Synthesis and physical properties of symmetrical and non-symmetrical triacylglycerols containing two palmitic fatty acids

Article abstract of DOI:10.1007/s11746-007-1173-y

A series of symmetrical (ABA) and non-symmetrical (AAB) triacylglycerol (TAG) isomers containing "A," palmitic (P; 16:0) acid, and "B," either oleic (O; 9c-18:1), elaidic (E; 9t-18:1), linoleic (L; 9c,12c-18:2) or linolenic (Ln; 9c,12c,15c-18:3) fatty acids were synthesized by esterification of the thermodynamically more-stable 1,3-di- or 1(3)-monoacylglycerols [1,3-DAG or 1(3)-MAG], respectively. 1,3- dipalmitoylglycerol (1,3P-DAG) was esterified with O, L or Ln acid to prepare the symmetrical TAG isomers POP, PLP and PLnP, while the O- E-, L- and Ln-1(3)MAG precursors, synthesized or obtained commercially, were esterified with P acid to prepare the non-symmetrical TAG isomers OPP, EPP, LPP and LnPP, respectively. The drop point(s), solid fat content and melting point values of the synthesized TAG were determined. The 1,3-dipalmitoylglycerol (1,3P-DAG) and 1(3)P-MAG precursors were prepared, in multi-gram quantities, by partial glycerolysis (glycerol/p-toluenesulfonic acid) of tripalmitin. After fractionation by silica gel chromatography, the 1(3)P-MAG and 1,3P-DAG isomers (ca. 80% of total MAG or DAG) were purified (>98%) by crystallization from acetone [silver ion-HPLC was utilized to determine the structural purities of the DAG (or MAG) precursors, and the synthesized TAG]. Esterification of the appropriate, thermodynamically more-stable MAG or DAG precursors was found to be a very versatile method for synthesis (in 80-90% yields) of multi-gram (3-5 g) quantities of symmetrical and non-symmetrical TAG isomers, in chemical and structural purities of >96 and 97-99%, respectively.

Full text of DOI:10.1007/s11746-007-1173-y

J Am Oil Chem Soc (2008) 85:99–104
DOI 10.1007/s11746-007-1173-y
ORIGINAL PAPER
Synthesis and Physical Properties of Symmetrical
and Non-symmetrical Triacylglycerols Containing
Two Palmitic Fatty Acids
R. O. Adlof Æ G. R. List
Received: 13 December 2006 / Revised: 24 September 2007 / Accepted: 13 November 2007 / Published online: 8 December 2007
Ó AOCS 2007
Abstract A series of symmetrical (ABA) and non-sym-
metrical (AAB) triacylglycerol (TAG) isomers containing
‘‘A,’’ palmitic (P; 16:0) acid, and ‘‘B,’’ either oleic (O; 9c-
18:1), elaidic (E; 9t-18:1), linoleic (L; 9c,12c-18:2) or li-
nolenic (Ln; 9c,12c,15c-18:3) fatty acids were synthesized
by esterification of the thermodynamically more-stable 1,3-
di- or 1(3)-monoacylglycerols [1,3-DAG or 1(3)-MAG],
respectively. 1,3-dipalmitoylglycerol (1,3P-DAG) was
esterified with O, L or Ln acid to prepare the symmetrical
TAG isomers POP, PLP and PLnP, while the O- E-, L- and
Ln-1(3)MAG precursors, synthesized or obtained com-
mercially, were esterified with P acid to prepare the non-
symmetrical TAG isomers OPP, EPP, LPP and LnPP,
respectively. The drop point(s), solid fat content and
melting point values of the synthesized TAG were deter-
mined. The 1,3-dipalmitoylglycerol (1,3P-DAG) and
1(3)P-MAG precursors were prepared, in multi-gram
quantities, by partial glycerolysis (glycerol/p-toluenesul-
fonic acid) of tripalmitin. After fractionation by silica gel
chromatography, the 1(3)P-MAG and 1,3P-DAG isomers
(ca. 80% of total MAG or DAG) were purified ([98%) by
crystallization from acetone [silver ion-HPLC was utilized
to determine the structural purities of the DAG (or MAG)
precursors, and the synthesized TAG]. Esterification of the
appropriate, thermodynamically more-stable MAG or DAG
precursors was found to be a very versatile method for
synthesis (in 80–90% yields) of multi-gram (3–5 g) quan-
tities of symmetrical and non-symmetrical TAG isomers, in
chemical and structural purities of [96 and 97–99%,
respectively.
