Part 5. Trienoic Fatty Acids

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Sybil Parrish
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1 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 infrmatin nly cncerning the duble bnd psitins in fatty acids. Hwever, 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 reasnably sure f the identity f a fatty acid. In cntrast t the situatin with mnenes and dienes, there are a few key ins that help t identify the cmmn plyenic fatty acids with methylene-interrupted ( hm-allylic ) unsaturatin, especially thse f the (n-6) and (n- 3) families, thugh these must be interpreted with cautin. Sme ther plyunsaturated fatty acids give distinctive fingerprint spectra, althugh the mechanisms f fragmentatin may nt always be prperly understd. Fr example, ur Archive illustrates mass spectra f ten natural 18:3 ismers (nt cunting Z/E ismers), all f which appear t be characteristic. A few f the spectra illustrated belw may have been published elsewhere (references cited if knwn), but sme are unique t this website. Methylene-Interrupted Trienes With methylene-interrupted trienes, as ppsed t tetraenes t hexaenes, there is usually a distinctive mlecular in tgether with a small in at [M-31/32] + fr lss f the elements f a methxyl grup (plus a hydrgen atm). The McLafferty in ( = 74) is always small. In the lwer mlecular weight regin, hydrcarbn ins f general frmula [C n H 2n-5 ] + tend t dminate the spectrum with the in at = as the base peak. There are tw relatively cmmn C 18 trienic acids and their mass spectra are illustrated belw, starting with methyl 6,9,12-ctadecatrienate (γ-linlenate r 18:3(n-6)) (Hlman and Rahm, 1971) W.W. Christie lipidlibrary.acs.rg 1

2 - and f methyl 9,12,15-ctadecatrienate (-linlenate r 18:3(n-3)) (Hallgren et al., 1959)) Bth spectra are rather similar, but there are features that make them smething mre than simply fingerprints fr identificatin purpses. Fr example, a peak at = 1 is characteristic fr methyl esters f plyunsaturated fatty acids with an n-6 terminal miety, while ne at = 8 defines an n-3 terminal grup (Hlman and Rahm, 1971; Brauner et al.,1982; Fellenberg et al., 1987)). Fr a 8 the minr (n-9) and (n-4) families the relevant ins 1 are at = 192 and 122, respectively. They are frmed by a cleavage in the psitins shwn. My impressin is that each f these ins ccurs with reasnable cnsistency in the spectra f fatty acid methyl esters frm these bichemical families, althugh they are nt necessarily unique t such acids. It shuld be nted that these ins, which are smetimes termed the mega ins, are relevant nly fr fatty acids with three r mre duble bnds, nt fr dienes. In additin, there are small ins frmed by a similar cleavage at the carbxyl end f the mlecule giving a fragment cntaining the first tw duble bnds and the secnd methylene grup (minus a prtn) that culd be termed the alpha in, as illustrated. Thus in the mass spectrum f methyl 6,9,12-ctadecatrienate, this in is at = 194, and it appears t be present in the spectra f all cnventinal plyenic acids t which we have access with the first duble bnd in psitin 6. The crrespnding in in the spectrum f methyl 9,12,15-ctadecatrienate is at a = 236. This in was first nted by 194 Hlman and Rahm (1971), but was studied mre systematically via 236 chemical inizatin methds in a paper CH by thers that appears t have been 3 OOC largely verlked (Brauner et a al.,1982). Analgus ins are seen in spectra f methyl esters f mst methylene-interrupted plyunsaturated fatty acids (three r mre duble bnds) as listed in Table 1. The alpha ins are nt always as easily distinguished as the mega ins, but they d appear always t be present ther than in sme f the fatty acids f the lesser knwn (n-1) family) (authr, unpublished bservatin). a W.W. Christie lipidlibrary.acs.rg 2

3 Table 1. Alpha ins in the mass spectra f methyl esters f plyunsaturated fatty acids. First duble bnds 4,7 5,8 6,9 7, 8,11 9,12,13 11,14 In () Fr example with the bilgically imprtant C analgues, such as 8,11,14-eicsatrienate (:3(n-6)) and the mass spectrum f the methyl ester, the mega in at = 1 is again prminent tgether with the alpha in fr a 8,11 duble bnd system at = The mass spectrum f the methyl ester f 16:3(n-6) is an exceptin in that it lacks the in at = 1, but it has a distinctive if uncharacterized in at = 147 (see the Archive pages). In the mass spectrum f methyl 11,14,17-eicsatrienate (:3(n-3)), the in at = 8 is similarly distinctive and the alpha in fr a 11,14 fatty acid at = 264 is small but clear Plyunsaturated fatty acids f the n-9 family are less cmmn in nature, but they d have bilgical relevance, especially in essential fatty acid deficiency in animals. W.W. Christie lipidlibrary.acs.rg 3

4 The mass spectrum f the imprtant ismer methyl 5,8,11-eicsatrienate (Mead's acid r :3(n-9)) In this instance, the distinctive mega in is that at = 192, which is frmed by an analgus type f cleavage t the key ins fr the n-6 and n-3 families, while the alpha 5,8 in is at = 1. With the minr (n-4) family f fatty acids frm fish ils, the diagnstic mega in is at = 122, as in the spectrum f methyl 8,11,14-ctadecatrienate, while the alpha 8,11 in is at = Trienes f the n-1 family f fatty acids have yet t be fund in nature, but the spectrum f methyl 11,14,17-ctadecatrienate (18:3(n-1)) frm a genetically mdified rganism is illustrated next t cmplete this sectin (Sayanva et al., 06). W.W. Christie lipidlibrary.acs.rg 4

