Part 5. Tetraenoic Fatty Acids

Methylene-Interrupted Tetraenoic Fatty Acids

As with trienes, the mass spectra of 3-pyridylcarbinyl esters of tetraenoic 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. Few model compounds are available, so most of the following spectra have been obtained from analyses of natural products, and fatty acids with a variety of chain-lengths are described. The widely used term 'picolinyl ester' is inaccurate and should now be avoided (see our web page on the saturated derivatives). References are listed when we are aware of prior formal publication of spectra in the scientific literature, but many of the following spectra will not have been published elsewhere.

For example, the mass spectrum of 3-pyridylcarbinyl 6,9,12,15-octadecatetraenoate (stearidonate or 18:4(n-3)) is illustrated below (Griffiths et al., 1996; Wolff et al., 1999) -

Mass spectrum of 3-pyridylcarbinyl 6,9,12,15-octadecatetraenoate

The gap of 26 amu between m/z = 312 and 338 locates the terminal double bond rather easily, and that between m/z = 272 and 298 locates than in position 12, but the first double bonds are not so readily identified. However, if the first double bond were in a different position, the spectrum would change (see that for 3-pyridylcarbinyl 5,9,12,15-octadecatetraenoate below). As with trienes, it is often easier to spot gaps of 40 amu for the double bond and an associated methylene group, in this instance from m/z = 178 to 218 to 258 to 298 to 338. Similar principles apply to interpretation of mass spectra of structurally related compounds.

3-Pyridylcarbinyl 8,11,14,17-octadecatetraenoate (18:4(n-1)), from a fatty acid that is occasionally found as a minor component of marine organisms -

Mass spectrum of 3-pyridylcarbinyl 8,11,14,17-octadecatetraenoate

In this instance, the presence of the terminal double bond causes rearrangements that confuse the interpretation (a problem with all derivatives of this type). However, the first two double bonds are easily located without dubiety, as indicated on the spectrum.

3-Pyridylcarbinyl 5,8,11,14-eicosatetraenoate (arachidonate or 20:4(n-6)) (Harvey, 1984; Wolff et al., 1999), a key essential fatty acid in animal tissues and precursor of families of eicosanoids or prostaglandins. The first double bond is not easily located, but the remaining three are from the fragment ions annotated. The same applies to the spectrum following this.

Mass spectrum of 3-pyridylcarbinyl 5,8,11,14-eicosatetraenoate

3-Pyridylcarbinyl 8,11,14,17-eicosatetraenoate (20:4(n-3)) -

Mass spectrum of 3-pyridylcarbinyl 8,11,14,17-eicosatetraenoate

On the other hand, when the double bonds are all relatively remote from the carboxyl group, as with 3-pyridylcarbinyl 9,12,15,18-tetracosanoate (24:4(n-6)), they are all relatively easy to locate.

Mass spectrum of 3-pyridylcarbinyl 9,12,15,18-tetracosanoate

Further relevant spectra are available in our Archive section (but without interpretation).

 

Bis- and Poly-Methylene-Interrupted Tetraenoic Fatty Acids

As described elsewhere for trienes, 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 3-pyridylcarbinyl 5,9,12,15-octadecatetraenoate from a Gymnosperm is typical (compare it with that of 5,9,12-18:3 in the web pages dealing with 3-pyridylcarbinyl esters of trienes), with the prominent ion at m/z = 219 representing cleavage between carbons 7 and 8, i.e. the centre of the bis-methylene-interrupted double bond system, as described first in the web pages on 3-pyridylcarbinyl esters of dienes. The remaining double bonds are easily located from the ions annotated.

Mass spectrum of 3-pyridylcarbinyl 5,9,12,15-octadecatetraenoate

3-Pyridylcarbinyl 5,11,14,17-eicosatetraenoate (also from a Gymnosperm) (Wolff et al., 1999) -

Mass spectrum of 3-pyridylcarbinyl 5,11,14,17-eicosatetraenoate

Again, the spectrum can be compared with that of the analogous triene, 3-pyridylcarbinyl 5,11,14-eicosatrienoate (Mass spectra of 3-pyridylcarbinyl esters - trienoic fatty acids), and distinctive ions serve to locate the double bonds.

3-Pyridylcarbinyl 5,9,19,23-triacontatetraenoate (5,9,19,23-30:4) (from a sponge - Joh et al., 1997).

Mass spectrum of 3-pyridylcarbinyl 5,9,19,23-triacontatetraenoate

This has two bis-methylene-interrupted double bond systems, defined by the ions at m/z = 219 and 410, while further useful ions serve to confirm the positions of the last two double bonds at least.

Further relevant spectra are available in our Archive section (but without interpretation).

 

References

  • Griffiths, G., Brechany, E.Y., Jackson, F.M., Christie, W.W., Stymne, S. and Stobart, A.K. Distribution and biosynthesis of stearidonic acid in leaves of Borago officinalis. Phytochemistry, 43, 381-386 (1996) (DOI: 10.1016/0031-9422(96)00305-6).
  • Harvey, D.J. Picolinyl derivatives for the structural determination of fatty acids by mass spectrometry. Applications to polyenoic acids, hydroxy acids, di-acids and related compounds. Biomed. Mass Spectrom., 11, 340-347 (1984) (DOI: 10.1002/bms.1200110705).
  • Joh, Y.G., Elenkov, I.J., Stefanov, K.L., Popov,S.S., Dobson, G. and Christie, W.W. Novel di-, tri-, and tetraenoic fatty acids with bis-methylene-interrupted double-bond systems from the sponge Haliclona cinerea. Lipids, 32, 13-17 (1997).
  • Wolff, R.L., Christie, W.W., Pedrono, F. and Marpeau, A.M. Arachidonic, eicosapentaenoic, and biosynthetically related fatty acids in the seed lipids from a primitive gymnosperm, Agathis robusta. Lipids, 34, 1083-1097 (1999) (DOI: 10.1007/s11745-999-0460-y).

Updated August 8, 2013 

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