Quasielastic light scattering by biopolymers. III. Effect of ionic strength on internal dynamics of DNA

Quasielastic light‐scattering data on DNA in 0.1, 0.5, and 1.0M NaCl, neutral pH, have been obtained in the scattering angle range 15 ⩽ θ ⩽ 50. The relaxation curves have been analyzed as a single exponential with a reciprocal relaxation time of the form 2DeffK2, where K is the fluctuation vector [proportional to sin(θ/2)] and Deff is the effective diffusion coefficient. The 1/τ vs sin2(θ/2) curves exhibit two regions that are proportional to sin2(θ/2), in accordance with the Lee‐Schurr and Lin‐Schurr theories for independent segment‐mean force and Rouse‐Zimm flexible polymers, respectively. In the region KR < 1, where R is a characteristic length, Deff is simply the translational diffusion coefficient, whereas Deff in the region KR > 1 represents the diffusion coefficient for a polymer segment. The data reported here indicate that the “transition” region between the two K2 extremes is strongly dependent on the ionic strength of the solvent. This observation, along with the salt variation of the segmental diffusion coefficient, suggests the internal dynamics of DNA are dependent on the ionic strength of the solvent. The relation of the present study to those of Lin and Schurr and of Caloin et al. is discussed.