Fast atom bombardment and tandem mass spectrometry for determining structural modification of fatty acids

Abstract

Previous studies showed that colllsionally activated gasphase carboxylate anlons of saturated fatty acids undergo losses of the elements of CH4, C2H6, C,H6, ... by way of a highly speclfk 1,4-eUmlnatbn of H, . These CnH2n+2 loses begln at the alkyl terminus and progress along the entire alkyl chaln. I n this paper we report that the presence of a substituent such as an alkyl branch, hydroxy group, cyclopropane rlng, cyclopropene ring, or epoxide rlng Interrupts this process in a characteristic fashlon that permits lndentlflcation of the substltuent and location of its position on the acid chaln. This method not only is useful for structural characterlzatlon of pure free fatty aclds but also Is applicable to the analysis of mixtures of carboxylic acids.Many lower forms of organisms such as bacteria, sponges, and certain plants possess the capability of producing a variety of structurally modified fatty acids. The structural modifications include branching, cyclopropane rings, cyclopropene rings, epoxy substituents, hydroxy and alkoxy groups, and unusual unsaturated acids. These acids are of interest because of their uniqueness and their utility for characterizing the organisms (1-5). Moreover, they may have substantial physiological implications for the organisms producing those substances and for organisms with which they come in contact.The problem of studying a specific fatty acid from a natural source is complicated because it must be isolated from a complex mixture of similar compounds. Classical methods of lipid analysis are described in several texts (4,6, 7). A wide range of techniques have been used for characterizing the purified compounds. They include comparison of chromatographic retention times with those of known compounds, degradative analysis, derivatization followed by analysis, synthesis of model compounds for comparison, and a variety of spectroscopic methods.Mass spectrometry has been especially useful for studying these compounds because pertinent structural information may be obtained from small samples, and gas chromatography/mass spectrometry (GC/MS) is an effective method for dealing with mixtures (8). The recent characterization of lipids of marine sponges by Djerassi and co-workers exemplifies the practical applicability of the method (ref 9 and 10 and references cited therein). Unfortunately, derivatization is necessary, as free acids are not amenable to GC/MS.A number of mass spectrometry methods have been applied to the study of structurally modified acids (11). Pioneering work in the field was done by Stenhagen and co-workers (12-16) who studied methyl esters of fatty acids by electron ionization (EI) mass spectrometry. Informative high mass ions are of low abundance and if double bonds are present, their location is obscured by rearrangments. Pyrrolidide (1 7-20) and more recently picolinyl derivatives of fatty acids (21-23) were established to be useful for locating the double bond position and other structural modifications. Chemical ionization (CI) mass spectrometry has been tip...

Location of functional groups in mycobacterial meromycolate chains; the recognition of new structural principles in mycolic acids

“…3(b) proximal. The fragment ion at m\z 363 had a stronger intensity than the corresponding one in the spectrum of a cis cyclopropane compound, as reported by Tomer et al (1986). The two pairs of ions due to the cyclopropane ring bond cleavage, observed in the spectrum of a cis cyclopropane ring (Fig.…”

“…A gap of 28 mu, observed between the ion at m\z 363, produced by cleavage of the cyclopropane\ alkyl carbon on the distal side, and the ion at m\z 391, demonstrated the presence of a methyl group on the distal side of the cyclopropane ring. The location of the hydroxy group was revealed by an ion at m\z 657, due to H # O loss (Tomer et al, 1986). As was the case for the distal methoxy group (Fig.…”

“…A methyl branch in the alkyl chain is characterized by a gap of 28 mu, as it involves almost simultaneous losses of the methyl and the methine to which the methyl is attached (Tomer et al, 1986). The spacing between m\z 225 and m\z 253 in the proximal spectra of Fig.…”

“…), and by a pair of ions of low intensities at m\z 445\447. The location of the proximal cis cyclopropane was shown by a characteristic gap of 40 mu between m\z 197 and m\z 237 (Tomer et al, 1986) along with two pairs of fragment ions at m\z 209\211 and m\z 223\225, due to the cleavage of the bonds in the cyclopropane ring.…”

Identification of Monohydroxy Fatty Acids by Electrospray Mass Spectrometry and Tandem Mass Spectrometry

“…18 O fatty acid derivatives were relatively easy to interpret, but revealed several unexsynthesized using slight modifications of the tech-pected product ions. The MS/MS spectrum of 2-OH niques of Wescott et al (36).…”