SCATTERING AND DIFFUSION OF MONONUCLEOSOMAL DNA - EFFECTS OF COUNTERION VALENCE AND SALT AND DNA CONCENTRATION

We have studied the counterion valence and salt and polyion concentration effects on the static and dynamic light scattering behavior of NaDNA and CaDNA in both the moderate salt, semidilute and the low salt ("extraordinary") regimes. We have compared our results with theoretical predictions for the effects of counterion valence and concentration on the second virial coefficient and diffusion properties of DNA and with predictions for the onset of the ordinary-extraordinary transition in flexible polyelectrolytes. Ours is the first study of the counterion valence dependence of the transition for a rodlike polyion, making this comparison possible. A higher counterion valence should screen polyion charges more effectively, decreasing electrostatic interactions between polyions. The "ordinary" diffusion coefficients are lower for CaDNA than for NaDNA and reflect the ratio of counterion valences, but the apparent linear charge densities are much lower than predicted by standard polyelectrolyte theories. The second virial coefficient, B2, Obtained from static light scattering at higher salt concentrations is also lower for CaDNA than NaDNA, consistent with more effective screening by Ca2+. For both NaDNA and CaDNA, the effective polyion charge needed to explain the magnitude of B2 is greater than predicted by polyelectrolyte theory (the opposite direction from the diffusion results). We observe a slow diffusion mode for CaDNA as well as for NaDNA, with an equal diffusion coefficient but smaller amplitude. The onset of the transition is not consistent with rules developed for flexible polyelectrolytes. We show that the slow mode could arise from small degrees of inherent polydispersity if the effective volume fraction is near unity due to an expanded DNA rod diameter at low ionic strength. We also estimate the size of a putative cluster from our diffusion results and show that the existence of a very small fraction of clustered (or aggregated) material is consistent with observations, though it is unclear how such clusters might be stabilized.