MASS SPECTRA OF DERIVATIVES OF ACETYLENIC FATTY ACIDS
| This section describes spectra under the headings - | |
As with other documents in this section, this is a subjective account of mass spectrometry of acetylenic fatty acids, detailing only those encountered during our research activities here and for which we have spectra available for illustration purposes. However, I trust that we have a wider range of spectra than are likely to be encountered elsewhere. Many have never been published formally. The review by Spitzer (Prog. Lipid Res., 35, 387-408 (1997)) contains much information on DMOX derivatives of natural acetylenic fatty acids. Spectra of methyl esters, picolinyl esters, DMOX derivatives and pyrrolidides are described in this document when they are available. Spectra of highly unsaturated acetylenic fatty acids are rarely easy to interpret, and a common failing is to attempt to see more in a spectrum than may be justified.
Monoynoic Fatty Acids
The mass spectrum of methyl octadec-9-ynoate (stearolate) is undistinguished. It differs in a number of ways from that of methyl linoleate, which has the same molecular weight.
Wrong spectrum published - replacement in preparation
The mass spectrum of the picolinyl ester of octadec-9-ynoate is much more useful. It resembles that of oleate superficially -
The molecular ion and others in the high mass range are of course 2 amu less than with oleate. The acetylenic bond is located by a gap of 24 amu between m/z = 234 and 258. If this is less than convincing, the gap of 38 amu for the triple bond and the proximal methylene between m/z = 220 and 258 is clear, and the two ions at m/z = 272 and 286 (resembling those in the mass spectrum of picolinyl oleate) are useful signposts.
The mass spectrum of the DMOX derivative of stearolate follows -
A similar rule to that devised for DMOX derivatives of monoenes should be applied to locate the triple bond, i.e.
If a mass separation of 10 instead of the regular 14 amu is observed between two neighbouring even-mass homologous fragments containing n-1 and n carbon atoms of the original acid moiety, a triple bond exists between carbons n and n+1 in the chain" (Zhang, J.Y. et al., J. Am. Oil Chem. Soc., 66, 256-259 (1989)).
As with monoenes, I suspect this will only hold true for the 7/8 to 15 positions, if standards become available to check. In the spectrum above, the gap of 10 amu between m/z = 196 and 206 is again unconvincing, but the gap of 38 amu between m/z = 182 and 220 is clear and characteristic.
It should be noted that the above rule was first promulgated for pyrrolidides (Valicenti, A.J., Heimermann, W.H. and Holman, R.T. J. Org. Chem., 44, 1068-1073 (1979)). Thus, similar features are seen in the spectrum of the pyrrolidide derivative of stearolate -
Diynoic Fatty Acids
We had access some years ago to a comprehensive series of dimethylene-interrupted diynoic fatty acids, prepared by M.S.F. Lie Ken Jie and colleagues and details of the mass spectra of the picolinyl ester derivatives (only) were published (Christie, W.W., Brechany, E.Y. and Lie Ken Jie, M.S.F. Chem. Phys. Lipids, 46, 225-229 (1988)). For most people, these are likely to be of limited academic interest, so two representative examples only are shown below. As cautioned earlier, spectra of highly unsaturated acetylenic fatty acids are rarely easy to interpret, and one should not attempt to see more in a spectrum than may be justified. It is likely that the triple bonds rearrange in complex ways under electron bombardment.
Mass spectrum of picolinyl 5,9-octadecadiynoate -
To someone who did not know the structure, it would be evident that there were eight hydrogens fewer than those present in a saturated compound, and that these must be before Carbon-11. Otherwise, the spectrum can only be useful as a fingerprint for comparison purposes.
Mass spectrum of picolinyl 9,13-octadecadiynoate -
Here, the triple bond in position 13 can be located from the gap of 38 amu between m/z =272 and 310, but that in position 9 cannot be located definitively. In this and many of the other related spectra, the ion for [M-1]+ is more abundant than the molecular ion.
