Part 1. Saturated and Branched-Chain Fatty Acids

Mass spectra f DMX derivatives. Part 1. Saturated and branched-chain fatty acids MASS SPECTRA F DMX DERIVATIVES Part 1. Saturated and Branched-Chain Fatty Acids a 4,4-Dimethylxazline (DMX) derivatives give excellent mass spectra f fatty acids with electrnimpact inizatin that frequently permit unequivcal a identificatin. Any dubiety that may remain can be remved if the analyst has access t authentic spectra, and many f thse illustrated belw (and in the ther dcuments n this site) shuld prve useful fr this purpse. The gas chrmatgraphic prperties f DMX derivatives are better than thse f 3-pyridinylcarbinl ( piclinyl ) esters and pyrrlidides, and many analysts cnsider them the best single derivative available fr identificatin f fatty acids in mass spectrmetric terms. This culd be debated, especially when there are functinal grups near the terminal end f the mlecule, but in much f ur research, we have used mre than ne type f derivative as they prvide useful cmplementary data. ne drawback f DMX derivatives is that they are nt very stable chemically; ring pening ccurs rapidly n strage in the presence f traces f misture, althugh this prblem can be amelirated by string them in the presence f desiccant. Hwever, there is n simple cleanup step, as is available fr ther derivatives. DMX derivatives can als be separated n a micr-preparative scale by reversed-phase HPLC if a base-deactivated statinary phase is emplyed (Christie, 1998)), and this can be a useful means f enriching minr cmpnents fr further analysis. DMX derivatives are very similar in their fragmentatin prperties in mass spectrmetry t pyrrlidides. Indeed, in spite f the differences in structure, the tw have identical mlecular weights and the McLafferty ins frm each are in the same place, fr example. Althugh pyrrlidides appear t have fallen ut f fashin, they are mre stable chemically and have advantages in sme circumstances, especially when functinal grups are near the terminal carbn atm. DMX derivatives are better with unsaturated fatty acids. Chinese scientists were respnsible fr much f the early wrk n DMX derivatives, and their papers are listed in the Bibligraphy Sectin f this website. A definitive review f the tpic, including mechanistic aspects, has been published by Spitzer (1997). References are listed when we are aware f prir frmal publicatin f spectra in the scientific literature, but many f the fllwing spectra will nt have been published elsewhere. Methds fr preparing the derivatives are described elsewhere in this website, in the separate dcument n Preparatin f derivatives. Straight-Chain Saturated Fatty Acids DMX derivatives are nt at their best with saturated fatty acids as the ins f higher mass tend t be f lw intensity, and rearrangement ins ccur in greater abundance than with unsaturated fatty acid derivatives. Hwever, they d at least give a usable mlecular in. This tgether with GC retentin data is usually sufficient fr definitive identificatin purpses. The mass spectrum f the DMX derivative f palmitic acid is illustrated first (fr example, see Zhang et al., 1988)). W.W. Christie lipidlibrary.acs.rg 1

Mass spectra f DMX derivatives. Part 1. Saturated and branched-chain fatty acids 2 294 72 83 98 1 168 182 2 224 252 2 294 9 0 1 1 1 1 0 2 2 2 2 0 The McLafferty in at = is usually the base peak (see the web pages n methyl esters f saturated fatty acids fr a mechanistic discussin), accmpanied by a prminent in at =. In general, the best apprach t interpretatin f the mass spectra f DMX derivatives is t start with the mlecular in and wrk backwards. The mlecular in ( = 9) is small but recgnizable in this instance, and then there is a gap f 15 amu t = 294, fllwed sequentially by ins 14 amu apart ( = 2,, 252, and s frth), which can be cnsidered simplistically as cleavage at successive methylene grups (but see the sectin n branched-chain acids belw). Similar features are seen in the spectra f related fatty acids, as illustrated fr the DMX derivatives f ddecanate (12:0) and dcsanate (22:0) 98 72 83 1 154 168 2 182 224 0 1 1 1 1 0 2 2 253 57 72 98 168 1 2 224 252 2 294 3 364 378393 8 336 0 1 1 1 1 0 2 2 2 2 0 3 3 3 3 W.W. Christie lipidlibrary.acs.rg 2

Mass spectra f DMX derivatives. Part 1. Saturated and branched-chain fatty acids We als have mass spectra n file fr the DMX derivatives f many mre saturated fatty acids (6:0 t :0), including a number labelled with stable istpes (Hamiltn, and Christie, 00). These are illustrated withut interpretatin in the Archive sectin f these web pages. Mn-Methyl-Branched-Chain Saturated Fatty Acids DMX derivatives are nt the best fr lcating methyl-branched in fatty acids, especially in the mst cmmn fatty acids f this type with is and anteis-branches, althugh they have been used fr the purpse by identifying lcal intensity minima (Yu et al., 1988). This is ne example where 3-pyridinylcarbinl esters (and pyrrlidides) are certainly much better. The explanatin lies in the fact that with saturated fatty acids, a methyl grup can be lst readily frm the hetercyclic ring t give tw series f ins, which further fragment and cnfund any interpretatin, especially in the higher mlecular weight regin f the spectrum (Hamiltn and Christie, 00). Hwever, if taken in cnjunctin with GC retentin data, these spectra may suffice fr identificatin. DMX derivative f 15-methyl-hexadecanate (is-methyl-16:0) - 57 83 98 8 154 168 2 182 2 224 252 294 0 1 1 1 1 0 2 2 2 2 0 323 DMX derivative f 14-methyl-hexadecanate (anteis-methyl-16:0) - 8 72 83 98 1 168 182 2 224 252 2 294 323 0 1 1 1 1 0 2 2 2 2 0 The spectra f the abve tw fatty acids resemble thse f the crrespnding saturated fatty acids, ther than minr changes in in intensity, with n features that lcate the branch pint, c.f. the spectra f the 3-pyridinylcarbinl esters r pyrrlidides n the apprpriate webpages. W.W. Christie lipidlibrary.acs.rg 3

