Structural studies of an immunostimulating gluco-arabinan from seeds of Caesalpinia bonduc

A water-soluble gluco-arabinan (PS-II, MW ∼ 62 kDa) isolated from the alkaline extract of the endosperm of Caesalpinia bonduc showed the presence of T-Glcp, (1→4)-Glcp, (1→2,3)-Glcp, T-Araf, (1→5)-Araf, (1→2,5)-Araf, and (1→2,3,5)-Araf in a relative proportion of approximately 2:2:2:3:2:1:1. The proposed repeating unit of the polysaccharide possessed a branched backbone of two (1→3)-α-d-glucopyranose followed by four (1→5)-α-l-arabinofuranose residues. In case of two (1→3)-α-d-glucopyranose, branching occurs at O-2 by a same residue terminated by another one at O-4 position. Out of four (1→5)-α-l-arabinofuranose residues, one residue is terminated at O-2 and O-3 by two arabinofuranose residues and another one situated at the adjacent position is terminated at O-2 with same residue, and two (1→5)-α-l-arabinofuranose residues are free from branching and located before and after the two branched arabinofuranose residues. This gluco-arabinan molecule and previously reported arabinan showed similar extent of splenocytes and thymocytes stimulation, but arabinan showed appreciable macrophage activations.