Ionic effects in semi-dilute biopolymer solutions: A small angle scattering study

Systematic investigations using neutron and X-ray small angle scattering in near-physiological salt solutions were made to reveal the effect of polymer concentration, pH, and calcium ion concentration on the structure of semi-dilute solutions of four model biopolymers [polyaspartic acid, DNA, chondroitin sulfate, and hyaluronic acid (HA)] representing typical backbone structures. In the low q range (<0.01 Å^-1), the scattering response I(q) is dominated by scattering from large clusters. In the intermediate q range, I(q) varies approximately as q ^-1, exposing the linear nature of the scatterers. In these polyelectrolyte solutions, the correlation length L displays a power law dependence on the polymer concentration c that resembles that of neutral polymer solutions. L increases with increasing calcium chloride concentration and with decreasing pH. The effect of the different divalent cations, Ba, Mg, Ca, Sr, and Mn, on the structure of DNA solutions is practically identical. However, in mixed salt conditions at the same ionic strength, the combined effect of mono- and divalent counter-ions on the structure of the polymer solutions deviates significantly from additivity. Anomalous small angle X-ray scattering observations on both DNA and HA solutions reveal that the divalent strontium counter-ions form a tight sheath around the polymer chain. The shape of the divalent ion cloud is similar in these two systems.