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

为了阐明壳聚糖与卤化氢复合物形成过程中涉及的分子间相互作用，基于同步加速器 X 射线光纤衍射数据（BL40B2，SPring-8，Hyogo，Japan）分析了壳聚糖与 HBr 和 HI 复合物的晶体结构。两种晶体都是同晶的，属于单斜空间群P2_1。 HBr 和 HI 复合物的晶胞常数为 a=9.299(9)、b=9.504(8)、c(纤维轴)=10.41(1) A 和 β= 106.93(8)°，以及 a=9.46(2)、b=9.79(2)、c(纤维轴)=10.33(2) A 和 β=105.1(2)°，分别。最终堆积模型通过连接原子最小二乘精修获得，HBr 络合物（93 个观察点）的 R 因子为 0.192，HI 络合物（44 个观察点）的 R 因子为 0.193。卤化物离子沿c轴以约5A的间隔排列，并被四个聚合物链包围。在不对称单元中，有两个卤离子。一个离子接受 NH3^ 基团（N2 氮）的三个氢键。另一个参与 N2 的一个氢键和伯羟基（O6 氧）的两个氢键。此外，壳聚糖链沿b轴方向通过N-H…O和C-H…O氢键连接。使用同步加速器 X 射线纤维衍射数据（BL38B1，Spring-8）重新分析了水合形式壳聚糖的晶体结构。壳聚糖链使片状结构平行于 bc 平面，并且这些片状片沿 a 轴堆叠。水分子在这些片材之间形成柱。复杂晶体中卤化物离子的位置与水合形式的水分子的位置相似。有人认为，水合形式的水的柱状结构对于复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.