Ionic effects on the elasticity of single DNA molecules
Ionic effects on the elasticity of single DNA molecules
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DOI：
10.1073/pnas.94.12.6185
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发表时间：
1997-06-10
影响因子：
11.1
通讯作者：
Bustamante, C
中科院分区：
文献类型：
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作者：
Baumann, CG;Smith, SB;Bustamante, C
我们使用测力激光镊子装置来确定在存在多价阳离子的情况下，波长-噬菌体DNA的弹性性质与离子强度的函数关系。正如标准的蠕虫状聚电解质模型所预测的那样，静电对持续长度P的贡献与单价盐中的离子强度成反比，然而，离子强度并不总是决定DNA弹性性质的主导变量，即使在相同的离子强度下，单价离子和多价离子也有很大不同的影响，多价离子导致P值低至250-300埃，远低于DNA在单价盐中的高盐值450-500A。在单价盐中，离子Mg2+和Co(NH3)(6)(3+)电荷集中，产生的P值低于多胺腐胺(2+)和亚精胺(3+)，后者的电荷是线性分布的。与宏观弹性理论的预测相反，DNA的弹性拉伸模数、S和P表现出与离子强度相反的趋势，在三价阳离子浓度能够引起缩合的情况下，DNA被很好地描述为蠕虫状的链，如果通过保持分子的拉伸来防止缩合，当分子伸展处存在允许分子内接触的多价阳离子时，就会出现拉伸力，这表明在拉伸的DNA中缩合是通过“热棘轮”机制发生的。
We used a force-measuring laser tweezers apparatus to determine the elastic properties of lambda-bacteriophage DNA as a function of ionic strength and in the presence of multivalent cations. The electrostatic contribution to the persistence length P varied as the inverse of the ionic strength in monovalent salt, as predicted by the standard worm-like polyelectrolyte model, However, ionic strength is not always the dominant variable in determining the elastic properties of DNA, Monovalent and multivalent ions have quite different effects even when present at the same ionic strength, Multivalent ions lead to P values as low as 250-300 Angstrom, well below the high-salt ''fully neutralized'' value of 450-500 A characteristic of DNA in monovalent salt, The ions Mg2+ and Co(NH3)(6)(3+), in which the charge is centrally concentrated, yield lower P values than the polyamines putrescine(2+) and spermidine(3+), in which the charge is linearly distributed. The elastic stretch modulus, S, and P display opposite trends with ionic strength, in contradiction to predictions of macroscopic elasticity theory, DNA is well described as a worm-like chain at concentrations of trivalent cations capable of inducing condensation, if condensation is prevented by keeping the molecule stretched, A retractile force appears in the presence of multivalent cations at molecular extensions that allow intramolecular contacts, suggesting condensation in stretched DNA occurs by a ''thermal ratchet'' mechanism.