A unique discrepancy in the thermal behavior exists between two species of linear polysaccharides of amylose with of α(1→4) linked d-glucan and of chitosan with β(1→4) linked d-glucosamine. Amylose and chitosan display thermal non-hysteresis and hysteresis phenomena, respectively. Amylose forms a blue-colored complex with iodine. Heating this complex to high temperatures causes the color disappear, and the complex turns white. After annealing the material, the blue color is restored. This behavior is different from that of another carbohydrate, chitosan. Chitosan forms a purple-colored complex with iodine. The color of the chitosan-iodine complex also disappears at high temperatures; however, the purple color is not restored after annealing. To explain the discrepancy in the thermal behavior between the two polysaccharides, molecular dynamics (MD) calculations were carried out to probe the effect of heating and annealing on amylose and chitosan conformations. The 12-mer oligosaccharide chains were used as a reference, and a wide exploration of the potential surfaces of both oligosaccharide 6-mers was made investigated under the restriction of a given end-to-end distance. For the amylose chain, it was predicted that amylose assumes a helical conformation as the lowest-energy structure at the bottom of a smooth-sided potential energy well. Therefore, the random coil structure of the amylose chain at high temperature can be restored to helical structure by annealing. On the other hand, chitosan was predicted to have a rugged shape potential energy surface, in which the difference between the energy values of the crystalline-like extended and hairpin conformations is small, but with a significant energy barrier between the two conformational states. The latter characteristics of chitosan result in a thermal hysteresis effect due to a long-term occupation of the trapped states at one of the hairpin conformations in the annealing process.
- Molecular dynamics
- Thermal hysteresis
ASJC Scopus subject areas
- Condensed Matter Physics
- Physical and Theoretical Chemistry