Internal residue loss produced by rearrangement of a novel cationic glycosphingolipid,glyceroplasmalopsychosine, in collision-induced dissociation

A novel plasmal conjugate of galactosylsphingosine (psychosine), Gro1(3)-O-plasmal-O-6Galbeta-sphingosine (glyceroplasmalopsychosine), was analyzed by electrospray ionization and liquid secondary ion mass spectrometry with low- or high-energy collision-induced dissociation (CID). In the product ion spectra of the M + H](+) ions, M + H - glycerol](+) ions arising from the loss of a glycerol were predominant. Unexpectedly, CID of the M + H - glycerol](+) ion produced an outstanding ion, (M + H - glycerol) - Hex](+), which required the loss of the galactose from inside the molecule. This ion was greatly reduced in the spectra of N,N-dimethyl derivatives, indicating that the (M + H - glycerol) - Hex](+) ion is formed from an intramolecular rearrangement with migration of the plasmal residue to the free amino group of sphingosine. It would be expected that the rearrangement occurs simultaneously with the elimination of glycerol or a rearranged M + H](+) ion leads to the elimination of glycerol, to form a Schiff base-type M + H - glycerol](+) ion, from which the terminal galactose could be removed by the normal mechanism of glycosidic cleavage. On the other hand, the M + Na - glycerol](+) ion derived from the sodiated molecule did not produce an ion corresponding to the rearrangement reaction, possibly owing to a higher stability of the sodiated ions against conformational changes.