Oxo (Keto) Fatty Acids

Mass Spectrometry

 

Elizabeth BrechanyAs with my other documents on mass spectrometry with electron-impact ionization, this is a subjective account that details only those fatty acids relevant to this topic encountered during my research activities and for which we have spectra available for illustration purposes. Spectra of methyl and 3-pyridylcarbinol ('picolinyl') esters (mainly) are described here, but I will only describe key diagnostic ions, as general features of each type of derivative are described elsewhere on this website. I have no spectra for DMOX or pyrrolidine derivatives, as I was not using these during my work at the time these spectra were obtained. Data are available elsewhere for the complete series of oxo stearates as pyrrolidides (Tulloch, 1980).

Only a few of the spectra illustrated below have been published elsewhere. All of these were obtained during studies of minor milk fatty acid constituents with my collaborator at that time, Elizabeth Brechany (in the picture) (Brechany and Christie, 1992 and 1994)

 

Methyl Ester Derivatives of Oxo Fatty Acids

It has long been known that the key diagnostic ions in mass spectra of methyl ester derivatives of oxo (keto) fatty acids are formed by cleavage both alpha and beta to the oxo group (Ryhage and Stenhagen, 1960; Kenner and Stenhagen, 1964). However, other rearrangement ions can confuse this simple picture, a common finding with mass spectra of methyl esters.

Mass spectrum of methyl 5-oxo stearate (octadecanoate) -

Mass spectrum of methyl 5-oxo stearate

The ions at m/z = 101, 129 and 211 are formed by cleavage alpha to the oxo group, while that at m/z = 144 is formed by beta cleavage between carbons 6 and 7 (and containing the carboxyl moiety). The ion at m/z = 112 is presumably formed from the last by elimination of the elements of methanol. The molecular ion is small but sufficient for identification purposes, as appears typical.

Mass spectrum of methyl 7-oxo stearate (see Ryhage and Stenhagen, 1960) -

Mass spectrum of methyl 7-oxo stearate

Here, the ion at m/z = 172 is formed by beta cleavage between carbons 8 and 9 (that at m/z = 140 represents loss of methanol from this), and those at m/z = 115 and 198 are for cleavage between carbons 5 and 6. The expected alpha cleavage ions (m/z = 129, 157 and 183) are less abundant but distinct.

Mass spectrum of methyl 12-oxo stearate -

Mass spectrum of methyl 12-oxo stearate

In this instance, the beta cleavage ions are at m/z = 185, 242 (with corresponding ions for loss of methanol at m/z = 153 and 210, respectively) and 128, with the alpha cleavage ions as marked.

The mass spectrum of the oxime derivative of methyl 12-oxo stearate is even more confusing, and it is illustrated below for record purposes only. The spectrum appears to be dominated by nitrogen-containing ions, with fragments from both ends of the molecule. Some of these may have lost the oxygen atom from the oxime group, so more detailed interpretation would be speculative.

Mass spectrum of the oxime derivative of methyl 12-oxo stearate

The mass spectrum of methyl 12-oxo-octadec-9-enoate -

Mass spectrum of methyl 12-oxo-octadec-9-enoate

In this instance, the double bond obviously has a major influence on fragmentation and the spectrum is dominated by the ion at m/z = 113, formed from the terminal end of the molecule by cleavage alpha to the oxo group and beta to the double bond, i.e. between carbons 11 and 12. I could speculate on the origins of the ions at m/z = 95 and 85, but I will leave that to the reader.

 

3-Pyridylcarbinol Esters of Saturated Oxo Fatty Acids

The only published spectra of 3-pyridylcarbinol esters of saturated oxo fatty acids are those of Brechany and Christie (1992, 1994). As with 3-pyridylcarbinol esters in general, interpretation is straight forward if we start with the molecular ion and count back for loss of successive methylene groups. When an oxo group is reached there is a gap of 28 amu for the loss of the C=O unit (alpha cleavage), followed by loss of successive methylene groups once more. The ions formed by cleavage beta to the oxo group tend to be especially abundant, and that of higher mass is frequently odd-numbered, presumably because it is protonated. Unlike spectra from methyl esters, only ions containing the carboxyl end of the molecule need be considered for interpretation purposes (see the web page on 3-pyridylcarbinol esters of saturated fatty acids).

Mass spectrum of 3-pyridylcarbinyl 5-oxo-stearate -

Mass spectrum of 3-pyridylcarbinyl 5-oxo-stearate

As is fairly common with 3-pyridylcarbinol esters when a functional group is close to the carboxyl moiety, the cleavage pattern is not always as expected. In this instance, the gap of 28 amu for alpha cleavage on either side of the oxo group between m/z = 178 and 206, is not immediately evident. However, the beta cleavage ions at m/z = 164 and 221 (the latter is unusual in being odd numbered) are distinctive.

Mass spectrum of 3-pyridylcarbinyl 7-oxo-stearate -

Mass spectrum of 3-pyridylcarbinyl 7-oxo-stearate

Now the gap of 28 amu between m/z = 206 and 234 is clearly seen, as are the beta cleavage ions at m/z = 192 and 249 (the latter is again odd numbered).

In the mass spectrum of 3-pyridylcarbinyl 10-oxo-stearate, the gap of 28 amu for alpha cleavage is between m/z = 248 and 276, while those for beta cleavage ions are at m/z = 234 and 291.

