Co-solvents (in water
solvent
A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...
) are defined as kosmotropic (order-making) if they contribute to the stability and structure of water-water interactions. In contrast,
chaotropic Chaotropicity describes the entropic disordering of lipid bilayers and other biomacromolecules which is caused by substances dissolved in water. According to the original usage and work carried out on cellular stress mechanisms and responses, ch ...
(disorder-making) agents have the opposite effect, disrupting water structure, increasing the solubility of nonpolar solvent particles, and destabilizing solute aggregates.
Kosmotropes cause water molecules to favorably interact, which in effect stabilizes intramolecular interactions in
macromolecules
A macromolecule is a very large molecule important to biophysical processes, such as a protein or nucleic acid. It is composed of thousands of covalently bonded atoms. Many macromolecules are polymers of smaller molecules called monomers. The ...
such as
proteins
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
.
Ionic kosmotropes
Ionic kosmotropes tend to be small or have high charge density. Some ionic kosmotropes are
,
,
,
,
,
and
. Large ions or ions with low charge density (such as
,
,
,
) instead act as
chaotropes. Kosmotropic
anions
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...
are more
polarizable and hydrate more strongly than kosmotropic
cations
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by con ...
of the same charge density.
A scale can be established if one refers to the
Hofmeister series or looks up the
free energy of
hydrogen bond
In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a ...
ing (
) of the salts, which quantifies the extent of hydrogen bonding of an ion in water.
For example, the kosmotropes
and
have
between 0.1 and 0.4
J/mol, whereas the chaotrope
has a
between −1.1 and −0.9.
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.
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.
Applications
Ammonium sulfate
Ammonium sulfate (American English and international scientific usage; ammonium sulphate in British English); (NH4)2SO4, is an inorganic salt with a number of commercial uses. The most common use is as a soil fertilizer. It contains 21% nitrogen a ...
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.
Nonionic kosmotropes
Nonionic kosmotropes have no net charge but are very soluble and become very hydrated. Carbohydrates such as
trehalose
Trehalose (from Turkish '' tıgala'' – a sugar derived from insect cocoons + -ose) is a sugar consisting of two molecules of glucose. It is also known as mycose or tremalose. Some bacteria, fungi, plants and invertebrate animals synthesize it ...
and
glucose
Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, using ...
, as well as
proline
Proline (symbol Pro or P) is an organic acid classed as a proteinogenic amino acid (used in the biosynthesis of proteins), although it does not contain the amino group but is rather a secondary amine. The secondary amine nitrogen is in the prot ...
and
''tert''-butanol, are kosmotropes.
See also
*
Chaotropic agent
A chaotropic agent is a molecule in water solution that can disrupt the hydrogen bonding network between water molecules (i.e. exerts chaotropic activity). This has an effect on the stability of the native state of other molecules in the solution, ...
and
guanidinium chloride
Guanidinium chloride or guanidine hydrochloride, usually abbreviated GdmCl and sometimes GdnHCl or GuHCl, is the hydrochloride salt of guanidine.
Structure
Guanidinium chloride crystallizes in orthorhombic space group ''Pbca''. The crystal stru ...
*
Protein precipitation, on ammonium sulfate "salting out"
References
External links
* {{cite journal, pmid=12554139, year=2003, last1=Polson, first1=C, last2=Sarkar, first2=P, last3=Incledon, first3=B, last4=Raguvaran, first4=V, last5=Grant, first5=R, title=Optimization of protein precipitation based upon effectiveness of protein removal and ionization effect in liquid chromatography-tandem mass spectrometry, volume=785, issue=2, pages=263–275, journal=Journal of Chromatography B , doi=10.1016/S1570-0232(02)00914-5
Chemical properties