Water structure and chaotropicity: their uses, abuses and biological implications

Abstract

The concept of "water structure" has been invoked to explain all manner of aqueous phenomena. Here we look at the origins of this tendency to understand solute hydration in terms of structural changes in bulk water, and consider the validity of one particular example: the classification of small solutes as chaotropic or kosmotropic, and the putative relation of this terminology to notions of structure-making and structure-breaking in the solvent. We doubt whether complex phenomena such as Hofmeister and osmolyte effects on macromolecules can be understood simply on the basis of a change in solvent structure. Rather, we argue that chaotropicity, if understood in the original sense, arises from the activities that solutes exert on macromolecular systems, as well as from deviations of solvation water from bulk-like behaviour. If applied judiciously, chaotropicity remains a potent, biologically pertinent parameter useful for classifying and understanding solution phenomena in all types of living system.

Modulating Nucleation by Kosmotropes and Chaotropes: Testing the Waters

“…On the other hand, guanidine and urea predominantly produce calcite. It is important to consider that the experimental observations are consistent with the original use of terms "chaotrope" and "kosmotrope," as previously discussed [3,69], and also that the chao-/kosmotorpic activities of salts and organic molecules can considerably deviate in solution mixtures. This discrepancy can originate from the non-additive effects of salts and organic molecules in inducing a net kosmotropic or chaotropic effect.…”

“…These studies suggest that the Hofmeister series emerges from interactions of the ions with macromolecules and the associated hydration, and not due to structural perturbations to the bulk solvent. Therefore the configurational and chemical properties of solute macromolecules are important factors that determine the consequences of ionic interactions [3,8,9].…”

“…It affects global processes such as erosion and climate as well as phenomena at atomic length scales such as molecular configurations and interactions. As a ubiquitous cellular constituent, water molecules influence biomolecular conformations and interactions [2,3]. However, the influences of hydration and bulk water on chemical processes are not completely understood.…”

“…Alternate explanations are provided for the observed effects of ionic species on macromolecules [3]. For instance, direct interactions of ions with macromolecules and their first shell of hydration play key roles in the hydrophobic collapse and solubility of poly(N-isopropylacrylamide) [5].…”

“…For this purpose, small molecules typically classified as chaotropes and kosmotropes are quantitatively evaluated as additives during the nucleation of CaCO 3 . The additives tested are betaine, ectoine, trehalose, sorbitol, mannitol, glycine, urea, thiourea, and guanidium.…”