NMR Spectroscopy of Fatty Acids and Their Derivatives
The nuclear magnetic resonance (NMR) phenomenon was first observed in late 1945/early 1946 by Edward Purcell at Harvard University and Felix Bloch at Stanford University. This achievement resulted in a shared Nobel Prize in Physics in 1952 for both researchers. Probably the first report on the 1H-NMR spectra of fatty compounds was published in 1959: C.Y. Hopkins and H.J. Bernstein; Applications of proton magnetic resonance spectra in fatty acid chemistry. Can. J. Chem., 37, 775-782 (1959). The basics of NMR spectroscopy are available from various textbooks as well as websites and therefore will not be discussed here. The present text assumes some basic knowledge of 1H-NMR spectroscopy.
The term "proton" is routinely used in 1H-NMR spectroscopy even though the atoms, the hydrogens, in the molecules are the species studied. In accordance with general NMR jargon, the term "proton" will be used here too. NMR spectroscopy has become an extremely valuable research tool in elucidating the structure of molecules as the chemical shifts (signals) of the protons are often very sensitive to minor changes in molecular structure. Such effects are visible in the spectra of fatty compounds as discussed here too. It is used routinely for determining the outcome of reactions but is also valuable for purposes of quantification. These uses are outlined in these web pages.
The 1H-NMR spectra of fatty acids and their derivatives are discussed here using the spectra of stearic (octadecanoic) acid and its methyl ester (methyl stearate; methyl octadecanoate) as reference compounds. Stearic acid and methyl stearate were selected for this purpose because many common fatty acids possess eighteen carbons, and therefore variations in the chain and resulting changes in the spectra can be easily discussed when introducing only one functional group "change" at a time. Major changes in the spectra caused by "introducing" a functional group will be pointed out, but all changes will not necessarily be discussed for all spectra. For some compounds or classes of compounds, pertinent literature is discussed and references are given. There is no claim that the corresponding data and reference lists are complete.
Readers who would like to suggest inclusion of more data or would like to have another section added or notice an error are encouraged to contact the author so that the website can be improved accordingly.
Most materials whose spectra are shown here were obtained from commercial sources. Some of them may contain minor impurities, which are visible in the spectra. The impurities and their signals are not discussed. Deuterated chloroform (CDCl3) is probably the most commonly used solvent in NMR experiments on fatty compounds and was used here to obtain the spectra depicted. All spectra were obtained at 500 MHz. For sake of consistency, the integration value of the terminal methyl protons in the fatty acid chains was chosen as reference in most spectra and assigned the value 3.00. Usually, the spectral range of 0-6 ppm is shown since it covers the range of chemical shifts in most fatty compounds. In some cases, not all integration values are shown, rather only integration values of "newly introduced" peaks are shown relative to the chosen standard peaks. Besides serving as solvent, CDCl3 is also the reference material for chemical shift values with residual chloroform giving a peak at 7.295 ppm, although the exact shift values of the same peaks can vary slightly from spectrum to spectrum. In accordance with IUPAC nomenclature, carbon atoms are counted from the carbon carrying the carboxylic acid or ester group in the fatty acid chain, with this carbon being C1.
Review Literature on 1H-NMR of Fatty Compounds:
- Mannina, L. and Segre, A. High resolution nuclear magnetic resonance: from chemical structure to food authenticity. Grasas y Aceites, 53, 22-33 (2002).
- Diehl, B.W.K. Multinuclear high-resolution nuclear magnetic resonance spectroscopy. Lipid Analysis in Oils and Fats (Conf. Proc.) (Ed: R.J. Hamilton, Blackie, London) (1998).
- Spitzer, V. High-resolution nuclear magnetic resonance spectroscopy of fatty acids and lipids. GIT Lab. J., 45-47 (1998).
- Lie Ken Jie, M.S.F. and Mustafa, J. High-resolution nuclear magnetic resonance spectroscopy - applications to fatty acids and triacylglycerols. Lipids, 32, 1019-1034 (1997).
- Gunstone, F.D. and Shukla, V.K.S. NMR of lipids. Annu. Rep. NMR Spectrosc., 31, 219-237 (1995).
Spectra and Peak Assignments
The assignments of the most common proton signals relevant to fatty compounds have been compiled in the literature (The Lipid Handbook, second edition, p. 516-517) and are given in Table 1.