MASS SPECTRA OF DERIVATIVES OF CYCLOPROPYL AD CYCLOPROPEYL FATTY ACIDS The dcument des nt aim t be a cmplete accunt f mass spectrmetry f cyclprpyl and cyclprpenyl fatty acids, but rather is a persnal accunt f ur experience f thse encuntered during ur research activities and fr which we have spectra available fr illustratin purpses. Spectra f methyl esters, 3-pyridylcarbinl ( piclinyl ) esters, DMOX derivatives and pyrrlidides are described in the same dcument. Where we are aware f prir illustratins f mass spectra in the literature, the apprpriate papers are cited. These ntes are a practical guide rather than a mechanistic accunt. The ccurrence and bilgical prperties f cyclic fatty acids were reviewed by Sébédi and Grandgirard (1989). Cyclprpyl Fatty Acids Cyclprpyl fatty acids are cmmn cnstituents f bacterial lipids and may accmpany cyclprpene fatty acids as minr cmpnents f certain seed ils. Mass spectra f methyl esters f cyclprpyl fatty acids are nt very infrmative as the ring appears t rearrange under electrn bmbardment in the mass spectrmeter t give a duble bnd. Spectra are thus indistinguishable frm thse f mnenic fatty acids with an alkyl chain ne carbn lnger (Christie and Hlman, 1966). The mass spectrum f methyl 9,-methylene-hexadecanate is illustrated as an example CH 3 OOC 69 83 97 111 2 125 152 166 8 194 221 233 0 1 1 1 1 0 2 2 2 282 The spectrum is in essence identical t that f methyl 9-heptadecenate. Althugh there have been suggestins in the literature that subtle differences exist between the mass spectra f cyclprpanes and mnenes that might serve fr diagnsis, these are nt very cnvincing t this authr. They may wrk with a limited range f pure mdel cmpunds, but are less likely t be f practical value in the analysis f cmplex mixtures, especially when instrumental variatins are taken int accunt. The GC retentin times f methyl esters f mnenic and cyclprpanic fatty acids are very different, and this can be useful infrmatin in deciding between pssible structures. W.W. Christie lipidlibrary.acs.rg 1
3-Pyridylcarbinl esters are by far the mst useful derivatives fr characterizatin f cyclprpane fatty acids, as they give distinctive cleavages that permit facile lcatin f the cyclprpane ring. As an example, the mass spectrum f 3-pyridylcarbinyl 9,-methylene-ctadecanate is illustrated next. 92 CH 2 OOC 247 8 164 247 288 386 69 133 151 1 4 2 274 2 344 3 372 0 1 1 1 1 0 2 2 2 2 0 3 3 3 There are the usual ins in the lwer mlecular weight regin at = 92, 8, 151 and 164, typical f a 3-pyridylcarbinl ester, but the [M-1] + in is mre abundant than the mlecular in ( = 387) itself. The distinctive in that permits lcatin f the ring is dd-numbered (uncmmn) at = 247, representing cleavage at the ring as shwn (Harvey, 1984). In the spectrum f 3-pyridylcarbinyl 11,12-methylene-ctadecanate (lactbacillic acid), illustrated next, the distinctive in has shifted 28 amu as expected t = 275. 92 CH 2 OOC 275 316 8 386 164 275 69 133 151 192 2 246 288 3 344 372 0 1 1 1 1 0 2 2 2 2 0 3 3 3 DMOX derivatives are f much less value fr the structural analysis f cyclprpyl fatty acids since they appear t underg a rearrangement t frm mnenes in a similar manner as ccurs with methyl esters (Zhang et al., 1987). This view f the mechanism is t simplistic but is serviceable. This is ne f nly a few examples where DMOX derivatives fail. W.W. Christie lipidlibrary.acs.rg 2
