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Ionic strength effects on macroion diffusion and excess light‐scattering intensities of short DNA rods

Quasielastic and static light‐scattering measurements were made on DNA isolated from chicken erythrocyte mononucleosomes as a function of ionic strength between 6 × 10−4 and 1.0M. A transition from single‐exponential autocorrelation functions to markedly non‐single‐exponential decays was observed around 10−2M ionic strength and was accompanied by a large decrease in the excess light‐scattering intensity. Autocorrelation functions recorded below 10−2M salt were well fit by the sum of two exponential relaxation which differed by as much as 100‐fold in time constants. Apparent diffusion coefficients for the fast and slow processes plateaued around 10−3M with numerical values approximately 10‐fold and 1/10, respectively, of the translational diffusion coefficient for mononucleosome DNA at high ionic strength. This behavior is similar to that observed with poly(L‐lysine), for which the slow decay has been associated with a transition to an extraordinary phase. The strong and complex salt dependence observed here illustrates potential difficulties in deriving structural information from scattering by polyions at low ionic strength.