Part 3. Monoenoic Fatty Acids

Size: px

Start display at page:

Download "Part 3. Monoenoic Fatty Acids"

Rosamond Andrews
5 years ago
Views:

1 MASS SPECTRA OF METHYL ESTERS OF FATTY ACIDS Part 3. Mnenic Fatty Acids As cautined in the Intrductin t these dcuments, the mass spectra f methyl esters btained with electrn-impact inizatin ften affrd limited infrmatin nly cncerning the structures f fatty acids. The mlecular weight is usually btainable, and this is an imprtant piece f infrmatin. If chrmatgraphic retentin data are added t this, it is ften pssible t be certain f the identity f a fatty acid. Unfrtunately, mass spectra f methyl esters f mst mnenic fatty acids cntain n infrmatin that helps t lcate the psitin r gemetry f duble bnds. While there have been suggestins that such infrmatin can be btained frm clse examinatin f certain minr peaks in the spectrum, I am dubtful f the value f such techniques with real samples, as small changes in instrumental parameters culd mask such effects. Hwever, chemical inizatin with acetnitrile as the reagent gas can enable bth duble bnd psitin and gemetry t be determined as described n a separate webpage. Nrmal Mnenic Fatty Acids The mass spectra f methyl esters f mnenic fatty acids under electrn-impact inizatin tend t be rather uninfrmative. Hwever, they d allw the mlecular weight t be determined and this cmbined with GC retentin data can be f practical value. The intrductin f a duble bnd changes the spectrum appreciably frm that f the crrespnding saturated ester. The mass spectrum f methyl leate (cis-9-ctadecenate) is illustrated first (t my knwledge it was first published by Hallgren et al., 1959) [M - 32] [M - 116] [M - ] The mlecular in ( = 2) is clearly seen, and ins representing lss f the elements f methanl ( = 264 r [M-32] + ), i.e. a methxyl grup plus a hydrgen atm, and the lss f the McLafferty in ( = ) are abundant, as is the McLafferty in per se ( = ) (see the web page n saturated acids). A characteristic in at [M-116] + ( = 1 in this instance), tgether with hmlgus ins at 166, 152, etc, are als diagnstic. The first f these ins is frmed by lss f a fragment cntaining the carbxyl grup by cleavage between carbns 5 and W.W. Christie lipidlibrary.acs.rg 1

2 6 with additin f a rearranged hydrgen atm. Thus, in cntrast t the spectra f methyl esters f saturated fatty acids, hydrcarbn ins (general frmula [C n H 2n-1 ] + ) dminate the spectrum, with = as the base peak usually. The relative abundances f all f these ins tends t be appreciably greater than in the mass spectra f dienes and plyenes. Hwever, there is n feature that permits lcatin f the duble bnd, because this can migrate t any psitin when the alkyl chain is inized in the mass spectrmeter. Thus, nearly all the cis- and trans-18:1 ismers have virtually identical spectra. The exceptin is the spectrum f methyl 2-ctadecenate (belw) in which the duble bnd and carbxyl grup presumably frm a relatively stable resnance structure. Amngst thers there is a distinctive in at = 113, which is believed t ccur via a cleavage between carbns 5 and 6 fllwed by frmatin f a stable 6-membered ring with the carbxyl grup (Ryhage et al., 11) [M+1] This particular fatty acid des nt ccur naturally t my knwledge, but it can be frmed artefactually by ver vigrus trans-esterificatin f the 3-ismer (trans-3-16:1 is an imprtant cmpnent f phtsynthetic tissues in plants, fr example). Hwever, sme ther natural fatty acids with α,β-unsaturatin are knwn. As further examples, the mass spectrum f methyl 5-eicsenate (frm meadwfam il) is W.W. Christie lipidlibrary.acs.rg 2

