On the reversibility of the acid denaturation of sodium desoxyribose nucleate

The current disagreement as to whether the acid denaturation of DNA produces reversible or irreversible changes of macromolecular configuration can be resolved by paying proper attention to the temperature dependence of the denaturation process. Light scattering measurements have been made on a sample of sodium desoxyribosenucleate taken to pH 2.6 and reneutralized at 0 and 25°C. in 0.1 M NaCl. Whereas the material, acid treated at 25°C., is irreversibly collapsed (radius of gyration = 1370 A., molecular weight unchanged), 0°C. titration leaves the size of the DNA molecule unchanged (ρ = 2800 A.). Viscosity measurements at 0°C. and pH 2.6 show that at this pH the DNA is nearly as fully extended as in neutral solution ([η] = 40 dl./g. compared to 63 dl./g. in neutral aqueous salt solution). However, collapse occurs at pH 2.6 between 0 and 25°C., and this thermal denaturation at pH 2.6 is irreversible. The light scattering properties of acid‐denatured DNA are interpreted in terms of a more heterogeneous distribution of macromolecular configurations than in the native material.