As an example, the mass spectrum f the DMOX derivative f 9,-methylene-ctadecanate is illustrated - O 72 83 98 1 168 2 * * 8 236 196 264 292 6 3 334 349 0 1 1 1 1 0 2 2 2 2 0 3 3 Althugh the structure can in thery be deduced frm the fact the rearranged duble bnd in psitin 9 gives a characteristic gap f 12 amu between = 196 and 8, this presuppses that the analyst is already aware that the fatty acid cntains the ring structure - nt a duble bnd (see the webpage dealing with DMOX derivatives f mnenes). On the ther hand, gas chrmatgraphic (GC) retentin times may indicate that an unusual fatty acid is present. The DMOX derivative f 11,12-methylene-ctadecanate (r lactbacillate) has the mass spectrum O 69 83 98 1 168 154 196 2224 * 264278 * 2 236 6 334 3 349 0 1 1 1 1 0 2 2 2 2 0 3 3 Again the psitin f the ring can be deduced frm the gap f 12 amu (between = 224 and 236), assuming we recgnize that it is a cyclprpyl acid nt a mnene. The mass spectra f pyrrlide derivatives f cyclprpane fatty acids clsely resemble thse f the DMOX derivatives, and like them give ambiguus spectra that are almst indistinguishable frm thse f mnenes ne carbn lnger in chain-length. The same caveats apply in interpreting the spectra. I am nt aware f publicatin elsewhere. W.W. Christie lipidlibrary.acs.rg 3
Fr example, the mass spectrum f the pyrrlidide f 9,-methylene-ctadecanate is almst identical t that f 9-nnadecenate, and the gap f 12 amu between = 196 and 8 is that expected fr a duble bnd in psitin 9 f the chain. OC 72 85 98 1 168 182 * 196 8 236 2 264 279 6 3 349 0 1 1 1 1 0 2 2 2 2 0 3 The mass spectrum f the pyrrlide f 11,12-methylene-ctadecanate (r lactbacillate) is OC 85 98 1 1 182 196 * 224 * 2 236 2 264 278 6 3 349 0 1 1 1 1 0 2 2 2 2 0 3 3 Althugh the psitin f the ring can in thery be deduced frm this frm first principles (frm the gap f 12 amu between = 224 and 236), in practice it may be better t regard it simply as a fingerprint. Again, GC retentin times f DMOX r pyrrlidide derivatives f mnenic and cyclprpanic fatty acids are very different, and this can be useful infrmatin in deciding between pssible structures. Alternative mass spectrmetric methds fr lcating cyclprpane rings in fatty acids invlve ring pening by vigrus hydrgenatin r by reactin with brn trifluride-methanl (see the nline Gas Chrmatgraphy and Lipids ). Cyclprpenyl Fatty Acids It was lng thught that GC and thus GC-MS f derivatives f cyclprpenyl fatty acid was impssible because f thermal degradatin n the GC clumn. Hwever, mdern capillary clumns W.W. Christie lipidlibrary.acs.rg 4
are relatively inert, and analysis by GC is straightfrward, prvided that apprpriate derivatizatin methds are emplyed, i.e. that acidic cnditins are avided. The mass spectra f methyl ester derivatives f cyclprpenid fatty acids tend t resemble thse f dienic fatty acids, s methyl sterculate (9,-methylene-ctadec-9-enate) has a spectrum (see belw) that differs in minr ways nly frm that f methyl nnadecadienate, and there are n bvius ins that serve t lcate the ring (Pawlwski et al., 1974). 81 95 67 CH 3 OOC 9 135 163 277 178 195 2 237 259 293 0 1 1 1 1 0 2 2 2 2 8 The mass spectrum f 3-pyridylcarbinyl sterculate is distinctive, hwever, and it is illustrated next (see als Spitzer et al., 1994). 