Mass Spectrometry of Dimethyloxazoline and Pyrrolidine Derivatives
Dicarboxylic Fatty Acids
Dicarboxylic fatty acids
are not often present as such in nature, although they are occasionally found in plants and especially in the suberin and cutin in plant cuticles
and are readily obtainable as major components of cork, i.e., a product of the outer layer of the bark of the cork oak (Quercus suber).
Short-chain dicarboxylic acids may be encountered in the oxidative degradation of lipids.
Commercial standards were used to obtain some of the mass spectra that follow, though others were obtained from cork and other cutin hydrolysates.
I am not aware of illustrations of any of these elsewhere, but relevant publications are cited where we know of them.
The web pages on pyrrolidides and DMOX
derivatives of monocarboxylic saturated fatty acids contains more introductory and mechanistic information,
together with links to pages dealing with additional practical methodology (sample concentration, derivative preparation, etc.).
Spectra for methyl esters and
3‑pyridylcarbinol esters of dicarboxylic acids are described in separate documents.
Provided that the analyst is aware of some of the oddities in the mass spectra of DMOX derivatives and has access to authentic spectra, these appear to be the best practical choice for the characterization of unsaturated dicarboxylic acids because of their lower polarity and molecular weights in comparison to pyrrolidides and 3-pyridylcarbinol derivatives.
4,4-Dimethyloxazoline (DMOX) Derivatives
Di-DMOX derivatives of dicarboxylic acids would be expected to have spectra that closely resemble those of the corresponding pyrrolidides, as is the case with most mono-carboxylic acids. By coincidence, DMOX derivatives and pyrrolidides have the same molecular weight despite the difference in structures, and the mass spectrometric fragmentation mechanisms are usually very similar. DMOX derivatives are easy to prepare and to subject to gas chromatography, but some aspects of their mass spectra are puzzling with dicarboxylic derivatives. The mass spectrum of the di-DMOX derivative of 1,9-nonanedioate (azeleate) is -
As there are two nitrogen atoms in the molecule, the molecular ion should be even-numbered, but in this instance, it is protonated and so is odd-numbered. The base ion at m/z = 182, equivalent to [M−112]+, represents the loss of the McLafferty ion, but there is no ion for [M−97]+ as in the spectrum of the analogous pyrrolidide. The ion at m/z = 279 represents the loss of a methyl group from the ring structures, and this can be a source of confusion when fatty acids have terminal functional groups (Hamilton and Christie, 2000). This ion is not present in the spectra of pyrrolidides (see below). Ions at m/z = 207 and 223, representing losses of 87 and 71 amu, respectively, from the molecular ion are not normally found in the spectra of DMOX derivatives, and they must be formed by complex rearrangements involving the ring structures, which have yet to be explained. I am content to let others speculate.
Comparable features are found in the spectra of the DMOX derivatives of all the other saturated dicarboxylic acids in our Archive pages, as illustrated for the di-DMOX derivative of 1,18-octadecanedioate –
The same is true for the di-DMOX derivatives of some 1,8-octenedioates, (described by Luthria and Sprecher (1993)), although these do have features that permit the location of the double bonds. The DMOX derivatives of longer-chain monoenoic dibasic acids have additional unusual features, and the spectrum of the di‑DMOX derivative of 1,18‑octadec-9-enedioate is -
The molecular ion tends to be more abundant for the unsaturated fatty acid derivatives, and the ion at [M−112]+ at m/z = 306 in this instance and representing the loss of the McLafferty ion is still very prominent. As expected for a double bond in position 9, there is a gap of 12 amu between m/z = 196 and 208 (see our web-page on DMOX derivatives of monoenes). The ions at m/z = 347, 361 and 375, equivalent to [M−71]+, [M−57]+, and [M−43]+, respectively, appear to be derived from rearrangements involving expulsion of hydrocarbon fragments from the chain, and that at [M−87]+ remains a puzzle.
