The stability of polyelectrolyte complexes of Calf-Thymus DNA and synthetic polycations: Theoretical and experimental investigations

Complexes of Calf‐Thymus DNA and some polycations such as PDADMAC, IONEN, and P4VP were formed and investigated with respect to their stoichiometry and stability. The central part of the study are two theoretical models, the E‐model and the T‐model. It is assumed that a complex molecule consists of two linear polyion strands connected by electrostatic forces. One strand is a DNA molecule and the other is built by the polycations. Both strands carry equal number of charges, so that the complex molecule is electrically neutral. In the E‐model the complex binding distance, r1, is a constant, while in the T‐model r1 depends on the system parameters, such as the concentration, CS, of added salt or the dielectric constant, ε, of the solvent. Both theories predict a critical salt concentration, CS,d where a complex molecule becomes instable and dissociates in its single strands. According to the E‐model CS,d is 0.6 mol/1 for a 1 : 1 salt if the solvent is water and the temperature 298.16 K. This value agrees quite well with those obtained experimentally, which are 0.5 mol/1 for LiCl, 0.6 mol/1 for NaCl, 0.56 mol/l for KCl, and 0.68mol/1 for CsCl. If the dielectric constant of the solvent decreases, CS,d should decrease. This is also confirmed by the experiment. The T‐model predicts that the complex binding distance, rl,d, at which a complex molecule dissociates, is correlated with CS,d r1,d is large at low CS,d and vice versa. Using the experimental data of CS,d the T‐model predicts that r1 increases from r1 ≈ 3.8 · 10−10 m when the complex is built to r1 = r1,d ≈ 8 · 10−10m when the complex dissociates. This value is nearly as large as the average distance of a counterion bound territorial to a DNA molecule. However, for the divalent cations, Ca2+ and Sr2+, r1,d is only 4 · 10−10 m. This may be unrealistic.