Structure and gel characterization of pectic polysaccharide from Nicandra physaloides (L.) Gaertn seeds

A pectic polysaccharide (named NPG) was isolated from the seed of Nicandra physaloides (L.) Gaertn by hot water extraction. Its structural characteristics were investigated and elucidated by HPLC, HPGPC, IR, and 1D ¹³C-NMR, while gelatinization properties were studied by atomic force microscopy (AFM) and transmission electron microscopy (TEM). The results indicated that NPG had a molecular weight of 1.6×10⁶ u and was mainly composed of galacturonic acid, glucose, galactose, and rhamnose at a molar ratio of 12:4:1:1. ¹³C-NMR spectrum analysis showed that NPG had a backbone composed of 1→4 linked galacturonic acid. NPG presented a regular spherical structure. In the presence of calcium ions, the spherical molecules of NPG linked end-to-end, forming a chain structure with branching nodes. The chain structure cross-linked further with an increase in calcium ions and integrated to form a network of fibrous structures. Subsequently, the fibrous structures were accumulated and dehydrated, thereby forming the gel.