Fast atom bombardment and tandem mass spectrometry of phosphatidylserine and phosphatidylcholine

Abstract

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 C_nH_2n+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 C_nH_2n+2 losses in a manner which permits determining the type of modification and its location on the fatty acid chain.