Keywords Triacylglycerols ꢀ cis ꢀ trans ꢀ Synthesis ꢀ
Palmitic ꢀ Oleic ꢀ Elaidic ꢀ Linoleic ꢀ Linolenic ꢀ
NMR ꢀ SFC
Abbreviations
TAG
DAG
MAG
P
Triacylglycerol
Diacylglycerol
Monoacylglycerol
Palmitic 16:0
O
Oleic cis-9-18:1
E
Elaidic trans-9-18:1
Linoleic cis-9,cis-12-18:2
Linolenic cis-9,cis-12,cis-15-18:3
L
Ln
Ag-HPLC Silver ion high performance
liquid chromatography
NMR
SFC
Nuclear magnetic resonance
Solid fat content
The 1- and 3- positions on the glycerol backbone of the MAG, DAG
and TAG molecules are assumed to be equivalent. Mention of trade
names or commercial products in this (publication) is solely for the
purpose of providing specific information and does not imply
recommendation or endorsement by the US Department of
Agriculture.
Introduction
Both the amounts of and specific structures of the triacyl-
glycerol (TAG) present in the oil phase of a margarine or
spread are considered responsible for both physical
(spreadability, resistance to water/oil loss) and sensory
(melting, graininess) properties. Thus cocoa butter (which
R. O. Adlof (&) ꢀ G. R. List
NCAUR, ARS, USDA, 1815 N. University Street,
Peoria, IL 61604, USA
e-mail: Richard.Adlof@ars.usda.gov
123

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J Am Oil Chem Soc (2008) 85:99–104
contains significant amounts of the symmetrical TAG iso-
mers SOS, POS and POP), is a solid at room temperature,
yet melts sharply at body temperature, and is thus highly
prized as a confectionary fat. POP, due to its ability to form
a variety of polymorphic compounds [1] and slow crys-
tallization properties, is an important component of cocoa
butter and palm oil. Interesterification of palm oil converts
some of the naturally occurring POP to the non-symmet-
rical TAG isomer OPP, which crystallizes much more
rapidly than POP. POP also tends to form large, coarse
crystals (the b form) leading to graininess in emulsified
spreads and shortenings [2, 3], while both EPP and PEP
crystallize in the b0 form, a more-desirable form for spreads
and shortenings [4].
Absorbance Detector (ISCO Inc., Lincoln, NE, USA) at a
wavelength of 206 nm were utilized, with data collection
via SS420X and ChromQuest 3.0 software (ThermoQuest
Corp., San Jose, CA, USA). The ChromSpherÓ Lipids
columns (4.6 mm I.D. 9 250 mm stainless steel; 5 micron
particle size; silver ion impregnated) were purchased from
Varian-Chrompack Int., (Middelburg, The Netherlands)
and used as received.
Thin-layer Chromatography
Formation of the mono- or dipalmitoyl, oleoylglycerol
intermediates and the final TAG were followed by TLC [7].
Samples (5–10 mg) were dissolved in 1.0 mL hexane and
applied (ca. 10 lL) to 100 9 300 silica gel TLC [K6] plates
(Whatman Inc., Clifton, NJ, USA). Eluting solvent:
80:20—hexane: ethyl ether (v/v; [11]); visualization by I2
vapor or by spraying with 10% CuSO4 in 8% H3PO4
solution/heating to 120 °C (hot plate).
To better understand the interrelationship of TAG
structures (symmetrical vs. non-symmetrical) and their
contribution to the functional properties of fats, oils [5] or
bakery shortenings [6], a number of symmetrical [tripal-
mitin (PPP), POP, PLP and PLnP] and non-symmetrical
(OPP, EPP, LPP and LnPP) TAG isomers were synthesized
by esterification of the appropriate 1,3-DAG or 1(3)-MAG
precursors, respectively [7, 8]—an extension of Kodali’s
synthesis [9]—and their melting/drop point and solid fat
content values determined.
