MASS SPECTRA OF PICOLINYL ESTERS
Part 2. Monoenoic Fatty Acids
Straight-Chain Monoenoic Fatty Acids
The mass spectra of picolinyl esters of monoenoic fatty acids are distinctive and permit facile location of the double bond. For example, that of picolinyl 9-octadecenoate (oleate) is illustrated below (Harvey, 1982) -
As before in the low molecular weight region of the spectrum, there are prominent ions at m/z = 92, 108, 151 and 164, which are all fragments about the pyridine ring. Then, the principle of interpretation is the same as that for saturated fatty acids (Check here), in that the simplest approach is to start with the molecular ion and progress downwards, as if one were unzipping the molecule one methylene group at a time. Thus, from the molecular ion (m/z = 373), there is loss of a methyl group to m/z = 358, followed by a series of ions 14 amu apart for loss of successive methylene groups.
When a double bond is reached, there is a gap of 26 amu (between m/z = 234 and 260. This gap can sometimes be difficult to locate precisely, and a fragmentation at the adjacent methylene group on the carboxyl side giving a gap of 40 amu between m/z = 220 and 260 in this instance is often easier to locate, especially with polyenes.
A further distinctive feature of clear diagnostic value is a doublet of abundant ions 14 amu apart, representing cleavage on the distal side of the double bond at m/z = 274 and 288 here. In practice, these or the equivalent two ions in other isomers can be picked out for identification purposes even when isomeric fatty acid derivatives are imperfectly resolved by gas chromatography. Formation of the ions has been rationalized in mechanistic terms as an initial abstraction of allylic hydrogen atoms on each side of the double bond with the production of conjugated diene systems, which form relatively stable ions.
Please note - the diagram showing the fragmentation points is an over-simplification of the processes that occur, but suffices for most practical purposes.
The geometry of the double bond makes no readily discernable difference, and the mass spectrum of picolinyl elaidate is identical to that of oleate.
Similar series of ions are see in spectra from picolinyl esters of most monoenes, but it is advantageous to have access to spectra of authentic standards when the double bond is close to either end of the molecule to avoid any confusion. Also, when interpreting the mass spectra of picolinyl esters of polyenoic fatty acids, the position of the first double bond is usually hardest to locate from first principles, and a comparison with spectra of standard monoenes can again be useful. Details of the spectra of the complete series of isomeric octadecenoates have been published (Christie, Brechany and Holman, 1987), but only a few of the spectra were depicted in the paper for practical reasons. All are now illustrated below, but please consult the original reference for detailed discussion. Spectra of other homologous fatty acid isomers displaying the same diagnostic features may have been described elsewhere in some instances.
Picolinyl 2-octadecenoate (2-18:1) -
In this and the next few isomers, fragments corresponding to cleavage at the double bonds (gaps of 26 amu) are not seen,, probably because the double bond forms a stable resonance structure with the carboxyl group. The McLafferty ions at m/z = 151 is relatively small, and the ion at m/z = 164 that is usually abundant is now of low intensity. However, the prominent doublet at m/z = 177 and 190 provides a distinctive fingerprint for identification purposes.
With picolinyl 3-octadecenoate (3-18:1) (Christie et al., 1987), the diagnostic doublet has moved by 14 amu to m/z = 190 and 204 -
Picolinyl 4-octadecenoate (4-18:1). In this instance the double bond in position 4 appears to assist the formation of the McLafferty rearrangement ion at m/z = 151, which is the base peak for this isomer only.
Picolinyl 5-octadecenoate (5-18:1). Although the gap of 26 amu for the double bond is not easy to locate, that of 40 amu between m/z = 164 and 204 serves instead. Only the second ion of the doublet, at m/z = 232, stands out with this isomer, but this also appears to be a diagnostic characteristic.
Picolinyl 6-octadecenoate or petroselinate (6-18:1). This spectrum is similar to that of the 5-isomer, except that all the important diagnostic ions are found 14 amu higher.
