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) 7 1 163 0 1 1 1 1 0 2 2 2 2 177 194 8 235 243 264 292 W.W. Christie lipidlibrary.acs.rg 1
- and f methyl 9,12,15-ctadecatrienate (-linlenate r 18:3(n-3)) (Hallgren et al., 1959)) 95 8 292 163173 236 191 3 223 249 261 0 1 1 1 1 0 2 2 2 2 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
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 () 166 1 194 8 222 236 2 264 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 = 222-7 1 163 177 192 222 249 263 289 3 0 1 1 1 1 0 2 2 2 2 0 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. 95 8 163 177 1 219 238 252 264 277 289 3 0 1 1 1 1 0 2 2 2 2 0 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
The mass spectrum f the imprtant ismer methyl 5,8,11-eicsatrienate (Mead's acid r :3(n-9)) 192 1 6 147 161 1 192 5 221 246 271 3 0 1 1 1 1 0 2 2 2 2 0 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 = 222-222 7 122 122 222 164 177 0 217 292 2 0 1 1 1 1 0 2 2 2 2 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
7 164 178 1 219 249 261 292 0 1 1 1 1 0 2 2 2 2 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). 81 141 1 95 9 136 1 141 161 177 191 221 243 7 261 292 0 1 1 1 1 0 2 2 2 2 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
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 81 9 141 1 194 235 247 275 289 318 345 411 386 428 4 0 1 0 2 0 3 0 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) - 81 218 95 7 133 218 147 161 1 243 261 278 292 0 1 1 1 1 0 2 2 2 2 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
69 81 91 95 7 163 173 163 0 1 1 1 1 0 2 2 2 2 191 191 223 213 223 243 69 261 263 292 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. 292 91 7 1 163 261 189 3 235 249 0 1 1 1 1 0 2 2 2 2 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
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, 161-164 (1982). Dbsn, G. and Christie, W.W. Mass spectrmetry f fatty acid derivatives. Eur. J. Lipid Sci. Technl., 4, 36-43 (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, 127-1 (1987). Hallgren, B., Ryhage, R. and Stenhagen, E. The mass spectra f methyl leate, methyl linleate and methyl linlenate. Acta Chem. Scand., 13, 845-847 (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, 36533-36541 (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