Part 1. Mono- and diynes

Mass Spectra of Acetylenic Fatty Acids

As with other documents in this section, this is a subjective account of mass spectrometry with electron-impact ionization 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, but I cite references to prior publications when these are known. Spectra of methyl esters, 3-pyridylcarbinol ('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) illustrated first is not especially distinguished. It was first published by Odham and Stenhagen (1972), but the most comprehensive description of mass spectrometry of methyl esters of acetylenic fatty acids is by Kleiman et al. (1976)), whose tabulated data are invaluable for interpretation of the spectra. This spectrum differs in significant ways from that of methyl linoleate, which has the same molecular weight.

Mass spectrum of methyl octadec-9-ynoate

For example, the molecular ion at m/z = 294 is barely detectable. However, there are several key ions that are reported to have diagnostic relevance. The ions at m/z = 152 and 196 are believed to have rearranged to form allenic structures (see our webpage on mass spectra of allenic fatty acids), with for example the latter as -   [CH3OOC(CH2)7CH=C=CH2]+
  - while the ions at m/z = 166 and 210 have rearranged to produce conjugated dienes, with as an example the second of these represented by -   [CH3OOC(CH2)7CH=CHCH=CH2]+.

The other naturally occurring mono-acetylenic acid is octadec-6-ynoic (tariric) acid, found in seed oils of the genus Picramnia. Methyl octadec-6-ynoate has the spectrum –

Mass spectrum of methyl octadec-6-ynoate

In this instance the defining ion is that at m/z = 154 (together with that at m/z =122, representing a further loss of methanol) from the favoured rearrangement to form the allenic fragment - [CH3OOC(CH2)3CH=C=CH2]+. In fact, this spectrum is extraordinarily similar to that of the allenic methyl octadeca-5,6-dienoate (see our web page on mass spectra of allenic fatty acids, where the ion at m/z = 94 is also discussed). Diene fragments are not significant in this instance.

The mass spectrum of the 3 pyridylcarbinol ester of octadec-9-ynoate also has features that serve to locate the triple bond as might be expected. It should be compared with that of 3 pyridylcarbinyl oleate, which it resembles superficially -

Mass spectrum of 3-pyridylcarbinyl octadec-9-ynoate

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 3-pyridylcarbinyl oleate) are useful signposts.

The mass spectrum of the DMOX derivative of stearolate follows -

Mass spectrum of the DMOX derivative of octadec-9-ynoate

Again the spectrum is superficially similar to that of the DMOX derivative of oleate, and 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 et al.1989).

In the spectrum above, the gap of 10 amu between m/z = 196 and 206 is once more not very convincing, but the gap of 38 amu between m/z = 182 and 220 is clear and characteristic and is arguably a more useful diagnostic aid.

As with monoenes, I suspect the above rule will only hold true for the 7/8 to 15 positions, if standards become available to check. It should be noted that the above rule was first promulgated for the less popular pyrrolidides in a paper by Valicenti et al. (1979))), who had access to a wide range of standards. To my eye, the ions corresponding to the above rule are not at all clear, but each isomer does have a unique fingerprint spectrum. Thus, similar if less pronounced features are seen in the spectrum of the pyrrolidide derivative of stearolate -

Mass spectrum of the pyrrolidide derivative of stearolate

The DMOX derivative of octadec-6-ynoate has a highly distinctive spectrum –

Mass spectrum of DMOX derivative of octadec-6-ynoate

The ion at m/z = 192 probably corresponds to formation of a stable allenic ion like that at m/z = 154 in the spectrum of the methyl ester derivative, but we can only speculate on how the ion at m/z = 178 is formed. The same ions are present in the spectrum of the analogous pyrrolidide, but with much lower intensity (Valicenti et al., 1979).

 

Diynoic Fatty Acids

We had access some years ago to a comprehensive series of bis-methylene-interrupted diynoic fatty acids, prepared by M.S.F. Lie Ken Jie and colleagues and details of the mass spectra of the 3-pyridylcarbinol ester derivatives (only) were published (Christie et al., 1988). For most people, these are likely to be of limited academic interest, so two representative examples only are shown below. Spectra of polyacetylenic fatty acids are rarely easy to interpret, as triple bonds rearrange in complex ways under electron bombardment.

