A universal measure of chaotropicity and kosmotropicity

Abstract

Summary: Diverse parameters, including chaotropicity, can limit the function of cellular systems and thereby determine the extent of E arth's biosphere. Whereas parameters such as temperature, hydrophobicity, pressure, pH , H ofmeister effects, and water activity can be quantified via standard scales of measurement, the chao‐/kosmotropic activities of environmentally ubiquitous substances have no widely accepted, universal scale. We developed an assay to determine and quantify chao‐/kosmotropicity for 97 chemically diverse substances that can be universally applied to all solutes. This scale is numerically continuous for the solutes assayed (from +361 kJ kg −1 mol −1 for chaotropes to −659 kJ kg −1 mol −1 for kosmotropes) but there are key points that delineate (i) chaotropic from kosmotropic substances (i.e. chaotropes ≥ +4; kosmotropes ≤ −4 kJ kg −1 mol −1 ); and (ii) chaotropic solutes that are readily water‐soluble (log P < 1.9) from hydrophobic substances that exert their chaotropic activity, by proxy , from within the hydrophobic domains of macromolecular systems (log P > 1.9). Examples of chao‐/kosmotropicity values are, for chaotropes: phenol +143, CaCl 2 +92.2, MgCl 2 +54.0, butanol +37.4, guanidine hydrochloride +31.9, urea +16.6, glycerol [> 6.5 M] +6.34, ethanol +5.93, fructose +4.56; for kosmotropes: proline −5.76, sucrose −6.92, dimethylsulphoxide (DMSO) −9.72, mannitol −6.69, trehalose −10.6, NaCl −11.0, glycine −14.2, ammonium sulfate −66.9, polyethylene glycol‐ (PEG‐)1000 −126; and for relatively neutral solutes: methanol, +3.12, ethylene glycol +1.66, glucose +1.19, glycerol [< 5 M] +1.06, maltose −1.43 (kJ kg −1 mol −1 ). The data obtained correlate with solute interactions with, and structure–function changes in, enzymes and membranes. We discuss the implications for diverse fields including microbial ecology, biotechnology and astrobiology.