Studies on Complex Formation Mechanisms of Chitosan Based on Synchrotron Fiber Diffication

为了阐明壳聚糖与卤化氢络合物形成过程中的分子间相互作用，利用同步x射线光纤衍射数据（BL40B2, SPring-8, Hyogo, Japan）分析了壳聚糖与HBr和HI络合物的晶体结构。两个晶体均为同晶，属于单斜空间群P2_1。HBr和HI配合物的单位胞常数分别为a=9.299(9), b=9.504(8), c（纤维轴）=10.41(1)a和β= 106.93(8)°，a=9.46(2), b=9.79(2), c（纤维轴）=10.33(2)a和β=105.1(2)°。通过键原子最小二乘优化得到最终的填充模型，得到HBr配合物（93个观测点）的r因子为0.192，HI配合物（44个观测点）的r因子为0.193。卤化物离子沿c轴以约5A的间隔排列，并被四条聚合物链包围。在不对称单元中，有两个卤化物离子。一个离子接受来自NH3^+基团（N2氮）的三个氢键。另一个参与一个来自N2的氢键和两个来自伯羟基（O6氧）的氢键。此外，壳聚糖链沿b轴方向由N-H…O和C-H…O氢键连接。利用同步加速器x射线纤维衍射数据（BL38B1, Spring-8）重新分析了壳聚糖水合形态的晶体结构。壳聚糖链使薄片结构平行于bc平面，这些薄片沿着a轴堆叠。水分子在这些薄片之间形成柱状结构。卤化物离子在复杂晶体中的位置与水合形式的水分子的位置相似。结果表明，水合态水的柱状结构对络合物的形成起着重要作用。

In order to elucidate the intermolecular interactions involved in complex formation of chitosan with hydrogen halides, crystal structures of chitosan complexes with HBr and HI were analyzed based on synchrotron X-ray fiber diffraction data (BL40B2, SPring-8, Hyogo, Japan). Both crystals are isomorphous and belong to the monoclinic space group P2_1. The unit cell constants are a=9.299(9), b=9.504(8), c(fiber axis)=10.41(1) A and β= 106.93(8)°, and a=9.46(2), b=9.79(2), c(fiber axis)=10.33(2) A and β=105.1(2)° for HBr and HI complexes, respectively. The final packing models were obtained by the linked-atom least-squares refinement, which gave R-factors of 0.192 for HBr complex (93 observed spots) and 0.193 for HI complex (44 observed spots). The halide ions are aligned along the c-axis at intervals of about 5A and are surrounded by four polymer chains. In an asymmetric unit, there are two halide ions. One ion accepts three hydrogen bonds from NH3^+ groups (N2 nitrogen). The other one participates in one hydrogen bond from N2 and two hydrogen bonds from primary hydroxyl groups (O6 oxygen). In addition, the chitosan chains are linked by N-H…O and C-H…O hydrogen bonds along the b-axis direction. The crystal structure of the hydrated form of chitosan was reanalyzed using synchrotron X-ray fiber diffraction data (BL38B1, Spring-8). The chitosan chains make sheet structures parallel to the bc-plane and these sheets stack along the a-axis. Water molecules form columns between these sheets. The sites of the halide ions in the complex crystals are similar to those of the water molecules in the hydrated form. It was suggested that the columnar structure of water in the hydrated form plays an important role for the complex formation.