Биологические науки/ Структурная ботаника и биохимия растений

Zhumabayeva S.¹, Gibadilova A. ², Poplavskiy N.¹

¹Kokshetau State University; ²Kokshetau Medical College, Kazakhstan

Study of rapeseed oil by NMR-spectroscopy method

Vegetable oils are important due to their nutritional value mainly. They are mandatory components of food and a source of energy and plastic material for humans. Biological properties of oils are determined by the fatty acid and triglyceride compositions, as well as by the presence of biologically active compounds (sterols, tocopherols, phospholipids, carotenoids, sterols, etc.).

In triglycerides OH-group of glycerol is bound to the COOH-group of fatty acid to form an ester bond. Fatty acids differ in molecular weight and degree of saturation. In the triglycerides they can be attached to glycerin in three different positions which defines a large variety of optical isomers and spatial triglycerides. Therefore the investigation of vegetable oils as a mixture of triglycerides is a difficult task [1].

To determine the fatty acid composition in triglycerides of vegetable oils classical methods of analysis such as chromatography and mass spectroscopy are applying. In the last decade, research is based on nuclear magnetic resonance (NMR) spectroscopy. This method is non-destructive and doesn’t require multi-step analysis which is convenient for the screening of large quantities of oil samples [2-4].

Recently, interest to rapeseed oil has grown, which, in addition to food purposes, is promising as a source of industrial oil, high protein animal feed and as a raw material for biodiesel production.

The aim of our research is comparative study of rapeseed oil by NMR spectroscopy method.

Methodology

Rapeseed oil was produced from seeds of 2012 years’ harvest. The extraction of rapeseed oil was performed with diethyl ether in a Soxhlet apparatus.

To perform the ¹H NMR analysis oil samples (0.5 ml) were dissolved in 0.5 ml of deuterated chloroform (CDCl₃). Records of spectra were done on the NMR spectrometer JNM-ECA 400 of Jeol company (Japan) with an operating frequency of 400 MHz. Chemical shifts are expressed in ppm.

Results and discussion

¹H NMR spectra of rapeseed oil we studied is shown in fig. 1. It is known that the main constituent of vegetable oils are glycerol esters of various saturated (palmitic, stearic) and unsaturated (oleic, lenoleic, lenolenic) fatty acids [1, 5-7].

Fig. 1. ¹H NMR spectra of rapeseed (B) oil. 400 MHz (solvent: CDCl₃).

Signal 1 indicates the presence of olefinic protons. In the current signal (5.15-5.20 ppm) we observe a broad multiplet, indicating the vinyl protons (-CH=CH-) of the double bonds in the chains of the all unsaturated fatty acids [6].

Table 1. Signals of 1H NMR spectra and the functional groups of rapeseed oil’s components

Signal	Functional groups	The multiplicity *	δ, ppm
1	-СН=СН-	t	5.12-5.20
2	-СН-О-СОR	t	5.05-5.10
3	-CH2–O–CO–C	dd	4.10-4.16
4	-CH2–O–CO–C	m	3.92-4.03
5	-СН=СН-СН2-СН=СН-	t	2.57-2.66 (m)
6	-СН2-СООН	t	2.08-2.15
7	-СН2-СН=СН-	m	1.82-1.93
8	-СН2-СН2-СООН	m	1.40-1.49
9	-(СН2)n-	d	1.09-1.20
10	-CН=СН-СН2-СН3	t	0.78-0.84
11	-СН2-СН2-СН2-СН3	t	0.69-0.76

* The multiplicity of the signal: s - single; d - doublet; t - triplet; m - multiplet.

Multiplets in the signals 2-4 (3.92-5.05 ppm) indicate protons of CH- and CH₂ groups of the glycerol moiety. Thus, the signal 2 shows the presence of two protons at the second carbon atom, signals 3 and 4 (Fig. 2) indicate the protons of the first and third carbon atoms of glycerin [6, 8].

Fig. 2. Signals 3 and 4 in the ¹H NMR spectrum of rapeseed oil

In the ¹H NMR spectrum in the areas of signal 5-11 (0.65-2.58 ppm) we observe signals for the CH₃-, CH₂- allyl protons and fragments of fatty acids.

A signal 5 as a multiplet indicates the presence of methylene (bis-allylic CH-CH=CH-CH=CH-) protons of lenolic and lenolenic acids. Signal 10 also points the presence of lenolenic acid in rapeseed oil [3, 7, 8].

Fig. 3. Signal area 5 ¹H -NMR spectrum of rapeseed oil.

The signals 6 and 8 in the ¹H-NMR spectra of oil indicate the protons of acyl-groups, and multiplet of signal 7 indicates the allyl protons of all unsaturated fatty acids. The signal 9 peaks show the methylene protons of saturated acyl groups in oleic acid, lenolic and, lenolenic acids [9]. Doublets of this signal are located in the area 1.09-1.20 (Fig. 1).

Conclusions

Thus, by using the ¹H NMR spectroscopy the high content of polyunsaturated acids (lenolic and lenolenic) in of rapeseed oil was determined.

The high accuracy levels together with the minimal requirements of sample preparation and short analyses time determine the NMR spectroscopy method as an ideal technique for rapid and reliable determination of the authenticity of product.

References

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