Conformation studies on the sodium and cesium salts of calf thymus deoxyribonucleic acid (DNA)

Solutions of calf thymus NaDNA in 0.2M NaCl and CsDNA in 0.2M CsCl (fragmented by sonic irradiation to a molecular weight of about 5 × 10⁵) were examined by electron microscopy, light scattering, and viscosity measurements. Electron microscopy showed that the molecular fragments are rodlike in shape and that the distribution of their lengths is sharp (L_w/L_n ～ 1.06). The weight-average number of nucleotides Z_w per DNA molecule derived from light scattering was found to be in very good agreement with the value L_w derived from electron microscopy. The z-average length L_z derived from light scattering at 25°C. was found to be about 89% of the corresponding value derived by electron microscopy. Similarly, the value of L_w derived from intrinsic viscosity at 25°C. on the basis of a rodlike model was found to be about 87% of the corresponding value derived by electron microscopy. It is concluded that the molecules are slightly flexible and do not, assume their full contour length in solution because of the disorienting effect of Brownian motion. The intrinsie viscosity was found to increase linearly with decreasing temperature toward a limiting value corresponding to the fully stretched length of the macromolecules at 0°K.: a reasonable value for the modulus of elasticity could be calculated. It was also found that, no differences in conformation could be delected in t he two systems CsDNA-0.2M CsCl and NaDNA-0.2M NaCl.