Counterions are important constituents for the structure and function of nucleic acids. Using ⁷Li and ¹³³Cs nuclear magnetic resonance (NMR) spectroscopy, we investigated how ionic radii affect the behavior of counterions around DNA through diffusion measurements of Li⁺ and Cs⁺ ions around a 15-bp DNA duplex. Together with our previous data on ²³Na⁺ and ¹⁵NH₄⁺ ions around the same DNA under the same conditions, we were able to compare the dynamics of four different monovalent ions around DNA. From the apparent diffusion coefficients at varied concentrations of DNA, we determined the diffusion coefficients of these cations inside and outside the ion atmosphere around DNA (D_b and D_f, respectively). We also analyzed ionic competition with K⁺ ions for the ion atmosphere and assessed the relative affinities of these cations for DNA. Interestingly, all cations (i.e., Li⁺, Na⁺, NH₄⁺, and Cs⁺) analyzed by diffusion NMR spectroscopy exhibited nearly identical D_b/D_f ratios despite the differences in their ionic radii, relative affinities, and diffusion coefficients. These results, along with the theoretical relationship between diffusion and entropy, suggest that the entropy change due to the release of counterions from the ion atmosphere around DNA is also similar regardless of the monovalent ion types. These findings and the experimental diffusion data on the monovalent ions are useful for examination of computational models for electrostatic interactions or ion solvation.

抗衡离子是核酸结构和功能的重要组成部分。使用⁷ Li 和¹³³ Cs 核磁共振 (NMR) 光谱，我们通过测量15 bp DNA 双链体周围的 Li ⁺和 Cs ^{+离子的扩散测量，研究了离子半径如何影响 DNA 周围反离子的行为。}结合我们之前在相同条件下相同 DNA 周围^{23 个}Na ⁺和^{15 个}NH ₄ ⁺离子的数据，我们能够比较 DNA 周围四种不同单价离子的动力学。根据不同 DNA 浓度下的表观扩散系数，我们确定了这些阳离子在 DNA 周围离子气氛内部和外部的扩散系数（分别为D _b和D _f）。我们还分析了离子氛围中与 K ⁺离子的离子竞争，并评估了这些阳离子对 DNA 的相对亲和力。有趣的是，通过扩散核磁共振波谱分析的所有阳离子（即Li ⁺、Na ⁺、NH ₄ ⁺和Cs ^{+ ）都表现出几乎相同的}D _b / D _f比率，尽管它们的离子半径、相对亲和力和扩散系数存在差异。这些结果以及扩散与熵之间的理论关系表明，无论单价离子类型如何，由于从 DNA 周围的离子气氛中释放抗衡离子而导致的熵变化也相似。这些发现和单价离子的实验扩散数据对于检查静电相互作用或离子溶剂化的计算模型很有用。