Simple Coacervation of Guanidinium-Containing Polymers Induced by Monovalent Salt

Abstract

Liquid-liquid phase separation (LLPS) of macromolecules,called coacervation, is induced by non-covalent intermolecular associationsin aqueous solutions, which is frequently observed in biological processesincluding cellular compartmentation, signaling, and regulation. Recently,associative interactions between pi-conjugated residues haveemerged to induce LLPS, along with electrostatic interaction. An arginineresidue having a positively charged guanidinium group plays a pivotalrole in protein phase behavior because the orientational amphiphilicityof guanidinium groups allows face-to-face pi-pi stackingdespite Coulombic repulsion. In this study, we investigate the simplecoacervates of guanidinium-functionalized polyelectrolytes (i.e., poly-arginine) in aqueous media as a function of saltconcentration, salt type, and temperature to understand the exclusiverole of pi-conjugated moieties. Contrary to ammonium-functionalizedpolyelectrolyte (i.e., poly-lysine) solutions, guanidinium-functionalizedpolyelectrolyte solutions become turbid by adding monovalent saltsand exhibit the upper critical solution temperature (UCST) behavior;the critical temperature is harnessed by salt concentration and salttype. Although a pi-pi stacked guanidinium pair canexist at a lower salt concentration, LLPS occurs when the number ofintermolecular guanidinium pairs goes beyond a critical value to producethe network structure. Furthermore, the enthalpically favored pi-pi interaction directly affects the bulk rheological behavior and theinterfacial property of the coacervates. Our findings provide insightsinto the underlying interactions of protein phase separation and shednew light on the critical role of pi-pi stackingin the biological process and material design.