Epoxy, Furanoid and Ethoxy Fatty Acids
As with my other documents on mass spectrometry, this is a subjective account that details representative examples only of those relevant fatty acids encountered during our research activities and for which we have spectra available for illustration purposes. Spectra of methyl esters, 3-pyridylcarbinol ('picolinyl') esters, pyrrolidides and DMOX derivatives are all described here (when available), but I will only discuss 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, and my general philosophy is to always to prepare more than one derivative. There are too many gaps in this account to make such a decision or even to discuss the topic 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 to our knowledge, but prior publications are cited when known.
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 -
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) (Ryhage and Stenhagen, 1960 ).
Mass spectra of 3-pyridylcarbinol esters of epoxy fatty acids were first described by Balazy and Nies (1989) . The mass spectrum of 3-pyridylcarbinyl 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 cyclopropyl 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. With methyl esters, many analysts have resorted to ring opening procedures prior to mass spectrometry as a better means of locating the epoxide ring.
The mass spectrum of 3-pyridylcarbinyl 12,13-epoxy-octadec-9-enoate or vernolate (Balazy and Nies, 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 3-pyridylcarbinol 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. Note that special methods are required for successful preparation of the derivative because of the sensitivity of the epoxyl moiety.
Spectra of DMOX derivatives of epoxy fatty acids have been described elsewhere (Marx and Classen, 1994 ). The mass spectrum of the DMOXderivative of 12,13-epoxy-octadec-9-enoateor 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 again best regarded simply as a fingerprint (see Kleiman and Spencer, 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
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 furanoid fatty acid, 8-(5-hexyl-2-furyl)-octanoic or 9,12-epoxy-octadec-9,11-dienic acid, has also been found in plants. It is formed also on oxidation of conjugated dienoic acids, e.g. 9-cis,11-trans-octadecadienoic acid and isomers. The material used for the first set of 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 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 3-pyridylcarbinol ester derivative of 8-(5-hexyl-2-furyl)-octanoate (spectrum not published elsewhere) -
As is usual with 3-pyridylcarbinol 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 3-pyridylcarbinol esters for characterization purposes.
That said, methyl ester spectra are surprisingly informative. For example, the furanoid ring in most of the natural furanoid fatty acids, including those present as minor components of fish oils from which the next examples are drawn, can contain either one or two methyl substituents in the furanoid ring. The spectrum of methyl 10,13-epoxy-11-methyl-octadecadienoate is –
- and that of methyl 10,13-epoxy-11,12-dimethyl-octadecadienoate is –
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). Glass et al. (1975)appear to be the first to illustrate such spectra. The position of the methyl branch in the mono-methyl compound cannot be determined from the spectrum, but this can be accomplished following hydrogenation.
Mass spectra of pyrrolidide and DMOX derivatives of natural furanoid acids also have useful mass spectra, (such spectra do not appear to have been published formally elsewhere). The mass spectrum of the pyrrolidide of 10,13-epoxy-11-methyl-octadecadienoateis illustrated next –
Unusually for a nitrogen-containing derivative the base ion is not the McLafferty ion at m/z = 113, but the fragment from the terminal region of the molecule at m/z = 165, as in the spectrum of the methyl ester derivative above. The ion at m/z = 109 is presumably the same as that in the spectrum of the methyl ester also. On the other hand, there are indeed ions that contain the pyrrolidine ring and locate the furanoid ring structure as marked on the spectrum. In addition, there is an intriguing ion at m/z = 248, which could be interpreted as a fragment containing carbons 10 and 11, the latter with its associated methyl group. Spectra of more isomers would be required to confirm this, but it may be a useful method for locating ring methyl groups in unknowns.
Essentially the same feature are seen in the spectrum of the DMOX derivative of 10,13-epoxy-11-methyl-octadecadienoate , except that ions for cleavage beta to the ring and containing the carboxyl moiety are now more prominent (that at m/z = 165 is no longer the base ion).
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.
3-Pyridylcarbinyl 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 3-pyridylcarbinol 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 3-pyridylcarbinol esters, because of the competing reaction for loss of a methyl group from the dimethyloxazoline ring (Hamilton and Christie, 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 3-pyridylcarbinyl 15-methoxy-hexadecanoate is unique in our experience of this type of derivative 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 3-pyridylcarbinyl 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 -- 3-pyridylcarbinol ('picolinyl') esters -- DMOX derivatives -- pyrrolidides.
- Balazy, M. and Nies, A.S. Characterization of epoxides of polyunsaturated fatty acids by mass spectrometry via 3-pyridinylmethyl esters.Biomed. Environ. Mass Spectrom., 18, 328-336 (1989) (DOI: 10.1002/bms.1200180508).
- Glass, R.L., Krick, T.P., Sand, D.M., Rahn, C.R. and Schlenk, H. Furanoid fatty acids from fish lipids . Lipids, 10, 695-702 (1975) (DOI: 10.1007/BF02532763).
- Hamilton, J.T.G. and Christie, W.W. Mechanisms for ion formation during the electron impact-mass spectrometry of picolinyl ester and 4,4-dimethyloxazoline derivatives of fatty acids . Chem. Phys. Lipids, 105, 93-104 (2000) (DOI: 10.1016/S0009-3084(99)00133-4).
- Kleiman, R. and Spencer, G.F. Gas chromatography-mass spectrometry of methyl esters of unsaturated oxygenated fatty acids . J. Am. Oil Chem. Soc., 50, 31-38 (1973) (DOI: 10.1007/BF02886864).
- Marx, F. and Classen, E. Analysis of epoxy fatty acids by GC-MS of their dimethyloxazoline derivatives . Fett/Lipid, 96, 207-211 (1994).
- Ryhage, R. and Stenhagen, E. Mass spectrometric studies. VI. Methyl esters of normal chain oxo-, hydroxy-, methoxy- and epoxy-acids . Arkiv Kemi, 15, 545-574 (1960).
- Yurawecz, M.P., Hood, J.K., Mossoba, M.M., Roach, J.A.G. and Ku, Y. Furan fatty acids determined as oxidation products of conjugated octadecadienoic acid.Lipids, 30, 595-598 (1995) (DOI: 10.1007/BF02536995).
Updated March 11, 2014