Gas Chromatography/Chemical Purities
Chemical purities of the 1(3)-MAGs or 1,3-DAGs, and of
the synthesized TAG were determined by gas chromatog-
raphy after conversion (5% HCl in methanol) to FAMEs
[12]. A Varian 3400 Gas Chromatograph (Varian Instru-
Experimental
Materials and Methods
ments, Palo Alto, CA, USA) equipped with
a
100 m 9 0.32 mm SP2380 (Supelco Inc., Bellefonte, PA,
USA) capillary column, flame ionization detector and He
as carrier gas (operating conditions: injector, 240 °C; split
ratio, 100:1; oven temperature programmed from 155 to
220 °C at 3 °C/min with an initial hold of 15 min; detector,
280 °C) was utilized.
Palmitic, elaidic, oleic, linoleic (L) and linolenic (Ln)
fatty acids and 1(3)-monoacyl (-palmitin, -olein, -linolein
and -linolenin) glycerols were purchased from Nu-Chek-
Prep, Elysian, MN, USA. Sodium methoxide was from
Harshaw Chem. Co. (Cleveland, OH, USA), N,N0-dicy-
clohexylcarbodiimide (DCCI) and p-toluenesulfonic acid
from Sigma-Aldrich (St Louis, MO, USA), 4,4-dimeth-
ylaminopyridine (DMAP) from Eastman Fine Chemicals
(Rochester, NY, USA) and Silica gel (60/200) from J.T.
Baker Chemical Co. (Phillipsburg, NJ, USA). All solvents
were either HPLC grade (acetone, acetonitrile, methanol)
or ACS grade [petroleum ether (PE), carbon tetrachloride,
ethyl ether (EE), toluene, benzene], and were used as
received.
Acylglycerol Structures
The structural purities of the isolated or purchased MAG
and DAG (after conversion to di- or monoacetate(s),
respectively) starting materials [13] and of the synthesized
TAG [14] were determined by Ag-HPLC [7]. Two Ag-
HPLC columns were connected in series, and the solvent
flow (isocratic conditions, 1.0% acetonitrile in hexane,
21 °C) standardized at 1.0 mL/min.
Silver Ion-HPLC
A Spectra-Physics P2000 solvent delivery system (Spectra-
Physics Analytical, San Jose, CA, USA/now Thermo-
Finnigan), a Rheodyne 7125 injector (Rheodyne Inc., Co-
tati, CA, USA) with a 20-lL injection loop, and ELSD
(Sedex Model 75; SEDERE, France) or an ISCO V4
Drop Point
Drop points were determined utilizing a Mettler (Colum-
bus, OH, USA) FP90 (incl. FP84 Dropping and Softening
Point Module), by AOCS Official Method Cc 18–80 [15].
123

J Am Oil Chem Soc (2008) 85:99–104
Percent Solids
101
Sodium methoxide (0.26 g) was added and the magnetically
stirred mixture was heated to 115 °C overnight. DAG for-
mation was followed by TLC. (After 7 h, another 0.1 g of
sodium methoxide was added.) After 24 h, the mixture was
cooled to room temperature, dissolved in 20 mL benzene
and eluted through a glass column (5 cm 9 70 cm) packed
with 200 g silica gel. The TAG (16.9 g) were eluted with
500 mL benzene, the DAG (19.0 g; ca. 40% yield) with
1.2 L 90:10 benzene: ether and the MAG/FA (4.8 g) with
1.7 L 100% ether (similar results were obtained when less-
toxic toluene was substituted for benzene). A portion
(14.4 g) of the DAG fraction was dissolved in 500 mL
acetone, and the solution was slowly cooled to 5 °C and
stored overnight. The crystals (6.5 g) were isolated by fil-
tration through a cold Buchner funnel, and the DAG
composition (conversion to the monoacetate/Ag-HPLC
[13]) determined to be[99% 1,3-dipalmitoylglycerol.
Percent solids were determined by pulsed nuclear magnetic
resonance (NMR). A Bruker Minispec NMR (Toronto, ON,
Canada) was utilized at a temperature range from 10 to
65 °C, according to AOCS Official Method Cd 16-81 [15].
Syntheses
See Table 1 for the chemical and structural purities of final
products.
