Abstract: In this paper, the single-molecule morphology of polysaccharides extracted from Nostoc sphaeroides Kütz. was studied by atomic force microscopy. As a result, the polysaccharides were found to consist of flexible and rigid molecular chains and mainly exist in the form of flexible long chain with multi-branches, rigid chains with less or without branches and a divergent structure composed of three intertwined chains. Besides, a single helix for some of the branch chains was also found. The polysaccharides mainly existed in the conformation with abundant branches whose length was generally about 200–600 nm, the diameter of single molecules was within 1.0 nm, and the height of intertwined chains was about 1.3–2.3 nm. The rheological properties showed that the polysaccharides could form a weak gel, indicating that the polysaccharide molecules could be only slightly crosslinked. All rheological properties confirmed that the polysaccharides had multiple flexible branches.

Key words: atomic force microscopy, Nostoc sphaeroides Kütz., polysaccharide, rheological properties, chain conformation