Chaotropic substance

Besides fhe application in chromatography, chaotropic substances are used for protein extraction, e.g., of recombinant proteins from inclusion bodies and for precipitation by salts or organic solvents... 

Denaturation of one or both partners of the affinity pair is done either by changing pH, by salts, or by chaotropic substances. This kind of desorption is easy to do, but there is danger of irreversible... 

Thus, aside from the covalently polymerized a-chain itself, the majority of protein structure is determined by weaker, noncovalent interactions that potentially can be disturbed by environmental changes. It is for this reason that protein structure can be easily disrupted or denatured by fluctuations in pH, temperature, or by substances that can alter the structure of water, such as detergents or chaotropes. 

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 separation selectivity of a mixture of acidic, basic, and neutral compounds can be altered with the addition of chaotropic mobile-phase additives (Figure 4-62). The retention of the basic compounds can be increased by addition of chaotropic counterions in the mobile phase, while the retention of neutral and acidic compounds is generally unaffected. This is particularly useful during the development of impurity profile methods in the pharmaceutical industry where the retention of a polar protonated basic impurity may be adjusted such that adequate separation selectivity is obtained when union-izable, acidic, or basic (in neutral form) impurities in the drug substance are present. In Figure 4-62 the retention of protonated basic compounds, metoprolol and labetalol, increase while the retention of phenol (in its neutral state) remains constant. 

Elution of bound substances is usually achieved by the use of a reagent that disrupts noncovalent bonds. These vary from mild" procedures, such as the use of high salt or high or low pH, to more drastic agents, such as 8M urea, 1% SDS, or 5Af guanidine hydrochloride. Chaotropic... 

Adsorption of the solute onto the gel matrix can also occur, but is not normally encountered in SEC. It may be counteracted by changing the eluent. The ideal eluent is a good solvent for both the solute and the substance forming the gel matrix. In addition, hydrogen-bond-breaking agents such as urea or guanidinium, or chaotropic ions, or surfactants may be added to eluents to improve SEC characteristics. 

A widely applicable, non-chaotropic elution technique in bioaffinity chromatography is electrophoretic desorption [60,74,75]. If the affinant is coupled to the solid support by an azo bond or by thiol- or alcohol-ester bonds, the complex of the affinant with the isolated substance can be detached from the solid matrix and then the affinity ligand separated by dialysis or gel filtration. 

Chaotropic The property of being able to disrupt the hydrogen bonding structure of water. Substances that are good hydrogen bonders, such as urea or guanidine hydrochloride, are chaotropic. Concentrated solutions of these substances tend to denature proteins because they reduce the hydrophobic effect. 

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