More On Hofmeister Ions

The Science & Technology article about Hofmeister ions certainly spotlights the current interest in the properties of these ions in aqueous solutions and at aqueous interfaces (C&EN, Nov. 26, 2007, page 45). Hofmeister effects, however, are first and foremost thermodynamic effects-for example, on the surface tension of water, the aqueous solubility of hydrocarbons or proteins, and on protein folding (Acc. Chem. Res. 1969, 2, 257)—and many significant thermodynamic contributions were overlooked.

The origin of the effect lies in the preferential interactions (ions versus water) with the surface under consideration, which determine whether the local concentration of a Hofmeister ion in the water at a surface (molecular, biopolymer, or macroscopic) is greater than, less than, or equal to its bulk concentration (Biochemistry 1982, 21, 6545; Biophys. J. 1996, 71, 2056; and Adv. Protein Chem. 1998, 51, 281). There must be a difference between local and bulk salt concentrations for there to be an effect of a Hofmeister salt on the thermodynamics of a process involving a change in water-accessible (molecular, biopolymer, or macroscopic) surface area (ASA); the larger the difference between local and bulk concentrations, the larger the effect.

Effects of both Hofmeister salts and the corresponding spectrum of nonelectrolytes (osmolytes, denaturants) on all processes in aqueous solution accompanied by a change in ASA can be quantitatively interpreted or predicted using a surface (local)-bulk partitioning model (Biophys. J. 1995, 68, 786 and J. Phys. Chem. B 2007, 111, 5411), which neatly bridges the gap (highlighted in the article) between the results of physical chemists and the needs of the biochemical/biological community.

Laurel Pegram Tom Record
Madison, Wis.