Guest Controlled Nonmonotonic Deep Cavity Cavitand Assembly State Switching
Guest Controlled Nonmonotonic Deep Cavity Cavitand Assembly State Switching
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DOI：
10.1021/acs.jpcb.7b09021
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发表时间：
2017-11-30
影响因子：
3.3
通讯作者：
Ashbaugh, Henry S.
中科院分区：
文献类型：
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作者：
Du Tang;Barnett, J. Wesley;Ashbaugh, Henry S.
八羧酸（OA）和四内甲基八羧酸（TEMOA）是水溶性的深空腔体，具有纳米大小的非极性口袋，易于结合互补客体，如正构烷烃。实验表明，随着烷烃链长度从甲烷到十四烷的增加，OA呈现出1:1到2:2再到2:1的主客体复合物（X:Y，其中X为宿主数，Y为客体数）。TEMOA与OA的不同之处在于，它在口袋的门脉上增加了四个甲基，随着客体长度的增加，其组装状态呈现出从1:1到2:2、从1:1到2:1的非单调进展。在这里，我们提出了一个系统的分子模拟研究，以解析分子和热力学决定因素，区分这些相似的宿主观察到的组装化学计量序列。主、客之间的平均力势，通过伞式抽样确定，用来表征联想自由能。这些自由能随后用于反应网络模型来预测组合的平衡分布。我们的模型准确地再现了实验观察到的趋势，表明TEMOA的内甲基单位收缩了结合的开放。口袋，限制构象可用于结合的客人和破坏单体配合物和二聚体胶囊之间的平衡。我们模拟的成功证明了它们在解释简单的化学修饰对超分子组装的影响方面的实用性，并突出了它们在帮助自下而上设计方面的潜力。
Octa-acid (OA) and tetra-endo-methyl octa-acid (TEMOA) are water-soluble, deep-cavity cavitands with nanometer-sized nonpolar pockets that readily bind complementary guests, such as n-alkanes. Experimentally, OA exhibits a progression of 1:1 to 2:2 to 2:1 host/guest complexes (X:Y where X is the number of hosts and Y is the number of guests) with increasing alkane chain length from methane to tetradecane. Differing from OA only by the addition of four methyl groups ringing the portal of the pocket, TEMOA exhibits a nonmonotonic progression of assembly states from 1:1 to 2:2 to 1:1 to 2:1 with increasing guest length. Here we present a systematic molecular simulation study to parse the molecular and thermodynamic determinants that distinguish the succession of assembly stoichiometries observed for these similar hosts. Potentials of mean force between hosts and guests, determined via umbrella sampling, are used to characterize association free energies. These free energies are subsequently used in a reaction network model to predict the equilibrium distributions of assemblies. Our models accurately reproduce the experimentally observed trends, showing that TEMOA's endo-methyl units constrict the opening of the binding. pocket, limiting the conformations available to bound guests and disrupting the balance between monomeric complexes and dimeric capsules. The success of our simulations demonstrate their utility at interpreting the impact of even simple chemical modifications on supramolecular assembly and highlight their potential to aid bottom-up design.