The Driving Force for the Complexation of Charged Polypeptides

TL;DR

This study uses molecular dynamics simulations to investigate the driving forces behind charged polypeptide complexation, revealing the dominant role of counterions and the influence of charge distribution on the thermodynamics of association.

Contribution

It demonstrates that the molecular nature of water is not crucial and highlights the importance of counterions in the entropic driving force for polypeptide complexation.

Findings

Counterions dominate the entropy of complexation.

Removing counterions makes complexation energetically driven.

Charge distribution affects the thermodynamics of association.

Abstract

The phase separation of oppositely-charged polyelectrolytes in solution is of current interest . In this work we study the driving force for polyelectrolyte complexation using molecular dynamics simulations. We calculate the potential of mean force between poly(lysine) and poly(glutamate) oligomers using three different forcefields, an atomistic force field and two coarse-grained force fields. There is qualitative agreement between all forcefields, i.e., the sign and magnitude of the free energy and the nature of the driving force are similar, which suggests that the molecular nature of water does not play a significant role. For fully charged peptides, we find that the driving force for association is entropic in all cases when small ions either neutralize the poly-ions, or are in excess. The removal of all counterions switches the driving force, making complexation energetic. This suggests that the entropy of complexation is dominated by the counterions. When only 6 residues of a 11-mer are charged, however, the driving force is enthalpic in salt-free conditions. The simulations shed insight into the mechanism of complex coacervation and the importance of realistic models for the polyions.