Nancy J. Jensen

Fast atom bombardment and tandem mass spectrometry of phosphatidylserine and phosphatidylcholine

Fast atom bombardment (FAB) desorption of phosphatidylserine and various phosphatidylcholines produces a limited number of very informative negative ions. Especially significant is the formation of (M-H)− ions for phosphatidylserine, a compound which does not yield informative high mass ions by other ionization methods. Phosphatidylcholines of not yield (M-H)− ions but instead produce three characteristic high mass ions, (M-CH3+_−, [M-HN(CH3)3+]− and [M-HN(CH33+-C2H2]−. Both classes of lipids also yield anions attributed to the carboxylate components of these complex lipids. FAB desorption in combination with collisional activation allows for characterization of fragmentation and determination of structural features. Collisional activation of the carboxylate anion fragments from the complex lipids is especially informative. Structural characterization of the fatty acid chain can be achieved as the released saturated carboxylate anions undergo a highly specific 1,4-elimination of H2, which results in the losses of the elements of CH4, C2H6, C3H8...in a fashion entirely consistent with the chemistry of carboxylate anions desorbed from free fatty acids. These CnH2n+2 losses begin at the alkyl terminus and progress along the entire alkyl chain. Modified fatty acids undergo a similar fragmentation; however, the modification affects the series of CnH2n+2 losses in a manner which permits determining the type of modification and its location on the fatty acid chain.

FAB MS/MS for phosphatidylinostitol,-glycerol,-ethanolamine and other complex phospholipids

Fast atom bombardment (FAB) of phosphatidylinositol, phosphatidylethanolamine, cardiolipin, phosphatidic acid and phosphatidylglycerol produces a limited number of very informative negative ions. Especially significant is the formation of (M−H)− ions and ions that correspond to the carboxylate portions of these molecules. FAB desorption in combination with collisional activation allows for characterization of fragmentation and determination of structural features. Collisional activation of the carboxylate anion from complex lipids is especially informative. Structural characterization of the fatty acids can be achieved as the released saturated carboxylate anions undergo highly specific charge remote fragmentations that are entirely consistent with the chemistry of carboxylate anions desorbed from free fatty acids. This permits both identification of the modification and assignment of its location on the acid chain. FAB-desorbed alkyl acetyl glycerophosphocholines (platelet-activating factor) do not produce (M−H)− ions. However, significant high mass ions are formed, and these can be collisionally activated for structural characterization.

Fast atom bombardment and tandem mass spectrometry for determing iso- and anteiso-fatty acids

Branched fatty acids can be distinguished from isomeric straight chain fatty acids by collisionally activating the (M-H)− ions desorbed by using fast atom bombardment (FAB) mass spectrometry (MS). In particular, an acid with iso- fatty branching can be readily distinguished from one containing anteiso- branching; the latter undergoes loss of the elements of CH4 and C2H6 but not C3H8. These decompositions are another example of remote charge site fragmentation. Mixtures of homologs and isomers can be investigated by using the combination of FAB and tandem mass spectrometry (MS-MS).

A charge-remote allylic cleavage reaction: Mechanistic possibilities

The collision-induced allylic cleavage reactions of deuterium-labeled [M − H + 2Li)+ and [M − H]- ions of monounsaturated fatty acids were investigated. Three concerted mechanistic possibilities were considered for this process: a l,4-elimination of a vinylic H, a retro-ene reaction, and a l,4-conjugate elimination. A fourth mechanistic possibility, a two-step radical version of the retro-ene and l,4-conjugate elimination reactions, was also considered. The radical reactions are in accord with the isotopic labeling results and offer certain mechanistic consistencies for cleavage of both C-C allyl bonds; they are expected, however, to have large activation energies. The lower-energy concerted alternatives, the retro-ene reaction for cleavage of the proximal and the l,4-conjugate elimination for cleavage of the distal C-C allyl bond, are also consistent with experimental results. The alternative of two different concerted mechanisms for cleavage of the two allyl bonds, however, is at odds with the charge-remote concept.