Cyclic Fatty Acids with 5/6-Membered Rings
Mass Spectrometry of Fatty Acid Derivatives
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, 3-pyridylcarbinol ('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 have been reviewed by Sébédio and Grandgirard (1989).
ω-Cyclopentenyl and Cyclopentyl 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 -
The base peak at m/z = 67 is presumed to be the ionized cyclopentene ring per se, but no corresponding fragment at m/z = 199 is detectable. Instead, an ion at m/z = 185 represents the remainder of the molecule with cleavage beta to the ring (together with an ion at m/z = 153 for loss of a methoxyl group from this ion). In the high mass range, the molecular ion is small but distinct, and there is an ion representing loss of a methoxyl group from this (at m/z = 235).
In the mass spectrum of methyl gorlate (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-cyclopentylundecanoate, i.e. with a saturated ring, is somewhat different –
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 (i.e. cleavage alpha to the ring). Otherwise, the spectrum resembles that of a normal saturated ester (other than the fact that the molecular ion is two units lower), and the McLafferty ion at m/z = 74 is now the base peak, for example.
Once more, in my opinion, 3-pyridylcarbinol esters are by far the best for characterization of fatty acids with terminal cyclopentenyl and cyclopentyl moieties, especially when these are on the terminal carbon (Christie et al., 1989). For example, the mass spectrum of 3-pyridylcarbinyl 11-cyclopentylundecanoate is illustrated next -
In the high mass region, the molecular ion (at m/z = 345) is followed by a relatively 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 -
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, 3-pyridylcarbinol esters work best with cyclopentenyl fatty acids, as illustrated with the mass spectrum of 3-pyridylcarbinyl 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 -
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), a problem with all terminally substituted fatty acids. 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 -
In the high mass region, the gap of 67 amu between m/z = 238 and 305 for cleavage of the cyclopentenyl 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 3-pyridylcarbinol esters are in our Archive pages (without interpretation). The only isomer where identification might be problematic is 3-pyridylcarbinyl 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 3-pyridylcarbinol 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, while the gap of 67 amu between m/z = 264 and 331 defines the cyclopentene ring..
DMOX derivative of 15-cyclopent-2-enylpentadec-9-enoate or hormelate -
With the last, 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, 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 is in position 8 rather than 9 of the chain. This does not occur with the 3-pyridylcarbinol 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.
- 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 from the fingerprint ions 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 -
In this example, both the positions of the ring and of the double bond can be located as indicated on the spectrum.
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 -- 3-pyridylcarbinol ('picolinyl') esters -- DMOX derivatives -- pyrrolidides.
ω-Cyclohexyl Fatty Acids
ω-Cyclohexylundecanoic acid is a minor component of cow's milk fat, although 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) -
The ion at m/z = 199 defines the position of the cyclohexane ring as illustrated, via fragmentation adjacent to the ring (as with the cyclopentyl fatty acids above). The other fragment ion at m/z = 83 is also prominent. The ion 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-phenyltridecanoate, which is usually the main component, is -
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, which is typical for spectra of aromatic compounds. The ion at m/z = 181 is presumably formed by loss of a tropylium from the ion representing [M-32]+.
3-Pyridylcarbinyl 13-phenyl-tridecanoate has the mass spectrum -
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.
When there is a double bond in the alkyl chain, the spectra of the methyl esters still have the tropylium ion or that plus a methyl group (m/z = 104) as the base ion, but as might be expected, there is nothing that helps to locate the double bond. This is possible with 3-pyridylcarbinol ester, DMOX and pyrrolidide derivatives (see the web pages dealing with the mass spectra of the appropriate monoenes for discussion).
Mass spectrum of 3-pyridylcarbinyl 15-phenyl-pentadec-9-enoate –
In addition to the ions that locate the phenyl moiety, the position of the double bond in the alkyl chain is easily determined from the ions highlighted.
Mass spectrum of the DMOX derivative 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 (to m/z = 278), as illustrated above, and the tropylium ion at m/z = 91 is distinct. However, the spectrum of the DMOX derivatives is confused 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 the aromatic ferulic and cafeic acids are in the various Archive pages, i.e. for methyl esters -- 3-pyridylcarbinol esters -- DMOX derivatives -- pyrrolidides.
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. 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. The chemistry and analyses of these compounds are discussed in our Frying Oil web pages.
- Christie, W.W. 13-Phenyltridec-9-enoic and 15-phenylpentadec-9-enoic acids in Arum maculatum seed oil. Eur. J. Lipid Sci. Technol., 105, 779-780 (2003) (DOI: 10.1002/ejlt.200300865).
- Christie, W.W., Brechany, E.Y. and Shukla, V.K.S. Analysis of seed oils containing cyclopentenyl fatty acids by combined chromatographic procedures. Lipids, 24, 116-120 (1989) (DOI: 10.1007/BF02535247).
- Christie, W.W., Rebello, D. and Holman, R.T. Mass spectrometry of derivatives of cyclopentenyl fatty acids. Lipids, 4, 229-231 (1969) (DOI: 10.1007/BF02532636).
- 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).
- Schogt, J.C.M. and Begemann, P.H. Isolation of 11-cyclohexylundecanoic acid from butter. J. Lipid Res., 6, 466-470 (1965).
- 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) (DOI: 10.1016/0163-7827(89)90003-9).
- Zhang, J.Y., Wang, H.Y., Yu, Q.T., Yu, X.J., Liu, B.N. and Huang, Z.H. The structures of cyclopentenyl fatty acids in the seed oils of Flacourtiaceae species by gas chromatography-mass spectrometry of their 4,4-dimethyloxazoline derivatives. J. Am. Oil Chem. Soc., 66, 242-246 (1989) (DOI: 10.1007/BF02546068).
Updated March 6, 2014