We also have unpublished mass spectra of some isomeric conjugated diynes (a gift from Professor M.S.F. Lie Ken Jie), such as that of methyl 9,11-octadecadiynoate -
The molecular ion is barely detectable, and the base peak at m/z = 91 presumably reflects a rearrangement to form a stable tropylium ion. Spectra of other isomers as the methyl esters are indistinguishable from this.
The mass spectrum of the picolinyl 9,11-octadecadiynoate is -
Spectra of other isomers with triple bonds in relatively central positions are all very similar to this, so I suspect complex rearrangements occur internally under electron bombardment to smooth out potential differences. No interpretation of the spectrum is offered, therefore.
The DMOX derivative of 9,11-octadecadiynoate -
Unusually, the base peak is at m/z = 126 (not 113) as in this example. The spectra of several positional isomers are again very similar, so no further interpretation is given here.
Methylene-Interrupted Ene-ynoic Fatty Acids
Crepenynic or octadec-9-en-12-ynoic acid is a major constituent of some seed oils and is important as a biosynthetic precursor of a family of secondary metabolites. The mass spectrum of methyl crepenynate is -
As with other methyl esters, little structural information can be gleaned from the spectrum. However, it is a distinctly different fingerprint from fatty acids with the same molecular weight, such as the various linolenic acid isomers. The ion at m/z = 236 (equivalent to [M-56]+) does appear to be distinctive. The same is true of the C20 homologue of crepenynate, methyl eicos-11-en-14-ynoate -
Picolinyl crepenynate (Christie, W.W. Chem. Phys. Lipids, 94, 35-41 (1999)) -
Interpretation is straightforward, as the double bond is recognized by the gap of 26 amu between m/z = 234 and 260, while the triple bond is characterized by the gap of 24 amu between m/z = 274 and 298.
Similarly for the C20 analogue, picolinyl eicos-11-en-14-ynoate , the equivalent diagnostic ions are 28 amu higher.
The DMOX derivative of crepenynic acid is not easy to prepare, as a cyclization reaction occurs at the high temperature normally employed, to give amongst other products an internal 6-membered ring with a conjugated double bond system. However, the derivative can be prepared by the relatively mild two-step procedure (see our web pages on Preparation of derivatives) (Christie, W.W. Chem. Phys. Lipids, 94, 35-41 (1999)) giving the spectrum illustrated -
Again, interpretation of the authentic spectrum is simple, as the double bond is recognized by the gap of 12 amu between m/z = 196 and 208, while the triple bond is characterized by the gap of 10 amu between m/z = 236 and 246.
The pyrrolidide of crepenynate is prepared by our standard method without difficulty and has a mass spectrum with the same key ions as in that of the DMOX derivative, though lower in abundance relative to the base peak.
Methyl octadeca-6,9-dien-12-ynoate has the spectrum -
Other than offering it as a fingerprint, little further interpretation is possible. The tropylium ion (m/z = 91) is now the base peak.
The picolinyl ester of octadeca-6,9-dien-12-ynoate has the spectrum -
The triple bond in position 12 is most easily recognized by the gap of 38 amu between m/z = 258 and 296, while the double bond in position 9 is located by the gap of 40 amu between m/z = 218 and 258. The position of the double bond in position 6 must be inferred. If it were in any other position, substantial changes to the spectrum would be expected.
Conjugated Ene-ynoic Fatty Acids
Octadeca-9-yn-11-trans-enoic (ximenynic or santalbic) acid is a component of certain seed oils. The methyl ester has the following spectrum -
The molecular ion is not very abundant, but the tropylium ion at m/z =91 does stand out. However, the really distinctive feature is the ion at m/z = 150, normally considered to be characteristic of fatty acids of the (n-6) family. It is presumably formed from the terminal end of the molecule by cleavage between carbons 7 and 8, since it is also prominent in the mass spectrum of the homologous methyl eicos-11-yn-13-trans-enoate (unpublished spectrum).
