:0-16:0-14:0
(b0-tending) and rac-16:0-16:0-14:0 (b-tending) [10].
Liquidus data was used to construct a phase dia-
gram for enantiomeric mixtures of sn-10:0-10:0-16:0 and
R. J. Craven � R. W. Lencki (&)
Department of Food Science, University of Guelph,
Guelph, ON N1G 2W1, Canada
e-mail: [email protected]
R. J. Craven
e-mail: [email protected]
123
J Am Oil Chem Soc
DOI 10.1007/s11746-011-1952-3
sn-16:0-10:0-10:0 (cf. Fig. 12 in Reference 9). One half of
the phase diagram was derived with the understanding that
the other half of the diagram is an identical mirror image.
Analysis of this data revealed that blends of sn-10:0-10:0-16:0
and sn-16:0-10:0-10:0 form a meta-stable eutectic (i.e.
conglomerate in the b0 polymorph) and a stable 1:1
molecular compound (i.e. racemic compound in the bpolymorph). In addition, the most thermodynamically-sta-
ble polymorph for enantiopure sn-10:0-10:0-16:0 and by
analogy sn-16:0-10:0-10:0 is b0. Thus, for the subject
compounds, molecules of opposite enantiomers occupy
separate unit cells in the b0-form and are matched in the
unit cell of the b-form [9]. These results are summarized
under the heading ‘‘crystalline tendency’’ in Tables 1, 2.
Stereochemistry of the Unit Cell
for b- and b0-Form Triacylglycerols
As mentioned above, the relative stereochemistry of mol-
ecules within the unit cell can also be determined by X-ray
spectroscopy (single-crystal and powder diffraction). This
information is expressed by the crystallographic space
group determined for the crystal structure [11, 12]. Crys-
tallographic space groups have been assigned for many
b- and b0-form TAG and as a result the relative stereo-
chemistry of their unit cells can be determined (Tables 1, 2).
Based on crystalline tendency and crystallographic space
group assignments (P�1 for the most part) the unit cell
of b-form TAG generally contains both stereoisomers
(Table 1). Not surprisingly, 1,2-dipalmitoyl-3-acetoyl-sn-
glycerol (sn-16:0-16:0-2:0) [14] is an exception as are many
acetoyl acylglycerols [15].
While the stereochemical composition of the unit cell for
b-form TAG is clear-cut, the situation appears more com-
plex for b0-form TAG (Table 2). However, if the role of
crystal twinning in TAG b0 forms is taken into account the
matter is simplified. For b0 forms of TAG, the determination
of crystal structure by X-ray (single-crystal and powder) is
frequently confounded by the growth of crystal twins [4];
thus, variations in crystal growth conditions coupled with
crystal twinning effects are a probable cause for the mul-
titude of b0 forms reported for numerous TAG including
tristearin (18:0-18:0-18:0) and tripalmitin (16:0-16:0-16:0).
‘‘[Crystal twinning is]… the growth of two or more dif-
ferently oriented domains of a single structure into a twin-
ned crystal… twinning can be described in terms of a
symmetry element, the twin-element, which, unlike normal
symmetry elements, does not occur in every unit cell but
relatively few times—or even only once—on a macroscopic
scale’’ [16]. When results from X-ray studies where crystal
twinning was negligible or was properly accounted for are
included with the crystalline tendency results the unit cell
for b0 form TAG contains only one stereoisomer (Table 2).
It appears that the unit cell of all b-form TAG contains
both stereoisomers (Table 1)—the exceptional polymor-
phism of acetoyl acylglycerols notwithstanding (viz.
Table 1 Unit cell stereochemistry for b-form triacylglycerols
Both stereoisomers in the unit cell
Crystalline tendency [9]
sn-10:0-10:0-16:0 and sn-16:0-10:0-10:0 (racemic mixture) form
a stable 1:1 molecular compound (racemic compound)
Single-crystal X-ray
10:0-10:0-10:0 P�1 [18], 12:0-12:0-12:0 P�1 [19], 18:0-18:0-18:0
[20], 10:0-C11Br:0-10:0 P�1 [21], 16:0-16:0-16:0 P�1 [22],
18:1t-18:1t-18:1t P�1 [23]
X-ray powder diffraction
14:0-14:0-14:0, 18:0-18:0-18:0 P�1 [24], 13:0-13:0-13:0,
15:0-15:0-15:0, 17:0-17:0-17:0, 19:0-19:0-19:0 P�1 [25],
18:0-18:1-18:0 P�1 [1], 14:0-18:1-14:0, 16:0-18:1-16:0,
18:0-18:1-18:0, 16:0-18:1-18:0, 18:0-18:1-20:0 P21/n [26],
16:0-18:1-16:0, 18:0-18:1-18:0, 16:0-18:1-18:0, 18:0-18:1-20:0
Cc [27], 18:0-18:0-18:1t, 16:0-18:0-18:0, 16:0-16:0-18:0,
16:0-16:0-18:1t, 14:0-14:0-16:0, 12:0-14:0-14:0, 12:0-12:0-14:0,
18:0-18:1t-18:0, 16:0-18:1t-16:0, 16:0-18:0-16:0 P�1 [5]
One stereoisomer in the unit cell
Single-crystal X-ray
sn-16:0-16:0-2:0 P21 [14]—exceptional polymorphic behavior
common for acetoyl acylglycerols
Table 2 Unit cell stereochemistry for b0-form triacylglycerols
One stereoisomer in the unit cell
Crystalline tendency [9]
Pure enantiomer (sn-10:0-10:0-16:0) is b-tending
Racemic mixture (sn-10:0-10:0-16:0 and sn-16:0-10:0-10:0)
forms a metastable conglomerate (eutectic)
Single-crystal X-ray
12:0-14:0-12:0 C2 [28], sn-16:0-16:0-14:0 C2 [10 ]
X-ray powder diffraction
12:0-14:0-12:0, 14:0-16:0-14:0 (at 250 K) I2 [29]
16:0-18:1t-16:0, 16:0-18:0-16:0, 16:0-16:0-18:1t,
16:0-16:0–18:0 I2 [6]
Both stereoisomers in the unit cell
Single-crystal X-ray
11:0-11:0-11:0 P21/c [30]—twinning observed but not taken into
account
X-ray powder diffraction
10:0-12:0-10:0, 14:0-16:0-14:0 (at 298 K), 16:0-18:0-16:0 Iba2
[29], 16:0-18:0-18:0 C2/c [6]—difficulties reported but
twinning not considered
J Am Oil Chem Soc
123
sn-16:0-16:0-2:0). Pure enantiomers must therefore assume
the b0 form because the unit cell for these compounds can
only contain one stereoisomer. Moreover, when the effects
of twinning are taken into account the current data indi-
cates that the unit cell for b0 forms of chiral and achiral
TAG (e.g. the CnCn?2Cn series) contain only one stereo-
isomer (Table 2).
This preliminary analysis demonstrates the key role unit
cell stereochemistry plays in TAG polymorphism (Fig. 1).
At present, crystallographic space group assignments,
drawn from X-ray data, are available for many b- and
b0-form TAG (Tables 1, 2). However, only one study to
determine the crystalline tendency of chiral TAG via the
binary phase behavior of enantiomeric mixtures has been
conducted (Tables 1, 2) [9]. Further work in this area will
indicate whether the trends noted here are system-specific
or whether they are the general rule. Given the role of
crystalline tendency in the polymorphism of chiral systems
[13] and new insights regarding conformational polymor-
phism [17] we hypothesize that these observations will
apply to the general case.
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