Liquid-liquid Phase Separation as the Second Step of Complex Coacervation

Liquid liquid phase separation (LLPS) mediated by pi-cation bonds between tyrosine and arginine residues are of biological importance. To understand the interactions between proteins in the condensed phase in close analogy to complex coacervation, we run multiple umbrella calculations between oligomers containing tyrosine (pY) and arginine (pR). We find pR-pY complexation to be energetically driven. Metadynamics simulations reveal that this energy of complexation comes primarily from pi-cation bonds. On running free energy calculation for the second binding step of complex coacervation, we find striking similarities between this process and pi-mediated LLPS. These calculations lead us to believe that contrary to the common notion, complex coacervation as whole, which involves an entropic complexation followed by an energetic aggregation is not invoked by proteins containing arginine and tyrosine residues. Rather, the latter step in itself, in which neutral polyion pairs aggregate together is the correct mechanism for pi-cation mediated LLPS.