Structural characterization of polysaccharides isolated from grape stalks of Vitis vinifera L.

The main structural polysaccharides of grape stalks are cellulose, heteroxylan, and glucan. Cellulose contributes 30.3% of grape stalk matter and has an unusually high degree of the crystallinity (75.4%). Among hemicelluloses, xylan was the most abundant one, contributing ∼12% to the weight. The heteroxylan was isolated from the corresponding peracetic holocellulose by DMSO extraction followed by precipitation in ethanol. The Mw of heteroxylan (19.0 kDa) was assessed by size exclusion chromatography (SEC) and the structure was inferred by methanolysis and methylation linkage analysis, as well as 1D and 2D nuclear magnetic resonance (NMR) spectroscopy. The heteroxylan is a partially acetylated (DS = 0.49) glucuronoxylan possessing the main backbone composed by β-(1→4)-linked d-xylopyranosyl units ramified with α-(1→2)-linked 4-O-methyl-α-d-glucuronosyl residues (MeGlcpA) at a molar ratio 25:1. The isolated heteroxylan contained concomitant β-glucan (ca. 15%), whose structure was elucidated by methylation linkage analysis and by NMR spectroscopy. The results obtained revealed mixed β-(1→3; 1→4)-d-glucan with a molar ratio of β-(1→3)- to β-(1→4)-linked glucopyranosyl units of 1:2.