Department of Physics, George Washington University, Washington, DC 20052, USA.
Proc Natl Acad Sci U S A. 2010 Dec 14;107(50):21482-6. doi: 10.1073/pnas.1003374107. Epub 2010 Nov 22.
Understanding and manipulation of the forces assembling DNA/RNA helices have broad implications for biology, medicine, and physics. One subject of significance is the attractive force between dsDNA mediated by polycations of valence ≥ 3. Despite extensive studies, the physical origin of the "like-charge attraction" remains unsettled among competing theories. Here we show that triple-strand DNA (tsDNA), a more highly charged helix than dsDNA, is precipitated by alkaline-earth divalent cations that are unable to condense dsDNA. We further show that our observation is general by examining several cations (Mg(2+), Ba(2+), and Ca(2+)) and two distinct tsDNA constructs. Cation-condensed tsDNA forms ordered hexagonal arrays that redissolve upon adding monovalent salts. Forces between tsDNA helices, measured by osmotic stress, follow the form of hydration forces observed with condensed dsDNA. Probing a well-defined system of point-like cations and tsDNAs with more evenly spaced helical charges, the counterintuitive observation that the more highly charged tsDNA (vs. dsDNA) is condensed by cations of lower valence provides new insights into theories of polyelectrolytes and the biological and pathological roles of tsDNA. Cations and tsDNAs also hold promise as a model system for future studies of DNA-DNA interactions and electrostatic interactions in general.
理解和操纵组装 DNA/RNA 螺旋的力对生物学、医学和物理学都有广泛的影响。一个重要的研究课题是由价态≥3 的聚阳离子介导的双链 DNA（dsDNA）之间的吸引力。尽管进行了广泛的研究，但在竞争理论中，“同电荷吸引”的物理起源仍未得到解决。在这里，我们表明三链 DNA（tsDNA）比 dsDNA 带更多的电荷，被不能凝聚 dsDNA 的碱性二价阳离子沉淀。我们进一步通过研究几种阳离子（Mg(2+)、Ba(2+)和 Ca(2+)）和两种不同的 tsDNA 构建体来证明我们的观察结果是普遍的。阳离子凝聚的 tsDNA 形成有序的六方排列，在加入单价盐时会重新溶解。通过渗透压测量的 tsDNA 螺旋之间的力遵循与凝聚 dsDNA 观察到的水化力相同的形式。通过用带有点状阳离子和 tsDNA 的定义良好的系统进行探测，带更多电荷的 tsDNA（与 dsDNA 相比）被低价阳离子凝聚的反直觉观察为聚电解质理论以及 tsDNA 的生物学和病理学作用提供了新的见解。阳离子和 tsDNA 也有望成为未来研究 DNA-DNA 相互作用和静电相互作用的模型系统。
