Urea effect on Cu2+-induced DNA structural transitions in solution

Cu2+ ion interaction with DNA in aqueous solutions containing urea (0-5 M) was studied by IR spectroscopy. It was shown that upon the Cu2+ ion binding DNA transition into a compact form occurs. This transition is of positive cooperativity. We suppose that the mechanism of Cu2+-induced DNA compaction in solutions containing urea is not completely electrostatic. Urea addition to the DNA solution decreases the Cu2+ ion concentration required to induce DNA compaction. As the urea content in solution rises, the binding constant of Cu2+ ions interacting with DNA increases, going through the maximum in the case of 2 M solution; further increase of the urea content in solutions leads to decrease of the binding constant. DNA transition into the compact form under the Cu2+ ion action is determined not only by the effects of the solution dielectric permeability but by the solvation effects; when changes of the dielectric permeability are small the solvation effects may prevail. Urea addition to the DNA solution also decreases cooperativity of the DNA compaction process. Perhaps, cooperativity of the DNA transition into the compact state depends on the ordered spatial structure of water adjacent to the macromolecule and decreases on the structure destruction. (C) 2004 Elsevier Inc. All rights reserved.