Chemical Properties and Stereoisomerism of Heterogeneous Long Chain Bases in Lysosphingolipids by Positive Ion Fast Atom Bombardment Mass Spectrometry and Carbon-13 NMR Spectroscopy1

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Abstract
Positive ion fast atom bombardment (FAB) mass spectrometry of galactopsychosine and glucopsychosine was capable of showing not only the pseudo molecular ion peaks, but also various fragment ion peaks such as protonated sphingosine and its fragment ions. The percent distribution of sphingosine and dihydrosphingosine in each lysosphingolipid was determined by GLC of the trimethyl-silylated derivatives of long chain bases after methanolysis and was comparable to the relative intensities of ion peaks derived from the sphingosine and dihydrosphingosine groups. The FAB mass spectra showed that during the fast atom bombardment the sphingosine more preferentially gave rise to one and/or two fragment ions by loss of one and/or two molecules of water than the dihydrosphingosine did. The stereoisomerism of sphingosylphosphorylcholine containing mainly L- Threo -sphingosine could be reconfirmed by carbon-13 NMR spectroscopy. Furthermore, although the carbon-13 NMR signals of sphingosine C-1, C-2, C-3, C-4, and C-5 showed significant chemical shift differences between D- erythro and L- Threo -sphingosines of lysosphingolipids, it was concluded that the signal position of sphingosine C-3 was the most important for the determination of D- erythro and L-threo configuration in the long chain base moieties of lysosphingolipids.