Chaotropic anions

In other enzymes such pockets are thought to be the sites for nucleotide binding142. A completely different type of agents which reversibly affect the calcium transport and calcium-dependent ATP splitting are substances like dimethylsulfoxid and ethyleneglycol143. They do not interfere with ATP binding as chaotropic anions do. 

The affinity is influenced by salt concentration in accordance with the Hofmeister series. It is increased in the presence of lyotropic anions such as sulphate and decreased in the presence of chaotropic anions at the other end of the series such as thiocyanate. Anions from the middle of the scale such as chloride have little effect on binding. 

The multilayered character of acetonitrile adsorption creates a pseudo-stationary phase of significant volume on the surface, which acts as a suitable phase for the ion accumulation. In the low organic concentration region (from 0 to 20 v/v% of acetonitrile), studied ions show significant deviation from the ideal retention behavior (decrease in ion retention with increase in acetonitrile composition) due to the formation of the acetonitrile layer, and significant adsorption of the chaotropic anions was observed. This creates an electrostatic potential on the surface in which there is an adsorbed acetonitrile layer, which provides an additional retentive force for the enhancement of the retention of protonated basic analytes. When the dielectric constant is lower than 42 [167], this favors the probability of ion pair formation in this organic enriched layer on top of the bonded phase. 

Mulitply Charged Species. Liophilic anions were also shown to have an effect on the retention of multiply charged peptides. Mono-, di-, tri-, and tetralysine were analyzed with and without the addition of chaotropic anion in the mobile phase. Mono-, di-, tri-, and tetralysine have two, three, four, and five positively charged residues, respectively. 

R. LoBrutto and Y. V. Kazakevich, Effect of chaotropic anions on analyte retention, in Proceedings, 22nd International Symposium on High Performance Liquid Phase Separations and Related Techniques, St. Louis, May 2-8,1998. 

DSC, reported that SCN, another chaotropic anion, stabilizes BSA in the same conditions. It is known that SCN binds strongly to BSA [191]. We observed [195] that Ca2+ and Mg2+ decrease QD and Tm of ribulose 1,5-diphosphate carboxylase (Rubisco) to a limited extent but in a definite and steady way as their concentration increases over the ionic strength range 0-0.3 mol/L Na+ and NH+4 had an opposite effect. In the case of Rubisco, specificity of the effect of Ca2+ and Mg2 + is supported by the fact that these cations are known to bind to the protein and to affect its enzymatic activity by inducing structural changes. Specific effects of Na+ and Ca2+ are also observed by DSC on the heat stability of a-lactalbumin [190] they result in a linear increase of QD with the cation/protein molar ratio up to 1, followed by a plateau (Fig. 7). 

Matrix ions (ions that may be present in an ABS but are not responsible for the formation of the ABS) may affect both system composition and the partitioning of solutes such as pertechnetate [56]. It appears from the results to date that the chaotropic anions that partition to the PEG-rich phase lower distribution ratios for pertechnetate, whereas water-structuring ions tend to increase E>tc, as observed above. 

Figure 11 Distribution ratios for three chaotropic anions in three different PEG-2000/(NH4)2SO/i ABSs versus their — 4Ghy,. ...

Recently, Liu et al. have broadened the field of ion-specific effects on polyelectrolyte multilayers by investigation of the deposition of PSS/ PDADMAC multilayers in mixed electrolyte solutions (PDADMAC poly(diallyldimethy-lammonium chloride)) [22]. It appeared that the effects of anions of a mixed electrolyte solution on the deposition of PEMs are nonadditive. Moreover, in a mixed electrolyte solution containing both chaotropic and cosmotropic anions, the multilayer buildup is dominated by chaotropic anions. 

The PSS/PDADMAC multilayer growth in a single electrolyte solution containing one type of chaotropic anions (e.g., Br, ClOs ) is nonlinear (increasing increment with increasing number of deposited layers). In contrast to this, in the presence of cosmotropic anions like E, CHaCOO, H2PO, and a linear growth... 

Kosmotropic anions tend to cause salting-out of polyelectrolytes, nonionic polymers, and other colloids and cosolutes. Chaotropic anions and kosmotropic cations tend to cause salting in (although at high concentrations most salts cause salting-out) [38, 45 9]. Positively charged macromolecular systems may show inverse Hofmeister behavior only at relatively low-salt concentrations, but revert to a direct Hofmeister series as the salt concentration is increased [50]. The topic of ion-specific effects is also dealt with in other sections of this encyclopedia. 

Kosmotropic anions SO4 HPO4 OAc cit OH Cl Br NO3 CIO3 BF4 I CIO4 SCN PFj Chaotropic anions... 

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. 

With its as5munetric cation enlarged with hydrophobic substituents and a chaotropic anion, l-butyl-3-methylimidazolium hexafluorophosphate appeared to be the most advantageous one. The isotherm of adsorption of this reagent presents a t5rpical Langmuir adsorption behavior. 

Chaotropic anions create an increase in entropy resulting from the disruption of water stracture around these ions in solution. This facilitates solubilization of hydrophobic proteins by water since many more water molecules are available. 

Equation (1.12) holds for salts within the concentration range of 0.01 to 0.2 M at pH 7. With certain salts, for example, tetraethylammonium chloride (2.4 M), the effect of GC content becomes negligible. Divalent cations have a much greater effect on T than do monovalent cations, whereas chaotropic anions depress in the order of CCI3COO > CNS > CIO4 > I" > Br > CH3COO . 

Harrison et undertook a particularly interesting ion-pair chromatography study. In contrast to most other investigations in this field, they varied the nature of the ion-pairing agent (IPA). Further to a classical ammonium-based IPA, the tetrabutylammonium ion (TBA), they used tetrabutylphosphonium (TBP) and tributylsulfonium (TBS). They found that TBP is the most chaotropic ion, whereas the TBS is the least chaotropic. Again in agreement with the rule like seeks like , the most chaotropic anions associate most with TBP and least with TBS. 

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