Cooperativity in Macromolecular Interactions as a Proximity Effect: NMR and Theoretical Study of Electrostatic Coupling of Weakly Charged Complementary Polyions

Abstract

Polyelectrolyte behavior and cooperativity of electrostatic coupling of two weakly charged polyions, poly(sodium methacrylate)-co-(acrylamide) containing 10 mol % ionic groups (A10) and poly(N-diallyldimethylammonium chloride)-co-(acrylamide) containing 8 mol % ionic groups (C8), was examined using 1H, 23Na, and 35Cl NMR and 1H PGSE NMR, ab initio SCF HF/6-31G(d) calculations, and simulations using a simplified theoretical model of insular equilibria. With the use of 23Na and 35Cl NMR relaxations, it was first shown that neither A10 nor C8 exhibits polyelectrolyte effects in the sense of counterion condensation in dilute aqueous solution. Accordingly, electrostatic binding of low-molecular-weight models of the charged groups, namely sodium pivalate (for A10) and tetramethylammonium chloride (for C8), to the complementary polyion is weakly populated and exhibits no cooperativity (as shown by 23Na and 35Cl NMR relaxations and 1H PGSE NMR). However, interaction of A10 with C8 in dilute solutions is distinctly cooperative and leads to relatively high coupling degrees. A simplified theoretical model based on a restriction imposed on the motion of the coupling groups by neighboring already coupled pairs, which we present here, explains this behavior and predicts experimental data in a semiquantitative way. The model can be improved by including a detailed description of motional constraints.