MASS SPECTRA OF PICOLINYL ESTERS


Part 4. Trienoic Fatty Acids


Methylene-Interrupted Trienoic Fatty Acids

The mass spectra of picolinyl esters of trienoic fatty acids permit location of the double bonds, but less easily than those of monoenes and dienes. The diagnostic ions often occur at the centres of clusters, and do not stand out well. However, different isomers tend to have very different spectra so that characterization is possible when spectra of authentic fatty acids can be compared. Unlike the dienes, few model compounds are available. Therefore, most of the following spectra have been gleaned from analyses of natural products, and fatty acids with a variety of chain-lengths are described. References are listed when we are aware of prior formal publication of spectra in the scientific literature. For example, that of picolinyl 6,9,12-octadecatrienoate (γ-linolenate or 18:3(n-6)) (from Wolff et al., 1999) is illustrated below -

Mass spectrum of picolinyl 6,9,12-octadecatrienoate

The principles of double bond location are described in our web page on picolinyl esters of monoenoic fatty acids. The gap of 26 amu between m/z = 272 and 298 locates the terminal double bond rather easily, and the gap between m/z = 232 and 258 locates that in position 9, but the first double bond is not easily seen. However, if the first double bond were in a different position, the spectrum would change (see that for picolinyl 5,9,12-octadecatrienoate below, for example). Often a gap of 40 amu for the double bond and an associated methylene group is easier to spot, as can be seen in the next spectrum.

Picolinyl 9,12,15-octadecatrienoate (α-linolenate or 18:3(n-3)) -

Mass spectrum of picolinyl 9,12,15-octadecatrienoate

Here the gaps of 40 discussed above are between m/z = 220 and 260, 260 and 300, and 300 and 340, as illustrated (Harvey, 1984). Similar principles apply to interpretation of spectra of structurally related compounds, including the following of which some are important biologically.

Picolinyl 4,7,10-hexadecatrienoate (16:3(n-6)) - diagnostic ions 28 amu less than for 6,9,12-18:3 above.

Mass spectrum of picolinyl 4,7,10-hexadecatrienoate

Picolinyl 6,9,12-hexadecatrienoate (16:3(n-4)) - the diagnostic ions are the same as those for 6,9,12-18:3 above.

Mass spectrum of picolinyl 6,9,12-hexadecatrienoate

Picolinyl 7,10,13-hexadecatrienoate (16:3(n-3)). The last two doubles bonds are easily located, but not the first. However, note that the 'fingerprint' is distinctly different from that of the other 16:3 isomers.

Mass spectrum of picolinyl 7,10,13-hexadecatrienoate

Now the biologically important 20:3 isomers, starting with picolinyl 5,8,11-eicosatrienoate (20:3(n-9)), an acid that  accumulates in essential fatty acid deficiency (Christie et al., 1986). Only the last two double bonds are easily located.

Mass spectrum of picolinyl 5,8,11-eicosatrienoate

Picolinyl 8,11,14-eicosatrienoate (20:3(n-6)) (from - Christie et al., 1986) -

Mass spectrum of picolinyl 8,11,14-eicosatrienoate

Picolinyl 11,14,17-eicosatrienoate (20:3(n-3)) (from - Christie et al., 1986). With this and the previous isomer, the double bonds can be located as marked without too much difficulty. Again, note that if considered only as a 'fingerprint', each mass spectrum is distinctive.

Mass spectrum of picolinyl 11,14,17-eicosatrienoate


Conjugated Trienoic Fatty Acids

Picolinyl 9t,11t,13t-octadecatrienoate or α-eleostearate is the only conjugated triene for which we have a spectrum (unpublished), other than the alpha-isomer in which the first double bond is of the cis configuration (spectrum identical). As with conjugated dienes, the molecular ion is the base peak. However, it would be difficult to locate the double bonds from first principles.

Mass spectrum of picolinyl 9,11,13-octadecatrienoate


Bis- and Poly-Methylene-Interrupted Trienoic Fatty Acids

As described elsewhere for picolinyl esters of dienes, it has become apparent that bis- and polymethylene-interrupted trienoic fatty acids are more common in nature than may have been supposed. In particular, fatty acids with a 5,9- double bond system or their chain elongation products are common in seed oils from Gymnosperms or in certain marine invertebrates such as sponges. The spectrum of picolinyl 5,9,12-octadecatrienoate from pine trees is typical (Christie, 1989) -

Mass spectrum of picolinyl 5,9,12-octadecatrienoate

As with the dienes, the abundant ion at m/z = 219 is very characteristic of a bis-methylene-interrupted double bond system in positions 5 and 9. The other ions marked provide confirmation of the structure.

Picolinyl 3,9,12-octadecatrienoate from Tanacetum zawadskii seed oil (Tsevegsuren et al., 2003). The double bonds in positions 9 and 12 are identified as for the previous spectrum. The characteristic fingerprint ions at m/z = 190/191 and 204, formed by cleavage with allylic rearrangements, locate the remaining double bond in position 3 (see our web pages on Picolinyl esters of monoenoic fatty acids).

Mass spectrum of picolinyl 3,9,12-octadecatrienoate

5,9,13-Eicosatrienoic acid from a sponge has two bis-methylene-interrupted double bond systems, so the mass spectrum of its picolinyl ester is especially distinctive (Joh et al., 1997)) -

Mass spectrum of picolinyl 5,9,13-eicosatrienoate

In this instance, the prominent ions at m/z = 219 and 272 are diagnostic for cleavage between carbons 7 and 8 and between11 and 12, respectively. The ions at m/z = 178, 204, 232, 258, 286 and 312 also locate the double bonds.

Picolinyl 5,11,14-eicosatrienoate (and the next fatty acid) is found in many plants of the Gymnosperm family.

Mass spectrum of picolinyl 5,11,14-eicosatrienoate

Here, interpretation is straightforward by comparison with the spectra of comparable 5-monoenes and 11,14-dienes, illustrated on the appropriate web pages.

Picolinyl 7,11,14-eicosatrienoate (from Wolff et al., 1997) -

Mass spectrum of picolinyl 7,11,14-eicosatrienoate

The spectrum can be considered as analogous to that of 5,9,12-18:3, with the diagnostic ions shifted 28 amu higher, i.e. the ion for the centre of the bis-methylene-interrupted double bond system is now at m/z = 247 instead of 219. The ions that locate the last two double bonds are the same as in the previous spectrum.

Spectra of picolinyl esters of many more bis-methylene-interrupted trienoic fatty acids are illustrated in our Archive section, but without interpretation. Many of these have not been formally published elsewhere. Most are from sponges and range from 5,9,17-24:3 to 5,9,27-34:3.


References



W.W. Christie

Updated: 25/4/2008

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Scottish Crop Research Institute (and MRS Lipid Analysis Unit), Invergowrie, Dundee (DD2 5DA), Scotland

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