Part 1. Normal Saturated Fatty Acids

Transcription

1 MASS SPECTRA OF METHYL ESTERS OF FATTY ACIS Prt 1. Norml Sturted Ftty Acids Methyl esters re by fr the most widely used ftty cid derivtives for nlysis in CH3 OOC generl, nd gret del of informtion is vilble on their chromtogrphic, physicl nd spectroscopic properties. However, s cutioned in the Introduction to these documents, mss spectrometry with electron-impct ioniztion of methyl esters ffords limited informtion only concerning the structures of ftty cids, especilly double bond positions. For exmple, some limited informtion on double bond positions in polyenoic ftty cids my be scertinble, but not for monoenes or dienes. No informtion cn be obtined on the geometry of double bonds. However, the moleculr weight is usully obtinble, nd this is n importnt piece of informtion. If gs chromtogrphic retention dt re dded to this, it is often possible to be certin of the identity of ftty cid. In ddition, the positions of structurl fetures, such s brnch points nd oxygented groups cn usully be deduced. In this document, mss spectr of liner sturted ftty cids only re described. The Spectr Methyl esters of stright-chin ftty cids hve chrcteristic spectr, nd some representtive exmples re illustrted below with brief detils of interprettion only. Those of the more common ftty cids were published in the erly dys of mss spectrometry (Ryhge, R. nd Stenhgen, E. Mss spectrometric studies. I. Sturted norml long-chin methyl esters. Arkiv Kemi, 13, (1959)). The mss spectrum of methyl plmitte is shown below [M-31] The moleculr ion t = 2 is clerly seen, s is n ion t 239 ([M-31] + ) representing loss of methoxyl group, nd confirming tht it is indeed methyl ester. An ion t = ([M-43] + ) W.W. Christie lipidlibrry.ocs.org 1

2 represents loss of C 3 unit (crbons 2 to 4), vi complex rerrngement, while tht t = is the McLfferty rerrngement ion. The ltter hs specil significnce (see below), not lest in tht it gives further confirmtion tht the spectrum is tht of methyl ester. An ion t = ([M-29] + ) is lso dignostic nd worthy of note. The long homologous series of relted ions (14 mu prt) t =, 1, 115,129,, 1, 199, etc. of generl formul [CH 3 -OCO(CH 2 ) n ] + is evidence tht there re unlikely to be other functionl groups in the chin. The origins of mny of these ions re discussed below. However, mss spectr of ftty cids with iso- nd nteiso-methyl brnches re esily confused with those of liner nlogues (see the web pge on methyl esters of brnched-chin ftty cids). In the mss spectr of methyl esters of unsturted ftty cids, hydrocrbon ions predominte, nd the McLfferty ion becomes much less distinctive. The mss spectrum of methyl docosnote (22:0) is shown next In essence, it is shows ll the sme fetures s tht of methyl plmitte except tht the moleculr ion nd tht for loss of the methyl group, etc. re shifted upwrds by 84 mu. Other stright-chin sturted esters would be expected to be comprble in ll essentils. For exmple, the sme fetures re seen in the mss spectr of methyl esters of ftty cids from methyl octnote (8:0) W.W. Christie lipidlibrry.ocs.org 2

