The Lipid Library

MASS SPECTRA OF DERIVATIVES OF CYCLOPROPYL AND CYCLOPROPENYL FATTY ACIDS

The document does not aim to be a complete account of mass spectrometry of cyclopropyl and cyclopropenyl fatty acids, but rather is a personal account of our experience of those encountered during our research activities and for which we have spectra available for illustration purposes. Spectra of methyl esters, picolinyl esters, DMOX derivatives and pyrrolidides are described in the same document. Where we are aware of prior illustrations of mass spectra in the literature, the appropriate papers are cited. These notes are a practical guide rather than a mechanistic account. The occurrence and biological properties of cyclic fatty acids were reviewed by Sébédio and Grandgirard (1989).

Cyclopropyl Fatty Acids

Cyclopropyl fatty acids are common constituents of bacterial lipids and may accompany cyclopropene fatty acids as minor components of certain seed oils. Mass spectra of methyl esters of cyclopropyl fatty acids are not very informative and the latter appear to rearrange under electron bombardment in the mass spectrometer to give a double bond. Spectra are thus indistinguishable from those of monoenoic fatty acids with an alkyl chain one carbon longer (Christie and Holman, 1966). The mass spectrum of methyl 9,10-methylene-hexadecanoate is illustrated as an example –

The spectrum is in essence identical to that of methyl 9-heptadecenoate. Although there have been suggestions in the literature that subtle differences exist between the mass spectra of cyclopropanes and monoenes that might serve for diagnosis, these are not very convincing to this author. They may work with a limited range of pure model compounds, but are less likely to be of practical value in the analysis of complex mixtures, especially when instrumental variations are taken into account.

Picolinyl esters are by far the most useful derivatives for characterization of cyclopropane fatty acids, as they give distinctive cleavages that permit facile location of the cyclopropane ring. As an example, the mass spectrum of picolinyl 9,10-methylene-octadecanoate is illustrated next.

There are the usual ions in the lower molecular weight region at m/z = 92, 108, 151 and 164, typical of a picolinyl ester, but the [M-1]⁺ ion is more abundant than the molecular ion (m/z = 387) itself. The distinctive ion that permits location of the ring is odd-numbered (uncommon) at m/z = 247, representing cleavage at the ring as shown (Harvey, 1984).

In the spectrum of picolinyl 11,12-methylene-octadecanoate (lactobacillic acid), illustrated next, the distinctive ion has shifted 28 amu as expected to m/z = 275.

DMOX derivatives are of much less value for the structural analysis of cyclopropyl fatty acids, as they appear to undergo a rearrangement to monoenes in a similar manner as occurs with methyl esters (Zhang et al., 1987). Such a simplistic view of the mechanism is improbable but serviceable. This is one of only a few examples where DMOX derivatives fail.

As an example, the mass spectrum of the DMOX derivative of 9,10-methylene-octadecanoate is illustrated -

Although the structure can in theory be deduced from the fact the rearranged double bond in position 9 gives a characteristic gap of 12 amu between m/z = 196 and 208, this presupposes that the analyst is already aware that the fatty acid contains the ring structure - not a double bond (see the webpage dealing with DMOX derivatives of monoenes). On the other hand, gas chromatographic (GC) retention times may indicate that an unusual fatty acid is present.

The DMOX derivative of 11,12-methylene-octadecanoate (or lactobacillate) has the mass spectrum –

Again the position of the ring can be deduced from the gap of 12 amu (between m/z = 224 and 236), assuming we recognize that it is a cyclopropyl acid not a monoene.

The mass spectra of pyrrolide derivatives of cyclopropane fatty acids closely resemble those of the DMOX derivatives, and like them give ambiguous spectra that are almost indistinguishable from those of monoenes one carbon longer in chain-length. The same caveats apply in interpreting the spectra. I am not aware of publication elsewhere.

For example, the mass spectrum of the pyrrolidide of 9,10-methylene-octadecanoate is almost identical to that of 9-nonadecenoate, and the gap of 12 amu between m/z = 196 and 208 is that expected for a double bond in position 9 of the chain.

