Conformation of LiDNA in solutions of LiCl

We have derived radii of gyratin, R_g, from the absolute intensity of the scattered light of mondisperse linear Col E₁ LiDNA in solution at various LiCl concentrations up to 5M. The second virial coefficients, A₂, decrease strongly with increasing LiCl concentration, and vanish between 3 and 5M LiCl. It was thus possible to calculate a limiting value at a high salt concentration of 28.5 nm for the persistence length, a₀, of LiDNA, without the necessity of applying excluded-volume corrections. The value obtained is in good agreement with the value previously obtained for NaDNA at high NaCl concentrations, and can be identified with the high salt limit of DNA flexibility, with long-range electrostatic interactions effectively screened. Sedimentation coefficients in the ultracentrifuge and apparent and translational diffusion coefficients (at finite and vanishing scattering vectors, respectively) from dynamic laser-light scattering have also been obtained up to 5M LiCl. From the sedimentation and apparent diffusion, D(90), (at 90° scattering angle only) above 5M, and up to 9M LiCl, it could be shown that the solutions are stable for reasonable periods of time, and the molecular parameters vary smoothly and moderately at high salt. Conformational transitions were not observed and precipitation occurs between 9 and 10M LiCl.