Lertworasirikul Amornrat, Noguchi Keiichi, Ogawa Kozo, Okuyama Kenji
Faculty of Technology, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan.
Carbohydr Res. 2004 Mar 15;339(4):835-43. doi: 10.1016/j.carres.2004.01.002.
Chitosan/HI type II salt prepared from crab tendon was investigated by X-ray fiber diffraction. Two polymer chains and 16 iodide ions (I(-)) crystallized in a tetragonal unit cell with lattice parameters of a = b = 10.68(3), c (fiber axis) = 40.77(13) A, and a space group P4(1). Chitosan forms a fourfold helix with a 40.77 A fiber period having a disaccharide as the helical asymmetric unit. One of the O-3... O-5 intramolecular hydrogen bonds at the glycosidic linkage is weakened by interacting with iodide ions, which seems to cause the polymer to take the 4/1-helical symmetry rather than the extended 2/1-helix. The plausible orientations of two O-6 atoms in the helical asymmetric unit were found to be gt and gg. Two chains are running through at the corner and the center of the unit cell along the c-axis. They are linked by hydrogen bonds between N-21 and O-61 atoms. Two out of four independent iodide ions are packed between the corner chains while the other two are packed between the corner and center chains when viewing through the ab-plane. The crystal structure of the salt is stabilized by hydrogen bonds between these iodide ions and N-21, N-22, O-32, O-61, O-62 of the polymer chains.
采用X射线纤维衍射法对由蟹腱制备的壳聚糖/ HI II型盐进行了研究。两条聚合物链和16个碘离子（I(-)）在一个四方晶胞中结晶，晶格参数为a = b = 10.68(3)，c（纤维轴）= 40.77(13) Å，空间群为P4(1)。壳聚糖形成四重螺旋，纤维周期为40.77 Å，以二糖作为螺旋不对称单元。糖苷键处的一个O-3... O-5分子内氢键通过与碘离子相互作用而减弱，这似乎导致聚合物采取4/1螺旋对称性而非伸展的2/1螺旋。发现螺旋不对称单元中两个O-6原子的合理取向为gt和gg。两条链沿c轴贯穿晶胞的角部和中心。它们通过N-21和O-61原子之间的氢键相连。从ab平面观察时，四个独立碘离子中的两个堆积在角部链之间，而另外两个堆积在角部链和中心链之间。该盐的晶体结构通过这些碘离子与聚合物链的N-21、N-22、O-32、O-61、O-62之间的氢键得以稳定。
