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Mass Spectrometry of Methyl Esters


Epoxy, Furanoid and Alkoxy Fatty Acids



As with my other documents on mass spectrometry, this is a subjective account that details only those relevant fatty acids encountered during our research activities and for which we have spectra available for illustration purposes. Methyl esters are often the first choice for identification of unknown fatty acids, including those with epoxy or similar functional moieties and they can afford definitive identifications, especially with structural features other than double bonds, although they are rarely the best for the purpose. 3-Pyridylcarbinol ('picolinyl') esters, pyrrolidides and DMOX derivatives also have their advantages, but I have no definite feelings on whether any of these derivatives are best for this purpose, simply because insufficient model spectra are available. My general philosophy is to always to prepare more than one derivative, when possible. In this document, the spectra of methyl esters are discussed, while mass spectra of 3-pyridylcarbinol esters and of DMOX and pyrrolidine derivatives are described in separate web pages, and all are intended as practical rather than mechanistic guides. Prior publications are cited here when they are known to me. There is an account of the natural occurrence of oxygenated fatty acids in the Lipid Essentials section.


Epoxy Fatty Acids

Mass spectra of three epoxy fatty acids that occur naturally in certain seed oils are described below. Note that care has to be taken in preparing the derivatives for mass spectrometry, as strong acids can cause ring opening.

The mass spectrum of methyl 9,10-epoxy-octadecanoate -

Mass spectrum of methyl 9,10-epoxy-octadecanoate

The molecular ion (m/z = 312) is just detectable, but the McLafferty ion at m/z = 104 is prominent, as expected for a saturated derivative (see our web page on methyl esters of saturated fatty acids). The most useful diagnostic ion is that at m/z = 155, which is the ion formed from the terminal part of the molecule after cleavage between carbons 8 and 9. The ion at m/z = 199 reflects cleavage between carbons 10 and 11 (including the carboxyl group) (Ryhage and Stenhagen, 1960). Although the relative abundances differ, these ions are also present in spectrum of the trimethylsilyl ether derivative of methyl 9,10-epoxy,18-hydroxy-octadecanoate from plant suberin in our Archive" section.

The mass spectrum of methyl 12,13-epoxy-octadec-9-enoate or vernolate, which was the first natural epoxy fatty acid to be characterized (by my mentor - the eminent lipid chemist F.D. Gunstone) follows; this sample was from the seed oil of Vernonia galamensis.

Mass spectrum of methyl 12,13-epoxy-octadec-9-enoate

Again, the molecular ion is very small. It is possible to speculate that the ions at m/z = 164 and 207 reflect cleavage on either side of the ring, after loss of the methanol group, but in general the spectrum is best regarded as a fingerprint. With methyl esters, many analysts have resorted to ring opening procedures prior to mass spectrometry as a better means of locating the epoxide ring.

Methyl 9,10-epoxy-octadec-12-enoate or coronarate, a natural isomer of vernolic acid from Xeranthemum annuum seed oil, has the mass spectrum -

Mass spectrum of methyl 9,10-epoxy-octadec-12-enoate

While it would be possible to speculate on the origin of some of the ions, I am reluctant to do this in print, and the spectrum is again best regarded simply as a fingerprint (see Kleiman and Spencer, 1973).

3-Pyridylcarbinol esters and DMOX derivatives give more useful spectra that permit definitive location of both the double bond and epoxy group.


Furanoid Fatty Acids

Furanoid fatty acids with methyl substituents on the ring occur naturally in small amounts in plant lipids including algae and certain seed oils, and via the food chain, they are present in small amounts in animal tissues and especially those of fish. The simplest natural furanoid fatty acid, 8-(5-hexyl-2-furanyl)-octanoic or 9,12-epoxy-octadeca-9,11-dienoic acid has also been found in plants and fish. In addition, it can be formed on autoxidation of conjugated dienoic acids, e.g. 9-cis,11-trans-octadecadienoic acid; the material used for the spectrum below was from Matreya Inc. (U.S.A.). The methyl ester has the spectrum illustrated next -

Mass spectrum of methyl 8-(5-hexyl-2-furanyl)-octanoate

The base peak at m/z = 165 presumably represents the ion formed by cleavage beta to the furanoid ring between carbons 7 and 8. Similarly, the ion at m/z = 237 is formed by a beta cleavage on the other side of the ring (Yurawecz et al., 1995). The ion at m/z = 95 is a fragment containing the furan ring and the two adjacent carbon atoms as illustrated. The full spectrum of a positional isomer of this fatty acid has been illustrated by Yurawecz et al. (1997).

