Water-Soluble 2,5-Anhydro-d-mannofuranose Chain End Chitosan Oligomers of a Very Low Molecular Weight: Synthesis and Characterization

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