Preparation of PPP
Palmitic acid (66.8 g, 0.25 mol) was combined with
glycerol (7.3 g, 0.08 mol) and 0.7 g p-toluenesulfonic acid
in a 250 mL, three-necked round-bottom flask fitted with
an argon inlet and thermometer. The contents were stirred
magnetically, under an argon atmosphere, and heated in an
oil bath to 115 °C. Water which condensed on the flask
walls during the reaction was removed by use of a heat
gun. Reaction progress was measured by TLC. After 5 h,
the reaction mixture was cooled and the solid residue dis-
solved in 150 mL PE and transferred to a glass column
(5 cm 9 60 cm) packed with 500 g Florisil. PPP was
isolated by elution with 1.5 L of 10% EE in PE, and the
solvents removed to yield 59.2 g (85%) of PPP.
Preparation of POP
POP was prepared following the method of Kodali et al.
[9]. 1,3-Dipalmitoylglycerol (5.8 g, 1.0 9 10-2 mol) in
150 mL carbon tetrachloride (CCl4) was transferred to a
heat-dried, three-necked, 250 mL round-bottom flask
equipped with a mechanical stirrer, thermometer and argon
inlet. Oleic acid (3.16 g, 1.1 9 10-2 mol, 10% excess) in
10 mL of CCl4 was added via syringe and the solution was
heated to 30 °C in an oil bath. DMAP (1.22 g) was added
in one portion followed by the dropwise addition of DCCI
(1.6 g) over a 30-min period (precipitate noted), and the
reaction was stirred under argon atmosphere for 3.5 h at 28
°C. The solution was then cooled to room temperature, the
precipitate removed by vacuum filtration, and the solvents
by rotary evaporator. The residue (10.3 g) was transferred
with 20 mL PE to a glass column (2.5 cm 9 70 cm)
packed with 30 g silica gel, and eluted with 3 9 200 mL
5% EE in PE. Evaporation of the solvents yielded 7.0 g
(84% yield) of POP.
Preparation of 1,3-Dipalmitin
Tripalmitin (44.5 g, 5.5 9 10-2 mol) and glycerol (2.84 g,
3.1 9 10-2 mol) were combined in a 250 mL round-bot-
tom flask equipped with reflux condenser and argon inlet.
Table 1 Chemical and structural purities, drop points, and solids loss
ranges of PPP, POP, OPP, PLP, LPP, EPP, PLnP, and LnPP
TAG
Puritiesa
Drop point
(°C; Mettler)
Solids loss
(by NMR)
Preparation of PLP
Chemical
Structural
PPP
POP
OPP
PLP
LPP
EPP
PLnP
LnPP
a
[99
98
[99
[98
99
68.1
36.0
30.6
26.0
29.3
46.1
27.3
30.0
65–70
See ‘‘Preparation of POP.’’ 1,3-dipalmitoylglycerol (5.8 g,
1.0 9 10-2 mol, in 150 mL carbon tetrachloride) was
esterified with L acid (3.13 g, 1.1 9 10-2 mol, 10%
excess, in 10 mL of CCl4), with DMAP (1.34 g) and DCCI
(1.6 g in 10 mL CCl4, added over 20 min). The solution
was stirred for 2 h at 29 °C (2.3 g more L acid was added
after 1 h), cooled, filtered and the solvents evaporated
[TLC indicated incomplete esterification (and more L acid
was added), but problem/low 70% yield may have been due
to presence of water]. The residue (12.1 g) was purified by
40–50
97
26.7–33
21.1–26.7
26.7–33
40–50
98
[97
99
97
97
98
97
99
26.7–33
26.7–33
96
99
All data are the means of duplicate determinations. Adapted form
Ref. [15]
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J Am Oil Chem Soc (2008) 85:99–104
elution (3 9 200 mL 5% EE in PE) through 60 g silica gel
(2.5 cm 9 70 cm glass column) to yield PLP (6.0 g; 71%
yield).
flask was esterified with palmitic acid (6.32, 2.47 9 10-2
mol, 10 mL CCL4), DMAP (1.4 g) and DCCI (5.64 g in
15 mL CCL4, over 30 min) and stirred under inert gas at
33 °C for 2.5 h. The solution was cooled, filtered and the
solvents evaporated. The residue (12.5 g) was purified by
elution (4 9 200 mL 5% EE in PE) through 60 g silica gel
(2.5 cm 9 70 cm glass column) to yield 8.5 g (89% yield)
of LPP.
