Salt Destabilization of Cationic Colistin Complexation within Polyanionic Microgels

Polyelectrolyte complexation, with and without additional salts such as NaCl, has been of long-standing interest across such areas as polyelectrolyte blends, layer-by-layer (LbL) thin films, and coacervates. To better understand how to control complexation strength, we study polyanionic microgels involving sulfonic or carboxylic acids that interact with colistin, a small multivalent cationic (+5) macroion. Complexation causes microgel deswelling, which can be followed by in situ optical microscopy. The release of complexed colistin causes microgel swelling and is triggered by threshold levels of NaCl in the surrounding buffer. Differences in these thresholds together with differences in the Na⁺doping of complexed microgels indicate that colistin complexation is stronger with poly(styrene sulfonate) (PSS) than it is with poly(acrylic acid) (PAA). Coarse-grained molecular dynamics simulations show that the free-energy decrease accompanying colistin-PSS complexation is significantly greater than that for colistin-PAA complexation. These simulations furthermore indicate that pendant aromatic groups in PSS play an important steric role as a spacer that creates polyanion conformations that maximize opportunities for both Coulombic and nonbonded Lennard-Jones interactions with colistin.