Polyunsaturated Fatty Acids

n-6 Polyunsaturated Fatty Acids

Information is given in Tables 1 and 2 for the chemical shifts (ppm) of three n-6 acids: 18:2, 18:3, and 20:4. The data relate to the natural all-cis acids and to some synthetic trans isomers. For comments see under n-3 acids below. Three further n-6 acids (19:3, 20:3 and 22:4) were reported in the earlier review (Gunstone, 1993).

n-3 Polyunsaturated Fatty Acids

Detailed information is given for three n-3 acids: 18:3 (ALA, Table 3), 20:5 (EPA, Table 4), and 22:6 (DHA, Table 5) covering both the natural cis isomers and some synthetic trans isomers. Data relating to 18:4, 20:3, 20:4, 22:5, and 24:6 are given in the earlier review (Gunstone, 1993).

Spectra of polyunsaturated fatty acids (PUFA) are marked by the large number of signals for olefinic and allylic carbon atoms. DHA has 22 carbon atoms of which 12 are olefinic and 7 are allylic leaving only C1, C2, and ω-1. The olefinic signals are difficult to interpret, and as the tabulated data show there is not always agreement on assignment. However, the allylic signals are more useful. The chemical shifts of allylic carbon atoms depend on three structural features:

• Whether the adjacent olefinic centres have cis or trans configuration.
• Whether the methylene function is adjacent to one or two olefinic groups.
• Whether the allylic function is mid-chain or close to the carboxyl group (C3/Δ4 or C4/Δ5) or to the end methyl group (ω-2/n-3).

The following chemical shifts (ppm) are taken from Tables 1-5:

• Mid-chain allylic groups adjacent to only one double bond have chemical shifts of 27.2 (cis) or 32.5 ppm (trans).
• Allylic groups adjacent to only one double bond but close to an end of the molecule have slightly different chemical shifts: C3 (Δ4c acid) 22.5 ppm, C4 (Δ5c acid) 26.5 ppm, ω-2 (n-3) 20.6 (c) and 26.0 (t), and ω-1 (n-3, homoallylic) 14.5 (c) and 14.1 (t).
• Mid-chain allylic groups between two double bond have chemical shifts of 25.5 (cis,cis), 30.5 (cis,trans), or 35.5 ppm (trans,trans).

Tables

Table 1. Chemical shifts (ppm) for the four stereoisomers of 9,12-18:2
	9c,12c (a)	9c,12t (a)	9t,12c (a)	9t,12t (a)	9c,12c (b)	9t,12c (c)
*1	173.17, 172.77	173.23, 172.82	173.22, 172.81	173.18, 172.78	180.54
2	34.025, 34.190	34.031, 34.199	34.028, 34.191	34.021, 34.184	34.15	34.0
3	24.860, 24.896	24.855, 24.897	24.856, 24.892	24.856, 24.895	24.70	24.9
4	29.107, 29.068	29.099, 29.061	29.081, 29.041	29.084, 29.042	29.08	28.5
5	29.204, 29.224	29.179, 29.202	28.990, 29.010	29.002, 29.022	29.12	28.5
6	29.138, 29.151	29.115, 29.130	29.142, 29.161	29.142, 29.159	29.40	28.5
7	29.635, 29.647	29.619, 29.630	29.487, 29.501	29.483, 29.496	29.63	29.5
8	27.213	27.102	32.543	32.550	27.22	32.5
*9	129.98, 129.95	130.27, 130.24	130.63, 130.61	130.86, 130.83	130.02	130.7
*10	128.10, 128.11	127.85, 127.87	128.44, 128.46	128.72, 128.74	128.12	128.1
11	25.655	30.474	30.458	35.665	25.67	30.3
*12	127.93, 127.92	128.26, 128.24	127.69, 127.68	128.55, 128.54	127.95	127.5
*13	130.18, 130.19	130.84, 130.85	130.46, 130.47	131.06, 131.07	130.21	130.5
14	27.226	32.564	27.102	32.568	27.25	27.0
15	29.382	29.260	29.364	29.252	29.19	28.5
16	31.557	31.437	31.527	31.447	31.58	31.5
17	22.610	22.632	22.590	22.571	22.62	22.5
18	14.094	14.087	14.090	14.089	14.09	14.0
(a) Lie Ken Jie and Lam (1995), triacylglycerols - figures refer to α and β chains, respectively. (b) Gunstone (1995)/Aursand and Grasdalen (1992), free acid; (c) Berdeaux et al. (1995), methyl ester. *These chemical shifts (ppm) are given to three decimal places in the original paper.

