Effect of electrostatic interactions on the structure and dynamics of a model polyelectrolyte. II. Intermolecular correlations

Abstract

The peak in the small angle x-ray scattering and the dynamic light-scattering slow mode for a 20 base-pair duplex oligonucleotide (“B-DNA”) are studied as functions of oligonucleotide and added-salt (NaCl) concentrations. Both the x-ray peak intensity and the relative intensity of the slow mode decrease as the added-salt concentration is increased. The hydrodynamic radius of the slow mode increases as the added-salt concentration is decreased. The x-ray peak gradually disappears with increasing salt while the slow mode decreases in intensity, but still has some residual intensity at the highest added-salt concentration studied. There is no abrupt change in either the peak or the slow mode with increasing salt. The existence and behavior of both the x-ray peak and the slow mode indicate local ordering in the solution due to electrostatic forces. The x-ray peak position for the oligonucleotide is correlated with the static light-scattering peak seen by other workers for dilute solutions of larger polyions. A simple model shows that the reduced electrostatic potential at the average distance between neighboring polyions is approximately the same for these polyelectrolytes. The slow mode has a hydrodynamic radius that increases with decreasing q at low added-salt concentrations, indicating a large correlation volume. The x-ray peak is a more local indicator of nearest-neighbor correlations among the polyions.