MASS SPECTRA OF DERIVATIVES OF ALICYCLIC FATTY ACIDS WITH 5- AND 6-MEMBERED RINGS



The document does not aim to be a complete account of mass spectrometry of all cyclic fatty acids, but rather is a personal account of our experience of those natural fatty acids 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. Natural fatty acids with 5- and 6-membered rings all have the ring structure in the omega or terminal position. The occurrence and biological properties of cyclic fatty acids were reviewed by Sébédio and Grandgirard (1989).


ω-Cyclopentyl and Cyclopentenyl Fatty Acids

Fatty acids with a terminal cyclopent-2-enyl moiety are found in high concentration in the seed oils of several species from the plant family Flacourtiaceae, though the corresponding cyclopentyl (saturated species) have been detected at low levels only. It is worth noting that although the nitrogen-containing derivatives permit location of most of the structural features with varying degrees of success, none fix the actual position of the double bond within the ring. For the latter purpose, chemical degradation, perhaps allied with mass spectrometry, is required (Christie et al., 1989).

Again the mass spectra of the methyl ester derivatives have only limited value for characterization purposes, although the ring structure itself can be detected and confirmed (Christie et al., 1969). Thus, the mass spectrum of methyl hydnocarpate (11-cyclopentenylundecanoate) is -

Mass spectrum of methyl hydnocarpate (11-cyclopentenylundecanoate)

The base peak at m/z = 67 is presumed to be the ionized cyclopentene ring per se. In the high mass range, the molecular ion is small but distinct and an ion at m/z = 185 represents cleavage beta to the ring (together with an ion at m/z = 153 for loss of a methoxyl group from this ion). There is also an ion representing loss of a methoxyl group from the molecular ion (at m/z = 235).

In the mass spectrum of methyl gorlate (13-cyclopent-2-enyltridec-6-enoate) -

Mass spectrum of methyl 13-cyclopent-2-enyltridec-6-enoate

- the base peak is again at m/z = 67, but the ion representing cleavage beta to the ring (at m/z = 210) is barely distinguishable in this instance. There is no feature that serves to locate the double bond in the alkyl chain.

The mass spectrum of methyl 11-cyclopentanylundecanoate, i.e. with a saturated ring, is somewhat different –

Mass spectrum of methyl 11-cyclopentanylundecanoate

An ion representing the ring fragment (m/z = 69) does stand out, as does an ion for the loss of the ring at m/z = 199. Otherwise, the spectrum resembles that of a normal saturated ester, and the McLafferty ion at m/z = 74 is now the base peak, for example.

Once more, in my opinion, picolinyl esters are by far the best for characterization purposes, both for cyclopentenyl and cyclopentyl fatty acids (Christie et al., 1989). For example, the mass spectrum of picolinyl 11-cyclopentylundecanoate is illustrated next -

Mass spectrum of picolinyl 11-cyclopentylundecanoate

In the high mass region, the molecular ion (at m/z = 345) is followed by a clear gap of 69 amu to an ion at m/z = 276, representing loss of the cyclopentane ring. Thereafter, there is a regular series of ions 14 amu part for cleavage at the successive methylene groups. The ion for the charged cyclopentane ring (m/z = 69) is also distinctive.

In contrast, there is the spectrum of the DMOX derivative of 11-cyclopentylundecanoate -

Mass spectrum of the DMOX derivative of 11-cyclopentylundecanoate

Here, one needs to use the eye of a true believer to see the gap of 69 amu, between the molecular ion and that at m/z = 238, for a terminal cyclopentane ring (Zhang et al., 1989). The problem is that the facile loss of methyl groups from the DMOX ring results in series of ions in the high mass range (m/z = 264, 278 and 292) that confound the picture (Hamilton and Christie, 2000).

Similarly, picolinyl esters work best with cyclopentenyl fatty acids, as illustrated with the mass spectrum of picolinyl hydnocarpate -

Mass spectrum of picolinyl hydnocarpate

Here the gap of 67 amu between the molecular ion and that at m/z = 276 is clearly diagnostic for loss of the cyclopentene ring, while the ionized cyclopentene ring (m/z = 67) is in fact the base peak.

