Monoenoic Acids

Monoolefinic acids and their esters have significantly different ¹³C-NMR spectra from their saturated homologues. The double bond has its own signals and also exerts a marked influence on the signals of nearby carbon atoms. Useful basic information was provided some time ago by Bus (1976, 1977), Gunstone (1993), and Lie Ken Jie (1995) and their colleagues on the acids and their methyl and glycerol esters. The following points have to be considered:

• The olefinic carbon atoms and their associated allylic groups have chemical shifts which depend on double bond position and configuration and, for glycerol esters, on whether the unsaturated centre is in the α or β chain.
• The double bond has a marked influence on the easily recognised C1 to C3 and ω1 to ω3 signals. This varies with double bond position and configuration.

If the double bond is sufficiently far from the acyl function and from the end methyl group, the two olefinic carbon atoms have the same chemical shift at ~129.9 and 130.4 ppm for cis and trans compounds, respectively. In other positions and more commonly Δ2 to 11 and ω2 to 4, the olefinic group has two chemical shifts. The difference between these is large for Δ2 acids and diminishes until it is not apparent at Δ12. The numerical difference between these two chemical shifts is one way of fixing double bond position and some typical values are quoted in Tables 1 and 2 (below). Knothe et al. (1995) have considered these olefinic shifts as rational values and have provided equations for cis and trans acids and their methyl and glycerol esters from which the olefinic shifts can be calculated.

The double bond has significant effects on nearby carbon atoms, and these are apparent in the C1-3 and ω1-3 signals of appropriate isomers (Table 4 below). For these isomers, the selected values provide another way of placing double bonds. It is of limited value for those isomers where the unsaturated centre is mid-chain.

At least three research groups (Bus et al., 1976, 1977; Gunstone et al., 1977) have quantified the effects of the double bonds. Average values are included in Table 1, which also indicates the effect of other functional groups. The largest difference is apparent at the a α position (allylic groups) with a very large difference between cis and trans isomers. The changes at the γ position, though smaller, are also useful and despite the very small difference between cis and trans isomers shown here, it has been exploited in the distinction between Δ12c and Δ12t octadecenoates in hydrogenated oil (Gunstone et al., 1977; Mazzola et al., 1997; Miyake, et al., 1998).

Knothe and Bagby (1995) report that the chemical shifts of unsaturated carbon atoms are proportional or inversely proportional to the 1st, 2nd, 3rd, and 4th powers of the position of unsaturation. They depend on other functional groups in the molecule and on the nature of the unsaturation. Bianchi et al. (1995) and Howarth et al. (1995) argue for the importance of through-bond rather than through-space interactions.

Table 1. Effects of various functional groups on nearby carbon atoms, which usually have a chemical shift of ~ 29.7 ppm
	α	β	γ	δ	other
Cis double bond	-2.5	0	-0.4	-0.2
Trans double bond	+2.9	-0.1	-0,5	-0.2
Hydroxy	+7.8	-4.0	+0.06	-0.06	-0.09, -0.05
Acetoxy	+4.4	-4.4	-0.20	-0.20	-0.10, -0.05
Oxo (keto)	+13.10	-5.75	-0.40	-0.25	-0.20, -0.08
Cis epoxy	+1.71	-2.93	-0.38
Trans epoxy	+2.57	-3.50	-0.23

 

Table 2. Differences in chemical shift (ppm) between the two olefinic carbon atoms in monoene acids and methyl esters
Double	Acids		Methyl esters
Bond	Cis (a)	Trans (a)	Cis (a)	Cis (b)	Trans (a)	Trans (b)
Δ2	-	-	31.28	31.80	-	28.85
Δ3	14.12	-	12.40	12.90	-	13.50
Δ4	4.89	-	4.01	4.30	-	4.30
Δ5	3.23	3.26	2.77	2.95	-	2.95
Δ6	1.60	1.64	1.30	1.50	-	1.50
Δ7	0.85	0.72	0.73	0.90	0.73	0.90
Δ8	0.50	0.54	0.45	0.50	0.43	0.50
Δ9	0.26	0.31	0.24	0.35	-	0.35
Δ10	0.17	0.16	-	0.20	-	0.20
Δ11	0.07	0.08	-	0.10	-	0.10
ω4	0.48	0.50	-	-	0.43	-
ω3	2.16	2.46	-	-	2.29	-
ω2	7.34	-	-	-	7.31	-
Note that although the differences for cis and trans isomers are very small, they are based on significantly different chemical shifts. (a) Gunstone et al. (1977), (b) Bus et al. (1976).

 

Table 3. Differences in chemical shift (ppm) between the two olefinic carbon atoms in monoene glycerol esters (these results are based on triacylglycerols of type GA3 where A is a range of acyl chains from Δ2-11:1 to Δ12-21; Lie Ken Jie et al., 1995)
Double	Cis		Trans
bond	α	β	α	β
Δ2	33.19	33.23	30.36	30.53
Δ3	13.79	13.78	14.42	14.39
Δ4	4.79	4.79	4.49	4.51
Δ5	3.13	3.13	3.20	3.19
Δ6	1.60	1.60	1.63	1.64
Δ7	0.87	0.91	0.88	0.90
Δ8	0.54	0.54	0.53	0.57
Δ9	0.30	0.34	0.31	0.34
Δ10	0.17	0.21	0.18	0.21
Δ11	0.11	0.12	0.11	0.13

 

Table 4. Effect of double bond position on the chemical shifts (ppm) of C1-C3 and ω1-ω3 signals of the Δ4-6 and ω3-6 acids. Only those isomers influencing these signals in the α or γ positions are included in this Table.
	C1	C2	C3	ω3	ω2	ω1
18:0	180.58	34.24	24.81	32.07	22.79	14.12
Δ4c	180.04#		22.66*
Δ5c		33.58#
Δ6c			24.39#
ω6c				31.64#
ω5c					22.40#
ω4c				29.69*		13.80#
ω3c					20.60*
Δ5t		33.47#
Δ6t			24.22#
ω6t				31.49#
ω5t					22.27#
ω4t				34.79*		13.64
ω3t					25.65*
*This group is α to the double bond (i.e. allylic). #This group is γ to the double bond.

