Study On Electrochemical Degradation Of Chitosan And Hyaluronan

Polysaccharides are abundant and can be regenerated in nature, which have extensive applications in pharmaceutical, chemical, food, cosmetics and other fields. There are many kinds of polysaccharides, such as chitosan, hyaluronan and so on. The performances of polysaccharides are related with their molecular weight. When the molecular weight of polysaccharides is reduced, their performances could be greatly improved. Especially the unique functions of polysaccharides are only obtained after their molecular weight is reduced to a certain extent. However, polysaccharides generally have high molecular weight which greatly limits their application. Therefore, the degradation of polysaccharides is of great significance.In this study chitosan and hyaluronan were chosen for degradation using reactive oxygen species which generated at the electrode surface. This treatment did not add chemical or biochemical reagents, facilitated the separation and purification of degraded products, was easy to control the reaction and environmentally friendly, and has a simple reactor and good prospect in industrial application. Therefore, the kinetics and mechanism of chitosan degradation using the above method was firstly discussed in detail in this paper. And then the degradation of hyaluronic acid was investigated using the same method in order to provide the theoretical and technical basis in the application of polysaccharides degradation. Main research contents and results are as follows:(1) Ti-based RuO2 (Ti/TiO2-RuO2) electrode and Sb-doped Ti-based SnO2 (Ti/Sb-SnO2) electrode could both reduce the molecular weight of chitosan. Ti/Sb-SnO2 electrode was much more effective than Ti/TiO2-RuO2 electrode on the degradation of chitosan. The effect of chitosan degradation using two kinds of electrodes both increased with the current density, reaction temperature, molecular weight of original chitosan and concentration of acetic acid, decreased with the initial concentration of chitosan, while the concentration of sodium acetate had a negligible effect and magnetic stirring had a slight increase on the degradation of chitosan. The chemical structure and degree of deacetylation of degraded chitosan was not obviously modified besides the new formed carboxylic or carboxyl side groups. The crystallinity and thermal stability of degradation products was slightly lower than that of original chitosan. (2) The degradation process using two kinds of electrodes (Ti/TiO2-RuO2 and Ti/Sb-SnO2) obeyed the zeroth-order reaction kinetics under the experimental conditions examined. The activation energys of the degradation process using two kinds of electrodes were significantly lower than that of that for H2O2 degradation and nitrous acid hydrolysis of chitosan. The apparent rate constant at Ti/TiO2-RuO2 electrode had the linear relationship with 1.28 power of current density,-1.04 power of initial concentration of chitosan and 0.3 power of concentration of acetic acid, while the concentration of sodium acetate had a negligible effect. The apparent rate constant at Ti/Sb-SnO2 electrode had the linear relationship with 1.13 power of current density,-1.36 power of initial concentration of chitosan and 0.19 power of concentration of acetic acid, while the concentration of sodium acetate had a negligible effect.(3) The degradation mechanism of chitosan by electrochemical process was present as follow:during electrolysis, the chemisorbed "active oxygen" (oxygen in the oxide lattice, MOx+1) and physically adsorbed "active oxygen" (adsorbed hydroxyl radicals,·OH) generated at the Ti/TiO2-RuO2 and Ti/Sb-SnO2 electrode surface, respectively, and then attacked the glycosidic bonds of chitosan chain, leading to chain scission of chitosan.(4) Low molecular weight hyaluronan could be prepared by electrochemical degradation method. In this degradation process, the chemical structure of degraded hyaluronan was not obviously modified besides the new formed C=O groups. The degradation mechanism of hyaluronan was similar to that of chitosan, and was present as follow:the glycosidic bond in the main chain of hyaluronan was attacked by reactive oxygen species generated at the electrode surface, leading to the chain scission of hyaluronan.