Preparation of PLnP
See ‘‘Preparation of POP.’’ 1,3-Dipalmitoylglycerol (5.5 g,
9.6 9 10-3 mol, 150 mL CCl4) was esterified with Ln acid
(3.11 g, 1.1 9 10-2 mol, 10% excess, in 10 mL of CCl4).
DMAP (1.22 g) and DCCI (1.6 g in 5 mL CCl4, dropwise
over a 30 min) were added and the reaction mixture stirred
at 30 °C for 2.5 h. After cooling to room temperature, the
precipitated solid was removed by vacuum filtration and
the solvents removed by rotary evaporation. The residue
(10.9 g) was purified by elution (4 9 200 mL portions 5%
EE in PE) through 60 g silica gel (2.5 cm 9 70 cm glass
column) to yield 6.9 g (87% yield) of PLnP.
Preparation of LnPP
See ‘‘Preparation of POP.’’ 1-Monolinolein (4.97 g,
1.41 9 10-2 mol, in 100 mL CCL4) in a 350 mL round-
bottom flask, was esterified with palmitic acid (8.02 g,
3.13 9 10-2 mol, 10 mL CCl4). DMAP (1.73 g) and
DCCI (7.02 g, 5 mL CCL4, batchwise over 20 min) were
added and the solution stirred under argon gas at 29 °C for
2.5 h. The solution was cooled, filtered, and the solvents
evaporated. The residue (15.3 g) was purified by elution
(4 9 150 mL 5% EE in PE) through 60 g silica gel
(2.5 cm 9 70 cm glass column) to yield 10.5 g (88%
yield) of LnPP.
Preparation of OPP
See ‘‘Preparation of POP.’’ PPO was prepared by esterifi-
cation of 1-monoolein (4.0 g, 1.12 9 10-2 mol, in 180 mL
CCl4) with palmitic acid (6.4 g, 2.36 9 10-2 mol, 10%
excess, in 10 mL CCl4), in 350 mL round-bottom flask in
the presence of DMAP (1.1 g) and DCCI (6.02 g in 15 mL
CCl4). After stirring at 33 °C for 2.5 h, the solution was
cooled, filtered and the solvents evaporated. The residue
(12.9 g) was eluted (4 9 200 mL 5% EE in PE) through
60 g silica gel (5 cm 9 40 cm glass column) to yield 8.1 g
(87% yield) of OPP.
Analyses
Chemical and structural purities, drop melting points
(Mettler) and solid loss ranges of PPP, POP, OPP, PLP,
LPP, EPP, PLnP, and LnPP are listed in Table 1, and the
percent solids by NMR in Table 2.
Results and Discussion
Preparation of EPP
Multi-gram (4–6 g) quantities of a variety of symmetrical
and non-symmetrical isomers can be readily prepared by
See ‘‘Preparation of POP.’’ 1(3)-monoelaidin (3.1 g, 8.7 9
10-3 mol, in 140 mL CCL4) was esterified with palmitic
acid (5.1 g, 1.95 9 10-3 mol, 10% excess, in 10 mL of
CCL4). DMAP (1.06 g, 8.2 9 10-3 mol, one batch) and
DCCI (4.2 g, dropwise over 30 min, precipitate formation
noted) were added. The reaction was stirred for 2 h at
39 °C. After cooling to room temperature, the precipitate
was removed by vacuum filtration, and the solvents
removed by rotary evaporator. The residue (10.1 g) was
eluted (5 9 200 mL 5% EE in PE) through 60 g silica gel
(2.5 cm 9 70 cm glass column), to yield EPP (6.5 g, 89%
yield).
Table 2 Percent solids by NMR (10–65 °C)
TAG
Solids (%) at temperature (°C)
10
21.1
26.7
33.3
40
60
65
PPP
99.5
98.8
95.4
86.6
85.4
99.2
86.8
86.3
99.5
97.9
93.5
78.2
80.8
99.0
67.8
82.6
99.1
95.1
75.0
0.4
99.1
81.1
0.3
98.6
9.2
98.1
0.0
0.0
0.0
0.0
0.0
0.0
0.0
75.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
POP
OPP
PLP
LPP
EPP
PLnP
LnPP
0.0
0.1
0.0
59.3
98.4
25.2
61.1
0.0
0.0
98.0
0.4
94.0
0.0
Preparation of LPP
0.0
0.0
See ‘‘Preparation of POP.’’ 1(3)-Monolinolein (4.0 g,
1.13 9 10-2 mol) in 100 mL CCL4/250 mL round-bottom
All data are the means of duplicate determinations. Adapted form Ref.
