The interactions of ions with a model peptide (a single melittin alpha-helix) in solutions
of either 0.1m tetrapropylammonium sulfate or 3m guanidinium chloride are examined
by molecular dynamics simulations. It is found that the tetrapropylammonium cation
shares the geometrical property of essentially flat faces with the previously examined
guanidinium cation, and that this leads to a strong preference for tetrapropylammonium
to interact in a similar stacking type conformation with flat non-polar groups such as the
indole sidechain of tryptophan. Sulfate is found to interact almost exclusively and
strongly with the cationic groups of the peptide, such that already in a 0.1m solution of
tetrapropylammonium sulfate the 6+ charge of the peptide is effectively neutralized. In
combination with previous simulations, neutron scattering, and biochemical experiments
on the conformational stability of model peptides the present results suggest that the
Hofmeister series may be explained in higher detail by splitting ions according to the
effect they have on hydrogen bonding, salt bridges, and hydrophobic interactions in the
protein and how these effects are altered by the counter-ion.