Structural studies of an arabinan from the stems of Ephedra sinica by methylation analysis and 1D and 2D NMR spectroscopy

Plant arabinan has important biological activity. In this study, a water-soluble arabinan (M-w similar to 6.15 kDa) isolated from the stems of Ephedra sinica was found to consist of (1 -> 5)-Araf, (1 -> 3,5)-Araf, T-Araf, (1 -> 3)-Araf and (1 -> 2,5)-Araf residues at proportions of 10:2:3:2:1. A tentative structure was proposed by methylation analysis, nuclear magnetic resonance (NMR) spectroscopy (H-1 NMR,C-13 NMR, DEPT-135, H-1-H-1 COSY, HSQC, HMBC and ROESY) and literature. The structure proposed includes a branched (1 -> 5)-alpha-Araf backbone where branching occurs at the O-2 and O-3 positions of the residues with 7.7% and 15.4% of the 1,5-linked alpha-Araf substituted at the O-2 and O-3 positions. The presence of a branched structure was further observed by atomic force microscopy. This polymer was characterized as having a much longer linear (1 -> 5)-alpha-Araf backbone as a repeating unit. In particular, the presence of alpha-Araf -> 3)-alpha-Araf-(1 -> 3)-(alpha-Araf-(1 -> attached at the O-2 is a new finding. This study may facilitate a deeper understanding of structure-activity relationships of biological polysaccharides from the stems of E. sinica. (C) 2014 Elsevier Ltd. All rights reserved.