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Mass Spectrometry of DMOX Derivatives


Tetra- to Hexaenoic Fatty Acids



As with trienes, the mass spectra of DMOX derivatives of polyenoic 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. The principles described in the earlier documents in this series apply, so we are in general looking for the diagnostic gaps of 12 amu between significant ions. When the first double bond is close to the carboxyl group, the general fragmentation rule for double bond location is no longer appropriate and the 'fingerprint' spectrum for the relevant monoene derivative is an invaluable guide to its identification.

Few model compounds are available, so most of the following spectra have been obtained from analyses of natural products as part of our research programme, and fatty acids with a variety of chain-lengths are described. As with other nitrogen-containing derivatives, the molecular ion is always clearly delineated, in contrast to the mass spectra of methyl esters, for example. References are listed when we are aware of prior formal publication of spectra in the scientific literature.


Methylene-Interrupted Tetraenoic Fatty Acids

For example, the mass spectrum of the DMOX derivative of 6,9,12,15-octadecatetraenoate (stearidonate or 18:4(n-3)), a minor component of fish and some seed oils, is illustrated first -

Mass spectrum of the DMOX derivative of 6,9,12,15-octadecatetraenoate

The double bond in position 6 is recognized by the characteristic fingerprint of ions at m/z = 152, 167 and 180 (DMOX derivatives of monoenoic fatty acids). The remainder are located by the gaps of 12 amu between m/z = 194 and 206, 234 and 246, and 274 and 286, i.e. for positions 9, 12 and 15, respectively, as can be deduced by extrapolation from our document on monoenes (Yu et al., 1989; Sayanova et al., 1997). Also, as with trienoic DMOX derivatives, the gaps of 40 amu between m/z = 194, 234 and 274, or between m/z = 206, 246 and 286, or best of all between m/z = 180, 220, 260 and 300, are useful diagnostic guides for the last three double bonds. Indeed, these gaps of 40 amu can sometimes be easier to detect than those of 12 amu.

The DMOX derivative of 8,11,14,17-octadecatetraenoate or 18:4(n-1) - a minor component of fish oils -

Mass spectrum of the DMOX derivative of 8,11,14,17-octadecatetraenoate

The double bonds in positions 8, 11 and 14 can be recognized by the appropriate gaps of 12 amu, as indicated. That in position 17 can only be inferred, although the gaps of 40 amu (m/z = 168 to 208 to 248 to 288 to 329) provide some additional evidence. However, terminal double bonds are not easily recognized with pyrrolidides and 3-pyridylcarbinol ('picolinyl') esters either.

The DMOX derivative of 5,8,11,14-eicosatetraenoate (20:4(n-6) or arachidonate), the important essential fatty acid -

Mass spectrum of the DMOX derivative of 5,8,11,14-eicosatetraenoate

The double bond in position 5 is confirmed by the fingerprint ion at m/z = 153, together with the relative abundance of the ion at m/z = 113, while the remainder are located by the gaps of 12 or 40 amu as indicated (Zhang et al., 1988; Yu et al., 1989; Wolff et al., 1999). The same diagnostic ions are present in the spectrum of the DMOX derivative of 5,8,11,14-18:4 or 18:4(n-4) in our Archive pages.

DMOX derivative of 8,11,14,17-eicosatetraenoate (20:4(n-3)) (Yu et al., 1989; Luthria and Sprecher, 1993). The diagnostic ions are marked.

Mass spectrum of the DMOX derivative of 8,11,14,17-eicosatetraenoate

We also have the mass spectra of the DMOX derivatives of several more methylene-interrupted tetraenes, including C16 and C22 isomers, on file in the Archive Section of the web pages, but without interpretation.


Tetraenoic Fatty Acids with Conjugated Double Bonds

We were unable to prepare the DMOX derivative of the common conjugated tetraene parinaric acid, possibly because it isomerized on derivatization, but we do have the spectrum of a fatty acid derived by microbial action on stearidonic acid with three double bonds in conjugation (stereochemistry not defined) and one isolated double bond, i.e. 6,8,10,15-octadecatetraenoate (Hennessy et al., 2012) -

Mass spectrum of the DMOX derivative of 6,8,10,15-octadecatetraenoate

The small ion at m/z = 152 is a fingerprint ion for the double bond in position 6, although this would not be convincing if other evidence were not available; the other double bonds are easily located by the ions marked. The large ion at m/z = 246 for a fragmentation beta to the conjugated double bond system is comparable to a distinctive ion in the spectrum of the DMOX derivative of 9-cis,11-trans,15-cis-octadecatrienoate.


