Ions, kosmotropic

Lipophilic ions tends to gather together at the surface of water, and hence are surface-active species that lower the surface tension of the solvent, at variance with small and strongly hydrated kosmotropic ions. 

Ion-specific effects on enzyme activity could be explained through their kosmo-tropicity (Hofmeister series) [80]. The Hofmeister series describes the strength of interaction of ions with water. Kosmotropic ions interact strongly with water and tend to produce ordered water structures, while chaotropic ions interact less with water than water itself The ability of ions to modify the water structure influences the protein hydration environment, and combined with direct interactions between ions and proteins, the protein structure can be affected. The suggested hypothesis is that kosmotropic anions and chaotropic cations stabilize enzymes, while chaotropic anions and kosmotropic cations destabilize them [80]. In a number of reports. 

Figure 8.6 Hofmeister series of kosmotropic and chaotropic ions.

The second attempt describes the influence of salt as an interaction of chaotropes and kosmotropes with layers of hydration on the surface of the respective ion. This interaction can be captured by measuring relative viscosity as a function of salt concentration or ionic strength (Jones, 1929) [Eq. (8.60)]. 

Here r/0 is the viscosity in salt-free medium and A and B are constants at high salt concentrations, the second term becomes irrelevant. The constant B, which is the second virial coefficient signifying ion-solvent interactions, is termed the Jones-Dole coefficient after the inventors (Jones, 1929). Chaotropes have a coefficient B which is less than zero, whereas kosmotropes are characterized by 1 > 0. 

The second indicator of kosmotropicity is the standard molar entropy of hydration. For all ions it is highly negative the higher its absolute value, the more water is ordered upon ionic hydration, and the higher the electrolyte kosmotropicity [2,21]. 

Based on the position of an ion in the Hoftneister series, it is possible to foretell the relative effectiveness of anions or cations in an enormous number of systems. The rank of an ion was related to its kosmotropicity, surface tension increments, and salting in and salting out of salt solutions (see below) [25]. A quantitative physical chemistry description of this phenomenon is not far off. Molecular dynamics simulations that considered ionic polarizability were found to be valuable tools for elucidating salt effects [26,27]. 

Even if anionic chaotropes are the most popular neoteric IPRs, polarizable cations such as sulfonium and phosphonium reagents showed single selectivity toward polarizable anions their behavior was rationalized on the basis of their chaotropicity. Probe anion retention generally increases in the order of tributylsulfonium < tetrabutylammonium < tetrabutylphosphonium. Interestingly, retention was found to be influenced by the kosmotropic/chaotropic character of both the IPR and the probe anion [93] and this confirms the peculiarities of hydrophobic ion-pairing. Quaternary phosphonium salts provided increased selectivity compared to ammonium in the IPC of heavy metal complexes of unithiol [112]. 

Sulfate is one of the Hofmeister series of anions, a ranking of the ability of various ions to precipitate proteins (Kunz et al. 2004). In simple terms, precipitation of proteins by kosmotropic anions occurs due to salting out - a competition between the anion and the protein for water of solvation resulting in a loss of water from the protein surface. This process is classically applied in ammonium sulfate precipitation as the first step in many protein purification schemes, although the levels employed are several fold higher than those in wine. In the particular case of white wine, this loss of water of solvation, even by a relatively low amount of sulfate anion, by a protein in a solution containing a variety of cations and other anions and... 

In principle, two different mechanisms have been proposed on how the ions influence protein stability. Firstly, it has been suggested that a modification of water s structure is the origin of the Hofmeister sequence (130). It has been hypothesized that some ions kosmotropes enhance the structure that surrounds the ions, which leads to a strengthening of the hydrophobic effect and thereby stabilizes the proteins (131). However, the ions that break the stmcture that surrounds the ions ( chaotropes )... 

Abstract This chapter discusses the potential usefulness of ionic liquids with respect to biocatalysis by illustrating the stability and activity of enzymes in ionic liquids in the presence or absence of water. Ionic liquids are a class of coulombic fluids composed of organic cations like alkyl-substituted imidazolium, pyrrolidin-ium, and tetraalkylammonium ions and anions such as halides, tetrafluoroborates, hexafluorophosphates, tosylates, etc. The possibility of tunable solvent properties by alternation of cations and anions has made ionic liquids attractive to study biocatalysis which warrants an understanding of enzyme stability and activity in ionic liquids. This chapter systematically outlines the recent studies on the stability of enzymes and their reactivity toward a wide range of catalytic reactions. A careful approach has been taken toward analysis of relationship between stabil-ity/activity of enzymes versus chaotropic/kosmotropic nature of cations and anions of ionic liquids. 

At the same time, activity of the enzyme was lost when very high concentrations of the ions were used. The optimum activity was observed when the ion concentration was within 0.3-0.8 M. But unlike stability and activity, enantioselectivity of protease seemed to markedly decrease with reduction in ee of protease with increasing kosmotropicity of both cations and anions of the ILs. 8 Value, i.e., the difference in viscosity B values of anion and cation, is indicative of overall kosmotropicity of cations and anions of ILs. Enantioselectivity of the enzymes increases in ILs having high 5 value [92,115]. 

Stability and activity of cyt c were also investigated in various ILs with various kosmotropic and chaotropic ions by superoxide reduction assay [49], It was observed that activity and thermodynamic stability of cyt c were reduced in the following order [BMIM][MeSO ]<[BMIM][lactate]<[BMIM][acetate]<[cho-line][Bu2PO ]<[BMPY][HjPO ]<[choline][H2PO ]. This trend agrees well with the kosmotropicity order of the anions present in the ILs indicated by the viscosity B coefficient of the anions [116] [MeSOJ < [lactate] < [acetate] <[BUjPO ] <[H,POJ-<[H/OJ-. 

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