Part 3. Dienoic Fatty Acids
Mass Spectra of Pyrrolidine Derivatives of Monoenoic Fatty Acids
Interpretation of the mass spectra of pyrrolidides of dienoic fatty acids is similar to that for monoenes, i.e. it is necessary to look for the gap of 12 amu that locates each double bond. However, it is not always easy to locate the first double bond especially, and then the 'fingerprint' of the appropriate monoene is a useful guide. As with the other pyrrolidides, it is always helpful to have access to an authentic spectrum, as I have never yet found two positional isomers with the same mass spectrum (in the form of the pyrrolidide). It is my general impression that pyrrolidides are less distinctive than DMOX or 3-pyridylcarbinol derivatives when trying to find the key diagnostic ions in an unknown, but they have their own merits. References are listed when we are aware of prior formal publication of spectra in the scientific literature.
Methylene-Interrupted Dienoic Fatty Acids
The mass spectrum of N-octadec-9,12-dienoylpyrrolidine (linoleate or 9,12-18:2 or 18:2(n-6)) follows -
The double bonds in positions 9 and 12 are located by the gaps of 12 amu between ions at m/z = 196 and 208, and 236 and 248, respectively (see our web page on Pyrrolidides of Monoenoic Fatty Acids for details of the rule that applies here). I have to confess that if I was faced with this as an unknown it would be difficult to spot the diagnostic ions. It is always helpful to have an authentic spectrum available.
Details of the mass spectra of the pyrrolidine derivatives of a comprehensive series of isomeric methylene-interrupted octadecadienoates (4,7- to 14,17-18:2) have been published, together with those of some trienoic acids, but data for most are in tabular form (Andersson et al., 1975). Spectra of many of these dienes are now illustrated below for the first time. When the first double bond is relatively close to the carboxyl group, we have to compare with the spectrum of the corresponding monoene but considered simply as a ‘fingerprint’ rather than attempting an interpretation from the fragmentation rule.
Pyrrolidide of 4,7-octadecadienoate (4,7-18:2) -
The double bond in position 4 is not readily located from first principles, although the ions at m/z = 139 and 159 are a useful fingerprint this position That in position 7 is detected by the gap of 12 amu between m/z = 166 and 178. As the following ions are all 14 amu apart, the first double bond must be in positions 3 to 5 at least.
Pyrrolidide of 5,8-octadecadienoate (5,8-18:2) -
The double bond in position 5 is located by the weak fingerprint ions at m/z = 140 to166, while a gap of 12 amu (barely discernible) between m/z = 180 and 192 locates the double bond in position 8. Again, all the ions in the higher mass range are 14 amu apart, confirming that there are no double bonds beyond carbon-9 in the molecule.
Pyrrolidide of 6,9-octadecadienoate (6,9-18:2) -
The double bonds in positions 6 and 9 are located by the gaps of 12 amu between m/z = 154 and 166, and 194 and 206, respectively.
Pyrrolidide of 7,10-octadecadienoate (7,10-18:2) -
The double bonds in positions 7 and 10 are located by the gaps of 12 amu between m/z = 168 and 180, and 208 and 220, respectively.
Pyrrolidide of 8,11-octadecadienoate (8,11-18:2) -
The double bonds in positions 8 and 11 are located by the gaps of 12 amu between m/z = 182 and 194, and 222 and 234, respectively.
Pyrrolidide of 10,13-octadecadienoate (10,13-18:2) -
The double bonds in positions 10 and 13 are located by the gaps of 12 amu between m/z = 210 and 222, and 250 and 262, respectively.
Pyrrolidide of 11,14-octadecadienoate (11,14-18:2) -
The double bonds in positions 11 and 14 are located by the gaps of 12 amu between m/z = 224 and 236, and 264 and 272, respectively, but they would not be easy to detect from first principles. Here it is again useful to have the authentic spectrum available. Indeed, from now on it is perhaps better to treat the spectra simply as fingerprints (no further interpretation is offered here). At least, it is easy to determine that there are no double bonds before carbon-12 from the regular series of ions 14 amu apart.
