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Nucleic Acids Research Advance Access originally published online on November 29, 2007
Nucleic Acids Research 2008 36(2):501-510; doi:10.1093/nar/gkm1038
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Nucleic Acids Research, 2008, Vol. 36, No. 2 501-510
© 2007 The Author(s)
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/2.0/uk/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Structural Biology

Interplay of ion binding and attraction in DNA condensed by multivalent cations

Brian A. Todd* and Donald C. Rau

Laboratory of Physical and Structural Biology, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892-0924, USA

*To whom correspondence should be addressed: Tel: +1 301 435 5803; Fax: +1 301 496 2172; Email: toddba{at}mail.nih.gov

Received July 20, 2007. Revised October 18, 2007. Accepted October 31, 2007.

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.


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