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Specific Ion Effects at Protein Surfaces: A Molecular Dynamics Study of Bovine Pancreatic Trypsin Inhibitor and Horseradish Peroxidase in Selected Salt Solutions

Luboš Vrbka,^† Pavel Jungwirth,*^† Pierre Bauduin,^‡ Didier Touraud,^‡ and Werner Kunz*^‡

Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, and Center for Biomolecules and Complex Molecular Systems, Flemingovo nm. 2, 16610 Prague 6, Czech Republic, and Institute of Physical and Theoretical Chemistry, University of Regensburg, D-93040 Regensburg, Germany

J. Phys. Chem. B, 2006, 110 (13), pp 7036–7043

DOI: 10.1021/jp0567624

Publication Date (Web): March 10, 2006

†

Institute of Organic Chemistry and Biochemistry, and Center for Biomolecules and Complex Molecular Systems, Flemingovo nám. 2.

In papers with more than one author, the asterisk indicates the name of the author to whom inquiries about the paper should be addressed.

‡

Institute of Physical and Theoretical Chemistry, University of Regensburg.

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 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, and 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.

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