Picolinyl 7-octadecenoate (7-18:1). From this until the 13-18:1 isomer, the 'typical' pattern for monoenes is observed, i.e. with the gap of 26 amu being clearly apparent, together with the pronounced doublet of ions at higher mass. For the 7-isomer, it is between m/z = 206 and 232, for the 8-isomer, between 220 and 246, and so on. However, the gap of 40 amu is often easier to locate unequivocally.
Picolinyl 8-octadecenoate (8-18:1) -
Picolinyl 9-octadecenoate (oleate or 9-18:1) - see the first spectrum of this document.
Picolinyl 10-octadecenoate (10-18:1) (Christie et al., 1987) -
Picolinyl 11-octadecenoate (cis-vaccenate or 11-18:1) (Harvey, 1982) -
Picolinyl 12-octadecenoate (12-18:1)
Picolinyl 13-octadecenoate (13-18:1) -
Picolinyl 14-octadecenoate (14-18:1). As the double bond nears the terminal end of the molecule, the gaps of 26 or 40 amu that locate the double bond are still easy to find, but only the first ion of the expected doublet (m/z = 344) is now distinctive.
Picolinyl 15-octadecenoate (15-18:1). Again the ions characteristic of the double bond are easy to locate, but now the distinctive doublet at higher mass is gone.
Picolinyl 16-octadecenoate (16-18:1) (Christie et al., 1987). With this and the 17-isomer that follows, it is not difficult to recognize that the double bond must be close to the terminal methyl group, but it helps to have the model spectra available to be sure of the exact position. Double bonds near the terminal position are a problem with all types of nitrogen-containing derivative.
Picolinyl 17-octadecenoate (17-18:1) -
Of course, if the 17-double bond is located more centrally in the chain, there is no difficulty with interpretation as with the spectrum of picolinyl 17-tetracosenoate (17-24:1) -
The double bond can be located by the gap of 26 amu between m/z = 346 and 372, or the gap of 40 amu between m/z = 332 and 372, with the doublet of ions at m/z = 386 and 400 providing valuable supporting evidence.
Spectra of picolinyl esters of many more monoenoic fatty acids are illustrated in our Archive section, but without interpretation. Many of these have not been formally published elsewhere.
Branched-Chain Monoenoic Fatty Acids
Monomethyl-branched-chain monoenoic fatty acids are not very common in nature, but we have mass spectra of picolinyl esters of a few fatty acids of this type from marine sources. Interpretation is usually straight forward. Positions of double bonds are confirmed as described above and those for methyl branches by gaps of 28 amu for loss of the carbon carrying the methyl branch (see the pages on Mass spectra of picolinyl esters - saturated and branched-chain fatty acids. None of these spectra have been published formally elsewhere to my knowledge.
Picolinyl 14-methyl-hexadec-6-enoate (anteiso-methyl-6-16:1) -
The methyl branch is located by the gap of 28 amu between m/z = 302 and 330, and the double bond in position 6 is most easily recognized by the ions around that at m/z = 246 (see the spectrum of picolinyl petroselinate above).
Picolinyl 15-methyl-hexadec-6-enoate (iso-methyl-6-16:1) -
Here, the methyl branch is located by the gap shifted to between m/z = 316 and 344, but the double bond is in the same position as the previous example.
Picolinyl 15-methyl-hexadec-9-enoate (iso-methyl-9-16:1) -
The methyl branch is located as in the previous spectrum, but the double bond in recognized by the characteristic group of peaks in the range m/z = 220 to 288 (see the spectrum of oleate above).
Spectra of picolinyl esters of more branched-chain monoenoic fatty acids are illustrated in our Archive section, but without interpretation. Only a few of these may have been published elsewhere.
References
- Christie, W.W., Brechany, E.Y. and Holman, R.T. Mass spectra of the picolinyl esters of isomeric mono- and dienoic fatty acids. Lipids, 22, 224-228 (1987).
- Harvey, D.J. Picolinyl esters as derivatives for the structural determination of long chain branched and unsaturated fatty acids. Biomed. Mass Spectrom., 9, 33-38 (1982).
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Updated: 24/4/2008 |
Scottish Crop Research Institute (and MRS Lipid Analysis Unit), Invergowrie, Dundee (DD2 5DA), Scotland
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