Solubility of the Proteinogenic α-Amino Acids in Water, Ethanol, and Ethanol-Water Mixtures

The addition of organic solvents to alpha-amino acids in aqueous solution could be an effective method in crystallization. We reviewed the available data on the solubility of alpha-amino acids in water, water-ethanol mixtures, and ethanol at 298.15 K and 0.1 MPa. The solubility of L-alanine, L-proline, L-arginine, L-cysteine, and L-lysine in water and ethanol mixtures and the solubility of L-alanine, L-proline, L-arginine, L-cysteine, L-lysine, L-asparagine, L-glutamine, L-histidine, and L-leucine in pure ethanol systems were measured and are published here for the first time. The impact on the solubility of amino acids that can convert in solution, L-glutamic acid and L-cysteine, was studied. At lower concentrations, only the ninhydrin method and the ultraperfomance liquid chromatography (UPLC) method yield reliable results. In the case of alpha-amino acids that convert in solution, only the UPLC method was able to discern between the different alpha-amino acids and yields reliable results. Our results demonstrate that alpha-amino acids with similar physical structures have similar changes in solubility in mixed water/ethanol mixtures. The solubility of L-tryptophan increased at moderate ethanol concentrations.