| Abstract
| - 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 dynamicssimulations with the aim to elucidate ion adsorption at the protein surface. Although the two proteins underinvestigation 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 forthe protein surfaces, and sodium ions have a weak surface affinity, while chloride has virtually no preferencefor the protein surface. In mixtures of all four ion species in protein solutions, the resulting distributions arealmost a superposition of the distributions of sodium sulfate and choline chloride, except that sodium partiallyreplaces choline close to the proteins. The present simulations support a picture of ions interacting withindividual ionic and polar amino acid groups rather than with an averaged protein surface. The results thusshow how subtle the so-called Hofmeister and electroselectivity effects are in salt solution of proteins, makingall simplified interaction models questionable.
|
