A novel cyclic trimer of a β-amino acid, trans-2-aminocyclohexylcarboxylic acid, was synthesized and its conformation and ability to form assemblies investigated. FT-IR and NMR measurements and computational calculations showed that this cyclic tri-β-peptide has a C3-symmetric conformation with trans amide groups. A notable feature of the conformation is a vertical and parallel orientation of the three amide groups to the cyclic skeleton. The cyclic tri-β-peptide was crystallized from a solution in trifluoroacetic acidâmethanol (or trifluoroacetic acidâwater) to yield a rod-shaped molecular assembly, as observed by TEM. The electron crystallography of the rod-shaped assembly both in suspension and in ultrathin cross-section revealed that the cyclic tri-β-peptides were stacked up to form molecular columns, and that a two-fold screw symmetry operation along the column direction was present in the unit cell, which contained two cyclic tri-β-peptides. This indicates that all the amide groups are oriented in the same direction. Since any two molecular columns are staggered by a quarter of a c-axis length and aligned parallel to each other, the dipole moments of the columns are aligned to enhance the strength additively in the whole assembly.
我们合成了一种δ-
氨基酸的新型环状三聚体--反式-2-
氨基环己基
羧酸,并研究了它的构象和形成集合体的能力。傅立叶变换红外光谱和核磁共振测量以及计算表明,这种环状三δ肽具有反式酰胺基团的 C3 对称构象。该构象的一个显著特点是三个酰胺基团与环状骨架的垂直和平行取向。环状三δ肽在
三氟乙酸-
甲醇(或
三氟乙酸-
水)溶液中结晶后,经
TEM 观察,生成了棒状分子组装体。对棒状组装体的悬浮液和超薄横截面进行电子晶体学分析后发现,环状三牺²肽堆叠在一起形成分子柱,在包含两个环状三牺²肽的单元胞中,沿分子柱方向存在两重螺旋对称操作。这表明所有的酰胺基团都朝向同一方向。由于任何两个分子柱都错开了四分之一的 c 轴长度并相互平行排列,因此分子柱的偶极矩排列在一起,从而增强了整个组装体的叠加强度。