Ion-induced changes in DNA gels.

Abstract

Small angle neutron scattering (SANS) measurements are reported for DNA gels under near physiological conditions in which the concentration of monovalent and divalent counter-ions and the pH are varied. The scattering intensity <i>I</i>(<i>q</i>) is described by a two-term equation, one due to osmotic concentration fluctuations and the other coming from static inhomogeneities frozen in by the cross-links. SANS in the low <i>q</i> range indicates the presence of large clusters and the size of which exceeds the resolution of the experiment. In the intermediate <i>q</i>-range, the intensity increases with the CaCl₂ concentration and the slope approaches -1, corresponding to linear (rod-like) scatterers. In the highest <i>q</i> region, the scattering response is governed by the local chain geometry. Screening of electrostatic interactions by sodium chloride causes a moderate increase in the SANS intensity that is accompanied by an increase in the mesh size <i>L</i> of the network. Addition of calcium chloride, or a decrease in pH, produces similar trends, and ultimately leads to phase separation. The scattering intensity at <i>q</i> = 0, estimated from independent measurements of the osmotic pressure <i>Π</i>, is in excellent agreement with <i>I</i>(0) from the SANS measurements. Anomalous small angle X-ray scattering (ASAXS) measurements on the uncross-linked DNA show that the monovalent ion cloud is only weakly influenced by the addition of divalent ions. Conversely, the divalent counter-ion cloud tightly follows the contour of polymer chains.