Diffusion of DNA at very low concentrations

The diffusion behavior of DNA samples of molecular weights between 1 × 106 and 25 × 106 Daltons was investigated under standard conditions at mean concentrations c̄ between 0.0009 and 0.017 g/dl. Special techniques described previously were used and supplemented. The sensitivity required was accomplished by multiple passage through the sample cells (effective path length of 10–45 cm) and application of the Gouy interference method. The maximum DNA refraction index difference has been determined more precisely from Gouy interference fringes by applying a systematic variation procedure and a linear‐plot criterion. Convection was prevented by a temperature constancy better than 0.002°C/day, vibrationless operation, and by application of a slight density gradient of heavy water, which also improved the boundary‐forming procedure. The corresponding optical HDO gradient was compensated. The concentration dependence of the DNA diffusion coefficient average DA was found to be positive and very small at extremely low concentrations, that is, below c̄ = 0.008 g/dl, for the sample of highest molecular weight investigated. With beginning penetration of different DNA molecules, DA increases markedly. The diffusion constant averages of our polydisperse samples will be corrected for monodisperse subfractions in a following paper. The resulting molecular weights M from diffusion and sedimentation constants (D0, s0) together with data from literature are the basis of new s0–M, D0 − M, and [η]–M relations for monodisperse DNA samples.