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

The peak in the small angle x-ray scattering and the dynamic light-scatteri ng slow mode for a 20 base-pair duplex oligonucleotide ("B-DNA'') are studi ed as functions of oligonucleotide and added-salt (NaCl) concentrations. Bo th the x-ray peak intensity and the relative intensity of the slow mode dec rease as the added-salt concentration is increased. The hydrodynamic radius of the slow mode increases as the added-salt concentration is decreased. T he x-ray peak gradually disappears with increasing salt while the slow mode decreases in intensity, but still has some residual intensity at the highe st added-salt concentration studied. There is no abrupt change in either th e 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 solut ion due to electrostatic forces. The x-ray peak position for the oligonucle otide is correlated with the static light-scattering peak seen by other wor kers for dilute solutions of larger polyions. A simple model shows that the reduced electrostatic potential at the average distance between neighborin g polyions is approximately the same for these polyelectrolytes. The slow m ode 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 polyi ons. (C) 1999 American Institute of Physics. [S0021-9606(99)50703-4].