3 - to methyl tricosnote (:0) - CH3OOC However, the moleculr ion cn be very smll nd is often difficult to discern in the mss spectr of sturted short-chin methyl esters. The ion representing [M-31] + must then be used for identifiction purposes. Mechnistic Aspects Although these pges re not intended to be tretise on mechnistic spects of mss spectrometry, the McLfferty rerrngement ion is centrl to the identifiction of ester derivtives of most ftty cids, so some digression in this direction seems desirble here. In fct, the McLfferty rerrngement is one of the most widely occurring nd thence most studied processes in mss spectrometry. The resulting ion is lwys importnt for identifiction purposes. CH 3 O H H O CH (CH 2 ) n CH 3 O rerrngement CH C CH 2-3 bond clevge CH 2 3 O CH 2 CH 2 = A site-specific rerrngement is involved in which hydrogen tom from position 4 of the liphtic chin migrtes to the crbo-methoxy group s illustrted, presumbly through sixmembered trnsition stte, which is stericlly fvoured. If one of the hydrogen toms on crbon 4 is substituted, then the McLfferty ion will be pprecibly lower in intensity thn expected. This my explin why it is less evident in the mss spectr of derivtives of unsturted ftty cids with incresing numbers of double bonds, which cn redily migrte to position 4 under electron bombrdment. If both hydrogens on crbon 4 re substituted, the McLfferty ion cnnot form (see for exmple the spectr of 4-thi ftty cids on the pproprite web pges s well s of the deuterted ftty cids below). If there is substituent on position 2, the vlue of the McLfferty ion will be incresed ccording to the size of the substituent. Of course, if the nture of the lcohol moiety vries, so will the size of the McLfferty ion, to = 88 for ethyl esters, 151 for 3-pyridylcrbinol ( picolinyl ) esters, nd 113 for dimethyloxzoline nd pyrrolidine derivtives, for exmple. W.W. Christie lipidlibrry.ocs.org 3

4 In ddition to the McLfferty ion, there is series of relted ions, formed by losses of neutrl liphtic rdicls, of generl formul [(CH 2 ) n COOCH 3 ] + of which tht t = is most bundnt, followed by 1, 115, 129, nd so forth. The ion t [M-43] + t = is believed to be formed vi rerrngement of the chin nd one hydrogen tom, followed by expulsion of propyl rdicl (crbons 2 to 4), gin vi sixmembered trnsition stte. Similrly, n ion t [M-29] + is presumed to rise in n nlogous mnner following n initil clevge between crbons 3 nd 4. These ions cn be useful dignosticlly when crbons 2 to 4 in ftty cid chin re substituted. Some of these fetures cn be seen in the mss spectr of methyl plmitte deuterted in specific positions, which cn be compred with tht of the norml ftty cid bove. In the first instnce, the spectrum of methyl 2,2-dideutero-hexdecnote is illustrted Both the moleculr ion nd the McLfferty rerrngement ion hve incresed by 2 units, the ltter to = 76. As would be nticipted, the ion representing the loss of methoxyl group is still t [M-31] + ( = ). On the other hnd, the ion representing loss of crbons 2 to 4 is now equivlent to the loss of 44 mu (t = 228), s is explined by the ccepted mechnism for the formtion of this ion. Next the spectrum of methyl 3,3-dideutero-hexdecnote W.W. Christie lipidlibrry.ocs.org 4

5 Now the McLfferty ion is t = gin, but the ion for [CH 3 OCO(CH 2 ) 2 ] +, formerly t = hs now incresed to 89. The ion representing loss of crbons 2 to 4 is now t [M-45] + ( = ). In this nd the previous spectrum, the ions resulting from the loss of crbons 2 nd 3 re now equivlent to [M-31] + nd coincide with tht for the loss of the methoxyl group ( = ) in low-resolution spectrum. Finlly, the spectrum of methyl 4,4-dideutero-hexdecnote The McLfferty ion is now t = 75 s one of the deuterium toms from crbon 4 hs been bstrcted during the rerrngement. The ion representing loss of crbons 2 to 4 is gin t [M-45] + ( = ), while tht for loss of crbons 2 nd 3 hs reppered t [M-29] + ( = 243). Spectr of mny more methyl esters of sturted ftty cids (4:0 to :0), including some lbelled with stble isotopes, cn be ccessed from our Archive pges (without interprettion). Willim W. Christie Jmes Hutton Institute (nd Mylnefield Lipid Anlysis), Invergowrie, undee (2 5A), Scotlnd Lst updted: Jnury 17 th, 13 W.W. Christie lipidlibrry.ocs.org 5