The Hofmeister series: salt and solvent effects on interfacial phenomena

Abstract

SUMMARY 241

1. INTRODUCTION 242

1.1 Milestones in the history of HS 244

1.2 Solvation forces 245

2. PHENOMENOLOGY OF THE HOFMEISTER SERIES 246

2.1 Extension of the properties of HS from inorganic to organic cosolutes 246

2.2 Effect of salts on the stability of biomolecules 247

2.3 Effect on enzyme activity 248

2.4 Influence of salts on the crystallization of biological macromolecules 249

2.5 Effects of salts on DNA–protein interactions 250

2.6 Interactions at receptor sites and effects on ion channels 251

2.7 Water-coordinating properties of salts as determined by gel chromatographic behaviour 254

3. INTERFACIAL FREE ENERGY 254

3.1 Types of forces 254

3.2 Distance dependence of the interactions 257

4. THE NATURE OF SURFACES AND LIQUIDS 259

4.1 Proteins 259

4.2 Adsorbing surfaces 260

4.3 Water 261

5. THE EFFECT OF COSOLUTES ON WATER 263

5.1 Electrochemistry 263

5.2 Neutron scattering 264

5.3 Vibrational spectroscopy 265

5.4 Nuclear magnetic resonance 265

6. COSOLUTE EXCLUSION FROM SURFACES 266

6.1 Cosolute enrichment 268

7. CONCLUSIONS AND OUTLOOK 268

8. ACKNOWLEDGEMENTS 269

9. REFERENCES 269

A. GLOSSARY 276

B. LIST OF PRINCIPAL SYMBOLS 276

Advances in experimental and computational methodologies have led to a recent renewed interest in the Hofmeister series and its molecular origins. New results are surveyed and assessed. Insights into the underlying mechanisms have been gained, although deeper molecular understanding still seems to be elusive. The principal reason appears to be that the Hofmeister series emerges from a combination of a general effect of cosolutes (salts, etc.) on solvent structure, and of specific interactions between the cosolutes and the solute (protein or other biopolymer). Hence every system needs to be studied individually in detail, a state of affairs which is likely to continue for some time. A deeper understanding of the Hofmeister series can be an extraordinarily valuable guide to designing experiments, including not only those probing the series per se, but also those designed to elucidate the adsorption, aggregation and stabilization phenomena which underlie so many biological events. The aim of this review is to provide an up-to-date framework to guide such understanding, consolidating recent advances in the many fields on which the Hofmeister series impinges.