The Hofmeister series: salt and solvent effects on interfacial phenomena

Abstract

SUMMARY 2411. INTRODUCTION 2421.1 Milestones in the history of HS 2441.2 Solvation forces 2452. PHENOMENOLOGY OF THE HOFMEISTER SERIES 2462.1 Extension of the properties of HS from inorganic to organic cosolutes 2462.2 Effect of salts on the stability of biomolecules 2472.3 Effect on enzyme activity 2482.4 Influence of salts on the crystallization of biological macromolecules 2492.5 Effects of salts on DNA–protein interactions 2502.6 Interactions at receptor sites and effects on ion channels 2512.7 Water-coordinating properties of salts as determined by gel chromatographic behaviour 2543. INTERFACIAL FREE ENERGY 2543.1 Types of forces 2543.2 Distance dependence of the interactions 2574. THE NATURE OF SURFACES AND LIQUIDS 2594.1 Proteins 2594.2 Adsorbing surfaces 2604.3 Water 2615. THE EFFECT OF COSOLUTES ON WATER 2635.1 Electrochemistry 2635.2 Neutron scattering 2645.3 Vibrational spectroscopy 2655.4 Nuclear magnetic resonance 2656. COSOLUTE EXCLUSION FROM SURFACES 2666.1 Cosolute enrichment 2687. CONCLUSIONS AND OUTLOOK 2688. ACKNOWLEDGEMENTS 2699. REFERENCES 269A. GLOSSARY 276B. LIST OF PRINCIPAL SYMBOLS 276Advances 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.