Association constants for the interaction of double-stranded and single-stranded DNA with spermine, spermidine, putrescine, diaminopropane, N1- and N8-acetylspermidine, and magnesium: determination from analysis of the broadening of thermal denaturation curves.

The effect of Mg2+, putrescine, diaminopropane, N1-acetylspermidine, N8-acetylspermidine, spermidine, and spermine on the thermal denaturation of calf thymus DNA was investigated. As in a previous study with magnesium [W.F. Dove and N. Davidson, (1962) J. Mol. Biol. 5, 467-478], these ligands were found to raise the thermal denaturation temperature of the DNA and to broaden the thermal denaturation curve dramatically at the point where 10 to 20% of the DNA charge had been neutralized. At higher levels of charge neutralization the curves became sharper again. This behavior was due to differential binding of the ligands to single- and double-stranded DNA. The broadening was used to determine the ratio of the association constants of each ligand to the two forms of DNA using either an independent sites model of binding or an excluded sites model. The results show that the primary mode of binding of the ligands to DNA is electrostatic but that important secondary, nonelectrostatic, effects are also present.