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Biophysical Journal 85:686-694 (2003)
© 2003 The Biophysical Society

Specific Ion Effects: Why the Properties of Lysozyme in Salt Solutions Follow a Hofmeister Series

M. Boström * {ddagger}, D. R. M. Williams * and B. W. Ninham * {dagger}

* Research School of Physical Sciences and Engineering, Institute of Advanced Studies, Canberra, Australia, 0200; {dagger} Department of Chemistry and CSGI, University of Florence, 50019 Sesto Fiorentino, Italy; and {ddagger} Department of Physics and Measurement Technology, Linköping University, SE 581 83 Linköping, Sweden

Correspondence: Address reprint requests to Mathias Boström, Institute of Advanced Studies, Canberra, Australia, 0200. Tel.: +61-2-61250171; Fax: +61-2-61250732; E-mail: mtb110{at}rsphysse.anu.edu.au.

Protein solubility in aqueous solutions depends in a complicated and not well understood way on pH, salt type, and salt concentration. Why for instance does the use of two different monovalent salts, potassium thiocyanate and potassium chloride, produce such different results? One important and previously neglected source of ion specificity is the ionic dispersion potential that acts between each ion and the protein. This attractive potential is found to be much stronger for SCN- than it is for Cl-. We present model calculations, performed within a modified ion-specific double-layer theory, that demonstrate the large effect of including these ionic dispersion potentials. The results are consistent with experiments performed on hen egg-white lysozymes and on neutral black lipid membranes. The calculated surface pH and net lysozyme charge depend strongly on the choice of anion. We demonstrate that the lysozyme net charge is larger, and the corresponding Debye length shorter, in a thiocyanate salt solution than in a chloride salt solution. Recent experiments have suggested that pKa values of histidines depend on salt concentration and on ionic species. We finally demonstrate that once ionic dispersion potentials are included in the theory these results can quantitatively be reinterpreted in terms of a highly specific surface pH (and a salt-independent pKa).




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