Mass spectra f DMX derivatives. Part 1. Saturated and branched-chain fatty acids DMX derivative f 11-methyl-ctadecanate. In this and ther ismers, where the methyl branch is mre central, it can be lcated by the gap f 28 amu between = 224 and 252, fr the lss f carbn 11 and the assciated methyl grup. Hwever, as the diagnstic ins are f relatively lw abundance, interpretatin can be difficult when faced with an unknwn. 224 252 72 83 1 168 182 224 252 294 8 322336 351 0 1 1 1 1 0 2 2 2 2 0 3 3 In the spectrum f the DMX derivative f -methyl-ctadecanate (tuberculstearate), the gap f 28 amu is nw between = 2 and - 2 72 85 1 168 182 2 336 252 8 322 351 294 0 1 1 1 1 0 2 2 2 2 0 3 3 The spectrum f the DMX derivative f 9-methyl-tetradecanate has the gap f 28 amu between = and 224. 224 57 72 98 1 154 168 182 2 224 252 295 0 1 1 1 1 0 2 2 2 2 W.W. Christie lipidlibrary.acs.rg 4

Mass spectra f DMX derivatives. Part 1. Saturated and branched-chain fatty acids The mass spectrum f the DMX derivative f 3-methyl-pentadecanate (unpublished) is unusual if nt unexpected in that the custmary in at = is missing and there is a gap f 27 amu between = t 1 t lcate the branch pint. 1 1 98 72 84 154 1 8 222 252 294 9 0 1 1 1 1 0 2 2 2 2 We have mass spectra n file fr the DMX derivatives f many mre branched-chain fatty acids. These are illustrated withut interpretatin in the Archive sectin f these web pages. Multi-Methyl-Branched-Chain Saturated Fatty Acids Isprenid fatty acids, derived frm phytl, are fund at lw levels in many animal species, but especially thse f marine rigin. Mass spectra f the three mst cmmn f these are nw illustrated. The DMX derivative f 4,8,12- trimethyl-tridecanate - 154 224 294 72 98 154 182 224 252 294 0 1 1 1 1 0 2 2 2 2 9 The branch pints are indicated by the gaps f 28 amu between the ins marked, althugh that fr the methyl grup in psitin 12 is nt distinct. The spectrum f the DMX derivative f 2,6,,14-tetramethyl-pentadecanate r pristanate is next. Althugh the diagnstic ins tend t be small, each f the branch-pints can be lcated frm the spectrum. The methyl branch in psitin 2 is easily seen as the expected ins at = and are shifted upwards t = 127 and 1, respectively. The remaining branches are lcated by the gaps between = 168 and, and and 8 and 336 (the rigin f the in at = 3 is nt knwn). W.W. Christie lipidlibrary.acs.rg 5

Mass spectra f DMX derivatives. Part 1. Saturated and branched-chain fatty acids 127 127 336 168 8 72 86 112 1 152 168 8? 3 336 351 0 1 1 1 1 0 2 2 2 2 0 3 DMX derivative f 3,7,11,15-tetramethyl-hexadecanate r phytanate - 1 2 2 3 182 252 322 1 57 71 182 3 83 2 224 252 2 294 322 365 0 1 1 1 1 0 2 2 2 2 0 3 3 In this instance, nly the in at = is shifted upwards by 14 amu (c.f. the spectrum fr 3- methyl-15:0 abve), and the gaps f 28 amu between = 182 and 2, 252 and 2, and 322 and 3 serve t lcate the methyl branches in psitins 7, 11 and 15, respectively. It is interesting that the is-methyl branch in the fatty acid can be clearly lcated, althugh this is nt pssible fr mn-methyl-substituted fatty acids. We als have the mass spectrum f the DMX derivative f a nn-isprenid dimethyl fatty acid (8,-dimethyl-16:0) in ur Archive sectin. References Christie, W.W. Gas chrmatgraphy-mass spectrmetry methds fr structural analysis f fatty acids. Lipids, 33, 343-353 (1998). Hamiltn, J.T.G. and Christie, W.W. Mechanisms fr in frmatin during the electrn impact-mass spectrmetry f piclinyl ester and 4,4-dimethylxazline derivatives f fatty acids. Chem. Phys. Lipids, 5, 93-4 (00). Spitzer, V. Structure analysis f fatty acids by gas chrmatgraphy - lw reslutin electrn impact mass spectrmetry f their 4,4-dimethylxazline derivatives - a review. Prg. Lipid Res., 35, 387-8 (1997). Yu, Q.T., Liu, B.., Zhang, J.Y. and Huang, Z.H. Lcatin f methyl branchings in fatty acids: Fatty acids in urpygial secretin f Shanghai ducks by GC-MS f 4,4-dimethylxazline derivatives. Lipids, 23, 4-8 (1988). W.W. Christie lipidlibrary.acs.rg 6

Mass spectra f DMX derivatives. Part 1. Saturated and branched-chain fatty acids Zhang, J.Y., Yu, Q.T., Liu, B.. and Huang, Z.H. Chemical mdificatin in mass spectrmetry IV. 2- Alkenyl-4,4-dimethylxazlines as derivatives fr duble bnd lcatin f lng-chain lefinic acids. Bimed. Envirn. Mass Spectrm., 15, 33-44 (1988). William W. Christie James Huttn Institute (and MRS Lipid Analysis service), Invergwrie, Dundee (DD2 5DA), Sctland September 11 th, 13 W.W. Christie lipidlibrary.acs.rg 7