In contrast, there is the spectrum of the DMOX derivative of hydnocarpate -

Mass spectrum of the DMOX derivative of hydnocarpate

In fairness, the gap of 67 amu (m/z = 238 to 305) for loss of the terminal ring is a little more convincing, although interpretation is again confounded by ions that result from loss of methyl groups from the DMOX ring (c.f. m/z = 290). The ion representing the cyclopentene ring itself (m/z = 67) is certainly more abundant in this instance.

However, the pyrrolidide derivative of hydnocarpic acid has a much more useful spectrum -

Mass spectrum of the pyrrolidide derivative of hydnocarpic acid

In the high mass region, the gap of 67 amu between m/z = 238 and 305 for cleavage of the cyclopentyl ring can be seen unambiguously. In all other respects, the spectrum is very similar to that of the corresponding DMOX derivative. We also have the spectrum of the C18 analogue in the Archive pages. Neither of these spectra appears to have been published formally elsewhere.

Three main isomers with one double bond in the chain are also found in nature and the spectra of the picolinyl esters are in our Archive pages (without interpretation). The only isomer where identification might be problematic is picolinyl 13-cyclopent-2-enyltridec-4-enoate -

Mass spectrum of picolinyl 13-cyclopent-2-enyltridec-4-enoate

The cyclopentene ring is located by the gap of 67 amu in the high mass region of the spectrum as before. The double bond in position 4 is recognized by the fingerprint ions at m/z = 205 and 218, together with the ion at m/z = 164 of low intensity relative to that at m/z = 151 (see the original reference (Christie et al., 1989) or the section of the mass spectrometry pages on picolinyl esters of monoenes.

Spectra of the DMOX derivatives of two of the natural cyclopentenyl fatty acids with double bonds in the chain acids follow, starting with that of 13-cyclopent-2-enyltridec-4-enoate. Fingerprint ions at m/z = 152 and 166 locate the double bond.

Mass spectrum of the DMOX derivative of 13-cyclopent-2-enyltridec-4-enoate

DMOX derivative of 15-cyclopent-2-enylpentadec-9-enoate or hormelate -

Mass spectrum of the DMOX derivative of 15-cyclopent-2-enylpentadec-9-enoate

Definitive location of the cyclopentene ring is again problematic, though it should be recognized that each derivative gives a distinctive fingerprint at least (Zhang et al., 1989). To define the location of the double bond in the chain, check the original reference or see the section of this web site on Mass spectrometry of DMOX derivatives of monoenes. However, with the last example, it would be easy to conclude from an apparent gap of 12 amu between m/z = 182 and 194 that the double bond was in position 8 rather than 9 of the chain. This does not occur with the picolinyl ester or pyrrolidide. This does not mean that one derivative is better than another, other than in one particular circumstance - but there are times when confirmatory evidence is desirable by using another derivative type.

The pyrrolidide derivatives of terminal cyclopent-2-enyl fatty acids give mass spectra that resemble those of the DMOX derivatives. However, the position of the ring is more clearly defined with the former. For example, in the spectrum of the pyrrolidide of 13-cyclopent-2-enyltridec-4-enoate.

Mass spectrum of the pyrrolide of 13-cyclopent-2-enyltridec-4-enoate

- the gap of 67 amu between m/z = 264 and 331 clearly locates the terminal ring structure, and there are no ions in the region that confound the picture. The double bond is located as in the analogous monoene (see our web pages on Pyrrolidine derivatives of monoenoic fatty acids).

Pyrrolidine derivative of 15-cyclopent-2-enylpentadec-9-enoate or hormelate -

Mass spectrum of the pyrrolidide of 15-cyclopent-2-enylpentadec-9-enoate

Spectra of further natural cyclic 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.

