  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.
