Structural analysis of an arabinan isolated from alkaline extract of the endosperm of seeds of Caesalpinia bonduc (Nata Karanja)

A water-soluble arabinan (Mw ∼2.0 × 102 kDa) isolated from the alkaline extract of the endosperm of Caesalpinia bonduc was found to consist of T-Araf, (1 → 5)-Araf, (1 → 2,5)-Araf, and (1 → 2,3,5)-Araf in a relative proportion of approximately 3:2:1:1. On the basis of acid hydrolysis, methylation analysis, and NMR experiments (1H, 13C, TOCSY, DQF-COSY, NOESY, ROESY, HMQC, and HMBC), the structure of the polysaccharide was established. The proposed repeating unit of the polysaccharide has a branched backbone composed of (1 → 5)-α-l-arabinofuranose residues where branching occurs at O-2, O-3 positions of one residue terminated with two arabinofuranosyl residues and O-2 of another one terminated with same residue situated at adjacent position.