The mass spectrum of the pyrrolide of 11,12-methylene-octadecanoate (or lactobacillate) is –

Although the position of the ring can in theory be deduced from this from first principles (from the gap of 12 amu between m/z = 224 and 236), in practice it may be better to regard it simply as a fingerprint.

Cyclopropenyl Fatty Acids

It was long thought that GC and thus GC-MS of derivatives of cyclopropenyl fatty acid was impossible because of thermal degradation on the GC column. However, modern capillary columns are relatively inert, and analysis by GC is straightforward, provided that appropriate derivatization methods are employed, i.e. that acidic conditions are avoided.

The mass spectra of methyl ester derivatives of cyclopropenoid fatty acids tend to resemble those of dienoic fatty acids, so methyl sterculate (9,10-methylene-octadec-9-enoate) has a spectrum (see below) that differs in minor ways only from that of methyl nonadecadienoate, and there are no obvious ions that serve to locate the ring (Pawlowski et al., 1974).

The mass spectrum of picolinyl sterculate is distinctive, however, and it is illustrated next (see also Spitzer et al., 1994).

In this instance, the diagnostic cleavages occur on either side of the ring and beta to it, giving distinctive ions at m/z = 220 and 286. There is also an ion that appears to be characteristic at m/z = 293 (though not found apparently by Spitzer et al.). As might be expected, the analogous diagnostic ions in the mass spectrum of picolinyl malvalate (8,9-methylene-heptadec-8-enoate) (not illustrated) are all 14 amu lower.

The mass spectrum of DMOX derivatives are less helpful, and resemble those of an acetylenic fatty acid with the triple bond in position 9 and one carbon longer, presumably because rearrangement occurs in the mass spectrometer (Spitzer, 1991)). For example, the mass spectrum of the DMOX derivative of sterculic acid is illustrated next. The key diagnostic ions are for a gap of 10 amu between m/z = 196 and 208.

The spectrum of the pyrrolidide of sterculate is very similar to this (unpublished), although the ions in the high mass range are less abundant, and it is illustrated below.

The key diagnostic ions are again for a gap of 10 amu between m/z = 196 and 208. The corresponding ions in the mass spectrum of the malvalate derivative are all 14 amu lower.

References

• Christie, W.W. and Holman, R.T. Mass spectrometry of lipids. 1. Cyclopropane fatty acids. Lipids, 1, 176-182 (1966).
• Harvey, D.J. Picolinyl derivatives for the characterization of cyclopropane fatty acids by mass spectrometry. Biomed. Environm. Mass Spectrom., 11, 187-192 (1984).
• Pawlowski, N.E., Eisele, T.A., Lee, D.J., Nixon, J.E. and Sinnhuber, R.O. Mass spectra of methyl sterculate and malvalate and 1,2-dialkylcyclopropenes. Chem. Phys. Lipids, 13, 164-172 (1974).
• Sébédio, J.L. and Grandgirard, A. Cyclic fatty acids: natural sources, formation during heat treatment, synthesis and biological properties. Prog. Lipid Res., 28, 303-336 (1989).
• Spitzer, V. GC-MS (chemical ionization and electron impact modes) characterization of the methyl esters and oxazoline derivatives of cyclopropene fatty acids. J. Am. Oil Chem. Soc., 68, 963-969 (1991).
• Spitzer, V., Marx, F., Maia, J.G.S. and Pfeilsticker, K. The mass spectra of the picolinyl ester derivatives of malvalic and sterculic acid. Fat Sci. Technol., 96, 395-396 (1994).
• Zhang, J.Y., Yu, Q.T. and Huang, Z.H. 2-Substituted 4,4-dimethyloxazolines as useful derivatives for the localisation of cyclopropane rings in long-chain fatty acids. Mass Spectrom., 35, 23-30 (1987).

Updated: 10/5/08 Credits/disclaimer	Scottish Crop Research Institute (and MRS Lipid Analysis Unit), Invergowrie, Dundee (DD2 5DA), Scotland