Effect of pH on the solubilities of divalent and trivalent amino acids in water at 298.15 K

The objective of this study is to report the effects of pH on the solubility of amino acids in water at 298.15 K. Two divalent amino acids, asparagine and glutamine, and three trivalent amino acids, tyrosine, aspartic acid and glutamic acid, were examined. The pH range studied was from 2 to 10. The solubility of amino acids in water is affected by the pH of the solution because amino acids dissociate into different ionic forms in aqueous solution: zwitterions, cations and anions. A chemical model was employed to describe the dissociation equilibria of all amino acid species with hydrogen ions in water. Moreover, for asparagine, glutamine and tyrosine, the chemical model indicated that the formation of cations and anions is insignificant in the isoelectric solution, with almost 100% of the amino acids exist as zwitterions. However, in the case of trivalent amino acids with acidic side chain groups, the zwitterion is not the only species present at the isoelectric point. Therefore, for aspartic acid and glutamic acid, at least three different ionic forms must be considered. To correlate the solubility data for amino acids in water, a non-random two liquid (NRTL) model was used to describe the non-idealities in the solution. The results calculated with two adjustable parameters were quite satisfactory, with an overall deviation of 2.21%.