Mass spectrum of 3-pyridylcarbinyl 10-oxo-stearate

In the mass spectrum of 3-pyridylcarbinyl 12-oxo-stearate, the key ions are shifted up from the previous by 28 amu -

Mass spectrum of 3-pyridylcarbinyl 12-oxo-stearate

- and in that of 3-pyridylcarbinyl 13-oxo-stearate, the key ions are shifted up by a further 14 amu -

Mass spectrum of 3-pyridylcarbinyl 13-oxo-stearate

In the mass spectrum of 3-pyridylcarbinyl 16-oxo-stearate, the key ions are shifted up by a further 42 amu, but the second of the beta cleavage ions (expected at m/z = 375) is not present.

Mass spectrum of 3-pyridylcarbinyl 16-oxo-stearate

 

3-Pyridylcarbinol Esters of Mono-Unsaturated Oxo Fatty Acids

In the mass spectrum of 3-pyridylcarbinyl 10-oxo-octadec-14-enoate, the ions expected for cleavage at the 10-oxo group are present (see the spectrum of the saturated analogue above).

Mass spectrum of 3-pyridylcarbinyl 10-oxo-octadec-14-enoate

In addition, there are ions that serve to locate the double bond in position 14, i.e. by the gap of 26 amu between m/z = 318 and 344. Also, the ion formed by cleavage beta to the double bond at m/z = 304 is especially abundant as shown below. See the web page on 3-pyridylcarbinol esters of monoenoic fatty acids for a full discussion of such fragmentations.

Fragmentation points in the mass spectrum of 3-pyridylcarbinyl 10-oxo-octadec-14-enoate

In the mass spectrum of the isomeric 3-pyridylcarbinyl 10-oxo-octadec-15-enoate in comparison (next), the same ions are present for fragmentation at the oxo group, but those for fragmentation at the double bond are shifted upwards by 14 amu to m/z = 332 and 358.

Mass spectrum of 3-pyridylcarbinyl 10-oxo-octadec-15-enoate

In the mass spectrum of 3-pyridylcarbinyl 12-oxo-octadec-5-enoate, the oxo group is located by the ions at m/z = 274, 288, 316 and 331, according to the principles described above. A double bond in position 5 is less easily located definitively, but the distinctive ion at m/z = 232 is a good guide. Note that in a superficial examination of the spectrum, the prominent ions at m/z = 274 and 288 might lead the observer to think that there was a double bond in position 9.

Mass spectrum of 3-pyridylcarbinyl 12-oxo-octadec-5-enoate

The next two isomers do indeed have a double bond in position 9, but with the oxo group in different positions. Both have the distinctive gap of 26 amu between m/z = 234 and 260 for the double bond, and the first has abundant ions at m/z = 274 and 288 that are also characteristic.

In the spectrum of 3-pyridylcarbinyl 16-oxo-octadec-9-enoate, the ions that serve to locate the oxo group are at m/z = 316, 330, 358 and 372, according to the principles set out above.

Mass spectrum of 3-pyridylcarbinyl 16-oxo-octadec-9-enoate

Finally, the spectrum of 3-pyridylcarbinyl 12-oxo-octadec-9-enoate is anomalous in that the main cleavage occurs alpha not beta to the oxo group because of the presence of the adjacent double bond -

Mass spectrum of 3-pyridylcarbinyl 12-oxo-octadec-9-enoate

When faced with this as an unknown, the potential confusion was resolved by deuteration and re-analysis, and the chemical basis for this is discussed in greater detail in the section of these web pages dealing with Mass spectra of fatty acid methyl esters - further derivatization. By eliminating the double bond while simultaneously 'fixing' it, its original position is confirmed and the position of the oxo-group is similarly clear, as is shown in the spectrum that follows, i.e. of 3-pyridylcarbinyl 9,10-deutero-12-oxo-octadecanoate. Thus, the ions at m/z = 234, 249 and 264 locate the double bond, while the base ion is at m/z = 264 for fragmentation beta to the oxo group.

Mass spectrum of 3-pyridylcarbinyl 9,10-deutero-12-oxo-octadecanoate

Spectra of further related fatty acids are available, but without interpretation, in the Archive Sections of these web pages, i.e. for methyl esters and 3-pyridylcarbinol ('picolinyl') esters.

 

References

  • Brechany, E.Y. and Christie, W.W. Identification of the saturated oxo fatty acids in cheese. J. Dairy Sci., 59, 57-64 (1992) (DOI: 10.1017/S0022029900030247).
  • Brechany, E.Y. and Christie, W.W. Identification of the unsaturated oxo fatty acids in cheese. J. Dairy Res., 61, 111-115 (1994) (DOI: 10.1017/S0022029900028107).
  • Kenner, G.W. and Stenhagen, E. Location of double bonds by mass spectrometry. Acta Chem. Scand., 18, 1551-1552 (1964) (DOI: 10.3891/acta.chem.scand.18-1551).
  • Ryhage, R. and Stenhagen, E. Mass spectrometric studies. VI. Methyl esters of normal chain oxo-, hydroxy-, methoxy- and epoxy-acids. Arkiv Kemi, 15, 545-574 (1960).
  • Tulloch, A.P. Mass spectrometry of pyrrolidides of oxo, hydroxy and trimethylsilyloxy octadecanoic acids. Lipids, 15, 881-888 (1980) (DOI: 10.1007/BF02534383).

Updated March 17, 2014

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