Table 2. Chemical shifts for 18:3 and 20:4 (n-6) acids
	6c9c12c-18:3	6c9c12t-18:3	5c8c11c14c-20:4	5c8c11c14t-20:4	5c8c11c14t-20:4
1	174.00	174.05	180.36	174.1
2	34.10	34.09	33.48	33.5
3	24.72	24.71	24.51	24.5
4	29.23	29.22	26.48	26.5
5	26.97	26.95	129.09		128.96
6	129.68	129.60	128.77		128.96
7	128.37	128.42#	25.63	25.5
8	25.77	25.69	128.27#		128.27
9	128.17	128.25#	128.12#		128.22
10	128.46	128.42#	25.63	25.5
11	25.77	30.54	127.88		127.93
12	127.70	128.02	128.60		128.64
13	130.54	131.15	25.63	30.2
14	27.11	32.36	127.57	127.62
15	-	-	130.52	131.2	130.54
16	-	-	27.26	32.1
17	22.40	22.36	29.37	29.0
18	14.05	14.03	31.57	31.0
19			22.63	22.3
20			14.12	14.0
#Tentative assignment. Rakoff (1984), Rakoff and Emken (1982, 1983) (methyl esters); Gunstone (1993)/Aursand and Grasdalen (1992) (acid); Sandri et al. (1997) (acid); Berdeaux et al. (1995) (methyl ester) unassigned olefinic signals at 128.9 (3), 128.2, 128.1 (2) and 127.8

Table 3. Chemical shifts for α-linolenic acid and its trans isomers
	ccc	cct	ctc	tcc	ttc	tct	ctt	ttt	tcc #	cct #
1									174.2	174.2
2									34.1	34.1
3									25.0
4									29.2
5									29.2
6									29.0
7									29.5	29.6
8	27.25	27.20	27.20	32.50	32.50	32.60	27.15	32.45	32.6	27.25
9	130.30	130.25	130.55	130.90	131.10	130.95	130.45	131.15	130.9	130.2
10	127.85	127.90	127.75	128.00	128.55	128.30	127.70	128.60	128.0	127.9
11	25.70	25.60	30.45	30.45	30.55	30.45	30.45	35.50	30.5	25.6
12	128.35	128.60	128.90	128.15	129.15	128.40	129.05	129.40	128.2	128.6
13	128.35	128.10	128.95	128.60	129.10	128.40	129.15	129.40	128.6	128.0
14	25.60	30.40	30.35	25.50	30.30	30.45	35.55	35.55	25.5	30.5
15	127.20	127.20	127.10	127.25	127.05	127.30	127.55	127.55	127.2	127.2
16	132.00	132.55	132.25	131.90	132.20	132.55	132.70	132.75	131.9	132.5
17	20.60	25.50	20.55	20.55	20.50	25.60	25.55	25.50	20.6	25.0
18	14.25	13.75	14.30	14.25	14.25	13.90	13.85	13.75	14.3	13.9
Rakoff (1984), Rakoff and Emkin (1982, 1983) (acids); Eynard et al. (1994) (methyl esters). #Olefinic signals assigned on the basis of Rakoff's results.

Table 4. Chemical shifts for 22:5(n-3) (EPA) and some trans isomers
	A	B	C	D	E	F	G
2			33.48	33.48	33.51	33.44
3			24.70	24.50	24.68	24.51
4			26.65	26.48	26.62	26.45
5	128.963	128.859	129.11	128.75	128.98	128.67	129.00
6	128.650	128.783	129.20	129.08	129.27	129.12	128.90
7			25.97	25.65	25.87	25.55
8	128.075	128.146	128.54	128.14	128.73	128.31	128.31
9	128.063	128.047	128.46	128.21	128.14	127.94	128.16
10			26.02	25.65	30.74	30.37
11	127.987	128.024	128.46	128.03	128.92	128.56	128.16
12	128.165	128.175	128.60	128.33	129.00	128.80	128.28
13			26.03	25.59	30.74	30.30
14	127.789	127.804	128.26	128.17	127.94	127.97	127.93
15	128.463	128.485	128.89	128.26	129.09	128.48	128.62
16			25.95	30.44	25.83	30.33
17	126.961	126.961	128.47	127.00	127.52	127.08	127.07
18	131.885	131.921	132.15	132.55	132.03	132.48	132.09
19			20.92	25.59	20.88	25.57
20			14.47	13.84	14.47	13.83
A, Sacchi et al. (1994) (acid); B, Sacchi et al. (1994) (methyl ester); C, Vatele et al. (1995) (acid); D, Vatele et al. (1995)17t (acid); E, Vatele et al. (1995) 11t (methyl ester); F, Vatele et al. (1995) 11t,17t (methyl ester); G, Sandri et al. (1997) (methyl ester).

