13C-NMR Spectroscopy of Fatty Acids and Their Derivatives
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.
- 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