Part 2. Ene-ynoic Systems

Mass Spectra of Acetylenic Fatty Acids

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, but references to prior publications are cited when these are known. The review by Spitzer (1997) contains much information on DMOX derivatives of natural acetylenic fatty acids. 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. Another potential problem is that such fatty acids can isomerize and oxidize readily on derivatization or if handled carelessly. Mass Spectrometry of acetylenic fatty acids - Part 1 deals with mono- and diynes.

 

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 -

Mass spectrum of methyl crepenynate

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, although it origin in unknown. The same is true of the C20 homologue of crepenynate, methyl eicos-11-en-14-ynoate in which the last ion is now 28 amu higher at m/z = 264 -

Mass spectrum of methyl eicos-11-en-14-ynoate

3-Pyridylcarbinyl crepenynate has a more informative spectrum (Christie, 1998) -

Mass spectrum of 3-pyridylcarbinyl crepenynate

Interpretation is straightforward, as the double bond is recognized by the gap of 26 amu between m/z = 234 and 260, as in the spectrum of 3-pyridylcarbinol oleate, for example, while the triple bond is characterized by the gap of 24 amu between m/z = 274 and 298, or better by the gap of 38 amu between m/z = 260 and 298.

With the C20 analogue, 3-pyridylcarbinyl eicos-11-en-14-ynoate, the equivalent diagnostic ions are 28 amu higher as expected.

Mass spectrum of 3-pyridylcarbinyl eicos-11-en-14-ynoate

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, 1998) giving the spectrum illustrated -

Mass spectrum of the DMOX derivative of crepenynic acid

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.

Mass spectrum of the pyrrolidide of crepenynate

Octadeca-6,9-dien-12-ynoic acid was produced from crepenynic acid by an incubation with a Δ6 desaturase. Methyl octadeca-6,9-dien-12-ynoate has the spectrum -

Mass spectrum of methyl octadeca-6,9-dien-12-ynoate

Other than offering it as a fingerprint, little further interpretation is possible. The tropylium ion (m/z = 91) is now the base peak, and there are abundant ions related to this (in 14 amu steps) at m/z = 105, 119, 113 and so forth.

The 3-pyridylcarbinol ester of octadeca-6,9-dien-12-ynoate has the spectrum -

Mass spectrum of 3-pyridylcarbinyl octadeca-6,9-dien-12-ynoate

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 from the fact that it were in any other position, substantial changes to the spectrum would be expected (by comparison with other spectra).

 

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 -

Mass spectrum of methyl octadeca-9-yn-11-trans-enoate

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 (not published elsewhere).

Mass spectrum of methyl eicos-11-yn-13-trans-enoate

The 3-pyridylcarbinol ester of ximenynic acid has the following mass spectrum -

Mass spectrum of 3-pyridylcarbinyl ximenynate

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 required for gas chromatography, or as part of the electron-impact ionization. The spectrum is best considered as a fingerprint, therefore.

The spectrum of the homologous 3-pyridylcarbinyl eicos-11-yn-13-trans-enoate is-

Mass spectrum of 3-pyridylcarbinyl eicos-11-yn-13-trans-enoate

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 et al., 1996) and follows -

Mass spectrum of the DMOX derivative of ximenynate

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 it is a useful fingerprint when a standard spectrum is required for comparison. 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 -

Mass spectrum of the DMOX derivative of eicos-11-yn-13-trans-enoate

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. Pyrrolidides tend to give much better spectra than DMOX derivatives when features at the terminal end of the molecule are relevant.

Mass spectrum of the pyrrolidide of ximenynate

The ion at m/z = 70, presumably representing the pyrrolidine ring, appears to be relatively prominent in spectra of pyrrolidides of acetylenic esters, and in that of the pyrrolidide of docosa-13-yn-trans-15-enoate from the seed oil of Ximenia americana (Christie, W.W., unpublished), it has become the base peak.

Mass spectrum of the pyrrolide of docosa-13-yn-trans-15-enoate

With the mass spectrum of 3-pyridylcarbinyl octadeca-8,10-dien-12-ynoate, from the seed oil of Tanacetum corymbosum, the picture is even more complicated -

Mass spectrum of 3-pyridylcarbinyl octadeca-8,10-dien-12-ynoate

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 3-pyridylcarbinol ester (Tsevegsuren et al., 1988) See also the previous webpage - Acetylenics Part 1.

The same was true of the DMOX derivative of octadeca-8,10-dien-12-ynoate -

Mass spectrum 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 -- 3-pyridylcarbinol ('picolinyl') esters -- DMOX derivatives -- pyrrolidides.

 

References

  • Christie, W.W. Mass spectrometry of fatty acids with methylene-interrupted ene-yne systems. Chem. Phys. Lipids, 94, 35-41 (1998) (DOI: 10.1016/S0009-3084(98)00016-4).
  • Liu, Y.D., Longmore, R.B. and Fox, J.E.D. Separation and identification of ximenynic acid isomers in the seed oil of Santalum spicatum RBr as their 4,4-dimethyloxazoline derivatives. J. Am. Oil Chem. Soc., 73, 1729-1731 (1996) (DOI: 10.1007/BF02517979).
  • Spitzer, V. Structure analysis of fatty acids by gas chromatography - low resolution electron impact mass spectrometry of their 4,4-dimethyloxazoline derivatives - a review. Prog. Lipid Res., 35, 387-408 (1997) (DOI: 10.1016/S0163-7827(96)00011-2).
  • 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).

Updated March 10, 2014

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