DNA conformation in N,N-dimethyl formamide-H2O solutions.

Optical density, viscosity and light scattering measurements for calf thymus DNA in water-N,N' dimethyl formamide (DMF) solutions are presented. DMF content varied between 0 and 60% (v/v) and DNA molecular weight varied between 15 x 10(6) and 0.5 x 10(6). Complementary measurements of the solubility of adenine, thymine, guanine and cytosine in H2O-DMF mixtures are presented. The denaturation temperature of DNA, manifested by about 35% increase of optical density, is gradually depressed by increasing DMF content. However, a significant increase of optical density occurs even before (and even after) the denaturation point, when DMF content is increased isothermally. The intrinsic viscosity also exhibits a large decrease when DMF content is decreased both before and after the denaturation point. Light scattering data for high-molecular-weight DNA in the predenaturation range indicate a decrease of the mean-square radius and a constant molecular weight on increasing DMF content. The resulta, interpreted in terms of the wormlike chain of Kratky and Porod, indicate a large decrease of the persistence length of DNA. For low-molecular-weight DNA, radius and molecular weight increase with DMF content, indicating intermolecular aggregation. The formation of compact structures of native DNA is discussed in terms of an increased solubility of uncharged bases, and a decreased solubility of phosphate and deoxyribose groups, when a less polar environment is provided by the addition of DMF.