[Effect of solvent structure on amide reaction with native DNA molecules. I. Aqueous mixtures of...

The influence of N,N-dimethylformamid (DMF) and urea (U) on the conformation of the native DNA molecule has been studied by methods of flow birefringence and viscometry. The investigations have been performed over a wide range of amides and supporting electrolyte concentrations. An essentially different influence of small additions of DMF and U on the macromolecule's dimensions was observed. At urea concentrations Cu < 0.03 mol% the DNA intrinsic viscosity [eta] decreases monotonously and the larger, the lower is the ionic strength mu of the solution. At DMF concentration CDMF approximately 1.5 mol% a cooperative decrease in [eta] of DNA, which also depends on mu was revealed. In the range of intermediate and large nonelectrolyte concentrations the influence of DMF on hydrodynamical and optical behaviour of the native DNA molecule resembles that of alcohols. The observed changes of [eta] are conditioned by conformational transitions in the DNA molecule. The constancy of macromolecule's optical anisotropy over a nonelectrolyte concentration range under investigation permits to interpret these transitions as occuring at the level of the tertiary structure of the native DNA molecule. The polyelecrolyte volume effects are considered on the basis of Manning's condensation model, and the structure singularities of water-organic solvents are taken into account. It has been shown that the nonelectrolyte influence on the native DNA molecule conformation may be direct (by means of ion-dipole interactions) or indirect (as result of changes in dielectric properties of solution). It is noted that the solvent structure is a very important factor which determines the interactions between low molecular organic compounds and the native DNA molecule.