方阱均聚物中螺旋的结构和稳定性
Structure and stability of helices in square-well homopolymers
摘要：
最近，已经证明[Magee等人，物理修订信函96，207802（2006）]，分离的方阱均聚物可以在足够低的温度下自发地破坏手征对称性并冻结成螺旋结构。这种行为是有趣的，因为方阱均聚物本身是非手性的。在这项工作中，我们使用事件驱动的分子动力学，结合一个优化的并行回火计划，在很宽的参数范围内研究这个聚合物模型。我们研究的螺旋结构是稳定的条件，并确定如何的相互作用参数的聚合物治理的螺旋结构的细节。发现方形井的宽度（与λ成比例）控制螺旋的半径，螺旋的半径随着井宽度的增加而减小，直到聚合物形成足够大的井的盘绕球体。螺旋被发现是稳定的，只有一个窗口的分子量。如果聚合物太短，螺旋就不会形成。如果聚合物太长，螺旋不再是最低能量结构，会形成其他折叠结构。该窗口的大小由链刚度决定，在该模型中，链刚度是单体大小与键长之比的函数。在这个窗口之外，聚合物在低温下仍然冻结成锁定结构，然而，除非链足够刚性，否则这种结构将不是唯一的，并且类似于玻璃态。
英文摘要：
Recently, it has been demonstrated [Magee et al., Phys. Rev. Lett. 96, 207802 (2006)] that isolated, square-well homopolymers can spontaneously break chiral symmetry and freeze into helical structures at sufficiently low temperatures. This behavior is interesting because the square-well homopolymer is itself achiral. In this work, we use event-driven molecular dynamics, combined with an optimized parallel tempering scheme, to study this polymer model over a wide range of parameters. We examine the conditions where the helix structure is stable and determine how the interaction parameters of the polymer govern the details of the helix structure. The width of the square well (proportional to lambda) is found to control the radius of the helix, which decreases with increasing well width until the polymer forms a coiled sphere for sufficiently large wells. The helices are found to be stable for only a window of molecular weights. If the polymer is too short, the helix will not form. If the polymer is too long, the helix is no longer the minimum energy structure, and other folded structures will form. The size of this window is governed by the chain stiffness, which in this model is a function of the ratio of the monomer size to the bond length. Outside this window, the polymer still freezes into a locked structure at low temperature, however, unless the chain is sufficiently stiff, this structure will not be unique and is similar to a glassy state.
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