Water soluble 2,5-anhydro-D-mannofuranose end chain chitosan oligomers of very low molecular weight : synthesis and characterization

Five chitosans with different acetylation degree (DA), molar masses and origin were depolymerized by nitrous acid treatment in acidic media leading to water soluble 2,5 anhydro-D-mannose end chain oligomers with DPn<20. The kinetic of the reaction was studied and the best work conditions were found to be 3h reaction at 50°C. It was shown that the DPn of oligomers only depends on the quantity of NaNO2 involved. Molar masses or DA do not have an impact on the depolymerization process when targeting oligomers with less than 20 units. This depolymerization process also leads to free AMF that might react with the free amines of obtained oligomers to form imines. This reaction is pH dependant and in acidic condition, leads to the formation of 5-hydromethyl-2-furfural (HMF). At the end, the oligomers were purified with dialysis to get rid of most free AMF (<5%) and other residual salts, and appeared to have no acute toxicity.