The picolinyl ester of ximenynic acid has the following mass spectrum -
It is evident from this that the unsaturation (loss of 6 H) must be somewhere between carbons 8 and 13 (m/z = 280 to 298), but other than this it would not be easy to locate the double and triple bonds more specifically, possibly because complex rearrangements occur, either during preparation of the derivative, at the high temperature of the GC or as part of the electron bombardment. The spectrum is best considered as a fingerprint, therefore.
The spectrum of the homologous picolinyl eicos-11-yn-13-trans-enoate is-
It exhibits many comparable features, but shifted 28 amu upwards.
The mass spectrum of the DMOX derivative of ximenynic acid has been published elsewhere (Liu, Y.D., Longmore, R.B. and Fox, J.E.D. J. Am. Oil Chem. Soc., 73, 1729-1731 (1996)) and follows -
Again faced with a compound of unknown structure, I am doubtful if the spectrum could be interpreted other than that the unsaturation (loss of 6 H) must be between carbons 8 and 12 (m/z = 182 to 246), but at least is a useful fingerprint. The spectrum of the DMOX derivative of the C20 homologue, eicos-11-yn-13-trans-enoate (not published elsewhere), has comparable features, but shifted 28 amu upwards -
As might be expected, the mass spectrum of the pyrrolidide derivative of ximenynate resembles that of the DMOX derivative, in that essentially the same ions are seen if in somewhat lower abundance. However, there is one significant difference in that an ion at m/z = 70, which is presumably formed by cleavage at the terminal part of the molecule, stands out in the spectrum of the pyrrolidide but is essentially absent in that of the DMOX derivative. Pyrrolidides tend to give much better spectra than DMOX derivatives when features at the terminal end of the molecule are relevant.
The ion at m/z = 70 becomes more prominent in the spectra of higher homologues, and in that of the pyrrolide of docosa-13-yn-trans-15-enoate from the seed oil of Ximenia americana (Christie, W.W., unpublished), it has become the base peak.
With the mass spectrum of picolinyl octadeca-8,10-dien-12-ynoate, from the seed oil of Tanacetum corymbosum, the picture is even more complicated -
It was only possible to identify the location of the double and triple bonds by isolating the fatty acid and performing deuteration prior to mass spectrometry of the picolinyl ester (Tsevegsuren, N., Christie, W.W. and Losel, D. Lipids, 33, 723-727 (1988)).
The same was true of the DMOX derivative of octadeca-8,10-dien-12-ynoate -
Spectra of further acetylenic fatty acids are available, but without interpretation, in the Archive Sections of these web pages, i.e. for methyl esters -- picolinyl esters -- DMOX derivatives -- pyrrolidides.
Alternative Derivatization Techniques
When faced with a mass spectrum of an acetylenic fatty acid that is not easily interpreted, perhaps the simplest technique is to perform deuteration with Wilkinson's catalyst. Four deuterium atoms are then added to triple bonds, and these can be identified easily by GC-MS. Note that the reaction must be performed on the methyl ester derivative, but this must be converted to the picolinyl ester (or DMOX derivative) for mass spectrometry. As noted in the previous section, this reaction helped us identify a conjugated acetylenic acid in T. corymbosum seed oil (Tsevegsuren, N., Christie, W.W. and Losel, D. Lipids, 33, 723-727 (1988)). See the section of this website on Mass spectra of methyl esters of fatty acids - further derivatization for a detailed protocol.
Another technique that appears that might be useful, at least with isolated acetylenic triple bonds, is mercuration-demercuration. The demercuration reaction is not reversible in this instance, and ketones are formed by addition of the elements of water (Bu'Lock, J.D. and Smith, G.N., J. Chem. Soc. (C) 332-336 (1967))) -
I have not tried this reaction myself, but it seems straightforward. Younger readers should note that reactions described over thirty years ago (before computerized data bases) may be just as useful as those published in the last few years.
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Updated: 28/8/2007 |
Scottish Crop Research Institute (and MRS Lipid Analysis Unit), Invergowrie, Dundee (DD2 5DA), Scotland
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