Copper(II)—DNA denaturation. II. The model of DNA denaturation

In a continuing study of the denaturation of DNA as brought abought about by Cu(II) ions, results are presented for the dependence of T m and τ (the terminal relaxation time) on ionic strength, pH, reactant concentrations, and temperature. Maximum stability of the double helix, as reflected by the longest relaxation times and highest T m values, was observed between pH 5.3 and 6.2. Outside this range both T m and τ decreased sharply. A second, faster relaxation time was deduced from the kinetic cureves. The apparent activation energies of the rapid and slow (“terminal”) relaxations were found to be 12 and 55 kcal/mole, respectively. Several lines of evidence led to the conclusions that ( 1 ) the rate‐determining step in DNA denaturation, when occurring in the transition region, is determined by chemical events and ( 2 ) the interactions which are disrupted kinetically in the rate‐determining step are those which account for the major portion of the thermal ( T m ) stability of helical DNA.