The mass spectrum of the di-DMOX derivative of 1,20-eicos-9-enedioate (a natural constituent of cork) has one interesting additional feature. The double bond is in position 9 relative to one of the carboxyl groups and position 11 relative to the other, so it would be expected to have diagnostic ions at m/z = 196 and 208 for the former, and at m/z = 224 and 236 for the latter. These ions are in fact present but largely cancel each other out and a gap of 12 amu appears between m/z = 210 and 222. Thus, without further experimental evidence, it could be concluded that the double bond was centrally located, i.e., in position 10.
The mass spectrum of the di-DMOX derivative of 1,18-octadeca-6,9-dienedioic acid (from cork) -
This fatty acid has presumably been formed by omega oxidation of linoleic acid. The double bond in position 6 is defined by characteristic ions at m/z = 167 and 180, while that in position 9 by the gap of 12 amu between ions at m/z = 194 and 206 (see our web page on DMOX derivatives of dienes). The ion representing the loss of the McLafferty ion ([M−112]+) is now at m/z = 304. Of course, this could also be considered to be a 9,12‑diene (counting from the other end), and the appropriate ions are present, if less abundant, but I will leave readers to sort them out.
More spectra of DMOX derivatives of dicarboxylic acids are available on our Archive page, but without interpretation.
Pyrrolidides
To my knowledge, no mass spectra of di-pyrrolidides of dicarboxylic acids have been published to date. Initially we found that pyrrolidides of dicarboxylic acids were difficult to prepare by our usual method (especially those of shorter chain-length), as they appeared to be too polar for extraction from the reaction medium with non-polar solvents. However, by using diethyl ether alone or diethyl ether-chloroform mixtures as extractant solvents, we obtained good yields of products. Because of the increase in polarity and molecular weight, relatively high GC column temperatures are required, and analysis is easier if non-polar stationary phases can be used.
With the modified derivatization procedure, we were able to obtain the mass spectra of pyrrolidides of several dibasic acids, including that of the di‑pyrrolidide of 1,9-nonanedioic (azelaic) acid illustrated next.
There is a respectable molecular ion (even-numbered at m/z = 294), but the spectrum is dominated by ions associated with the pyrrolidine ring with the base ion at m/z = 113, equivalent to the McLafferty ion, tending to be the most abundant in spectra of most pyrrolidides and accompanied by the expected ions at m/z = 98 and 126. Our web page on the mass spectrometry of pyrrolidides of normal saturated fatty acids has a more detailed explanation for these ions. In addition, in the higher mass range, there are ions for loss of 97/8 and 112/3 from the molecular ion at m/z = 197 and 182, respectively. The ion at m/z = 70 and one representing [M−70]+ (m/z = 224) represent ions consisting of the pyrrolidine ring and loss of this, respectively. Both ions are usually present in the spectra of pyrrolidides of mono-carboxylic acids, but with lower relative abundance.
The mass spectrum of the di-pyrrolidide of 1,18-octadecanedioate shows essentially the same features, although the ion at m/z = 308, equivalent to [M−112]+, is now the base ion.
The intermediate ions (m/z = 140 to 294) represent radical induced cleavage at each successive methylene group as in the spectra of more conventional pyrrolidides.
The mass spectrum of the dipyrrolidide derivative of 1,18-octadec-9-enedioate has ions in the high mass range with a similar origin to the previous, but it has the gap of 12 amu between m/z = 196 and 208 expected for a double bond in position 9 (see our web-page on pyrrolidides of monoenes).
More spectra of pyrrolidides of dicarboxylic acids are available on our Archive page, but without interpretation.
References
- 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.
- Luthria, D.L. and Sprecher, H. 2-Alkenyl-4,4-dimethyloxazolines as derivatives for the structural elucidation of isomeric unsaturated fatty acids. Lipids, 28, 561-564 (1993); DOI.
I can 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.
| © Author: William W. Christie |  |
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