Polyunsaturated Fatty Acids

13C-NMR Spectroscopy

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-18:36c9c12t-18:35c8c11c14c-20:45c8c11c14t-20:45c8c11c14t-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
 ccccctctctccttctctcttttttcc #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
 ABCDEFG
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
 ABCDEF
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).