Fatty acid composition of Prosopis cineraria seeds

Full text of DOI:10.1007/s10600-009-9425-8

Chemistry of Natural Compounds, Vol. 45, No. 5, 2009
FATTYACID COMPOSITION OF Prosopis cineraria SEEDS
*
S. Gangal, S. Sharma, and A. Rauf
UDC 547.915
The genus Prosopis belongs to the family Fabaceae, subfamily Mimosaceae and comprises 44 species distributed
mainly in arid and semi-arid, tropical, and subtropical countries [1]. In many desert regions of the world the seeds of Prosopis
species have been proposed as a source for agro-industrial development of economic foods for humans and animals [2, 3].
Furthermore, the seeds and pods have been used in traditional medicine for treating a broad range of diseases and illness [4, 5].
Sugars, flavonones, fatty acids, tannins from the heartwood [1], flavones, glycoside patulitrin from flowers [6], and piperidine-
3-ol alkaloid [7] were previously reported.
In continuation of our previous studies on various seed oils and the identification of hydroxy fatty acids [8], a study
of seed oils of P. cineraria has been carried out for their component fatty acids and to confirm the presence or absence of
hydroxy fatty acid in this seed oil.
The analytical values of oil and seeds (Table 1) were determined according to the procedure recommended by the
AOCS [9]. The seed oil did not respond to the Halphen test [10], the 2,4-dinitrophenyl hydrazine thin layer chromatography
test [11], and the picric acid TLC test [12], thereby indicating the absence of cyclopropenoid, keto and epoxy fatty acids. The
IR spectra of the oil, as well as its methyl ester, showed the presence of the hydroxy group. The infrared spectrum also showed
–1
characteristic absorption bands at 756 and 1600 cm , indicating the presence of a cis-double bond. However, IR and UV
spectra of oils showed no evidence for trans-unsaturation or the presence of conjugation. TLC analysis of the methyl esters
showed two spots that correspond to standard usual and monohydroxy methyl esters.
Quantitation of component acids was made by GLC analysis of the methyl esters as their trimethylsilyl (TMS) derivatives
and data are given in Table 2. The P. cineraria seed oil has been found to contain total saturated fatty esters (28.6%), total
unsaturated fatty esters (68.3%), and methyl hydroxy fatty ester (3.1%). The seed oil is rich in linoleic acid (32.1%) along with
oleic acid (31.3%).
To determine the absolute identity of hydroxy ester, freshly prepared mixed fatty acid methyl esters were
chromatographed over a column of silica-gel. Elution with petroleum ether–diethyl ether (70:30 v/v) gave pure hydroxy fatty
–1
ester. IR of the isolated ester again showed the hydroxyl band at 3425 cm and the absence of trans-unsaturation. The
elemental analysis of the ethyl ester corresponded to the molecular formula C H O , suggesting an 18-carbon fatty acid.
19 36
3
1
The H NMR spectrum exhibited signals at δ 5.43 (2H, m, -CH=CH-), 3.75 (1H, br.s, -CH-OH), 3.61 (3H, s, -COOCH ),
3
2.71 (1H, br.s, -CH-OH, disappearing upon addition of D O), 2.32 (2H, m, α to -CH -CO-), 2.04 (4H, m, -CH -CH=CH-CH ),
2
2
2
2
13
1.27 (20H, br.s, chain CH ), and 0.88 (3H, dist.t , terminal CH ). The C NMR spectrum of the methyl ester of hydroxy fatty
2
3
acid was also helpful in assigning the structure. It showed a characteristic signal at δ 172.2 for carbonyl carbon, 130.1 attributed
to C-9 and C-10, 29.4 (C-8), 31.5 (C-11), 71.5 (C-12), 33.8 (C-13), and 51.4 (OCH ).
3
–1
The acetate derivative of the pure hydroxy ester showed a strong band at 1235 cm and no hydroxyl absorption band
1
in its IR spectrum. The H NMR spectrum (acetate derivative) showed no unusual features apart from two expected but
significant signals at δ 4.7 as multiplet for one methine proton (CH-OAc) and at δ 1.9 as singlet for methyl protons (-OCOCH ).
3
The disappearance of the signal for the hydroxyl group confirmed the original acid as a hydroxy acid.
Catalytic hydrogenation [13] of the hydroxy-olefinic ester gave a saturated hydroxy ester melting at 54–55°C, which
was identified as methyl 12-hydroxystearate with an authentic sample.
Department of Chemistry, Aligarh Muslim University, Aligarh 202001, India, e-mail: abduloafchem@gmail.com.
Published in Khimiya Prirodnykh Soedinenii, No. 5, p. 592–593, September–October, 2009. Original article submitted
January 14, 2008.
0009-3130/09/4505-0705 ©2009 Springer Science+Business Media, Inc.
705

