The distribution of sodium, choline, sulfate, and chloride ions around two proteins,
horseradish peroxidase (HRP) and bovine pancreatic trypsin inhibitor (BPTI) is
investigated by means of molecular dynamics simulations with the aim to elucidate
ion adsorption at the protein surface. Although the two proteins under investigation
are very different from each other, the ion distributions around them are remarkably
similar. Sulfate is always strongly attached to the proteins, choline shows a
significant, but unspecific propensity for the protein surfaces, sodium ions have a
weak surface affinity, while chloride has virtually no preference for the protein
surface. In mixtures of all four ion species in protein solutions, the resulting
distributions are almost a superposition of the distributions of sodium sulfate and
choline chloride, except that sodium partially replaces choline close to the proteins.
The present simulations support a picture of ions interacting with individual ionic and
polar amino acid groups rather than with an averaged protein surface. The results thus
show how subtle the so-called Hofmeister and electroselectivity effects are in salt
solution of proteins making all simplified interaction models questionable.