92 293 CH 2 OOC 2 286 67 8 135 151 164 192 2 234 272 0 1 1 1 1 0 2 2 2 2 0 3 3 3 286 293 314 328 356 385 In this instance, the diagnstic cleavages ccur n either side f the ring and beta t it, giving distinctive ins at = 2 and 286. There is als an in that appears t be characteristic at = 293 (thugh nt fund apparently by Spitzer et al., 1994). This has nw been identified as cntaining the fatty acyl chain withut the 3-pyridylcarbinyl miety, i.e. it represents a distinctive type f fragmentatin that appears t be unique t and diagnstic fr cyclprpene rings (Knthe et al., 11). As might be expected, the analgus diagnstic ins in the mass spectrum f 3- pyridylcarbinyl malvalate (8,9-methylene-heptadec-8-enate) are all 14 amu lwer. W.W. Christie lipidlibrary.acs.rg 5
The mass spectrum f DMOX derivatives are less helpful, and resemble thse f an acetylenic fatty acid with the triple bnd in psitin 9 and ne carbn lnger, presumably because rearrangement ccurs in the mass spectrmeter (Spitzer, 1991)). Fr example, the mass spectrum f the DMOX derivative f sterculic acid is illustrated next. The key diagnstic ins are fr a gap f amu between = 196 and 6. O 67 81 95 168 182 1 152 196 6 234 248 262 276 2 318 332 347 0 1 1 1 1 0 2 2 2 2 0 3 The spectrum f the pyrrlidide f sterculate is very similar t this (unpublished), althugh the ins in the high mass range are less abundant, and it is illustrated belw. OC 72 81 98 1 154 168 196 6 * * 234 248 262 276 4 318 347 0 1 1 1 1 0 2 2 2 2 0 3 The key diagnstic ins are again fr a gap f amu between = 196 and 6. The crrespnding ins in the mass spectrum f the malvalate derivative are all 14 amu lwer. The simplest alternative mass spectrmetric methd fr lcating cyclprpene rings in fatty acids invlves additin f methane thil acrss the duble bnd (see the nline Gas Chrmatgraphy and Lipids ). Mass spectra f mre derivatives f cyclprpyl and cyclprpenyl fatty acids are available in ur Archive pages. W.W. Christie lipidlibrary.acs.rg 6
References Christie, W.W. and Hlman, R.T. Mass spectrmetry f lipids. 1. Cyclprpane fatty acids. Lipids, 1, 176-182 (1966). Harvey, D.J. Piclinyl derivatives fr the characterizatin f cyclprpane fatty acids by mass spectrmetry. Bimed. Envirnm. Mass Spectrm., 11, 187-192 (1984). Knthe, G., Rashid, U., Yusup, S. and Anwar, F. Fatty acids f Thespesia ppulnea: mass spectrmetry f piclinyl esters f cyclprpene fatty acids. Eur. J. Lipid Sci. Technl.,, 9 984 (11). Pawlwski,.E., Eisele, T.A., Lee, D.J., ixn, J.E. and Sinnhuber, R.O. Mass spectra f methyl sterculate and malvalate and 1,2-dialkylcyclprpenes. Chem. Phys. Lipids, 13, 164-172 (1974). Sébédi, J.L. and Grandgirard, A. Cyclic fatty acids: natural surces, frmatin during heat treatment, synthesis and bilgical prperties. Prg. Lipid Res., 28, 3-336 (1989). Spitzer, V. GC-MS (chemical inizatin and electrn impact mdes) characterizatin f the methyl esters and xazline derivatives f cyclprpene fatty acids. J. Am. Oil Chem. Sc., 68, 963-969 (1991). Spitzer, V., Marx, F., Maia, J.G.S. and Pfeilsticker, K. The mass spectra f the piclinyl ester derivatives f malvalic and sterculic acid. Fat Sci. Technl., 96, 395-396 (1994). Zhang, J.Y., Yu, Q.T. and Huang, Z.H. 2-Substituted 4,4-dimethylxazlines as useful derivatives fr the lcalisatin f cyclprpane rings in lng-chain fatty acids. J. Mass Spectrm. Sc. Japan, 35, 23- (1987). William W. Christie James Huttn Institute (and Mylnefield Lipid Analysis), Invergwrie, Dundee (DD2 5DA), Sctland Last updated: March 6 th, 14 W.W. Christie lipidlibrary.acs.rg 7