3 - and that f methyl 13-dcsenate (als frm meadwfam il) is Bth f these shw the same features as in the spectrum f methyl leate, especially ins representing [M-32] +, [M-] + and [M-116] +. The mass spectrum f the shrter-chain methyl 9-ddecenate (frm milk fat) is Analgus ins t thse in the previus spectra are present, althugh the relative intensities are smewhat different as might be expected. Nte that the presence f an alicyclic ring reduces the mlecular weight f a fatty acid by 2 amu in cmparisn t the crrespnding straight-chain saturated fatty acid, i.e. the same amunt as a duble bnd. Methyl esters f cyclprpyl fatty acids have mass spectra that are virtually identical t thse f mnenes with the same ttal number f carbn atms (see the web-page n Mass spectra f cyclic fatty acids ). Where there is dubt, GC retentin data are an imprtant aid t identificatin. Branched-Chain Mnenic Fatty Acids Unsaturated branched-chain fatty acids are ften encuntered as minr cmpnents f marine lipid samples. The mass spectrum f methyl 15-methyl-hexadec-9-enate (frm a spnge) is - W.W. Christie lipidlibrary.acs.rg 3

4 The ins at = 235 ([M-47] + ) and 227 ([M-] + ) may be related t the presence f the ismethyl grup, but there is presumably nthing t assist lcatin f the duble bnd (althugh the rigin f the ins at = 177 and 195 is intriguing). Apart frm these last tw ins, the spectrum is typical f that f mst ther mnenes. The mass spectrum f methyl 7-methyl-hexadec-6-enate (frm a jellyfish in this instance, but ften present in fish frm warm seas) is Unlike ther mnenes, ins representing [M-32] +, [M-] + and [M-116] + are small. It is tempting t suggest that the ins at = and 167 result frm the simple fragmentatins between carbns 5 and 6 as illustrated, while thse at = 138 and 151 result frm a rearrangement fllwing a fragmentatin f the hydrcarbn prtin f the mlecule. Evidence supprting this interpretatin cmes frm the spectrum f the ethyl ester, where the ins at = 138, 151 and 167 remain, but that at = is replaced by ne at = 129. Subsequent t publicatin here, this spectrum was illustrated in a paper by Fardin-Kia, A.R et al. (13), tgether with that f methyl 7-methyl-ctadec-6-enate. The latter cntains ins that supprt the abve interpretatins. W.W. Christie lipidlibrary.acs.rg 4

5 The first f these spectra may nt have been illustrated elsewhere. We als have the mass spectrum f the isprenid methyl 3,7,11,15-tetramethylhexadec-trans-2-enate (phytenate) n file in the Archive sectin. Dimethyldisulfide adducts: T get rund the prblem f lcatin f duble bnds, it is pssible t prepare specific derivatives f unsaturated fatty acids that fix the duble bnd. Very many have been described, but the nly ne t have std the test f time is the dimethyl disulfide adduct, as this has excellent mass spectrmetric prperties and is prepared in a simple ne-pt reactin (see the sectin n Mass spectra f methyl esters f fatty acids further derivatizatin ). Alternatively, 3-pyridinylcarbinl ( piclinyl ) esters r DMOX r pyrrlidine derivatives can be utilized t lcate duble bnds, as described in sme detail elsewhere in these web pages. Spectra f many mre methyl esters f mnenic fatty acids can be accessed frm ur Archive pages (withut interpretatin). References Fardin-Kia, A.R., Delmnte, P., Kramer, J.K.G., Jahreis, G., Kuhnt, K., Santercle, V. and Rader, J.I. Separatin f the fatty acids in menhaden il as methyl esters with a highly plar inic liquid gas chrmatgraphic clumn and identificatin by time f flight mass spectrmetry. Lipids, 48, (13). Hallgren, B., Ryhage, R. and Stenhagen, E. The mass spectra f methyl leate, methyl linleate and methyl linlenate. Acta Chem. Scand., 13, (1959). Ryhage, R., Ställberg-Stenhagen, S. and Stenhagen, E. Methyl esters f α,β-unsaturated lng-chain acids. On the structure f C27-phthienic acid. Arkiv. Kemi, 18, (11). William W. Christie James Huttn Institute (and Mylnefield Lipid Analysis), Invergwrie, Dundee (DD2 5DA), Sctland Last updated: August 13 th, 13 W.W. Christie lipidlibrary.acs.rg 5

Part 5. Trienoic Fatty Acids

Part 5. Trienoic Fatty Acids MASS SPECTRA OF METHYL ESTERS OF FATTY ACIDS Part 5. Trienic Fatty Acids As cautined in the Intrductin t these dcuments, the mass spectra f methyl esters btained under electrn-impact inizatin affrd limited