Mechanistic insights into protein precipitation by alcohol

Ethanol is used to precipitate proteins during various processes, including purification and crystallization. To elucidate the mechanism of protein precipitation by alcohol, we have investigated the solubility and structural changes of protein over a wide range of alcohol concentrations. Conformation of hen egg-white lysozyme was changed from native to α-helical rich structure in the presence of ethanol at concentrations above 60%. The solubility of lysozyme was reduced with increasing ethanol concentration, although gel formation at ethanol concentrations between 60% and 75% prevented accurate solubility measurements. SH-modified lysozyme showed largely unfolded structure in water and α-helical structure in the presence of ethanol. More importantly, solubility of the chemically modified lysozyme molecules decreased with increasing ethanol concentration. There is no indication of increased solubility upon unfolding of the lysozyme molecules by ethanol, indicating that any favorable interaction of ethanol with the hydrophobic side chains, if indeed occuring, is offset by the unfavorable interaction of ethanol with the hydrophilic side chains and peptide bonds.