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Biophys. J. BioFAST: First Published June 24, 2005. doi:10.1529/biophysj.105.065060
© 2005 by the Biophysical Society.

A more recent version of this article appeared on September 1, 2005.
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NUCLEIC ACIDS

Incomplete ion dissociation underlies the weakened attraction between DNA helices at high spermidine concentrations

Jie Yang 1 and Donald C. Rau 2*

1 Univ of Vermont
2 National Institutes of Health

* To whom correspondence should be addressed. E-mail: raud{at}mail.nih.gov.

Submitted on April 22, 2005
Revised on May 17, 2005
Accepted on 13 June 2005


  Abstract
We have investigated the salt sensitivity of the hexagonal to cholesteric phase transition of spermidine condensed DNA. This transition precedes the resolubilization of precipitated DNA that occurs at high spermidine concentration. The sensitivity of the critical spermidine concentration at the transition point to the anion species and the NaCl concentration indicates that ion pairing of this trivalent ion underlies this unusual transition. Osmotic pressure measurements of spermidine salt solutions are consistent with this interpretation. Spermidine salts are not fully dissociated at higher concentrations. The competition for DNA binding among the fully charged trivalent ion and the lesser charged complex species at higher concentrations significantly weakens attraction between DNA helices in the condensed state. This is contrary to the suggestion that the binding of spermidine at higher concentrations causes DNA overcharging and consequent electrostatic repulsion.

Key Words: DNA, assembly, cholesteric, ion-pairing, spermidine, spermine




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Copyright © 2005 by the Biophysical Society.