Chemical Synthesis and Structural Determination of the Inositol Glycan Head of Plant Sphingolipid GIPC in Brassicaceae

Glycosylinositol phosphoceramides (GIPCs), a major class of plant sphingolipids, are key components of the plasma membrane and play important roles in plants. These molecules consist of a ceramide 1-phosphate tail attached to an inositol glycan (IG) head, which typically contains an d-glucuronic acid (GlcA)-α(1→2)-myo-inositol structure. The second monosaccharide linked to the O-4 position of the GlcA residue differs depending on plant species and cell tissues. In Arabidopsis, the predominant second monosaccharide has been identified as hexose (Hex) using LC-MS/MS analysis. However, the stereochemistry and anomeric configuration of the Hex have not yet been determined using spectroscopic techniques. In this study, we chemically synthesized three different Hex-type IGs in which the GlcA residue binds to d-glucose, d-galactose, or d-mannose (Man) as the second monosaccharide. Using the synthetic standards, we determined the IG head structure of naturally occurring GIPCs isolated from cabbage by using several analytical methods. We successfully confirmed that Hex as the second monosaccharide linked to the GlcA residue is d-Man with an α-glycosidic bond.