MASS SPECTRA OF METHYL ESTERS OF FATTY ACIDS
Part 4. Dienoic Fatty Acids
As cautioned in the Introduction to these documents, the mass spectra of methyl esters afford limited information only concerning the structures of unsaturated fatty acids. However, the molecular weight is usually obtainable, and this is a significant piece of information. If chromatographic retention data are added to this, it is often possible to be 90% certain of the identity of a fatty acid. In addition, there are a few key ions that help to identify specific di- or polyunsaturated fatty acids, though these must be applied with caution. A few of the spectra illustrated below may have been published elsewhere, but it would be an onerous task to establish priority for all. Most will be unique to this site.
Methylene-Interrupted Dienes
As with monoenes, mass spectra of methyl esters of the common methylene-interrupted dienoic fatty acids do not allow ready interpretation in terms of double bond positions. However, small differences in spectra of other isomers are sometimes useful as fingerprints, which can be related to some structural aspect. The mass spectrum of methyl linoleate (9,12-18:2 or 18:2(n-6)) is illustrated below (Hallgren et al., 1959) -
It has an abundant molecular ion (m/z = 294) and a prominent ion for loss of the McLafferty ion (m/z = 220), although the McLafferty ion per se (m/z = 74) is small. The ion representing [M-31]+ is higher than that for [M-32]+ (although this may not be true for all isomers). Hydrocarbon ions of general formula [CnH2n-3]+ dominate in the lower mass range (m/z = 67, 81, 95, 109, 123, etc). As with the monoenes, there is little evidence for any ions that might serve to locate the double bonds. For example, an ion at m/z = 150 is a useful diagnostic aid for methylene-interrupted polyenoic (3 to 6 double bonds) fatty acids with an (n-6) or ω6 structure (see the web pages on methyl esters of trienes), but not for dienes.
Mass spectra of the methyl esters of all the C18 methylene-interrupted dienes have been described, but only the the 2,5-isomer differed significantly from the rest (Christie and Holman, 1967).
For example, the spectrum of methyl 11,14-octadecadienoate is illustrated -
This spectrum resembles that of methyl linoleate above except that the ions in the higher mass range are of lower intensity. Such differences could even be due to instrumental factors, for example when the source was last cleaned.
The mass spectrum of methyl 7,10-hexadecadienoate (or 16:2(n-6)) is illustrated next for comparison.
This mass spectrum closely resembles the previous one except that the key ions of higher mass are at 28 amu less.
One further diene of this type is of biological interest, i.e. methyl 11,14-eicosadienoate (20:2(n-6)), which has the next spectrum. Again this has no diagnostic features to aid double bond location.
Bis-Methylene-Interrupted Dienes
Fatty acids with 5,9-double bond systems are common in sponges and conifers. The mass spectrum of methyl 5,9-octadecadienoate, i.e. with two methylene groups between the double bonds, is similar to that of the previous dienes in many ways. However, it does have distinctive ions at m/z = 141 and 245 ([M-49]+), which appear to serve as fingerprints for identification. The first of these is presumably formed from the carboxyl end of the molecule by cleavage between carbons 7 and 8, i.e. at the centre of the double bond system. It is present in all homologous 5,9-dienoates, as is an ion at m/z = 109, which is equivalent to that at m/z = 141 less the elements of methanol (minus 32 amu). A loss of 49 amu from the molecular ion is presumably due first to the lost of both the methoxyl group and the elements of water from the molecular ion, but why it should be especially prominent in this instance has yet to be determined. This ion appears in spectra of homologous 5.9- and certain other dienoates, but apparently not in conventional methylene-interrupted dienes. The base peak is usually at m/z = 81 rather than 67. While the spectrum below has not been published elsewhere to my knowledge, the distinctive fragmentation has been discussed in connection with the spectrum of 5,9,12-18:3 (Dobson and Christie, 2002).
