MASS SPECTRA OF OXYGENATED FATTY ACIDS
Part 2. Heterocyclic 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 here and for which we have spectra available for illustration purposes. Spectra of methyl esters, picolinyl esters, pyrrolidides and DMOX derivatives are all described here (when available), but I will only describe key diagnostic ions as general features of each type of derivative are described elsewhere on this website. I have no definite feelings on whether any of these derivatives are best for the purpose. There are too many gaps in this account to make such a decision or even to discuss the problem systematically. However, DMOX derivatives are not suitable when the functional group is near the terminal end of the molecule. Only a few of the spectra have been published elsewhere.
Epoxy Fatty Acids
Mass spectra of three epoxy fatty acids that occur naturally in certain seed oils are described below.
The mass spectrum of methyl 9,10-epoxy-octadecanoate -
The molecular ion (m/z = 312) is just detectable. 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).
Mass spectra of picolinyl esters of epoxy fatty acids were first described by Balazy and Nies (Biomed. Environ. Mass Spectrom., 18, 328-336 (1989)). The mass spectrum of picolinyl 9,10-epoxy-octadecanoate is -
The ring structure is between the ions at m/z = 234 and 276, but that at m/z = 247 (unusual in being odd-numbered) is an invaluable diagnostic guide. The last ion is also found in the spectra of analogous cyclopropane fatty acids (see the section of this website on Mass spectra of cyclic fatty acids). Also, the ion at m/z = 290 formed by cleavage beta to the ring is distinctive.
Methyl 12,13-epoxy-octadec-9-enoate or vernolate -
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.
The mass spectrum of picolinyl 12,13-epoxy-octadec-9-enoate or vernolate (Balazy, M. and Nies, A.S. Biomed. Environ. Mass Spectrom., 18, 328-336 (1989)) -
The gap of 26 amu between m/z = 234 and 260, and of 40 amu between m/z = 220 and 260, serve to locate the double bond in position 9. Also, the two abundant ions at m/z = 274 and 288 are akin to those in picolinyl esters of 9-monoenes. The first of these is also formed by cleavage alpha to the ring and this may explain why it is the base peak. In this instance, the ion at m/z = 288 is part of the ring structure, and that at m/z = 316 represents cleavage adjacent to the ring.
Spectra of DMOX derivatives of epoxy fatty acids have been described elsewhere (Marx, F. and Classen, E. Fat Sci. Technol., 96, 207-211 (1994)). Note that special methods are required for successful preparation of the derivative because of the sensitivity of the epoxyl moiety. The mass spectrum of the DMOX derivative of 12,13-epoxy-octadec-9-enoate or vernolate is -
The ions at m/z = 236 and 278 serve to locate the oxirane ring. The gap of 12 amu for the double bond in position 9 is between m/z = 195 and 207 (not 196 and 208 as might have been expected).
The pyrrolidine derivative of vernolic acid is much easier to prepare by starting from the methyl ester. As expected, the mass spectrum (below) is superficially similar to that of the DMOX derivative, but the diagnostic ions are more clearly seen. For example the gap of 12 amu between m/z = 196 and 208 serves to locate the double bond in position 9, while the ring structure is defined by the gap of 42 amu between m/z = 236 and 278.
Methyl 9,10-epoxy-octadec-12-enoate or coronarate, a natural isomer of vernolic acid, has the mass spectrum -
While it would be possible to speculate on the origin of some of the ions, the spectrum is best regarded simply as a fingerprint (see Kleiman, R. and Spencer,G.F. J. Am. Oil Chem. Soc., 50, 31-38 (1973)).
The pyrrolidine derivative of 9,10-epoxy-octadec-12-enoate has a mass spectrum with some unexpected features, as the mode of fragmentation is different from that of vernolate. There appear to be ions that locate the epoxy ring as illustrated below, but not for the double bond.
Furanoid Fatty Acids
The furanoid fatty acid, 8-(5-hexyl-2-furyl)-octanoic or 9,12-epoxy-octadec-9,11-dienic acid, occurs naturally in small amounts in certain seed oils and can be formed on oxidation of conjugated linoleic acid, 9-cis,11-trans-octadecadienoic acid. The material used for the spectra below was purchased from Matreya Inc. (U.S.A.). I am not aware of publication of some of these spectra elsewhere. The methyl ester derivative has the spectrum illustrated next -
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, M.P., Hood, J.K., Mossoba, M.M., Roach, J.A.G. and Ku, Y., Lipids, 30, 595-598 (1995)).
