Part 2. Monoenoic Fatty Acids

Transcription

1 MASS SPECTRA OF 3-PYRIDYLCARBIOL ESTERS Part 2. Monoenoic Fatty Acids Straight-Chain Monoenoic Fatty Acids The mass spectra of 3-pyridylcarbinol esters of monoenoic fatty acids are distinctive and permit facile location of the double bond. The widely used term 'picolinyl ester' is inaccurate and should now be avoided (see our webpage on the saturated derivatives). As the first example, the mass spectrum of 3-pyridylcarbinyl 9-octadecenoate (oleate) is illustrated below (Harvey, 1982) As before in the low molecular weight region of the spectrum, there are prominent ions at =, 8, and, which are all fragments about the pyridine ring. Then, the principle of interpretation is the same as that for saturated fatty acids (see the pages dealing with Mass spectra of 3-pyridylcarbinol esters saturated and branched-chain fatty acids), in that the simplest approach is to start with the molecular ion and progress downwards, as if one were unzipping the molecule one methylene group at a time. Thus, from the molecular ion ( = ), there is loss of a methyl group to =, followed by a series of ions 14 amu apart for loss of successive methylene groups, i.e. =, 3, and so forth. a When a double bond is reached, there is a gap of 26 amu (between = 234 and 2. This gap can sometimes be difficult to locate precisely, and a fragmentation at the adjacent methylene group on the carboxyl side giving a gap of amu between = 2 and 2 in this instance is often easier to locate, especially with polyenes. W.W. Christie lipidlibrary.aocs.org 1

2 A further distinctive feature of clear diagnostic value is a doublet of abundant ions 14 amu apart, representing cleavage on the distal side of the double bond at = and in this example. In practice, these or the equivalent two ions in other isomers can be picked out for identification purposes even when isomeric fatty acid derivatives are imperfectly resolved by gas chromatography. Formation of the ions has been rationalized mechanistically as an initial abstraction of allylic hydrogen atoms on each side of the double bond with the production of conjugated diene systems, which form relatively stable ions. Please note that diagrams showing the fragmentation points over-simplify the processes that occur, but suffice for many practical purposes. The geometry of the double bond makes no readily discernible difference, and the mass spectrum of 3-pyridylcarbinyl elaidate is identical to that of oleate. Similar series of ions are seen in spectra from 3-pyridylcarbinol esters of most monoenes, but it is advantageous to have access to spectra of authentic standards when the double bond is close to either end of the molecule to avoid any confusion. Also, when interpreting the mass spectra of 3-pyridylcarbinyl esters of polyenoic fatty acids, the position of the first double bond is usually hardest to locate from first principles, and a comparison with spectra of standard monoenes can again be useful. Details of the spectra of the complete series of isomeric octadecenoates have been published (Christie, Brechany and Holman, 1987), but only a few of the spectra were depicted in the paper for practical reasons. All are now illustrated below with the minimum of discussion, but please consult the original reference for detailed interpretation. Spectra of other homologous fatty acid isomers displaying the same diagnostic features may have been described elsewhere in some instances. 3-Pyridylcarbinyl 2-octadecenoate (2-18:1) In this, fragments corresponding to cleavage at the double bonds (gaps of 26 amu) are not seen, probably because the double bond forms a stable resonance structure with the carboxyl group. The ion at = is relatively small, and that at =, which is usually abundant, is now of low intensity. However, the prominent doublet at = 177 and 1 provides a distinctive fingerprint for identification purposes. The ions regularly spaced 14 amu apart further down the chain confirm that there cannot be a double bond anywhere else in the molecule. Similarly with the next few isomers, the gap of 26 amu for the double bond is not seen but other features can be used for identification. W.W. Christie lipidlibrary.aocs.org 2

3 With 3-pyridylcarbinyl 3-octadecenoate (3-18:1) (Christie et al.,1987), the diagnostic doublet has moved by 14 amu to = 1 and 4 CH 2OOC Pyridylcarbinyl 4-octadecenoate (4-18:1). In this instance the double bond in position 4 appears to assist the formation of the ion at =, which is the base peak for this isomer only Pyridylcarbinyl 5-octadecenoate (5-18:1). Although the gap of 26 amu for the double bond is not easy to locate, that of amu between = and 4 serves instead. Only the second ion of the doublet, at =, stands out with this isomer, but this feature also appears to be a diagnostic characteristic W.W. Christie lipidlibrary.aocs.org 3