ω-Cyclohexyl Fatty Acids

ω-Cyclohexylundecanoic acid is a minor component of cow's milk fat, though it probably originates in rumen bacteria, and this was the source for the mass spectrum of methyl 11-cyclohexylundecanoate (first published by Schogt and Begemann, 1965) -

Mass spectrum of methyl 11-cyclohexylundecanoate

The ion at m/z = 199 defines the position of the cyclohexane ring as illustrated, via fragmentation adjacent to the ring. That at m/z = 239 ([M-43]+) is common in saturated esters and represents a complex rearrangement involving expulsion of carbons C2 to C4, while that at m/z = 251 is produced by the loss of the methoxyl group.

ω-Phenyl Fatty Acids

Fatty acids with a terminal phenyl moiety are found in seed oils of the subfamily Aroideae of the Araceae and certain species of bacteria. Some of the available data have been published (Christie, 2003). The mass spectrum of methyl 13-phenyl-tridecanoate, which is usually the main component, is -

Mass spectrum of methyl 13-phenyl-tridecanoate

There is a distinct molecular ion at m/z = 304, and an abundant ion at m/z = 272 reflects the loss of methanol. However, the base peak at m/z = 91 is a tropylium ion that is typical for the spectra of aromatic compounds. The ion at m/z = 181 is presumably formed by loss of tropylium from the ion representing [M-32]+.

Formula of a tropylium ion

Picolinyl 13-phenyl-tridecanoate has the mass spectrum -

Mass spectrum of picolinyl 13-phenyl-tridecanoate

The distinctive and diagnostic feature is a gap of 91 amu between m/z = 290 and 381 (M+) for loss of the terminal phenyl group together with carbon-13, presumably as the stable tropylium ion. Thereafter, there are sequential gaps of 14 amu for loss of successive methylene groups in the aliphatic chain. The tropylium ion per se is the base peak.

Mass spectrum of picolinyl 15-phenyl-pentadec-9-enoate –

Mass spectrum of picolinyl 15-phenyl-pentadec-9-enoate

Mass spectrum of the DMOX derivative of 15-phenyl-pentadec-9-enoate –

Mass spectrum of the DMOX derivative of 15-phenyl-pentadec-9-enoate

Mass spectrum of the pyrrolidide of 15-phenyl-pentadec-9-enoate –

Mass spectrum of the pyrrolidide of 15-phenyl-pentadec-9-enoate

Mass spectra of pyrrolidides and DMOX derivatives also exhibit the gap of 91 amu between the molecular ion and the next abundant ion in the high mass region, as illustrated above, although the ion at m/z = 91 is not as marked. However, the spectrum of the DMOX derivatives is a little confusing in the high mass range, because of fragmentations involving the methyl group on the heterocyclic ring.

When there is a double bond in the aliphatic chain in addition to the terminal benzene ring, it can be located by means of the nitrogen-containing derivatives, or by preparing dimethyl disulfide derivatives (c.f. the spectra of the appropriate monoenes). However, in this instance the spectrum of the pyrrolidide could be confusing as a gap of 12 amu between m/z = 182 and 194 suggests a double bond in position 8 rather than 9. Again, this illustrates the value of using more than one derivative type for identification purposes.

Spectra of more fatty acids of this type together with those of derivatives of ferulic and cafeic acids are in the various Archive pages.

Cyclic Fatty Acids Formed in Frying Oils

Fatty acids with internal five- and six-membered ring structures are formed when vegetable oils are heated to the high temperatures attained during frying or the physical refining process. The mechanism of cyclization is now believed to involve concerted thermal rearrangements with loss of one of the double bonds (Destaillats and Angers, 2005). As a host of basic structures can be formed from different fatty acid precursors, and cis-trans isomerization of double bonds and/or about the ring also takes place, the range of cyclic products is very great. With my colleagues and collaborators, we have characterized most of these, and of course others have worked on the problem. However, interpretation of the spectra is likely to be a minority interest for the moment (reviewed by Dobson (1998)).


References


W.W. Christie

Updated: 11/5/2008

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Scottish Crop Research Institute (and MRS Lipid Analysis Unit), Invergowrie, Dundee (DD2 5DA), Scotland

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