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
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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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acids. J Am Oil Chem Soc 84:427–431
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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
positional isomers by silver ion HPLC. J Chromatogr A 741:135–
138
14. Adlof RO (1995) Analysis of triacylglycerol positional isomers
by silver ion high performance liquid chromatography. J High
Resolut Chromatogr 18:105–107
While the melting points of POP, PLP, PLnP, OPP, PEP
and EPP have been reported in the literature [17–23], the
values for EPP, PLnP and LnPP have not. OPP reportedly
melts at 34–35.2 °C [24]. (PPO, unlike POP and most TAG
123