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: rlencki@uoguelph.ca

R. J. Craven

e-mail: craven@uoguelph.ca

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

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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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