Kosmotropic

Solutes are defined as kosmotropic if they contribute to the stability and structure of water-water interactions. Kosmotropes cause water molecules to favorably interact, which also stabilizes intermolecular interactions in macromolecules such as proteins.

Ionic Kosmotropes

Ionic kosmotropes tend to be small or have high charge density. Some ionic kosmotropes are sulfate, phosphate, magnesium(2+), lithium(1+), zinc (2+) and aluminium (+3). Large ions or ions with low charge density (such as bromide, iodide, potassium(1+), caesium(1+)) instead act as chaotropes.

A scale can be established if one refers to the Hofmeister series or looks up the free energy of hydration (ΔG_hydr) of the salts. The more negative ΔG_hydr, the more kosmotropic the salt.

Applications

Ammonium sulphate is the traditional kosmotropic salt for the salting out of protein from an aqueous solution. The chaotropic agents like Guanidinium chloride or urea are used to solubilise all proteins.

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.

See also

• Chaotropic agent and Guanidinium chloride
• Protein precipitation, on aluminium sulphate "salting out"

External links

• Kosmotropes and Chaotropes (from Water Structure and Behavior)
• 'Zink sulphate' C. Polson et al., "Optimization of protein precipitation based upon effectiveness of protein removal and ionization effect in liquid chromatography–tandem mass spectrometry," Journal of Chromatography B, Volume 785 (263:275 )