Table 5. Chemical shifts for 22:6(n-3) (DHA) and some trans isomers
	A	B	C	D	E	F
1					179.38
2			33.81	34.09	34.02
3			22.45	22.72	22.54
4	129.515	129.156	129.41	127.96	129.64	127.93
5	127.488	129.321	127.70	129.68	127.58	129.38
6			25.65	25.92	25.63
7	128.244	127.993	128.13	128.39	128.01	128.14
8	127.915	127.813	128.29	128.57	128.35	128.14
9			25.75	26.01	25.68
10	128.177	128.134	128.22	128.45	128.28	128.32
11	128.030	128.039	128.29	128.59	128.31	128.18
12			25.78	26.04	25.68
13	128.018	128.003	128.20	128.41	128.13	128.18
14	128.203	128.177	128.34	128.66	128.13	128.32
15			25.78	26.00	25.57
16	127.820	127.795	128.00	128.59	127.92	127.93
17	128.495	128.472	128.62	128.54	128.60	128.63
18			25.68	30.83	25.57
19	126.981	126.956	127.22	127.49	127.08	127.06
20	131.932	131.900	131.89	132.67	132.03	132.09
21			20.66	25.99	20.59
22			14.78	14.08	14.27
A, Sacchi et al. (1994) (acid); B, Sacchi et al. (1994) (methyl ester); C, Vatele et al. (1995) (acid); D, Vatele et al. (1995) (19t acid); E, Aursand and Grasdalen (1992) (acid); F, Sandri et al. (1997) (methyl ester).

References

• Aursand, M. and Grasdalen, H. Interpretation of the ¹³C-NMR spectra of omega-3 fatty acids extracted from the white muscle of Atlantic salmon (Salmo salar). Chem. Phys. Lipids, 62, 239-251 (1992).
• Berdeaux, O., Vatele, J.-M., Eynard, T., Nour, M., Poullain, D., Noel, J.-P. and Sébédio, J.-L. Synthesis of (9Z,12E)- and (9E,12Z)-[1-¹⁴C]linoleic acid and (5Z,8Z,11Z,14E)-[1-¹⁴C]arachidonic acid. Chem. Phys. Lipids, 78, 71-80 (1995).
• Eynard, T., Vatele, J.-M., Poullain, D., Noel, J.-P., Chardigny, J.-M. and Sébédio, J.-L. Synthesis of (9Z,12Z,15E)- and (9E,12Z,15Z)-octadecatrienoic acids and their [1-¹⁴C]-radio-labelled analogs. Chem. Phys. Lipids, 74, 175-184 (1994).
• Gunstone, F.D. High resolution of ¹³C-NMR spectroscopy of lipids. pp. 1-68. In: Advances in Lipid Methodology - Two (ed. W.W. Christie, Oily Press, Dundee) (1993).
• Lie Ken Jie, M.S.F. and Lam, C.C. ¹³C-Nuclear magnetic resonance spectroscopic studies of triacylglycerols of type AAA and mixed triacylglycerols containing saturated, acetylenic and ethylenic acyl groups. Chem. Phys. Lipids, 78, 1-13 (1995).
• Rakoff, H. Synthesis of deuterated methyl 6,9,12-octadecatrienoate geometric isomers. Chem. Phys. Lipids, 35, 117-125 (1984).
• Rakoff, H. and Emken, E.A. Synthesis and properties of methyl 9,12,15-octadecatrienoate geometric isomers. Chem. Phys. Lipids, 31, 215-225 (1982).
• Rakoff, H. and Emken, E.A. Synthesis and properties of mono-, di- and trienoic fatty esters containing a 12,13 double bond. J. Am. Oil Chem. Soc., 60, 546-552 (1983).
• Sacchi, R., Medina, I., Paolillo, L. and Addeo, F. High-resolution ¹³C-NMR olefinic spectra of DHA and EPA acids, methyl esters and triacylglycerols. Chem. Phys. Lipids, 69, 65-73 (1994).
• Sandri, J., Soto, T., Gras, J.-L. and Viala, J. Chemical shifts of ethylenic carbons in polyunsaturated fatty acids and related compounds, Mag. Res. Chem., 35, 785-794 (1997).
• Vatele, J.M., Doan, H.D., Chardigny, J.-M., Sébédio, J.-L. and Grandgirard, A. Synthesis of methyl (5Z,8Z,11Z,14Z,17E)-eicosapentaenoate and methyl (4Z,7Z,10Z,13Z,16Z,19E)-docosahexaenoate. Chem. Phys. Lipids, 74, 185-193 (1974).
• Vatele, J.M., Doan, H.D., Fenet, B., Chardigny, J.-M. and Sébédio, J.-L. Synthesis of methyl (5Z,8Z,11E,14Z,17Z)- and (5Z,8Z,11E,14Z,17E)-eicosapentaenoate (EPA Δ11t and Δ11t,17t). Chem. Phys. Lipids, 78, 65-70 (1995).
• Vatele, J.M., Fenet, B. and Eynard, T. Complete C-13 assignments and structural elucidation of n-3 polyunsaturated fatty acids by the use of a new 2D NMR technique: SAPHIR-HSQC. Chem. Phys. Lipids, 94, 239-250 (1998).