NMR characterization of endogenously O-acetylated oligosaccharides isolated from tomato (Lycopersicon esculentum) xyloglucan Original Research Article

Eight oligosaccharide subunits, generated by endoglucanase treatment of the plant polysaccharide xyloglucan isolated from the culture filtrate of suspension-cultured tomato (Lycopersicon esculentum) cells, were structurally characterized by NMR spectroscopy. These oligosaccharides, which contain up to three endogenous O-acetyl substituents, consist of a cellotetraose core with α-d-Xylp residues at O-6 of the two β-d-Glcp residues at the non-reducing end of the core. Some of the α-d-Xylp residues themselves bear either an α-l-Arap or a β-d-Galp residue at O-2. O-Acetyl substituents are located at O-6 of the unbranched (internal) β-d-Glcp residue, O-6 of the terminal β-d-Galp residue, and/or at O-5 of the terminal α-l-Arap residue. Structural assignments were facilitated by long-range scalar coupling interactions observed in the high-resolution gCOSY spectra of the oligosaccharides. The presence of five-bond scalar coupling constants in the gCOSY spectra provides a direct method of assigning O-acetylation sites, which may prove generally useful in the analysis of O-acylated glycans. Spectral assignment of these endogenously O-acetylated oligosaccharides makes it possible to deduce correlations between their structural features and the chemical shifts of diagnostic resonances in their NMR spectra.