[15]
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J Am Oil Chem Soc (2008) 85:99–104
103
esterification of the appropriate, thermodynamically more-
stable 1,3-DAG or 1(3)-MAG, respectively. POP, PLP and
PLnP were prepared by esterification of 1,3-dipalmitoyl-
glycerol (1,3-diP-DAG) with oleic, L or Ln acid,
respectively. The diP-DAG precursors were readily syn-
thesized by partial glycerolysis of PPP [10], the DAG
fraction (ca. 80% 1,3- and 20% 1,2-di-DAG isomer) sep-
arated by silica gel chromatography, and the 1,3-diP-DAG
component isolated in 99+% chemical and structural purity
by low-temperature crystallization from acetone. The
thermodynamically less-stable (and, if commercially
available, significantly more expensive) 1,2P-DAG, present
at only 20–25% in the original diP-DAG fraction, could be
isolated in purities of ca. 85% [16], but further purification/
removal of the 1,3-DAG ‘‘impurities’’ by repeated crys-
tallizations, resulted in losses of [50% of the desired
1,2-isomer. Utilization of the less-pure 1,2P-DAG precur-
sor for preparation of non-symmetrical TAG isomers such
as PPO, PPL or PPLn required extensive, and time-con-
suming, purification of the final product by semi-
preparative silver ion TLC or HPLC. We found Ag-HPLC
to be a rapid and reproducible method to analyze the final
structured TAG, with a detection limit of \0.5% for the
‘‘undesired’’ isomer [8].
isomers, does not have a b crystalline form [24, 25]; its
most-stable form is b0. Hydrogenation of palm oil is
reported to improve its crystallization and melt properties
due to conversion of POP to the elaidenized form, PEP.
The reported [26] melting points of POP range from 35.1 to
38.5 °C, which is in agreement with our data of 36 °C
(Mettler dropping point). Our drop point value for EPP was
ca. 46 °C, (lower than the value of 50.2 °C obtained by
Wesdorp [23]), compared to a value of 53.5 °C for the
symmetrical PEP isomer [22]. Care must therefore be uti-
lized when applying Official or Standard Methods to the
characterization of structured TAG, whether as pure sam-
ples, mixtures or in commercial formulations.
References
1. Engstrom L (2006) Triglyceride systems forming molecular
compounds. Fat Sci Technol 94:173–191
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458
4. Sato K (2001) Molecular aspect of fat polymorphism in crystal-
lization and solidification. In: Widlak N, Hartel R, Narine S (eds)
Properties of lipids. AOCS, Champaign, pp 1–16
A similar situation was noted during preparation and
isolation of the 1(3)P-MAG. While the 16:0, 18:0 and 18:1-
MAG isomers could be readily isolated, the more highly
unsaturated (18:2, 18:3, etc.) 1(3)-MAG isomers were more
difficult to prepare in the quantities and structural purities
([98%) desired for synthesis of such non-symmetrical TAG
isomers as PPL and PPLn. The more-stable 1(3)-MAGs,
like the 1,3-DAG analogues, were usually readily available
in multi-gram quantities from commercial sources. The
2-MAG isomers, like the 1,2-DAG, were also very difficult
to synthesize and were often not commercially available,
except by ‘‘special (and expensive) synthesis.’’
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The chemical and structural purities, drop points and
solids losses of the TAGs synthesized in this study are
listed in Table 1, and the percent solids values (NMR;
10–65 °C) in Table 2. All TAGs were found to be at least
97% chemically and structurally (ABA vs. BAA) pure.
Melting points were determined by Mettler dropping point.
Insertion of oleic or elaidic acid into the molecule lowered
the melting point of PPP by ca. 47% for the symmetrical
POP, 55% for OPP, but only 32% for EPP. Our NMR data
shows that PPP, POP, OPP and EPP are essentially solids at
10 °C. In general, the NMR data indicate a high degree of
purity, since the solid fat values approach 0 at temperatures
near the Mettler melting points.
13. Adlof RO (1996) Analysis of fatty acid mono- and diacylglycerol
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