The Helix-Coil Transition in DNA: Effects of the Interactions with Small-Ion and of the Composition of DNA

Year: 1962

DOI: 10.7907/SYPJ-C611

We have studied the effects of ion binding and DNA composition on the helix-coil denaturation of the DNA macromolecule. Among the systems studied, none gave a marked increase in the composition dependence of the denaturation conditions, an increase which would
allow extensive fractionation of a compositionally disperse DNA sample.

The binding of protons to DNA is extensive even before hyperchromicity and irreversible changes in intrinsic viscosity are observed, and includes protonation of the cytosine heterocycle. It is intriguing that this cytosine protonation seems to require the breaking of hydrogen
bonds in the Watson-Crick structure.

The binding of Mg^(++), Co^(++), or Ag^+ at low ionic strengths markedly stabilizes the helical conformation. We have evaluated the extent to which high concentrations of Na^+ ion reduce the binding of Mg^(++) and H^+ ions.

Broad transitions are observed at low ionic strengths of Na^+ ion, and in the presence of the equivalent ratios r=0.2 Ag^+, r=0.5 Co^(++), or r=0.5 Mg^(++). In the latter two cases, this broadening maybe due to the selective binding of cations to native DNA, but the broadening observed at low Na^+ concentrations may be due to a change in Zimm's parameters σ_o and/or σ_j.

The inactivation of the transforming ability of pneumococcal
DNA for three characteristics was studied at 0.1 and 3x10^(-4) ionic strengths. Differences in denaturation temperature were observed which could not be correlated with the compositions deduced by Rolfe and Ephrussi-Taylor from density differences. At low ionic strengths, extremely broad inactivation curves were observed, and can be explained by the renaturation made possible by the existence of short helical regions in low ionic strength denaturations.