Studies on nucleic acids. VIII. Changes in the stability of DNA secondary structure by interaction with divalent metal ions

The melting temperature of a natural DNA is decreased in the presence of increasing amounts of copper ions, whereas other divalent metal ions stabilize the DNA secondary structure at low ionic strength. At 1.28 × 10 −4 M , Cu 2+ produces a decrease of T m depending on base composition. At very low Cu 2+ concentrations (0.5 Cu 2+ /2 DNA‐P) a stabilization of the DNA conformation appears due to an interaction between Cu 2+ and phosphate groups of the DNA molecule. In this case the normal trend of GC dependence of T m exists similar to that with Na + and Mg 2+ as counterions. If copper ions are in excess, the observed destabilization is stronger for DNAs rich in guanine plus cytosine than for those rich in adenine plus thymine. A sharp decrease of T m occurs between 0.5–0.8 Cu 2+ /2 DNA‐P and 1.5 Cu 2+ /2 DNA‐P. The breadth of the transition decreases at high Cu 2+ concentration with further addition of copper ions. Denaturation and renaturation experiments indicate that Cu 2+ ions exceeding the phosphate equivalents interact with the bases and reduce the forces of the DNA helix conformation. Evidence is presented, that the destabilization effect produced by Cu 2+ is possibly due to an interaction with guanine sites of the DNA molecule.