MASS SPECTRA OF PICOLINYL ESTERS
Part 3. Dienoic Fatty Acids
Here, the electron-impact mass spectra of picolinyl esters of those dienoic acids that are most likely to be encountered in nature, together with some useful model compounds, are described. There is a separate document dealing with Mass spectra of allenic fatty acids.
Methylene-Interrupted Dienoic Fatty Acids
The mass spectra of picolinyl esters of dienoic fatty acids, like those of monoenes, tend to be distinctive and permit location of the double bond. As an example, the mass spectrum of picolinyl 9,12-octadecadienoate (linoleate or 18:2(n-6)), the most important dienoic fatty acid in nature, is illustrated below (Harvey, 1982) -
Ions corresponding to cleavage on either side of the terminal double bond are seen at m/z = 300 and 274 (26 amu apart), there is then a gap of 14 amu to m/z = 260, then one of 26 amu for the internal double bond to m/z = 234. When examining the spectra of unknowns, however, it is often easier to locate gaps of 40 amu for the double bond and the associated methylene group on the carboxyl side, i.e. in this instance between m/z = 300 and 260, and between 260 and 220. Regular series of ions 14 amu apart on either side of the double bond confirm that there are no further functional groups in these regions of the molecule.
Similar series of ions are see in spectra from picolinyl esters of most methylene-interrupted dienes, but it is advantageous to have access to spectra of authentic standards when the double bond is close to either end of the molecule to avoid any confusion. Details of the spectra of the complete series of isomeric methylene-interrupted octadecadienoates have been published (Christie, Brechany and Holman, 1987a), but only a few of the spectra were depicted in the paper for practical reasons. All are now illustrated below, but please consult the original reference for tabulated data. Some of these may not occur in nature, but a surprising number do. Spectra of other homologous fatty acid isomers displaying the same diagnostic features may have been described elsewhere in some instances. Spectra for the complete series follow.
Picolinyl 2,5-octadecadienoate (2,5-18:2) (Christie et al., 1987) -
The molecular ion is the base peak, presumably because of the double bond in conjugation with the carboxyl group. Ions that identify the position of the double bond are not apparent, but the doublet of ions at m/z = 214 and 230 are a useful indicator (see the web pages on picolinyl esters of monoenes). The regular series of ions 14 amu apart between m/z = 214 and the molecular ion confirm that there are no functional groups in this part of the alkyl chain.
Picolinyl 3,6-octadecadienoate (3,6-18:2) -
In this spectrum, the gaps of 26 amu between m/z = 151 (the McLafferty ion) and 177, and 190 and 216 serve to locate the double bonds in positions 3 and 6, respectively.
Picolinyl 4,7-octadecadienoate (4,7-18:2). Some of the marked ions that locate the double bonds would be difficult to identify from first principles, so a model spectrum is essential.
Picolinyl 5,8-octadecadienoate (5,8-18:2) – a major component of human sebum lipids. As the double bonds move away from the carboxyl group, the expected fragmentation patterns for the double bonds emerge as marked.
Picolinyl 6,9-octadecadienoate (6,9-18:2). Here, and with many of the subsequent spectra in this series, the double bonds are easily located.
Picolinyl 7,10-octadecadienoate (7,10-18:2).
Picolinyl 8,11-octadecadienoate (8,11-18:2) -
Picolinyl 9,12-octadecadienoate - see the start of this web page.
Picolinyl 10,13-octadecadienoate (10,13-18:2) -
Picolinyl 11,14-octadecadienoate (11,14-18:2) (Christie et al., 1987a) -
Picolinyl 12,15-octadecadienoate (12,15-18:2) -
Piolinyl 13,16-octadecadienoate (13,16 -18:2) -
Picolinyl 14,17-octadecadienoate (14,17-18:2). It is possible even with this last of the isomers to locate the double bonds by the specific fragments marked.
Spectra of picolinyl esters of many more dienoic fatty acids of this type of differing chain lengths are illustrated in our Archive section, but without interpretation. Many of these have not been formally published elsewhere.
Conjugated Dienoic Fatty Acids
Picolinyl esters may not be the best derivatives for fatty acids with conjugated double bond systems, and for example, DMOX derivatives may be better in this instance (see Mass spectra of DMOX derivatives - dienoic fatty acids), or MTAD adducts of methyl esters are uniquely valuable, especially for the complex mixtures found in commercial ‘conjugated linoleic acid’ (CLA) and some natural samples. However, picolinyl esters can be useful if authentic standards are available for comparison, especially when isomers are well resolved on the GC column.
The mass spectrum of picolinyl 9-cis,11-trans-octadecadienoate, the most abundant natural isomer (e.g. in cows' milk fat) follows -
There is a gap of 52 amu between m/z = 234 and 286 at either end of the conjugated double bond system, with intervals of 14 amu on either side for cleavage at successive methylene groups. The molecular ion at m/z = 371 is the base peak.
Picolinyl 10-trans,12-cis-octadecadienoate -
In this instance, the diagnostic ions are shifted up by 14 amu from the previous spectrum, but not very convincingly if a definitive identification were to be required. These spectra do not appear to have been published formally elsewhere.
Bis-Methylene-Interrupted Dienoic Fatty Acids
It is becoming apparent that bis- and polymethylene-interrupted dienoic fatty acids are more common in nature than may have been supposed. In particular, fatty acids with a 5,9- double bond system or their chain elongation products are common in seed oils from Gymnosperms or in certain marine invertebrates such as sponges. The spectrum of 5,9-18:2 and of some synthetic fatty acids with the bis-methylene-interrupted double bonds (useful for confirming fragmentation mechanisms) follow next (Christie et al., 1987b).