5 The mega in is expected t be at = but this is present in the spectra f all methyleneinterrupted plyunsaturated fatty acids s is n lnger characteristic. The alpha in wuld be expected at = 264, but is nt present. This spectrum culd be said t be distinct frm the thers nly in the sense that there appears t be nthing distinctive. Unusual Trienes The rules develped fr methylene-interrupted trienes d nt apply when there is mre than ne methylene grup between the duble bnds. Fr example, 5,9,12-ctadecatrienate is a cmmn cnstituent f cnifer lipids and its methyl ester has the spectrum shwn next (Dbsn and Christie, 02) It has the in at = 1 fr a (n-6) duble bnd system, but als the ins at = 141, 9 and [M-49] + ( = 243) characteristic f 5,9-dienes. We als have mass spectra n file fr f 5,11,14-:3, 7,11,14-:3 (frm cnifers), and 5,9,21-28:3, 5,9,22-29:3 and 5,9,23-:3 (frm spnges) see ur Archive pages. Sme f these fatty W.W. Christie lipidlibrary.acs.rg 5

6 acids have the characteristic ins fr a 5,9-duble bnd system als, and and the spectrum f the last f them, methyl 5,9,23-triacntatrienate, is illustrated Thus, the key ins are at = 141 and fr [M-49] + ( = 411). Of curse, it is unlikely that there are any diagnstic ins fr the duble bnd in psitin 23. 3,9,12-Octadecatrienic acid (usually with the first duble bnd f the trans cnfiguratin, but here cis) is ccasinally fund in seed ils frm the Chrysanthemum and related families. The methyl ester has the spectrum (Dbsn. and Christie, 02) It is very different frm the spectra f ther C 18 trienes, and the in at = 218 is pssibly frmed frm a fragmentatin as illustrated, i.e. after lss f the McLafferty in. Trienic fatty acids in which nly tw f the duble bnds are in cnjugatin are rare in nature, and the best knwn is cis-9,trans-11,cis-15 ctadecatrienic acid. This is frmed as an intermediate in the bihydrgenatin f -linlenic acid by mirrganisms in the rumen, and is a minr cmpnent f meat and dairy prducts frm ruminant animals, such as sheep and cws. The mass spectrum f its methyl ester fllws. W.W. Christie lipidlibrary.acs.rg 6

7 In this instance, the spectrum is quite infrmative. The in at = 223 represents a fragmentatin at the centre f the bis-methylene-interrupted system cntaining the carbxyl grup (the ther expected in at = 69 is present but hidden amng the lw mass hydrcarbn ins). The in at = 191 represents lss f a methxyl grup frm the in at = 223, while that at = 173 represents a further fragmentatin in which the elements f water are lst. The ins at = and 163 are frmed frm the hydrcarbn tail f the mlecule invlving cleavages beta and gamma, respectively, t the first duble bnd. Cnjugated trienes are imprtant cnstituents f certain seed ils f cmmerce. The mass spectrum f the methyl ester f the cnjugated triene, -elestearate r methyl 9-cis,11-trans,13- trans-ctadecatrienate is illustrated next There are tw main distinctive features, a particularly high mlecular in, and a trpylium rearrangement in at = 91 (discussed in ur web dcument n tetraenes, etc). Bth f these ins are characteristic f highly cnjugated duble bnd systems. There are n ins indicative f duble bnd psitins. Other cnjugated trienes have mass spectra that are very similar t this. Mass spectra f the MTAD adducts f the methyl ester f the cnjugated triene, punicic acid, are described in anther dcument (see the sectin n Mass spectra f methyl esters f fatty acids further derivatizatin ). W.W. Christie lipidlibrary.acs.rg 7

8 We have spectra f many mre methyl esters f trienic fatty acids n file, and they can be accessed (but withut interpretatin) frm ur Archive pages. References Brauner, A., Budzikiewicz, H. and Bland, W. Studies in chemical inizatin mass spectrmetry. 5. Lcalizatin f hmcnjugated triene and tetraene units in aliphatic cmpunds. Org. Mass Spectrm., 17, (1982). Dbsn, G. and Christie, W.W. Mass spectrmetry f fatty acid derivatives. Eur. J. Lipid Sci. Technl., 4, (02). Fellenberg, A.J., Jhnsn, D.W., Puls, A. and Sharp, P. Simple mass spectrmetric differentiatin f the n-3, n-6 and n-9 series f methylene interrupted plyenic acids. Bimed. Envirn. Mass Spectrm., 14, (1987). Hallgren, B., Ryhage, R. and Stenhagen, E. The mass spectra f methyl leate, methyl linleate and methyl linlenate. Acta Chem. Scand., 13, (1959). Hlman, R.T. and Rahm, J.J. Analysis and characterizatin f plyunsaturated fatty acids. Prg. Chem. Fats Other Lipids, 9, 15- (1971). Sayanva, O., Haslam, R., Guschina, I., Llyd, D., Christie, W.W., Harwd, J.L. and Napier, J.A. A bifunctinal 12, 15-desaturase frm Acanthameba castellanii directs the synthesis f highly unusual n-1 series unsaturated fatty acids. J. Bil. Chem., 281, (06). William W. Christie James Huttn Institute (and Mylnefield Lipid Analysis), Invergwrie, Dundee (DD2 5DA), Sctland Last updated: May 1 st, 13 W.W. Christie lipidlibrary.acs.rg 8

Part 3. Monoenoic Fatty Acids

Part 3. Monoenoic Fatty Acids 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