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Mass Spectrometry of 3-Pyridylcarbinol Esters


Saturated Branched-Chain Fatty Acids



3-Pyridylcarbinol ('picolinyl') esters are especially useful for structure determination of branched-chain fatty acids, especially the iso- and anteiso-methyl-branched forms, i.e. those most likely to be encountered in natural samples, where for example DMOX derivatives are less suitable. Our web page on mass spectrometry of 3-pyridylcarbinol esters of saturated fatty acids contains more introductory and mechanistic information, together with links to pages dealing with additional practical methodology (sample concentration, derivative preparation, etc). An early review article by D.J. Harvey (1992) is still very valuable. All the spectra illustrated were obtained incidentally from natural samples during my own research work, so there are some obvious gaps. Most will not have been illustrated elsewhere.


Iso- and anteiso-Methyl-Branched Fatty Acids

Most naturally occurring monomethyl-branched fatty acids have saturated alkyl chains, and the most common of these are the iso- and anteiso-methyl isomers. They are produced by bacteria mainly, but they can enter animal tissues via the food chain, for example. The mass spectrum of 3-pyridylcarbinyl iso-methyl-octadecanoate (17-methyl-18:0) follows (Harvey, 1982) -

Mass spectrum of 3-pyridylcarbinyl iso-methyl-octadecanoate

The spectrum resembles that of a straight-chain saturated fatty acid except for the very obvious gap of 28 amu between m/z = 346 and 374, which represents loss of carbon-17 and its attached methyl group, locating it definitively. The remaining ions are uniformly 14 amu apart. This is one example where 3-pyridylcarbinol esters have a distinct advantage over other types of derivative, as with this and the next spectrum there are no 'noise' peaks in the gap.

In the mass spectrum of 3-pyridylcarbinyl anteiso-methyl-octadecanoate (16-methyl-18:0), this gap is shifted to between m/z = 332 and 360 as -

Mass spectrum of 3-pyridylcarbinyl anteiso-methyl-octadecanoate


Fatty Acids with Centrally Located Methyl Branches

Similar principles apply for the location of methyl branches in other positions. For example, in the mass spectrum of 3-pyridylcarbinyl 15-methyloctadecanoate, the gap is shifted to m/z = 318 to 346 as -

Mass spectrum of 3-pyridylcarbinyl 15-methyloctadecanoate

With that of 3-pyridylcarbinyl 11-methyloctadecanoate, the gap is between m/z = 262 and 290 as -

Mass spectrum of 3-pyridylcarbinyl 11-methyloctadecanoate

With that of 3-pyridylcarbinyl 10-methyloctadecanoate (tuberculostearate), the gap is between m/z = 248 and 276 -

Mass spectrum of 3-pyridylcarbinyl 10-methyloctadecanoate

The mass spectrum of 3-pyridylcarbinyl 3-methylpentadecanoate is interesting in that the usual ion at m/z = 164 is shifted to 178, and there is a gap of 27 amu between m/z = 151 and 178 to locate the branch point.

Mass spectrum of 3-pyridylcarbinyl 3-methylpentadecanoate

It is also easy to locate methyl-branch points in spectra of very-long-chain fatty acids as with that of 3-pyridylcarbinyl 22-methyl-octacosanoate -

Mass spectrum of 3-pyridylcarbinyl 22-methyl-octacosanoate

Of course, prediction of the diagnostic features in the spectra of other methyl-branched isomers or homologues not illustrated or archived here is straight-forward.


Di- and Polymethyl-Branched-Chain Saturated Fatty Acids

Similar principles to the above apply in the interpretation of mass spectra of 3-pyridylcarbinol esters of di- and polymethyl-branched-chain fatty acids. For example, the 3-pyridylcarbinol ester of an unusual dimethyl-branched fatty acid from a sponge, i.e. 8,10-dimethyl-hexadecanoate follows (Nechev et al., 2002).

Mass spectrum of 3-pyridylcarbinyl 8,10-dimethyl-hexadecanoate

Gaps of 28 amu between m/z = 220 and 248, and between m/z = 262 and 290, locate the branch points.

Polymethyl-branched fatty acids containing isoprene units and derived from phytol are more common in nature, especially as minor components of fish oils or in ruminant tissues. For example, the spectrum of the 3-pyridylcarbinol ester of 4,8,12-trimethyltridecanoate is -

Mass spectrum of 3-pyridylcarbinyl 4,8,12-trimethyltridecanoate

There are clear gaps of 28 amu between ions at m/z = 164 and 192, 234 and 262, and 304 and 332, which locate the methyl groups on carbons 4, 8 and 12, respectively (Christie, 1997).

In the spectrum of the 3-pyridylcarbinol ester of the homologous fatty acid 5,9,13-trimethyl-tetradecanoate, the diagnostic gaps are all 14 amu higher as anticipated -

Mass spectrum of 3-pyridylcarbinyl 5,9,13-trimethyl-tetradecanoate

The mass spectrum of 3-pyridylcarbinyl 2,6,10,14-tetramethyl-pentadecanoate (pristanate) has some similar features -

Mass spectrum of 3-pyridylcarbinyl 2,6,10,14-tetramethyl-pentadecanoate

- for example, the methyl branches in positions 6, 10 and 14 are located by gaps of 28 amu between m/z = 206 and 234, 276 and 304, and 346 and 374, respectively. However, the presence of the 2-methyl group is recognized as the ion usually found at m/z = 151 is shifted to 165, while the ion normally at m/z = 164 is shifted to 178.

In comparison, in the spectrum of the homologous 3,7,11,15-tetramethylhexadecanoate (phytanate), the gaps for the methyl branches on carbons 7, 11 and 15 are shifted up by 14 amu as expected (Christie, 1989).

Mass spectrum of 3-pyridylcarbinyl 3,7,11,15-tetramethylhexadecanoate

The methyl group on carbon 3 is located by the fact that the ion at m/z = 151 is again substantial, while that normally at m/z = 164 has shifted to 178.

We have spectra of many more 3-pyridylcarbinol esters of branched-chain saturated fatty acids on file, and they can be accessed (but without interpretation) from our Archive page. Only a few of these have been published elsewhere.


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: July 14th, 2017 Author: William W. Christie LipidWeb icon