TABLE 1. Analytical Values of P. cineraria Seed Oil
Refractive index, n3D0
Halphen test
Oil content in seeds, %
Protein Content, %
Iodine value, %
10.6
16.5
87.3
1.4885
–ve
–ve
Picric-acid TLC test
Saponification value
195.4
2,4-DNPH TLC test
–ve
TABLE 2. Composition of Silylated Fatty Acid Methyl Esters (Percentage of Total Silylated Fatty Acid Methyl Esters) of
Prosopis cineraria Seed Oil
Component
Area, % by GLC
Component
Area, % by GLC
14:0
16:0
16:1
18:0
18:1
18:2
1.5
15.2
0.8
2.2
31.3
32.1
18:3
20:0
20:1
22:0
24:0
2.9
4.4
1.2
4.1
1.2
3.1
18:1-OH
.
+
OTMS
OCH
3
CH (CH )
2 5
CH CH
CH CH (CH )
2 7
C
3
2
O
m/z187
m/z299
The oxidative degradation of the original hydroxy acid obtained from P. cineraria seed oil was performed [14].
Formation of the azelaic acid showed that the double bond was at C-9 and that the hydroxyl function was not located between
the double bond and the carbonyl group. The other product, 3-hydroxypelargonic acid, placed the hydroxyl position at C-12.
The MS of the TMS derivative of the hydroxy-olefinic ester was identical to the TMS derivative of the authentic methyl
ricinoleate. Structure-revealing ions were observed at m/z 187 and 299, and a TMS rearrangement ion [15] at m/z 270
unequivocally established the position of hydroxyl at C-12 and indicated the double bond at C-9.
On the basis of these physical and chemical evidences, the hydroxy acid was characterized as (Z)-12-hydroxyoctadec-
9-enoic acid, commercially known as ricinoleic acid.
Plant Material and Extraction of Seed Oil. The seed were purchased from Pratap nursery, Dehradun (U.P.). The
air-dried seeds were powdered and extracted thoroughly with light petroleum ether (bp 40–60°C) in a Soxhlet extractor for 8 h
to yield 10.6% of oil.
Isolation of Hydroxy Fatty Ester. Saponification of the seed oil was carried out by refluxing it with 0.5 N alcoholic
KOH. The unsaponifiable material was removed by diethyl ether extraction. The methyl esters of fatty acids were prepared by
Fischer esterification.
Preparation of Trimethylsilyl Derivatives of Methyl Ester. Silylation of the methyl ester from P. cineraria was
done by treating it with hexamethyldisilazane and trimethylchlorosilane [16]. The silylated methyl esters were subjected to
GLC analysis using silylated methyl esters of castor oil (Ricinus communis) as a reference standard.
Acetylation of Hydroxy Methyl Ester. The hydroxy methyl ester (50 mg) was dissolved in acetic anhydride in
pyridine (2 mL, 5:1, v/v) and left at room temperature overnight. The solvent was removed in a stream of nitrogen with gentle
warming, and the acetylated ester was purified by TLC on silica gel G layers.
Instrumentation. IR spectra were obtained on Shimadzu 8201 PC FT-IR apparatus as liquid films. The ultraviolet
1
13
(UV) measurements were made on a methanolic solution with a Cintra 5 UV-vis. H NMR and C NMR spectra were recorded
in CDCl on a Bruker Avance II-400 instrument using CDCl as a solvent. Chemical shifts were measured in ppm downfield
3
3
from internal tetramethylsilane (δ = 0). Mass spectra were obtained in a Shimadzu QP-2000 spectrometer. The GLC analysis
were carried out using a Varian Vista 6000 instrument equipped with an FID (290°C) detector using a stainless steel column
packed with 15% OV-275 on chromosorb-W (80–100 mesh). Separations were carried out at programmed temperature of
–1
140–200°C (10°C min ). The machine recorded directly the area% of individual peaks. The peaks were identified by comparing
their retention times with those of standard reference samples under similar conditions.
706

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