Furanoid fatty acids, with one or two methyl groups attached to the furanoid ring, occur as minor components of fish oils, but they almost certainly originate in algae as part of the marine food chain. They have also been reported as minor component of the lipids of higher plants. The spectrum of methyl 10,13-epoxy-11-methyl-octadecadienoate is -

Mass spectrum of methyl 10,13-epoxy-11-methyl-octadecadienoate

- and that of methyl 10,13-epoxy-11,12-dimethyl-octadecadienoate is –

Mass spectrum of methyl 10,13-epoxy-11,12-dimethyl-octadecadienoate

Perhaps surprisingly, the mass spectra of the methyl esters are sufficiently informative for most practical purposes, as they define the position of the ring and the number of attached methyl groups. The positions of the rings are clearly delineated by fragmentations beta to the rings as illustrated, while the presence of one or two methyl groups is determined by ions at m/z = 109 or 123, respectively (c.f. the corresponding ion at m/z = 95 in the spectrum of the unbranched compound above). The ion at m/z = 123 is often accompanied by a small ion at m/z = 109, but not vice versa. Glass et al. (1975) appear to be the first to illustrate and discuss such spectra, but they have been followed by many others. The position of the methyl branch in the mono-methyl compound cannot be determined from the spectrum, but this can be accomplished following hydrogenation.

Methyl 10,13-epoxy-11-methyl-hexadecadienoate has the spectrum -

Mass spectrum of methyl 10,13-epoxy-11-methyl-hexadecadienoate

The ions at m/z = 137 and 265 locate the furanoid ring, while that at m/z = 109 confirms that there is only one methyl group attached to the ring.

In the spectrum of the analogous fatty acid with two methyl groups in the ring, i.e. of methyl 10,13-epoxy-11,12-dimethyl-hexadecadienoate, ions at m/z = 151 and 279 locate the furanoid ring, while that at m/z = 123 confirms that there are now two methyl groups attached to the ring.

Mass spectrum of methyl 10,13-epoxy-11,12-dimethyl-hexadecadienoate

As a practical point when searching for furanoid fatty acids in complex marine samples, it can be helpful to do a selective ion search for the ions at m/z = 137, 151, 165 and 179, which in my experience should enable the analyst to find those most often encountered. It also helps to have authentic spectra available for comparison, and there are more available on our Archive page, but without interpretation.


Alkoxy Fatty Acids

Methoxy fatty acids are often found in the lipids of marine invertebrates and in some bacteria, but rarely elsewhere. The only one for which we have a spectrum on file was synthesised from methyl ricinoleate of castor oil, i.e. methyl 12-methoxy-octadec-9-enoate -

Mass spectrum of methyl 12-methoxy-octadec-9-enoate

There is a very simple cleavage pattern between carbons 11 and 12, probably facilitated by the presence of the adjacent double bond, to yield the ion at m/z = 129, which presumably loses the elements of methanol from the 12-methoxyl group to give the ion at m/z = 97. Supporting evidence comes from the spectrum of the pyrrolidide derivative, which contains the same ions.


References


I also recommend - Christie, W.W. and Han, X. Lipid Analysis - Isolation, Separation, Identification and Lipidomic Analysis (4th edition), 446 pages (Oily Press, Woodhead Publishing and now Elsevier) (2010) - from Science Direct.

Credits/disclaimer Updated: March 3rd, 2017 Author LipidWeb icon

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