【作者】 蔡泉源；

【导师】 宋洪昌；

【作者基本信息】 南京理工大学 ， 材料加工工程， 2011， 博士

【摘要】 聚多糖在自然界储量丰富并可再生,广泛应用于医药、化工、食品、化妆品等领域。聚多糖种类繁多,常见的有壳聚糖、透明质酸等。聚多糖的应用性能与相对分子质量密切相关。当相对分子质量降低后,其性能有着显著的提高,特别是许多独特功能只有在其相对分子质量降至一定程度后才能表现出来。然而聚多糖的相对分子质量往往比较高,极大限制了其应用,因此聚多糖的降解具有重要的意义。本课题以壳聚糖和透明质酸为研究对象,设想通过有催化活性的电极产生活性氧使之降解,该方法无需加入化学或生化反应试剂,利于产物分离提纯,反应易控制,反应装置简单,环境友好,具有良好的工业化应用前景。基于此,本文首先详细探讨了该方法对壳聚糖的降解规律、降解动力学及其机理。在此基础上,采用同样的方法降解透明质酸,旨在为该方法在聚多糖降解中的应用提供理论和技术基础。主要研究内容及结果如下：(1)采用钛基氧化钌(Ti/TiO2-RuO2)和钛基锡锑(Ti/Sb-SnO2)电极皆可有效降低壳聚糖的相对分子质量,其中Ti/Sb-SnO2电极降解壳聚糖的效果明显高于Ti/TiO2-RuO2电极。两种电极的降解效果都随着电流密度、反应温度、原料相对分子质量、乙酸浓度的增大而提高,随壳聚糖初始浓度的增大而减小,基本不受乙酸钠浓度的影响,有磁力搅拌而略微提高；两种电极的降解都基本不改变其产物的主链结构、脱乙酰度,仅在其产物的端基处生成羰基或羧基,略微降低其产物的结晶度和热稳定性。(2)在本实验条件下,Ti/TiO2-RuO2和Ti/Sb-SnO2两种电极的电化学降解反应均为零级反应动力学,两种电极降解壳聚糖的反应活化能皆明显低于H202降解以及亚硝酸水解的反应活化能。Ti/TiO2-RuO2电极的表观速率常数与电流密度的1.28次方、原料初始浓度的-1.04次方、乙酸浓度的0.3次方成线性关系,基本不受乙酸钠浓度的影响；Ti/Sb-SnO2电极的表观速率常数与电流密度的1.13次方、原料初始浓度的-1.36次方、乙酸浓度的0.19次方成线性关系,也基本不受乙酸钠浓度的影响。(3)在电化学降解壳聚糖过程中,其反应历程为：Ti/TiO2-RuO2和Ti/Sb-SnO2两种电极首先分别在其电极表面电解水产生化学吸附的活性氧(晶格中高价态氧化物的氧)和物理吸附的活性氧(吸附的羟基自由基),进而攻击壳聚糖主链上的糖苷键,导致壳聚糖断链。(4)采用电化学法可降解制得低分子量透明质酸。在降解过程中,透明质酸化学结构基本不变,只在其端基位置生成C=O双键。与壳聚糖的降解反应历程相似,透明质酸主链上的糖苷键也受电极表面生成活性氧的攻击,导致透明质酸断链。更多还原

【Abstract】 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.更多还原