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Mass Spectrometry of Fatty Acid Pyrrolidides


Dienoic Fatty Acids. Part 1. Methylene-Interrupted Dienes



Interpretation of the mass spectra of pyrrolidides of dienoic fatty acids is similar to that discussed in some detail in the web page here dealing with 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. The web page on pyrrolidides of saturated fatty acids contains more introductory and mechanistic information, together with links to pages dealing with additional practical methodology (sample concentration, derivative preparation, etc). Part 2 describes the spectra of the pyrrolidides of most other dienoic acids.


Methylene-Interrupted Octadecadienoates

Fortunately, we had available a series of positional isomers of methylene-interrupted octadecadienoic acids (4,7- to 14,17-18:2), which had been prepared some years ago by total synthesis (Christie and Holman, 1967). Information on the mass spectra of the pyrrolidides of these have been published, together with details of some trienoic acids, but data for most are in tabular form and a few spectra only were illustrated (Andersson, B.A. et al., 1975). Therefore, spectra of many of these dienes are now illustrated below for the first time. The mass spectrum of N-octadec-9,12-dienoylpyrrolidine (linoleate or 9,12-18:2 or 18:2(n-6)) follows -

Mass spectrum of N-octadec-9,12-dienoylpyrrolidine

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.

When the first double bond is relatively close to the carboxyl group, we have to compare with the spectrum of the corresponding monoene while considering it simply as a ‘fingerprint’ rather than attempting an interpretation from the fragmentation rule.

Pyrrolidide of 4,7-octadecadienoate (4,7-18:2) -

Mass spectrum of the pyrrolide of 4,7-octadecadienoate

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 for 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) -

Mass spectrum of the pyrrolide of 5,8-octadecadienoate

The double bond in position 5 is located by the weak fingerprint ions at m/z = 140 to 166, 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) -

Mass spectrum of the pyrrolide of 6,9-octadecadienoate

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) -

Mass spectrum of the pyrrolide of 7,10-octadecadienoate

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) -

Mass spectrum of the pyrrolide of 8,11-octadecadienoate

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) -

Mass spectrum of the pyrrolide of 10,13-octadecadienoate

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) -

Mass spectrum of the pyrrolide of 11,14-octadecadienoate

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. 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. From now on it is perhaps better to treat the spectra simply as fingerprints (no further interpretation for the next two at least is offered here).

Pyrrolidide of 12,15-octadecadienoate (12,15-18:2) -

Mass spectrum of the pyrrolide of 12,15-octadecadienoate

Pyrrolidide of 13,16-octadecadienoate (13,16-18:2) -

Mass spectrum of the pyrrolide of 13,16-octadecadienoate

Pyrrolidide of 14,17-octadecadienoate (14,17-18:2) -

Mass spectrum of the pyrrolide of 14,17-octadecadienoate

In this last example, the terminal double bond is defined by the ion beta to the double bond at m/z = 292, as in the spectra of pyrrolidides of terminal monoenes.


Some Other Methylene-Interrupted Dienes

Mass spectra of pyrrolidides of methylene-interrupted dienes of different chain-lengths are of course interpreted in the same way as the next examples show. The diagnostic ions are indicated on the spectra, and it should be evident that they are the same as on the corresponding C18 analogues. For example, the pyrrolidide of 9,12-hexadecadienoate (9,12-16:2 or 16:2(n-4)) -

Mass spectrum of the pyrrolidide of 9,12-hexadecadienoate

The pyrrolidide of 11,14-eicosadienoate (11,14-20:2 or 20:2(n-6)) -

Mass spectrum of the pyrrolidide of 11,14-eicosadienoate

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.


References


I also recommend - Christie, W.W. and Han, X. Lipid Analysis - Isolation, Separation, Identification and Lipidomic Analysis (4th edition), 446 pages (Oily Press, Woodhead Publishing and now Elsevier) (2010) - at Science Direct.


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