The picolinyl ester derivative of 8-(5-hexyl-2-furyl)-octanoate (spectrum not published elsewhere) -
As is usual with picolinyl esters, there is a distinctive fingerprint spectrum. However, interpretation is not as straightforward as I have come to expect with such derivatives. A gap of 66 amu between m/z = 234 and 300 for cleavage on either side of the ring might have been expected, but this is hardly obvious. On the other hand, the ions formed by cleavage beta to the ring (m/z = 220 and 314) are more distinct, as with other derivatives of this acid. I presume that the ring must open under electron bombardment so that a series of alternative fragmentations occurs. The large ion at m/z = 328 is formed by cleavage between carbons 14 and 15, possibly with formation of a stable conjugated system via formation of a third double bond.
The mass spectrum of the DMOX derivative of 8-(5-hexyl-2-furyl)-octanoate -
In this instance, interpretation is relatively straightforward, if somewhat unexpected in view of what was known of fragmentation mechanisms of DMOX derivatives. Here, the ion at m/z = 290 is presumably formed in the same manner as that at m/z = 328 in the previous spectrum. The ions for cleavage immediately adjacent to either side of the ring are not apparent, but those at m/z = 182 and 276 are formed by cleavage beta to it. In this instance, DMOX derivatives appear to be better than picolinyl esters for characterization purposes.
Ethoxy and Methoxy Fatty Acids
The fatty acids whose spectra are illustrated here were formed as artefacts while attempting to hydrolyse or methylate brominated fatty acids produced from brominated hydrocarbons by microorganisms (with Dr Jack Hamilton of Queens University, Belfast). None of these spectra have been published elsewhere.
Picolinyl 17-ethoxy-heptadec-9-enoate -
The double bond in position 9 is easily recognized by the gap of 26 amu between m/z = 234 and 260 and the doublet of ions at m/z = 274 and 288 (see the section on picolinyl esters of monoenes in this website). Similarly the terminal ethoxyl group is easily characterized by the gap of 29 amu for the loss of the ethyl moiety, between the molecular ion (m/z = 403) and that at m/z = 374; a further gap of 16 amu to m/z = 358 represents the loss of the oxygen.
DMOX derivative of 17-ethoxy-heptadec-9-enoate -
In this instance, the double bond in position 9 is easily located by the gap of 12 amu between m/z = 196 and 208 (see the section on DMOX derivatives of monoenes in this website). However, the fragmentations around the terminal ethoxyl moiety are less clear cut than with picolinyl esters, because of the competing reaction for loss of a methyl group from the dimethyloxazoline ring (Hamilton, J.T.G. and Christie, W.W., Chem. Phys. Lipids, 105, 93-104 (2000)). However, the gaps of 29 amu and 16 amu from the molecular ion for the ethyl and oxygen moieties, respectively, can be discerned as marked on the spectrum.
The spectrum of picolinyl 15-methoxy-hexadecanoate is unique in our experience in that the base peak does not contain the pyridine moiety, but consists of the terminal fragment (m/z = 59).
After the molecular ion (m/z = 377), there is a gap of 15 amu to m/z = 362 for loss of the methyl group from the methoxyl moiety followed by a gap of 16 amu to m/z = 346 for the loss of the oxygen atom. Then, there is a gap of 28 amu for the loss of carbons 15 and 16. The spectrum thus resembles that of a methyl-branched acid with the methoxyl group considered as part of the linear region of the molecule.
Similarly, with the mass spectrum of picolinyl 17-methoxy-octadec-9-enoate -
The ion at m/z = 59 is again the base peak, and from the molecular ion, gaps of 15, 16 and 28 amu locate the methyl group attached to oxygen, the oxygen atom, and the 'branch-point', respectively. The double bond can be located by the gap of 26 amu between m/z = 234 and 260.
The mass spectrum of the DMOX derivative of 17-methoxy-octadec-9-enoate is -
As with the previous spectrum, following the molecular ion, there is a gap of 15 amu for the loss of the methyl group from the methoxyl moiety to m/z = 350, followed by a gap of 16 amu for the oxygen atom. Thereafter, the spectrum is somewhat complicated, presumably because of competing fragmentations in which methyl groups are lost from the oxazoline ring. The double bond is located by the gap of 12 amu between m/z = 196 and 208.
Spectra of further related 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.
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Updated: 29/8/2007 |
Scottish Crop Research Institute (and MRS Lipid Analysis Unit), Invergowrie, Dundee (DD2 5DA), Scotland
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