Protonation constants of chitosan with different molecular weight and degree of deacetylation

The protonation constants of chitosans with different molecular weight (Mw) and degree of deacetylation were demonstrated by the modified Henderson-Hasselbalch model and Hِgfeldtʹs three-parameter model. The protonation constants pKa of chitosan showed a slightly decreasing from 6.51 to 6.39 when the molecular weight changing from 1370 to 60 kDa. The degree of deacetylation showed a greatly effect on pKa values, which were increased from 6.17 to 6.51 with the degree of deacetylation decreasing from 94.6 to 73.3%. The degree of deacetylation influenced the balance of hydrophobic interactions and hydrogen bondings on chitsoan. The fitting of the titration data to both two methods showed that Hِgfeldtʹs three-parameter model provided a better agreement. The results obtained in this paper allowed to a better understanding of some physicochemical mechanisms and biological properties of chitosan.