AFM的拉伸和给体-受体低聚紫杉烷的折叠：现象学和解释
AFM pulling and the folding of donor-acceptor oligorotaxanes: phenomenology and interpretation
摘要：
通过平衡等轴测单分子力谱AFM实验的分子动力学模拟，阐明了一类给体-受体寡紫杉烷二级结构自组装的热力学驱动力.低聚紫杉烷由环双（百草枯- \emph{p} -亚苯基）环组成，该环通过聚醚连接到1，5-二氧基萘的低聚物上。在高介电介质中使用MM 3作为力场进行模拟。所得到的力与延伸等温线显示了一个机械不稳定的区域，其中分子展开，并为选定的扩展，在高力和低力制度之间的力测量闪烁。利用加权直方图分析方法，从力与伸长数据中重建了平均力的分子势，并将其分解为能量和熵的贡献。模拟结果表明，低聚紫杉烷的折叠是积极的青睐，但熵惩罚，与精力充沛的贡献克服熵惩罚和有效地驱动自组装。此外，由AFM针尖和热力学理论的一阶相变驱动的单分子折叠/展开事件之间的类比进行了讨论和一般条件下，在分子和悬臂，出现的机械不稳定性和闪烁的力测量平衡等距拉伸实验。特别是，它示出的机械稳定性在扩展过程中观察到的性能是密切相关的力测量的波动。
英文摘要：
The thermodynamic driving force in the self-assembly of the secondary structure of a class of donor-acceptor oligorotaxanes is elucidated by means of molecular dynamics simulations of equilibrium isometric single-molecule force spectroscopy AFM experiments. The oligorotaxanes consist of cyclobis(paraquat-\emph{p}-phenylene) rings threaded onto an oligomer of 1,5-dioxynaphthalenes linked by polyethers. The simulations are performed in a high dielectric medium using MM3 as the force field. The resulting force vs. extension isotherms show a mechanically unstable region in which the molecule unfolds and, for selected extensions, blinks in the force measurements between a high-force and a low-force regime. From the force vs. extension data the molecular potential of mean force is reconstructed using the weighted histogram analysis method and decomposed into energetic and entropic contributions. The simulations indicate that the folding of the oligorotaxanes is energetically favored but entropically penalized, with the energetic contributions overcoming the entropy penalty and effectively driving the self-assembly. In addition, an analogy between the single-molecule folding/unfolding events driven by the AFM tip and the thermodynamic theory of first-order phase transitions is discussed and general conditions, on the molecule and the cantilever, for the emergence of mechanical instabilities and blinks in the force measurements in equilibrium isometric pulling experiments are presented. In particular, it is shown that the mechanical stability properties observed during the extension are intimately related to the fluctuations in the force measurements.
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