NMR structural elucidation of the arabinan from Prunus dulcis immunobiological active pectic polysaccharides

The structure of the arabinan moiety of a purified arabinan-rich pectic polysaccharide from the cell walls of Prunus dulcis seeds was investigated by NMR spectroscopy. This polymer, which is able to induce a lymphocyte stimulatory effect, had an arabinan glycosidic-linkage composed of T-Araf: (1 → 5)-Araf: (1 → 3,5)-Araf: (1 → 2,3,5)-Araf in the relative proportions of approximately 3:2:1:1. Based on the 1H, 13C, COSYPR, gHSQC and gHMBC spectra of analysed arabinan, a tentative structure is presented, showing that it is composed by a very branched and, possibly, repetitive structure of seven residues. The observed (1 → 5) direct linkage between (1 → 2,3,5)-Araf and (1 → 3,5)-Araf residues support the occurrence of these contiguous branched residues in the arabinan backbone.