Mass spectrum of 3-pyridylcarbinyl 5,9-octadecadiynoate -

Mass spectrum of 3-pyridylcarbinyl 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 3-pyridylcarbinyl 9,13-octadecadiynoate -

Mass spectrum of 3-pyridylcarbinyl 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 -

Mass spectrum 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 (see the web page on mass spectra of methyl esters of polyenes). Spectra of other isomers as the methyl esters are indistinguishable from this.

The mass spectrum of the 3-pyridylcarbinyl 9,11-octadecadiynoate is -

Mass spectrum of 3-pyridylcarbinyl 9,11-octadecadiynoate

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 -

Mass spectrum of the DMOX derivative of 9,11-octadecadiynoate

Unusually, the base peak in this spectrum is at m/z = 126, not 113 as is normal. The spectra of several positional isomers are again very similar, so no further interpretation is given here.

 

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 then be identified easily by GC-MS. Note that the reaction must be performed on the methyl ester derivative, and this must be converted subsequently to the 3-pyridylcarbinol ester (or DMOX derivative) for mass spectrometry. As noted in the webpage Acetylenic acids Part 2, this reaction helped us identify a conjugated acetylenic acid in T. corymbosum seed oil (Tsevegsuren et al., 1998). See the section of this website on Mass spectra of methyl esters of fatty acids - further derivatization for a detailed protocol.

Deuteration with Wilkinson's catalyst

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 (in contrast to the reaction with double bonds), and ketones are formed by addition of the elements of water (Bu'Lock and Smith, 1967).

Reaction of mercury salts with triple bonds

I have not tried this reaction myself, but it seems straightforward. Kleiman et al. (1976) recommend additional steps, i.e. to reduce the ketones to hydroxyls with sodium borohydride and conversion to trimethylsilyl ethers prior to GC-MS.

Spectra of further acetylenic fatty acids are available, but without interpretation, in the Archive Sections of these web pages, i.e. for methyl esters -- 3-pyridylcarbinol esters -- DMOX derivatives -- pyrrolidides. Our web document Mass Spectrometry of acetylenic fatty acids - Part 2 deals with eneyne systems.

 

References

  • Bu'Lock, J.D. and Smith, G.N. The origin of naturally-occurring acetylenes. J. Chem. Soc. (C), 332-336 (1967).
  • Christie, W.W., Brechany, E.Y. and Lie Ken Jie, M.S.F. Mass spectra of the picolinyl ester derivatives of some isomeric dimethylene-interrupted octadecadiynoic acids. Chem. Phys. Lipids, 46, 225-229 (1988) (DOI: 10.1016/0009-3084(88)90025-4).
  • Kleiman, R., Bohannon, M.B., Gunstone, F.D. and Barve, J.A. Mass spectra of acetylenic fatty acid methyl esters and derivatives. Lipids, 11, 599-603 (1976) (DOI: 10.1007/BF02532872).
  • Odham, G. and Stenhagen, E. Fatty acids. In: Biochemical Applications of Mass Spectrometry, pp. 211-228 (Ed. G. R. Wallace, Wiley, N.Y.) (1972).
  • Tsevegsuren, N., Christie, W.W. and Lösel, D. Tanacetum (Chrysanthemum) corymbosum seed oil: a rich source of a novel conjugated acetylenic acid. Lipids, 33, 723-727 (1998) (DOI: 10.1007/s11745-998-0262-2).
  • Valicenti, A.J., Heimermann, W.H. and Holman, R.T. Mass spectrometry location of triple bonds in fatty acids and fragmentation mechanisms of N-acylpyrrolidides. J. Org. Chem., 44, 1068-1073 (1979).
  • Zhang, J.Y., Yu, X.J., Wang, H.Y., Liu, B.N., Yu, Q.T. and Huang, Z.H. Location of triple bonds in the fatty acids from the kernel oil of Pyrularia edulis by gas chromatography-mass spectrometry of their 4,4-dimethyloxazoline derivatives. J. Am. Oil Chem. Soc., 66, 256-259 (1989) (DOI: 10.1007/BF02546071).

Updated August 28, 2013

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