[Effect of the pH of the medium on DNA conformation]

The hydrodynamical and optical properties of DNA were investigated in the wide-range of pH by the methods of streaming birefringence, viscometry and spectrophotometry for the different ionic strengths of environment. The measurements of the intrinsic viscosity as a function of pH allow us to determine the compactization of protonated DNA without the destruction of double-helical conformation. This transition is accompanied by a decrease in the optical anisotropy of DNA and the coefficient of molar extinction E260 (P). The increase of volume and persistence length of DNA was observed in the alkaline range of pH. Analyses of experimental data lead to an assumption that the predominant cause of these effects is the change of flexibility of DNA as a result of ionization of its bases. The data obtained were compared with those for polycationic molecules.