Picolinyl 5,9-octadecadienoate (5,9-18:2) (Hierro et al., 1996) -
The truly distinctive feature is the ion at m/z = 219 (unusual in being odd-numbered) representing cleavage at the centre of the bis-methylene-interrupted double bond system. In addition, gaps of 26 amu, between m/z = 178 and 206 and m/z = 232 and 258 help to locate the double bonds in positions 5 and 9, respectively.
Picolinyl 2,6-octadecadienoate (2,6-18:2) -
In this spectrum (unpublished), the ion at m/z = 177 representing cleavage at the centre of the bis-methylene-interrupted double bonds is the base peak in fact. The expected ion at m/z = 164 is missing, but the ions at m/z = 190 and 216 locate the double bond in position 6.
Picolinyl 6,10-octadecadienoate (6,10-18:2) (Christie et al., 1987b) -
All the ions that locate the double bonds (marked) are easily located, and the same is true for the 7,11-isomer (not illustrated). It appears that ions diagnostic for the positions of the double bonds become easier to locate as they move further from the carboxyl group.
Picolinyl 8,12-octadecadienoate (8,12-18:2) (Christie et al., 1987b) -
Note that the ion representing cleavage at the centre of the bis-methylene-interrupted double bonds is now even numbered (m/z = 260). The same is true of the 9,13-isomer (not illustrated).
Spectra of picolinyl esters of many more bis-methylene-interrupted dienoic fatty acids are illustrated in our Archive section, but without interpretation. Many of these have not been formally published elsewhere.
Poly-Methylene-Interrupted Dienoic Acids
Fatty acids with more than two methylene groups between double bonds are perhaps less common than bis-methylene-interrupted isomers in nature, but some examples of mass spectra of picolinyl esters of synthetic and natural fatty acids of this type are illustrated below. Spectra of two synthetic fatty acids are available (Christie et al., 1987b)), and the first of these has been found naturally in a sponge (Carballeira and Maldona, 1989).
Picolinyl 6,11-octadecadienoate (6,11-18:2) has three methylene groups between the double bonds -
There are prominent ions for cleavages between the double bonds, i.e. at m/z - 233 and 246 (one odd-numbered), and the double bonds themselves can be located by the usual means as marked.
Picolinyl 7,12-octadecadienoate (7,12-18:2) -
The spectrum is very similar to the previous, except that the key ions are all shifted by 14 amu upwards, as might be expected.
Picolinyl 5,11-octadecadienoate (5,11-18:2) has four methylene groups between the double bonds and is present in the seeds of Gingko biloba. Its spectrum has been published (Wolff et al., 1999) -
In this instance, the double bonds are located in the same way as for isolated monoenes, with that for position 5 being determined by the gap of 26 amu between m/z = 178 and 204, and for position 11 by that between m/z = 260 and 286. Direct comparison with the mass spectrum of 5-18:1 is invaluable here, for confirmation of the position of the first double bond. 5,11-Eicosadienoate has a very similar spectrum (see the above reference).
Picolinyl 6,12-octadecadienoate (6,12-18:2) -
This spectrum resembles that of the 5,11-isomer except that the diagnostic ions are all 14 amu higher.
My colleagues and I have encountered a few long-chain fatty acids with more than four (six) methylene groups between double bonds and some examples from marine invertebrates follow.
Picolinyl 5,13-eicosadienoate (Christie et al., 1988)). As with the previous, interpretation of the spectrum is similar to those of isolated monoenes, and this is also true of the next spectrum.
Picolinyl 9,19-tetracosadienoate (9,19-26:2) (Christie et al., 1992) -
Spectra of picolinyl esters of many more dienoic fatty acids, including some branched-chain isomers, are illustrated in our Archive section, but without interpretation. Many of these have not been published formally elsewhere.
References
- Carballeira, N.M. and Maldona, M.E. On the isolation of the new fatty acid 6,11-eicosadienoic (20:2) and related 6,11-dienoic acids from the sponge Euryspongia rosea. Lipids, 24, 665-668 (1989).
- Christie, W.W., Brechany, E.Y. and Holman, R.T. Mass spectra of the picolinyl esters of isomeric mono- and dienoic fatty acids. Lipids, 22, 224-228 (1987a).
- Christie, W.W., Brechany, E.Y., Gunstone, F.D., Lie Ken Jie, M.S.F. and Holman, R.T. Mass spectra of the picolinyl esters of some non-methylene-interrupted octadecadienoic acids. Lipids, 22, 664-666 (1987b).
- Christie, W.W., Brechany ,E.Y. and Stefanov, K. Silver ion high-performance liquid chromatography and gas chromatography-mass spectrometry in the analysis of complex fatty acid mixtures: application to marine invertebrates. Chem. Phys. Lipids, 46, 127-136 (1988).
- Christie, W.W., Brechany, E.Y., Stefanov, K. and Popov, S. The fatty acids of the sponge Dysidea fragilis from the Black Sea. Lipids, 27, 640-644 (1992).
- Harvey, D.J. Picolinyl esters as derivatives for the structural determination of long chain branched and unsaturated fatty acids. Biomed. Mass Spectrom., 9, 33-38 (1982).
- Hierro, M.T.G., Robertson, G., Christie, W.W. and Joh, Y.-G. The fatty acid composition of the seeds of Ginkgo biloba. J. Am. Oil Chem. Soc., 73, 575-579 (1996).
- Wolff, R.L., Christie, W.W. and Marpeau, A.M. Reinvestigation of the polymethylene-interrupted 18:2 and 20:2 acids of Ginkgo biloba seed lipids. J. Am. Oil Chem. Soc., 76, 273-276 (1999).
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Updated: 24/4/2008 |
Scottish Crop Research Institute (and MRS Lipid Analysis Unit), Invergowrie, Dundee (DD2 5DA), Scotland
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