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

为了阐明参与壳聚糖与卤化氢的复合物形成的分子间相互作用，基于同步辐射X射线纤维衍射数据（BL 40 B2，SPring-8，Hyogo，Japan）分析了壳聚糖与HBr和HI的复合物的晶体结构。这两种晶体是同晶的，属于单斜空间群P2_1。HBr和HI配合物的晶胞常数分别为a=9.299（9），B=9.504（8），c（纤维轴）=10.41（1）A和β = 106.93（8）°;通过连接原子最小二乘精修获得最终的堆积模型，其对于HBr复合物（93个观察点）和HI复合物（44个观察点）给出0.192的R因子和0.193的R因子。卤离子沿c轴沿着以约5埃的间隔排列，并被四个聚合物链包围。在不对称单元中，存在两个卤离子。一个离子接受来自NH3^+基团（N2氮）的三个氢键。另一个参与来自N2的一个氢键和来自伯羟基（O 6氧）的两个氢键。此外，壳聚糖分子链沿b轴方向以N-H. O和C-H. O氢键沿着连接。使用同步加速器X射线纤维衍射数据（BL 38 B1，Spring-8）重新分析壳聚糖的水合形式的晶体结构。壳聚糖链形成平行于bc平面的片结构，并且这些片沿着a轴堆叠。水分子在这些薄片之间形成柱状。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，水的柱状结构在水化形式中起着重要作用的复杂的形成。

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.