Agents which contribute to stability of water-water interactions
Co-solvents (in water
solvent) are defined as kosmotropic (order-making) if they contribute to the stability and structure of water-water interactions. In contrast,
chaotropic (disorder-making) agents have the opposite effect, disrupting water structure, increasing the solubility of nonpolar solvent particles, and destabilizing solute aggregates.[1] Kosmotropes cause water molecules to favorably interact, which in effect stabilizes intramolecular interactions in
macromolecules such as
proteins.[1]
Ionic kosmotropes
Ionic kosmotropes tend to be small or have high charge density. Some ionic kosmotropes are
CO2− 3,
SO2− 4,
HPO2− 4,
Mg2+ ,
Li+ ,
Zn2+ and
Al3+ . Large ions or ions with low charge density (such as
Br− ,
I− ,
K+ ,
Cs+ ) instead act as
chaotropes.[2] Kosmotropic
anions are more
polarizable and hydrate more strongly than kosmotropic
cations of the same charge density.[3]
A scale can be established if one refers to the
Hofmeister series or looks up the
free energy of
hydrogen bonding () of the salts, which quantifies the extent of hydrogen bonding of an ion in water.[4] For example, the kosmotropes
CO2− 3 and
OH− have between 0.1 and 0.4
J/mol, whereas the chaotrope
SCN− has a between −1.1 and −0.9.[4]
Recent simulation studies have shown that the variation in solvation energy between the ions and the surrounding water molecules underlies the mechanism of the
Hofmeister series.[5][6] Thus, ionic kosmotropes are characterized by strong solvation energy leading to an increase of the overall cohesiveness of the solution, which is also reflected by the increase of the viscosity and density of the solution.[6]
Applications
Ammonium sulfate is the traditional kosmotropic salt for the salting out of protein from an aqueous solution. Kosmotropes are used to induce protein aggregation in pharmaceutical preparation and at various stages of protein extraction and purification.[7][citation needed]
Nonionic kosmotropes
Nonionic kosmotropes have no net charge but are very soluble and become very hydrated. Carbohydrates such as
trehalose and
glucose, as well as
proline and
tert-butanol, are kosmotropes.
^Yang Z (2009). "Hofmeister effects: an explanation for the impact of ionic liquids on biocatalysis". Journal of Biotechnology. 144 (1): 12–22.
doi:
10.1016/j.jbiotec.2009.04.011.
PMID19409939.
^M. Adreev; A. Chremos; J. de Pablo; J. F. Douglas (2017). "Coarse-Grained Model of the Dynamics of Electrolyte Solutions". J. Phys. Chem. B. 121 (34): 8195–8202.
doi:
10.1021/acs.jpcb.7b04297.
PMID28816050.
^
abM. Adreev; J. de Pablo; A. Chremos; J. F. Douglas (2018). "Influence of Ion Solvation on the Properties of Electrolyte Solutions". J. Phys. Chem. B. 122 (14): 4029–4034.
doi:
10.1021/acs.jpcb.8b00518.
PMID29611710.
Polson, C; Sarkar, P; Incledon, B; Raguvaran, V; Grant, R (2003). "Optimization of protein precipitation based upon effectiveness of protein removal and ionization effect in liquid chromatography-tandem mass spectrometry". Journal of Chromatography B. 785 (2): 263–275.
doi:
10.1016/S1570-0232(02)00914-5.
PMID12554139.