Abstract
The work examines the structural transitions of DNA under the action of Cu^2+^ and Ca^2+^ ions in aqueous solution at temperatures of 29 and 45°C by ir spectroscopy. Upon binding to the divalent ions studied, DNA transits into the compact state both at 29 and 45°C. In the compact state DNA remains in B‐form limits. The compaction process is of high positive cooperativity. As temperature increases the divalent metal ion concentration required to induce DNA compaction decreases in the case of Cu^2+^‐induced compaction and increases in the case of Ca^2+^‐induced compaction. It is suggested that the mechanism of the temperature effect on DNA compaction in the presence of Cu^2+^ ions possessing higher affinity for DNA bases differs from that of the temperature influence on Ca^2+^‐induced DNA compaction. In the case of copper ions the determining factor is the increase of binding constants of the Cu^2+^ ions interacting with the denatured parts formed on DNA while in the case of calcium ions it is the decreased screening action of counterions upon the increase of their hydration with temperature. The efficiency of divalent metal ions studied in inducing DNA compaction depends on hydration of counterions. DNA compaction occurs in a narrow interval of Cu^2+^ concentrations. As the Cu^2+^ ion concentration increases, DNA compaction is replaced with Cu^2+^‐induced DNA aggregation. At elevated temperatures Cu^2+^‐induced DNA compaction could acquire a phase transition character. © 2005 Wiley Periodicals, Inc. Biopolymers 77: 315–324, 2005