Interface Counterion Localization Induces a Switch between Tight and Loose Configurations of Knotted Weak Polyacid Rings despite Intermonomer Coulomb Repulsions

Abstract

Stochastic simulations have been used to investigate the conformational behavior of knotted weak polyacid rings as a function of pH. Different from the commonly expected ionization–repulsion–expansion scheme upon increasing pH, theoretical results suggest a nonmonotonic behavior of the gyration radius R g 2. Polyelectrolyte recontraction at high ionization is induced by the weakening of Coulomb repulsion due to counterions (CIs) localizing at the interphase between the polymer and solvent, and the more marked it appears, the more complex is the knot topology. Compared with strong polyelectrolytic species of identical ionization, weak polyacids present tighter knots due to their ability to localize neutral monomers inside the tangled part. Increasing the solvent Bjerrum length enhances CIs localization, lowering the pH at which polyacids start decreasing their average size. A similar effect is also obtained by increasing the amount of “localizable” cations by adding salts.

Tunable Knot Segregation in Copolyelectrolyte Rings Carrying a Neutral Segment

“…We show that by varying their relative lengths it is possible to modulate the effective interactions of the essential crossings and thus tune the contour length and location of the knot in ways that, to our knowledge, have no analog in other polymeric systems. In particular, we show that the knot size has a striking nonmonotonic dependence on the ring composition, as it has with respect to the polyelectrolyte ionization degree …”

“…In particular, we show that the knot size has a striking nonmonotonic dependence on the ring composition, as it has with respect to the polyelectrolyte ionization degree. 33 The simulated system consists of a knotted block co-PE ring in solution with neutralizing counterions. The bead−spring ring has either a 3 1 or a 5 1 topology and is made of N = 120 monomers of which N neu forms the neutral segment (NS) and the rest carry a monovalent negative charge (see Figure 1).…”

“…Short-ranged attractive interactions, such as the ones mediated by coordinating counterions, also represent a promising avenue for the possibility to localize the knot essential crossings in the charged region. 41 Other relevant physical properties could be tuned along with chemical composition to influence the barriers for pinning or localizing knots, such as the total contour length of the ring, 42 the background ionic strength 33 of the solution, or the solvent quality. Besides, intrinsically short-ranged attraction in a neutral block (e.g., the ones due to a limited solvophilicity or the ability to form intrachain chemical specific interactions 43,44 ), may strengthen or even induce 41 the tendency to knot pinning, provided the NS is sufficiently long to position a few of its monomers in the regions facing each other in an essential crossing.…”

Impact of Chemically Specific Interactions between Anions and Weak Polyacids on Chain Ionization, Conformations, and Solution Energetics

“…The shortening of trains, and the simultaneous freeing of portion of chains or arms, upon increasing λ should be expected to substantially impact on the conformation properties of the studied polyelectrolytes, especially considering that it happens concomitantly with the ionization of the latter, a phenomenon well-known to be the driving force for polymer straightening, ,− ring widening, and the tightening or localization of knots . To gauge the quantitative aspects of such behavior, Figure shows the average “end-to-end” distance for chains or arms in the polymeric systems as a function of λ.…”

“…Thus, the “end-to-end” distance in linear strong polyelectrolytes tends to decrease upon increasing the salt concentration ( C s ), as it does the average gyration radius. , If chains represent the bristles of polymeric brushes, this effect results in a decrease in brush thickness ,, and, potentially, in a reduction in wettability of surface brushes . A decrease in gyration radius upon increasing C s is witnessed also in knotted and nonknotted ring polyelectrolytes, the latter stimulus potentially modifying also the number of monomers involved in the knotted portion due to a reduction in Coulomb repulsion between essential crossings . Divalent and trivalent salt ions, as well as hydrophilic macroions, bearing charges of opposite sign than the polyelectrolyte affect the latter more markedly than monovalent ones, so that packaging in tight vessels such as capsids may become possible …”