Pyrrolidide of 12,15-octadecadienoate (12,15-18:2) -
Pyrrolidide of 13,16-octadecadienoate (13,16-18:2) -
Pyrrolidide of 14,17-octadecadienoate (14,17-18:2) -
Conjugated Dienoic Fatty Acids
The mass spectrum of the pyrrolidide of the conjugated diene 9-cis,11-trans-octadecadienoate (a 'CLA' isomer) is illustrated next (Iversen et al., 1984)-
In this instance, the ions in the high mass range have not been magnified, and there is an especially abundant molecular ion. However, it is not easy to locate the double bonds with confidence using the established rules. All the isomers we have examined have very similar mass spectra. DMOX derivatives are certainly much more useful with conjugated fatty acids.
Bis-Methylene-Interrupted Dienoic Fatty Acids
Fatty acids with double bonds in the 5,9-position, i.e. with two methylene groups between the double bonds (or metabolites of these), are common in marine sponges and in plants of the Gymnosperm family. They display rather distinctive fragmentations that serve to locate the double bonds. Many of these spectra have not been published formally elsewhere
The mass spectrum of the pyrrolidide of 5,9-hexadecadienoate (5,9-16:2) (Carballeira and Maldonado, 1986)-
As with all derivatives with a double bond in position 5, the ions in the higher mass range are not very abundant, but a distinctive ion at m/z = 180 is formed by cleavage at the centre of the bis-methylene-interrupted double bond system. An analogous fragmentation is found with all the nitrogenous derivatives of fatty acids. The gap of 12 amu between the ions at m/z = 194 and 206 confirm also that there is a double bond in position 12.
The mass spectrum of the pyrrolidide of 7,11-octadecadienoate (7,11-18:2) -
The diagnostic ion for cleavage at the centre of the bis-methylene-interrupted double bond system has now shifted up 28 amu to m/z =208. It might be easy to confuse the location of the other double bonds by looking for the gaps of 12 amu, but that hardly matters with such a distinctive spectrum.
The mass spectrum of the pyrrolidide of 9,13-eicosadienoate (9,13-20:2) -
The diagnostic ion for cleavage at the centre of the bis-methylene-interrupted double bonds is now at m/z = 236, with further evidence for location of the double bonds in positions 9 and 13 being provided by gaps of 12 amu between m/z = 196 and 208, and 250 and 262, respectively.
Poly-Methylene-Interrupted Dienoic Fatty Acids
Fatty acids with multiple methylene groups between the double bonds are relatively rare in nature, but they do occur, especially in marine organisms and some seed oils. The mass spectrum of the pyrrolidide of 5,13-docosadienoate (5,13-22:2) from meadowfoam oil is -
Only by magnifying the ions in the higher mass region can the ions at m/z = 140, 153 and 166 characteristic of a double bond in position 5 be seen. The fact that magnification is necessary is in itself typical for a 5 double bond. The gap of 12 amu between m/z = 250 and 262 locates the double bond in position 13.
Mass spectrum of the pyrrolidide of 7,15-docosadienoate (7,15-22:2) from a marine invertebrate, Rapana thomasiana, -
The gaps of 12 amu between m/z = 168 and 180, and between m/z = 278 and 290, locate the double bonds in positions 7 and 15, respectively.
Mass spectra of pyrrolidides of many more dienoic acids of various chain lengths can be found in our Archive Section, but without interpretation. Most of these have not been published elsewhere.
- Andersson, B.A., Christie, W.W. and Holman, R.T. Mass spectrometric determination of positions of double bonds in polyunsaturated fatty acid pyrrolidides. Lipids, 10, 215-219 (1975) (DOI: 10.1007/BF02532483).
- Carballeira, N.M. and Maldonado, L. Identification of 5,9-hexadecadienoic acid in the marine sponge Chondrilla nucula. Lipids, 21, 470-471 (1986) (DOI: 10.1007/BF02536406).
- Iversen, S.A., Cawood, P., Madigan, M.J., Lawson, A.M. and Dormandy, T.L. Identification of a diene conjugated component of human lipid as octadeca-9,11-dienoic acid. FEBS Letts, 171, 320-324 (1984) (DOI: 10.1016/0014-5793(84)80512-8).
Updated November 14, 2013