Structural determination of glycosphingolipids as lithiated adducts by electrospray ionization mass spectrometry using low-energy collisional-activated dissociation on a triple stage quadrupole instrument

Structural characterization of glycosphingolipids as their lithiated adduct s using low-energy collisional-activated dissociation (CAD) tandem mass spe ctrometry with electrospray ionization (ESI) is described. The tandem mass spectra contain abundant fragment ions reflecting the long chain base (LCB) , fatty acid, and the sugar constituent of the molecule and permit unequivo cal identification of cerebrosides, di-, trihexosyl ceramides and globoside s. The major fragmentation pathways arise from loss of the sugar moiety to yield a lithiated ceramide ion, which undergoes further fragmentation to fo rm multiple fragment ions that confirm the structures of the fatty acid and LCB. The mechanisms for the ion formation and the possible configuration o f the fragment ions, resulting from CAD of the lithiated molecular ions ([M + Li](+)) of monoglycosylceramides are proposed. The mechanisms were suppo rted by CAD and source CAD tandem mass spectra of various cerebrosides and of their analogous molecules prepared by H-D exchange. Constant neutral los s and precursor ion scannings to identify galactosylceramides with sphingos ine or sphinganine LCB subclasses, and with specific N-2-hydroxyl fatty aci d subclass in mixtures are also demonstrated. (C) 2001 American Society for Mass Spectrometry.