Chaotropic effect and preferential binding in a hydrophobic interaction model

Abstract

Chaotropic substances such as urea and guanidinium chloride, which tend to increase the solubility of hydrophobic particles in aqueous solutions, are used frequently to destabilize aggregations of nonpolar solute particles and micelles, or to denature proteins. Their important applications have generated a growing interest in the physical origin of the chaotropic effect, which to date remains unclear. In this study, the two-state Muller–Lee–Graziano model for water is adapted to describe the ternary system of water, chaotropic cosolvents, and hydrophobic particles in order to analyze the effect of chaotropic substances on hydrophobic interactions. A mean-field approximation confirms the destabilizing effect of chaotropic substances on aggregates of hydrophobic solute particles. In agreement with a pair approximation, detailed Monte Carlo simulations of a three-dimensional system show preferential binding of chaotropic substances to the nonpolar particles and an increase in solubility of the latter due to the cosolvent. The modification of effective hydrophobic interactions in the presence of chaotropic substances is shown to be reproduced within a simple model where the ternary system is described only in terms of the induced alterations in hydrogen bonding between solvent molecules.

Kosmotropes and chaotropes: modelling preferential exclusion, binding and aggregate stability

“…[36]. Here we provide only a brief review of the results of this analysis, with emphasis on the differences and similarities in comparison with the kosmotropic effect, which we have discussed at greater length in Sec.…”

“…The adapted MLG model has been extended to provide a minimal model for the study of chaotropic phenomena in ternary water/solute/cosolvent systems [36] shown that this framework yields a successful description of preferential binding, and of the resulting destabilisation of aggregates of solute particles as a consequence of the role of chaotropic cosolvents in reducing the formation of water structure. We begin with a brief review of the model to summarise its physical basis and to explain the modifications required for the inclusion of cosolvent effects.…”

Solvent-induced micelle formation in a hydrophobic interaction model

“…The respective degeneracies, normalized to a non-degenerate ordered shell conformation, are taken to be q ds = 49, q db = 40, q ob = 10 and q os = 1 [31]. These relative values have been found to be appropriate for reproducing the phenomenology of hydrophobic interactions [22,31], protein denaturation [33,34], swelling of biopolymers [35], and cosolvent effects on solubility of hydrophobic particles [23,36]. Precise values of the microscopic parameters may in fact be refined by comparison with experimental measurements to yield semi-quantitative agreement for different solutions [22,31,35,36].…”

“…Full details of the foundations and qualitative properties of the model can be found in Refs. [22] and [23].…”

“…However, the results of the calculations to follow are not particularly sensitive to the exact values of the differences between these parameters, and are of course independent of their absolute values. We have used the energy values E ds = 1.8, E db = 1.0, E ob = −1.0, and E os = −2.0, which are thought to be qualitatively representative for aqueous solutions, and which have been successful in describing different types of solution [22,23,31,35]. The respective degeneracies, normalized to a non-degenerate ordered shell conformation, are taken to be q ds = 49, q db = 40, q ob = 10 and q os = 1 [31].…”