 

Table 5. Chemical shifts (ppm) for cis and trans triacylglycerols containing 5-14:1 and 9-18:1 (are taken from more extensive data provided by Lie Ken Jie et al. (1995) and by Mannina et al. (1999)
	5-14:1 cis	5-14:1 trans	9-18:1 cis	9-18:1 trans
1	173.112, 172.718	173.154, 172.751	173.224, 172.819	173.281, 172.869
2	33.443, 33.631	33.348, 33.515	34.041, 34.206	34.055, 34.219
3	24.828, 24.894	24.705, 24.773	24.876, 24.914	24.877, 24.917
4	26.513, 26.497	31.388, 31.839	29.124, 29.084	29.157, 29.110
5	128.168, 128,172	128.674, 128.678	29.217, 29.239	29.087, 29.110
6	131.290, 131.283	131.867, 131.859	29.147, 29.162	28.998, 29.017
7	27.277	32.618, 32.627	29.742, 29.755	29.613, 29.628
8	-	-	27.202	32.591
9	-	-	129.720, 129.693	130.196, 130.173
10	-	-	130.014, 130.029	130.502, 130.517
11	-	-	27.254	32.642
ω7	29.731, 29.743	29.595, 29.611	29.809	29.690
ω6	29.362	29.248	29.370	29.224
ω5	29.560, 29.567	29.524	29.577	29.525
ω4	29.345	29.349	29.370	29.350
ω3	31.935	31.944	31.956	31.937
ω2	22.710	22.712	22.726	22.710
ω1	14.129	14.128	14.138	14.128
Where two shifts are cited, these refer to the α and β chains, respectively; where there is only one figure, this relates to both chains.

 

References

• Bianchi, G., Howarth, O.W., Samuel, C.J. and Vlahov, G. Long range σ-inductive interactions through saturated C-C bonds in polymethylene chains. J. Chem. Soc, Perkin Trans. 2, 1427-1432 (1995).
• Bus, J., Sies, I. and Lie Ken Jie, M.S.F. ¹³C-NMR of methyl, methylene, and carbonyl carbon atoms of methyl alkenoates and alkynoates. Chem. Phys. Lipids, 17, 501-518 (1976).
• Bus, J., Sies, I. and Lie Ken Jie, M.S.F. ¹³C-NMR of double and triple bond carbon atoms of unsaturated fatty acid methyl esters. Chem. Phys. Lipids, 18, 130-144 (1977).
• Gunstone, F.D., Pollard, M.R., Scrimgeour, C.M. and Vedanayagam, H.S. ¹³C-Nuclear magnetic resonance studies of olefinic fatty acids and esters. Chem. Phys. Lipids, 18, 115-129 (1977).
• Gunstone, F.D.The composition of hydrogenated fats by high resolution ¹³C-nuclear magnetic resonance spectroscopy. J. Am. Oil Chem. Soc., 70, 965-970 (1993).
• Johns, S.R., Leslie, D.R., Willing, R.J. and Bishop, D.G. Studies on chloroplast membranes. I. ¹³C chemical shifts and longitudinal relaxation times of carboxylic acids. Austral. J. Chem., 30, 813-822 (1977).
• Howarth, O.W., Samuel,C.J. and Vlahov, G. The σ-inductive effect of C=C and C≡C bonds: predictability of NMR shifts of sp² carbon in non-conjugated polyene acids, esters, and glycerides. J. Chem. Soc. Perkin Trans. 2, 2307-2310 (1995).
• Knothe, G. and Bagby, M.O. ¹³C-NMR spectroscopy of unsaturated long-chain compounds; an evaluation of the unsaturated carbon signals as rational functions. J. Chem. Soc. Perkin Trans. 2, 615-620 (1995).
• Knothe, G., Lie Ken Jie, M.S.F., Lam, C.C. and Bagby, M.O. Evaluation of the ¹³C-NMR signals of the unsaturated carbons of triacylglycerols. Chem. Phys. Lipids, 77, 187-191 (1995).
• 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).
• Lie Ken Jie, M.S.F. and Lam, C.C. ¹³C-NMR studies of polyunsaturated triacylglycerols of type AAA containing (Z)- and (E)-monoethylenic acyl groups. Chem. Phys. Lipids, 78, 15-27 (1995).
• Mannina, L., Luchinat, C., Emanuele, M.C. and Segre, A. Acyl positional distribution of glycerol tri-esters in vegetable oils: a ¹³C-NMR study. Chem. Phys. Lipids, 103, 47-55 (1999).
• Mazzola, E.P., McMahon, J.B., McDonald, R.E., Yurawecz, M.P., Sehat, N., and Mossoba, M.M. ¹³C-Nuclear magnetic resonance spectral confirmation of Δ6 and Δ7-trans-18:1 fatty acid methyl ester positional isomers. J. Am. Oil Chem. Soc., 74, 1335-1337 (1997).
• Miyake, Y., Yokomizo, K. and Matsuzaki, N. Determination of unsaturated fatty acid composition by high-resolution nuclear magnetic resonance spectroscopy. J. Am. Oil Chem. Soc., 75, 1091-1094 (1998).