Quantifying accumulation or exclusion of H+, HO−, and Hofmeister salt ions near interfaces

Abstract

Recently, surface spectroscopies and simulations have begun to characterize the non-uniform distributions of salt ions near macroscopic and molecular surfaces. The thermodynamic consequences of these non-uniform distributions determine the often-large ion-specific effects of Hofmeister salts on a very wide range of processes in water. For uncharged surfaces, where these nonuniform ion distributions are confined to the first few layers of water at the surface, a two-state approximation to the distributions of water and ions, called the salt ion partitioning model (SPM) has both molecular and thermodynamic signiicance. Here, we summarize SPM results quantifying the local accumulation of H+, exclusion of HO−, and range of partitioning behavior of Hofmeister anions and cations near macroscopic and molecular interfaces. These results provide a database to interpret or predict Hofmeister salt effects on aqueous processes in terms of structural information regarding amount and composition of the surface exposed or buried in these processes.

Why Hofmeister effects of many salts favor protein folding but not DNA helix formation

“…For example, the predicted Hofmeister m-value∕RT for KF (3.9 m −1 ) is 70% larger than the observed value of 2.3 m −1 . Deviations at the highly excluded end of the spectrum of salts could be explained by an effect of the salt identity on the unfolded ensemble, possibly analogous to what we previously observed for the model process of micelle formation, where better agreement was observed between calculated and observed salt effects when more headgroups were assumed to be buried in excluded salt solutions (13). Here, compaction of the unfolded state in excluded salt solutions could result in the larger discrepancies observed for those salts.…”

“…We previously quantified Hofmeister ion exclusion from, or accumulation at, hydrocarbon (C) and amide nitrogen and oxygen (N, O) surfaces and showed how the net exclusion or accumulation of salt ions affects the solubility of model hydrocarbons, peptides, and micelles (13,17). Our analysis of these studies (13,17) quantified the conclusion drawn by earlier investigators (15,18) that the interactions of Hofmeister salts with amide surface are favorable and relatively nonspecific. Thus, the observed Hofmeister ordering of salt effects on solubility of small peptides is primarily due to the ∼75% of the surface that is hydrocarbon.…”

“…Hofmeister ions have often been discussed as either water structure breakers ("chaotropes") or structure makers ("kosmotropes"), and Hofmeister effects have been attributed to the "ordering" or "disordering" of bulk water structure effected by a particular ion; however, proposals of long-range ordering (or disordering) of water in concentrated salt solutions are not supported by water activity data (44) (4,5,28). The idea that direct interactions of ions (in competition with water) with nonpolar surface (e.g., air-water and molecular hydrocarbon) are the source of the Hofmeister effect has gained traction in recent years (13,17,(49)(50)(51). Our work quantifying the accumulation or exclusion of Hofmeister ions near various biochemically relevant surfaces (13,17,52,53) has allowed us to make qualitative predictions about the difference between Hofmeister effects on protein folding and DNA melting.…”

“…In 1888, Franz Hofmeister discovered that the effectiveness of salts for protein precipitation generally followed a specific order, regardless of the protein being investigated (7). Since then, the so-called Hofmeister series of salt effects has been observed in physical properties of aqueous salt solutions (e.g., surface tension and surface potential) (8,9), as well as salt effects on a variety of macromolecular processes (e.g., micelle formation, "salting out" nonpolar compounds, and protein folding) (10)(11)(12)(13). The general ranking of ions, in decreasing order of effectiveness (best to worst) in driving processes where surface area is buried (e.g., folding and precipitation) or macroscopic surface is lost (transfer of water from the air-water interface to bulk), is as follows (14):…”

The inverse and direct Hofmeister series for lysozyme

“…As a lysozyme solution is lowered below its cloud-point temperature, the formation of coacervate droplets rapidly reduces the area of the protein/ bulk water interface. Moreover, the partitioning of anions to this interface modulates the interfacial tension and thereby the thermodynamics of protein aggregation (34,48). It is well known that interfacial tension for the air/water and many hydrocarbon/water interfaces varies linearly with salt concentration up to at least moderate ionic strengths (37-39).…”