The functioning of a wide variety of charged macromolecules, from DNA to fuel cell membranes, is dependent on how the counterions surrounding them are arranged. In order to decrease Coulombic repulsion, some of the fixed charges on these molecules are neutralized by a fraction of the counterions─this phenomenon is called counterion condensation. The nature of counterion condensation can be only be inferred indirectly from traditional experiments such as X-ray scattering and modern experiments such as single molecule electrometry. The prevalent conclusion in the literature, based on both theory and experiment, is that the distribution of counterions is peaked right next to the macromolecule, i.e., condensation results in the formation of contact ion pairs. In this study, cryogenic electron microscopy (cryo-EM) was used to study the arrangement of condensed halide counterions near a positively charged polypeptoid nanofiber. The locations of both condensed and fixed charges were determined directly from atomic-scale images. Our experimentally determined counterion distributions were peaked at distances of about 5 Å away from the fixed positive charge, indicating the presence of a layer of water molecules between condensed ion pairs. We posit that this distribution is driven by the entropy of the condensed ions.

中文翻译：

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凝聚态反离子的原子级成像


从 DNA 到燃料电池膜，各种带电大分子的功能取决于它们周围的反离子的排列方式。为了减少库仑排斥力，这些分子上的一些固定电荷被一小部分反离子中和——这种现象称为反离子凝聚。反离子凝聚的性质只能从传统实验（如 X 射线散射）和现代实验（如单分子电学）中间接推断出来。基于理论和实验的文献中普遍得出的结论是，反离子的分布在大分子旁边达到峰值，即缩合导致接触离子对的形成。在这项研究中，低温电子显微镜 （cryo-EM） 用于研究带正电荷的聚肽纳米纤维附近凝聚卤化物反离子的排列。凝聚电荷和固定电荷的位置都是直接从原子尺度图像中确定的。我们实验确定的反离子分布在距离固定正电荷约 5 Å 处达到峰值，表明凝聚离子对之间存在一层水分子。我们假设这种分布是由凝聚离子的熵驱动的。