4 3-Pyridylcarbinyl 6-octadecenoate or petroselinate (6-18:1). This spectrum is similar to that of the 5-isomer, except that all the important diagnostic ions are found 14 amu higher Pyridylcarbinyl 7-octadecenoate (7-18:1). From this until the 13-18:1 isomer, the typical pattern for monoenes is observed, i.e. with the gap of 26 amu being clearly apparent, together with the pronounced doublet of ions at higher mass. For the 7-isomer, it is between = 6 and, for the 8-isomer, between 2 and, and so on. However, the gap of amu is often easier to locate unequivocally Pyridylcarbinyl 8-octadecenoate (8-18:1) W.W. Christie lipidlibrary.aocs.org 4

5 3-Pyridylcarbinyl 9-octadecenoate (oleate or 9-18:1) - see the first spectrum of this document. ote that with the later isomers the ion doublets after the double bond become more distinctive until the 14-isomer is reached. 3-Pyridylcarbinyl -octadecenoate (-18:1) (Christie et al.,1987) Pyridylcarbinyl 11-octadecenoate (cis-vaccenate or 11-18:1) (Harvey, 1982) M Pyridylcarbinyl 12-octadecenoate (12-18:1) W.W. Christie lipidlibrary.aocs.org 5

6 3-Pyridylcarbinyl 13-octadecenoate (13-18:1) Pyridylcarbinyl 14-octadecenoate (14-18:1). As the double bond nears the terminal end of the molecule, the gaps of 26 or amu that locate the double bond are still easy to find, but only the first ion of the expected doublet ( = ) is now distinctive Pyridylcarbinyl 15-octadecenoate (15-18:1). Again the ions characteristic of the double bond are easy to locate, but now the distinctive doublet at higher mass is gone W.W. Christie lipidlibrary.aocs.org 6

7 3-Pyridylcarbinyl 16-octadecenoate (16-18:1) (Christie et al., 1987). With this and the 17-isomer that follows, it is not difficult to recognize that the double bond must be close to the terminal methyl group, but it helps to have the model spectra available to be sure of the exact position. Double bonds near the terminal position are a problem with all types of nitrogen-containing derivative Pyridylcarbinyl 17-octadecenoate (17-18:1) Of course, if the 17-double bond is located more centrally in the chain, there is no difficulty with interpretation as with the spectrum of 3-pyridylcarbinyl 17-tetracosenoate (17-24:1) - 8 CH 2OOC W.W. Christie lipidlibrary.aocs.org 7

8 The double bond can be located by the gap of 26 amu between = 346 and 372, or the gap of amu between = 332 and 372, with the doublet of ions at = 386 and 0 providing valuable supporting evidence. Mass spectra of 3-pyridylcarbinol esters of many more monoenoic fatty acids are illustrated in our Archive Section (but without interpretation). Only a few of these have been formally published elsewhere. Branched-Chain Monoenoic Fatty Acids Monomethyl-branched-chain monoenoic fatty acids are not very common in nature, but we have mass spectra of 3-pyridylcarbinol esters of a few fatty acids of this type from marine sources. Interpretation is usually straight forward. Positions of double bonds are confirmed as described above and those for methyl branches by gaps of 28 amu for loss of the carbon carrying the methyl branch (see Mass spectra of 3-pyridylcarbinol esters saturated and branched-chain fatty acids). one of these spectra have been published formally elsewhere to my knowledge. 3-Pyridylcarbinol 14-methyl-hexadec-6-enoate (anteiso-methyl-6-16:1) The methyl branch is located by the gap of 28 amu between = 2 and 3, and the double bond in position 6 is most easily recognized by the ions around that at = (see the spectrum of 3-pyridylcarbinyl petroselinate above). 3-Pyridylcarbinol 15-methyl-hexadec-6-enoate (iso-methyl-6-16:1) W.W. Christie lipidlibrary.aocs.org 8

9 Here, the methyl branch is located by the gap shifted to between = and, but the double bond is in the same position as the previous example. 3-Pyridylcarbinol 15-methyl-hexadec-9-enoate (isomethyl-9-16:1) The methyl branch is located as in the previous spectrum, but the double bond in recognized by the characteristic group of peaks in the range = 2 to (see the spectrum of oleate above). Mass spectra of 3-pyridylcarbinol esters of more branched-chain monoenoic fatty acids are illustrated in our Archive Section (but without interpretation). Only a few of these may have been published elsewhere. References o o Christie, W.W., Brechany, E.Y. and Holman, R.T. Mass spectra of the picolinyl esters of isomeric monoand dienoic fatty acids. Lipids, 22, (1987). Harvey, D.J. Picolinyl esters as derivatives for the structural determination of long chain branched and unsaturated fatty acids. Biomed. Mass Spectrom., 9, (1982). William W. Christie James Hutton Institute (and Mylnefield Lipid Analysis), Invergowrie, Dundee (DD2 5DA), Scotland Last updated: August 5 th, 13 W.W. Christie lipidlibrary.aocs.org 9