Analogous ions are present in the mass spectra of the methyl esters of 5,9-16:2, 5,9-17:2, 5,9-22:2, 5,9-23:2, 5,9-24:2, 5,9-25:2, 5,9-26:2, 5,9-27:2 and 5,9-28:2 (all unpublished and obtained from analyses of sponge fatty acids). As an example, the mass spectrum of the last of these, methyl 5,9-octacosadienoate is illustrated -
As noted earlier, the ions at m/z = 141 and 109, and that for [M-49]+ (m/z = 385) are useful diagnostically. There is also a prominent ion at m/z = 150 in this and the previous isomer, although there is no double bond in the n-6 position.
The mass spectrum of methyl 6,10-octadecadienoate (prepared by total synthesis in the laboratory of Professor M.S.F. Lie Ken Jie in Hong Kong) is -
By analogy with the spectrum of the 5,9-analogue, we might expect a characteristic ion at m/z = 155. This is not distinctive but the ion at 32 amu less (m/z = 123) is presumably formed from this by the loss of a methanol equivalent. In this instance too, an ion representing [M-49]+ (m/z = 245) is present.
Polymethylene-Interrupted Dienes
The mass spectrum of methyl trans-3,cis-9-octadecadienoate, a minor component in some seed oils of the Compositae family, is -
The differences from the spectrum of methyl linoleate above are trivial, mainly in the relative intensities of certain ions. For example, the molecular ion is smaller and the [M-74]+ ion is larger, but there is nothing that suggests the location of any of the double bonds.
The mass spectrum of methyl 6,11-octadecadienoate (prepared by total synthesis in the laboratory of Professor M.S.F. Lie Ken Jie in Hong Kong) is -
Again, it seems best to treat this simply as a fingerprint spectrum, as no interpretation in terms of double bond position seems possible. The same is true of the spectrum of methyl 5,13-docosadienoate (from meadowfoam oil) -
Note that both this and the previous spectra have small ions equivalent to [M-49]+, as in the spectra of bis-methylene-interrupted dienes. We have spectra of methyl esters of 5,12- and 7,12-18:2 (synthetic) on file in the Archive section also.
Conjugated Dienes
The mass spectrum of the methyl ester of a fatty acid with a conjugated diene system is illustrated, i.e. methyl 9-cis,11-trans-octadecadienoate. This is a natural component of all ruminant fats, as a product of the biohydrogenation process in the rumen, and it is a major component of commercial ‘conjugated linoleic acid’ (CLA) preparations.
Here, the only distinguishing feature from the spectrum of linoleate seems to be a more abundant molecular ion. No useful information on the positions of the double bonds can be gleaned from the spectrum, and that of methyl 10-trans,12-cis-18:2 is identical to this (not illustrated). However, double bond positions can be determined after a further derivatization step, i.e. the preparation of 4-methyl-1,2,4-triazoline-3,5-dione (MTAD) adducts (see the section on Mass spectra of methyl esters of fatty acids - further derivatization).
We have spectra of many more methyl esters of dienoic fatty acids on file, and they can be accessed (but with no interpretation) from our Archive page.
References
- Christie, W.W. and Holman, R.T. Synthesis and characterization of the complete series of methylene-interrupted cis,cis-octadecadienoic acids. Chem. Phys. Lipids, 1, 407-423 (1967).
- Dobson, G. and Christie, W.W. Mass spectrometry of fatty acid derivatives. Eur. J. Lipid Sci. Technol., 104, 36-43 (2002).
- Hallgren, B., Ryhage, R. and Stenhagen, E. The mass spectra of methyl oleate, methyl linoleate and methyl linolenate. Acta Chem. Scand., 13, 845-847 (1959).
- Iversen, S.A., Cawood, P., Madigan, M.J., Lawson, A.M. and Dormandy, T.L. Identification of a diene conjugated component of human lipid as octadeca-9,11-dienoic acid. FEBS Letts, 171, 320-324 (1984).
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Updated: 14/5/2008 |
Scottish Crop Research Institute (and MRS Lipid Analysis Unit), Invergowrie, Dundee (DD2 5DA), Scotland
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