Protein Chemistry. Structure, Properties, Research Methods - Shendryk A.N. 2022

Protein Structure
Behavior of Proteins in Solutions
Protein Solubility and Factors Determining It - Dependence of Protein Solubility on Solvent Polarity

Water-miscible Solvents, such as ethanol or acetone, reduce the solubility of most Proteins in water. Increasing the concentration of an organic solvent in the solution leads to protein precipitation. Quantitative studies have shown that Protein solubility at fixed pH values and Ionic strength depends on the Dielectric Constant of the medium and the ability of the non-aqueous solvent to decrease the degree of Hydration of ionic groups. Since the dielectric constants (s) of organic solvents are significantly lower than that of water (see Table 2.2), they substantially reduce the ionizing capacity of the medium.

The reason for the decrease in ionizing capacity is an increase in the force of Coulombic electrostatic attraction between counterions. It is determined by the expression:

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from which it can be seen that as s decreases, the attractive force between counterions will increase. This promotes the association of ions into molecules. For this very reason, water is one of the unique ionizing solvents.

Table 2.2 Dielectric constants of certain solvents at T = 20∘С