Mass Spectra of Thia Fatty Acids
As with my other documents on mass spectrometry, this is a subjective account that details only those relevant fatty acids encountered during my research activities and for which we have spectra available for illustration purposes. These are synthetic fatty acids, prepared by my friend and collaborator Professor Marcel Lie Ken Jie of Hong Kong University, and they do not occur in nature. However, these and related hetero-atom fatty acids are being considered for therapeutic purposes and have biochemical applications. Their mass spectra may be of interest from the standpoint of mechanistic mass spectrometry, but this is not my primary concern here.
Spectra of methyl and 3-pyridylcarbinol ('picolinyl) esters are described in this document, 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. Details of the spectra were first published in -
Christie, W.W., Lie Ken Jie, M.S.F., Brechany, E.Y. and Bakare, O. Mass spectral characterization of 3-picolinyl and methyl ester derivatives of isomeric thia fatty acids. Biomed. Environm. Mass Spectrom., 20, 629-635 (1991) (DOI: 10.1002/bms.1200201007).
Here, I will describe spectra of mono-thia-stearates only for illustrative purposes, but information on a series of thia-laureates and dithia-stearates is available in the above paper.
Methyl esters of thia stearates give distinctive spectra from which the position of the sulphur atom is easily deduced. The relative proportions of these ions vary from isomer to isomer, but in general the ions A1 and A2 are most abundant, while an ion representing loss of methanol from A1 is also diagnostic. In a high proportion of the isomers, either ion A1 or A2 is the base ion. I n all the isomers, cleavage occurs on either side of the sulfur atom to give two principal fragments in each instance, as shown in the figure below. If ion B1 loses the elements of methanol, it will give an ion numerically identical to ion A1 in low-resolution mass spectrometry. Ion B2 is usually small and can be ignored for all practical purposes.
It is hard to locate the molecular ion (m/z = 316) with methyl 2-thia stearate only, and in this instance other potentially useful ions, for example that for loss of methanol ([M-32]+) together with the McLafferty ion (at m/z = 92 for this isomer) are not very abundant. The McLafferty ion in its usual position at m/z = 74 is present in most of the remaining isomers, except for those with the sulfur atom in positions 4 and 6. With the first of these, the absence of a hydrogen atom on C4 explains the loss. Data are listed in the table below. The mass spectra of the selected isomers that follow are offered without further comment.
The mass spectra not illustrated (including 6-thia-laureate and ten dithia-stearates) are available in the Archive section of these web pages.
The 3-pyridylcarbinol esters of thia fatty acids tend to give straightforward spectra with simple fragmentations that permit location of the sulfur atom. In addition, there are the expected ions at m/z = 92, 108, 151 and 164 for fragmentation near the carboxyl group, except in special circumstances (see the web page on 3-pyridylcarbinol esters of saturated fatty acids). Thus, the McLafferty ion in the 2-thia isomer is at m/z = 169 rather than 151. It is missing from the 4- and 6-thia-isomers, with the former because a hydrogen atom is required on carbon 4 for its formation. In all the isomers, cleavage occurs on either side of the sulfur atom to give the two fragments containing the carboxyl (or picolinyl) end of the molecule, as shown in the figure.
The relative proportions of these ions vary from isomer to isomer. In the early isomers, ion A tends to be more abundant, but with the 7- and later isomers ion B is more important (often the base peak).
On either side of these key ions are ions for cleavage at successive methylene groups, i.e. 14 amu apart. The mass spectra of the selected isomers that follow are offered without further comment.
The mass spectra of 3-pyridylcarbinol ('picolinyl') esters of such fatty acids not illustrated here (including the thia-laurates and dithia-stearates) are available in the Archive section of these web pages.
Updated April 4, 2011