MASS SPECTRA OF ALLENIC FATTY ACIDS
Fatty acids with allenic fatty acid systems are rather uncommon in nature, and we have not encountered them in our work. However, we do have spectra from a series of synthetic allenes, both as the picolinyl and methyl esters. As these spectra are unique, selected examples are illustrated with a minimal commentary. Further details are available in the published paper (Christie, W.W., Brechany, E.Y., Lie Ken Jie, M.S.F. and Wong, C.F. Mass spectrometry of derivatives of isomeric allenic fatty acids. Biol. Mass Spectrom., 21, 267-270 (1992)).
Picolinyl Esters
Interpretation is not straightforward in all instances, but the main diagnostic fragmentation is beta to the allenic moiety on either side, as illustrated.
Picolinyl 5,6-octadecadienoate -
Picolinyl 7,8-octadecadienoate -
Picolinyl 9,10-octadecadienoate. Note that from this isomer onwards, cleavage alpha to the double bond system on the distal side (at m/z = 244 in this instance) becomes more prominent.
As the double bond system moves further from the carboxyl group, rearrangement ions within the double bond system stand out more, e.g. that at m/z = 258 in this example.
Picolinyl 11,12-octadecadienoate -
Picolinyl 13,14-octadecadienoate -
We were unable to prepare the picolinyl ester of a 3,4-dienoate isomer.
Methyl Esters
Allenic systems are more stable and less prone to migrate than conventional double bonds under electron-impact ionization, so that mass spectra of methyl esters of allenic acids are also informative. Three useful ions are formed by cleavage on the far side of the molecule beta to the double bond system, at least for the 3,4- to 10,11-isomers, although the relative intensities of the relevant ions vary with the distance from either end of the molecule. They include all or part of the carboxyl group, as shown in the figure (ions a, b and c).
For example, in the spectrum of methyl 3,4-tetradecadienoate -
- ions a, b and c are at m/z = 66, 94 and 126, respectively, although only the last stands out from the background.
However, with methyl 5,6-octadecadienoate, all three ions are distinctive.
Methyl 7,8-octadecadienoate -
In the mass spectrum of methyl 9,10-octadecadienoate, an ion that probably represents a fragment from the terminal part of the molecule (at m/z = 152) now becomes a useful diagnostic aid, while ions a, b and c are now less abundant.
Methyl 11,12-octadecadienoate -
Methyl 13,14-octadecadienoate - now only the ion from the distal portion of the molecule is significant.
We have spectra on file for further allenic fatty acids, both as the methyl and picolinyl ester derivatives, and these can be found in our Archive pages, but without interpretation - Archive of methyl esters --- Archive of picolinyl esters. In addition, the spectra of ethyl 2,3- and 3,4-tetradecadienoates and 3,4-tetradecadienoic acid are available. The first of these is especially important as no other derivative is available for this isomer.
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Scottish Crop Research Institute (and MRS Lipid Analysis Unit), Invergowrie, Dundee (DD2 5DA), Scotland. |
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Updated: 18/5/2008 |
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