优化条件下过氧化氢降解壳聚糖的光谱学研究

在最佳反应条件下制备过氧化氢对壳聚糖的降解产物，通过测定原料及降解产物(Mw=1 500～166.53 kDa)的氮、碳含量，分析它们的红外光谱、核磁共振碳谱、X射线衍射谱及圆二色谱，考察各产物是否保持其天然结构。结果表明：各降解产物与原料的碳含量，氮含量及脱乙酰度的最大相对偏差分别为2.4%，2.3%与6.9%，且后两者随着反应时间的延长而减小;各降解样品的红外光谱及核磁共振碳谱与原料壳聚糖的相似，说明降解过程几乎不改变原料的化学结构;经相同方法处理过的原料及降解产物在2θ=10.4°和19.8°处均存在X射线衍射峰，在210 nm处均存在一个负的圆二吸收峰，表明产物的固态结构与其在溶液中的构象均没有发生变化。说明在优化的降解条件下过氧化氢氧化降解没有改变壳聚糖的天然结构。

Spectra Analyses of Chitosans Degraded by Hydrogen Peroxide under Optimal Conditions

Chitosans with different average molecular weight (1 500-156 kDa) were prepared under optimal conditions based on our previous research by the depolymerization of initial chitosan using hydrogen peroxide, with maintaining the native structure of natural chitosan as principal consideration. Nitrogen and Carbon components of initial chitosan and degraded products were analyzed by BCHI fully automatic nitrogen determination system and Liquid total organic carbon (TOC) analyzer respectively. Fourier transform infrared (FTIR), 13C nuclear magnetic resonance (13C NMR), X-ray diffraction and circular dichroism (CD) analyses were used to characterize the structure properties of samples. The results indicated that the maximum standard deviations of carbon content and nitrogen content and the degree of deacetylation (DD) of degraded products with the corresponding values of the initial chitosan are 2.4%, 2.3% and 6.9% respectively. Besides, the differentiations of the nitrogen content and DD value of the degraded product with the corresponding values of the initial chitosan are narrowed with the increase in the reaction time. FTIR spectra of resulting products are similar to that of initial chitosan in terms of peak number and position, indicating that there are no other functional groups formed during the degradation. 13C-NMR analyses of initial chitosan and degraded products revealed that the chemical structures of resulting chitosans are not changed to any noticeable extent. X-ray diffraction patterns of initial chitosan and degraded chitosans are alike and show characteristic peaks at 2θ=10.4°and 19.8°under the condition that the initial chitosan was disposed as degraded chitosans. Circular dichroism analyses showed that all the samples exhibit a broad negative band located at about 210 nm assigned to n→π* electronic transition of the —NH—CO— chromophore on a glycosidic ring in acidic media, which demonstrated that degraded chitosans maintain their natural conformation in liquid state substantially. All these confirmed that the degraded chtiosans maintain their natural structure and conformation, and the breakage of β-1,4-glucoside bonds in macromolecule is the basic process under optimal degradation conditions.