Todd Brian A, Rau Donald C
Laboratory of Physical and Structural Biology, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892-0924, USA.
Nucleic Acids Res. 2008 Feb;36(2):501-10. doi: 10.1093/nar/gkm1038. Epub 2007 Nov 29.
We have measured forces generated by multivalent cation-induced DNA condensation using single-molecule magnetic tweezers. In the presence of cobalt hexammine, spermidine, or spermine, stretched DNA exhibits an abrupt configurational change from extended to condensed. This occurs at a well-defined condensation force that is nearly equal to the condensation free energy per unit length. The multivalent cation concentration dependence for this condensation force gives the apparent number of multivalent cations that bind DNA upon condensation. The measurements show that the lower critical concentration for cobalt hexammine as compared to spermidine is due to a difference in ion binding, not a difference in the electrostatic energy of the condensed state as previously thought. We also show that the resolubilization of condensed DNA can be described using a traditional Manning-Oosawa cation adsorption model, provided that cation-anion pairing at high electrolyte concentrations is taken into account. Neither overcharging nor significant alterations in the condensed state are required to describe the resolubilization of condensed DNA. The same model also describes the spermidine3+/Na+ phase diagram measured previously.
我们使用单分子磁镊测量了多价阳离子诱导的DNA凝聚所产生的力。在六氨合钴、亚精胺或精胺存在的情况下，拉伸的DNA会出现从伸展到凝聚的突然构型变化。这发生在一个明确的凝聚力处，该凝聚力几乎等于每单位长度的凝聚自由能。这种凝聚力对多价阳离子浓度的依赖性给出了凝聚时结合到DNA上的多价阳离子的表观数量。测量结果表明，与亚精胺相比，六氨合钴的较低临界浓度是由于离子结合的差异，而不是如先前认为的凝聚态静电能的差异。我们还表明，只要考虑到高电解质浓度下的阳离子 - 阴离子配对，凝聚DNA的再溶解可以用传统的曼宁 - 大泽阳离子吸附模型来描述。描述凝聚DNA的再溶解不需要过度充电或凝聚态的显著改变。同一模型也描述了先前测量的亚精胺3 + /Na +相图。