Bis- and Polymethylene-Interrupted Tetraenoic Fatty Acids

We have a few relevant spectra on file garnered from natural sources. For example, the DMOX derivative of 5,11,14,17-eicosatetraenoate (5,11,14,17-20:4) from the seed oil of a gymnosperm (Berdeaux and Wolff, 1996).

Mass spectrum of the DMOX derivative of 5,11,14,17-eicosatetraenoate

Again, the double bond in position 5 can be deduced from the characteristic fingerprint ion at m/z = 153, together with the abundance of the ion at m/z = 113 relative to that at m/z = 126, while the remaining double bonds are located by the appropriate gaps of 12 or 40 amu, as indicated on the spectrum.

DMOX derivative of 5,9,19,23-tricosatetraenoate (5,9,19,23-30:4) -

Mass spectrum of the DMOX derivative of 5,9,19,23-tricosatetraenoate

This unusual fatty acid from a sponge has two bis-methylene-interrupted double bond systems, recognized most easily by the characteristic fragmentations at the centre of each, i.e. at m/z = 180 and 372, for the 5,9- and 19,23-double bond systems, respectively.

We also have the mass spectrum of the DMOX derivative of 5,9,21,25-32:4 on file in the Archive Section of the web pages, but without interpretation.


Pentaenoic Fatty Acids

Only a few natural fatty acids of this type exist, but they are all of great biological importance. Interpretation of these spectra is the same as for DMOX derivatives of other polyunsaturated fatty acids.

DMOX derivative of 5,8,11,14,17-eicosapentaenoate (20:5(n-3) or EPA) (Yu et al., 1989; Wolff et al., 1999) -

Mass spectrum of the DMOX derivative of 5,8,11,14,17-eicosapentaenoate

The double bond in position 5 can be deduced from the characteristic fingerprint ion at m/z = 153, while all the remaining double bonds are located by the appropriate gaps of 12 or 40 amu as indicated on the spectrum.

DMOX derivative of 4,7,10,13,16-docosapentaenoate (22:5(n-6)) (Yu et al., 1989) -

Mass spectrum of the DMOX derivative of 4,7,10,13,16-docosapentaenoate

The double bond in position 4 is defined by the fingerprint ion at m/z = 152, but only the later double bonds are clearly delineated otherwise. The relative abundance of the ion at m/z = 113 in comparison to that at m/z = 126 is a further aid to identification of the double bond in position 4.

DMOX derivative of 7,10,13,16,19-docosapentaenoate (22:5(n-3)) (Yu et al., 1989). When the first double bond is a little further from the carboxyl group, as in this example, it is not difficult to locate each of the double bonds in the spectrum as indicated.

Mass spectrum of the DMOX derivative of 7,10,13,16,19-docosapentaenoate

We also have the mass spectrum of the DMOX derivative of 6,9,12,15,18-heneicosapentaenoate (21:5(n-3)), a minor but ubiquitous component of fish oils, and of 5,8,11,14,17-octadecapentaenoate (18:5(n-1)) on file in the Archive Section of the web pages, but without interpretation.


Hexaenoic Fatty Acids

4,7,10,13,16,19-Docosahexaenoate (22:6(n-3) or 'DHA') is a vital component of animal cell membranes, especially those in brain and nervous tissue, and of fish oils. The mass spectrum of its DMOX derivative (Yu et al., 1989; Zhang et al., 1988) is -

Mass spectrum of the DMOX derivative of 4,7,10,13,16,19-docosahexaenoate

The double bond in position 4 is defined by the fingerprint ion at m/z = 152, and the subsequent double bonds are located by the appropriate gaps of 12 amu, or those of 40 amu between analogous ions.

6,9,12,15,18,21-Tetracosahexaenoate (24:6(n-3)) is a biosynthetic precursor of the previous fatty acid, but is only occasionally encountered in tissues in significant amounts (in this instance from a jelly fish). The mass spectrum of its DMOX derivative (Nichols et al., 2003) is -

Mass spectrum of the DMOX derivative of 6,9,12,15,18,21-tetracosahexaenoate

The double bond in position 6 is recognized by the characteristic fingerprint of ions at m/z = 152, 167 and 180, while the remainder are located by the gaps of 12 amu, or those of 40 amu between analogous ions. The ion at m/z = 126 is now more abundant than that at m/z = 113.

In all of these spectra, the tropylium ion at m/z = 91 is small but significant, whereas with the corresponding methyl ester derivatives this can be a dominant ion (see the web page on mass spectra of methyl esters of tetraenoic acids, for example). Presumably this reflects the more orderly fragmentation that occurs with nitrogen-containing derivatives.


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.


Credits/disclaimer Updated: April 21st, 2017 Author: William W. Christie LipidWeb icon