Flexible, charged biopolymers in monovalent and mixed-valence salt: Regimes of anomalous electrostatic stiffening and of salt insensitivity,Physical Review E

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Flexible, charged biopolymers in monovalent and mixed-valence salt: Regimes of anomalous electrostatic stiffening and of salt insensitivity
Physical Review E ( IF 2.2 ) Pub Date : 2021-07-19 , DOI: 10.1103/physreve.104.014504
Sarah N Innes-Gold ₁ , David R Jacobson ₂ , Philip A Pincus ₃ , Mark J Stevens ₄ , Omar A Saleh ₅

Affiliation

The conformations of biological polyelectrolytes (PEs), such as polysaccharides, proteins, and nucleic acids, affect how they behave and interact with other biomolecules. Relative to neutral polymers, PEs in solution are more locally rigid due to intrachain electrostatic repulsion, the magnitude of which depends on the concentration of added salt. This is typically quantified using the Odijk-Skolnick-Fixman (OSF) electrostatic-stiffening model, in which salt-dependent Debye-Hückel (DH) screening modulates intrachain repulsion. However, the applicability of this approach to flexible PEs has long been questioned. To investigate this, we use high-precision single-molecule elasticity measurements to infer the scaling with salt of the local stiffness of three flexible biopolymers (hyaluronic acid, single-stranded RNA, and single-stranded DNA) in both monovalent and mixed-valence salt solutions. In monovalent salt, we collapse the data across all three polymers by accounting for charge spacing, and find a common power-law scaling of the electrostatic persistence length with ionic strength with an exponent of

0.66 \pm 0.02

. This result rules out simple OSF pictures of electrostatic stiffening. It is roughly compatible with a modified OSF picture developed by Netz and Orland; alternatively, we posit the exponent can be explained if the relevant electrostatic screening length is the interion spacing rather than the DH length. In mixed salt solutions, we find a regime where adding monovalent salt, in the presence of multivalent salt, does not affect PE stiffness. Using coarse-grained simulations, and a three-state model of condensed, chain-proximate, and bulk ions, we attribute this regime to a “jacket” of ions surrounding the PE that regulates the chain's effective charge density as ionic strength varies. The size of this jacket in simulations is again consistent with a screening length controlled by interion spacing rather than the DH length. Taken together, our results describe a unified picture of the electrostatic stiffness of polyelectrolytes in the mixed-valence salt conditions of direct relevance to cellular and intercellular biological systems.

中文翻译：

单价和混合价盐中的柔性带电生物聚合物：异常静电硬化和盐不敏感性机制

生物聚电解质 (PE) 的构象，例如多糖、蛋白质和核酸，会影响它们的行为方式以及与其他生物分子的相互作用。相对于中性聚合物，由于链内静电排斥，溶液中的 PE 更局部刚性，其大小取决于添加盐的浓度。这通常使用 Odijk-Skolnick-Fixman (OSF) 静电硬化模型进行量化，其中盐依赖性 Debye-Hückel (DH) 筛选调节链内排斥。然而，这种方法对柔性 PE 的适用性长期以来一直受到质疑。为了对此进行研究，我们使用高精度单分子弹性测量来推断三种柔性生物聚合物（透明质酸、单链 RNA、和单链 DNA）在单价和混合价盐溶液中。在单价盐中，我们通过考虑电荷间距来折叠所有三种聚合物的数据，并找到静电持久长度与离子强度的共同幂律标度，指数为

0.66 \pm 0.02

. 该结果排除了静电硬化的简单 OSF 图片。大致兼容Netz和Orland开发的修改后的OSF图片；或者，如果相关的静电屏蔽长度是离子间距而不是 DH 长度，我们假设指数可以被解释。在混合盐溶液中，我们发现在多价盐存在的情况下添加单价盐不会影响 PE 刚度的方案。使用粗粒度模拟和凝聚离子、链状离子和本体离子的三态模型，我们将这种状态归因于 PE 周围的离子“外套”，随着离子强度的变化，它调节链的有效电荷密度。模拟中该护套的尺寸再次与由中间间距而不是 DH 长度控制的屏蔽长度一致。综合起来，

更新日期：2021-07-19

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