Collision-induced decomposition of sphingomyelins for structural elucidation

Fast atom bombardment (FAB) and collision-induced decomposition (CID) are used to determine the structures of sphingomyelins. CID of [M + H]+ ions; [M + Cat]+ ions, in which Cat = Li+, Na+, K+, Rb+ or Cs+; and [M – 15]−, [M – 60]−, and [M – 86]− ions are investigated. CID of the anions reveals fragmentations that allow simultaneous characterization of both the ceramide and the phosphorylcholine portions of sphingomyelins. Consequently, information regarding the lengths of the sphingoid base and fatty acyl chain, and the locations of double bonds on the fatty acyl chain can be directly obtained. Structures of individual components of complex mixtures can also be determined without prior chemical derivatization or gas chromatographic separation. The detection limits for obtaining acceptable FAB and CID spectra of the [M – 15]− ions of N-oleoyl-(4E)-sphingenine phosphorylcholine are 4 and 30 pmol, respectively.