Garric Joachim, Léger Jean-Michel, Huc Ivan
Université Bordeaux 1 - ENITAB - CNRS UMR5248, Institut Européen, de Chimie et Biologie, 2 rue Robert Escarpit, 33607 Pessac, France.
Chemistry. 2007;13(30):8454-62. doi: 10.1002/chem.200700640.
Three aromatic oligoamides have been prepared that have alternating 1,6-diaminopyridine and 1,6-pyridinedicarboxylic acid units at the center of the sequence and two 8-amino-2-quinolinecarboxylic acid units at each extremity. The three oligomers differ in the number--3, 5, or 7-of pyridine units in the sequence. They were designed to adopt helically folded conformations in solution and in the solid state. The sequence of monomers was chosen so that the diameter of the helix is larger in the center than at each extremity, and hence they resemble helically wrapped apple peels. According to modeling studies, the pyridine units were expected to define a polar hollow within the helix that is large enough to accommodate small polar guests, whereas the quinoline units at each end of the oligomeric sequences were expected to completely cap the hollow and transform the helix cavities into a closed shell that may act as a capsule. Crystallographic studies demonstrate that the oligomers do fold into helices that define a cavity isolated from the surrounding medium in the solid state. Depending on the number of pyridine rings, one or two water molecules are bound within the capsules. The crystal structure of a capsule fragment shows that MeOH can also be hosted by the largest oligomer. Solution NMR studies confirm that binding of water also occurs in solution with the same stoichiometry as observed in the solid state. The capsules have distinct signals depending on whether they are empty, half-full, or full, and these species are in slow exchange on the NMR timescale at low temperature. Indeed, the binding and release of water molecules requires a significant conformational distortion of the helix that slows down these processes. The addition of small polar molecules such as methanol, hydrazine, hydrogen peroxide, or formic acid to the largest capsule leads to the observation of new sets of NMR signals of the capsules that were assigned to complexes with these guests. However, water appears to be the preferred guest.
已制备出三种芳香族低聚酰胺，它们在序列中心具有交替的1,6 - 二氨基吡啶和1,6 - 吡啶二甲酸单元，在每个末端具有两个8 - 氨基 - 2 - 喹啉羧酸单元。这三种低聚物在序列中吡啶单元的数量上有所不同，分别为3、5或7个。它们被设计为在溶液和固态中采用螺旋折叠构象。选择单体序列是为了使螺旋的直径在中心比在每个末端更大，因此它们类似于螺旋包裹的苹果皮。根据模型研究，吡啶单元预计会在螺旋内定义一个极性空洞，其大小足以容纳小的极性客体，而低聚物序列两端的喹啉单元预计会完全封闭这个空洞，并将螺旋腔转化为一个可作为胶囊的封闭壳。晶体学研究表明，这些低聚物在固态中确实折叠成螺旋，形成一个与周围介质隔离的腔。根据吡啶环的数量，一个或两个水分子结合在胶囊内。一个胶囊片段的晶体结构表明，甲醇也可以被最大的低聚物容纳。溶液核磁共振研究证实，在溶液中也会发生水的结合，其化学计量与在固态中观察到的相同。根据胶囊是空的、半满的还是满的，会有不同的信号，并且这些物种在低温下的核磁共振时间尺度上处于缓慢交换状态。实际上，水分子的结合和释放需要螺旋发生显著的构象扭曲，这会减缓这些过程。向最大的胶囊中加入小的极性分子，如甲醇、肼、过氧化氢或甲酸，会观察到胶囊的新的核磁共振信号集，这些信号被归因于与这些客体的复合物。然而，水似乎是首选的客体。
