ln (S/Sw) = (A/RT) (1/Dw - 1/D)
where S is the solubility of the protein in the organic solvent, Sw is the solubility in water, D is the dielectric constant of the medium when organic solvent is added and Dw is the dielectric constant of the water. R is the gas constant, T is the absolute temperature, and A is a constant.
From the equation , it is clear that if the dielectric constant of the medium decreases with the addition of organic solvent, then the solubility should also decrease and we can expect precipitation. But, organic solvents have a affinity for the hydrophobic surfaces of the protein and this results in denaturation of the protein along with precipitation. Hence, the concentration of organic solvents is usually kept low, although some organic solvents like 2 methyl-2,4-pentane diol (MPD), Dimethyl Sulfoxide (DMSO) and ethanol can be used in high concentrations.
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