Hydration Effects Turn a Highly Stretched Polymer from an Entropic into an Energetic Spring
Hydration Effects Turn a Highly Stretched Polymer from an Entropic into an Energetic Spring
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DOI：
10.1021/acsnano.6b07071
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发表时间：
2017-01-01
期刊：
影响因子：
17.1
通讯作者：
Netz, Roland R.
中科院分区：
文献类型：
--
作者：
Liese, Susanne;Gensler, Manuel;Netz, Roland R.
聚乙二醇（PEG）是一种结构简单、无毒的水溶性高分子，在医学和制药领域被广泛用作分子连接基和间隔基。在这些应用中，PEG对构象变形的弹性响应是其功能的关键。根据教科书知识，聚合物通过熵的减少对其端到端分离的拉伸做出反应，这是由于可用构象的减少，这就是为什么聚合物通常被称为熵弹簧。通过结合单分子力谱实验与分子动力学模拟在明确的水，我们表明，熵水合作用几乎完全补偿链构象熵损失在高拉伸。我们的模拟表明，这种熵补偿是由于拉伸诱导的水分子释放，在放松状态下形成双氢键与PEG。因此，PEG的拉伸响应主要是能量的，而不是熵的，起源于高的力和水化效应引起的，而PEG骨架变形只起次要作用。这些研究结果表明，水合作用的力学大分子的重要性，并构成一个案例，揭示了拮抗相互作用的构象和水合自由度。
Polyethylene glycol (PEG) is a structurally simple and nontoxic water-soluble polymer that is widely used in medical and pharmaceutical applications as molecular linker and spacer. In such applications, PEG's elastic response against conformational deformations is key to its function. According to text-book knowledge, a polymer reacts to the stretching of its end-to-end separation by a decrease in entropy that is due to the reduction of available conformations, which is why polymers are commonly called entropic springs. By a combination of single-molecule force spectroscopy experiments with molecular dynamics simulations in explicit water, we show that entropic hydration effects almost exactly compensate the chain conformational entropy loss at high stretching. Our simulations reveal that this entropic compensation is due to the stretching-induced release of water molecules that in the relaxed state form double hydrogen bonds with PEG. As a consequence, the stretching response of PEG is predominantly of energetic, not of entropic, origin at high forces and caused by hydration effects, while PEG backbone deformations only play a minor role. These findings demonstrate the importance of hydration for the mechanics of macromolecules and constitute a case example that sheds light on the antagonistic interplay of conformational and hydration degrees of freedom.