Two different molecular conformations found in chitosan type II salts.

Area of Science:

• Biopolymer structural analysis
• Materials science
• Chitin and chitosan chemistry

Background:

• Chitosan, a derivative of chitin, is a versatile biopolymer with applications in various fields.
• Understanding the solid-state structure of chitosan derivatives is crucial for optimizing their properties.
• Previous studies have characterized certain chitosan salt structures, but variations exist.

Purpose of the Study:

• To elucidate the type II solid-state structure of chitosan acidic salts derived from crab tendon.
• To investigate the helical conformations adopted by these chitosan salts.
• To compare the structural findings with previously reported chitosan salt conformations.

Main Methods:

• X-ray fiber diffraction technique was employed to analyze the solid-state structure.
• Linked-atom least-squares (LALS) refinement was used for structural modeling.
• Cylindrical Patterson method was applied to confirm molecular conformation.

Main Results:

• Two distinct helical conformations were identified for type II chitosan acidic salts.
• A relaxed twofold helix, with a tetrasaccharide asymmetric unit, was observed in chitosan hydrochloride (chitosan.HCl) salt.
• A novel fourfold helix, featuring a disaccharide asymmetric unit, was newly discovered in chitosan hydroiodide (chitosan.HI) salt.

Conclusions:

• Chitosan type II acidic salts exhibit structural polymorphism.
• The helical conformation is dependent on the specific acidic salt form.
• The discovery of the fourfold helix in chitosan.HI salt expands the understanding of chitosan structural diversity.