Solubilities of Amino Acids in the Presence of Chaotropic Anions

This work addresses the effect of chaotropic anions (thiocyanate and tosylate) on the solubility of glycine, L-leucine, L-phenylalanine, and L-aspartic acid in aqueous solutions at 298.2 K. The salts used were NaSCN, KSCN, NH4SCN, and NaC7H7SO3 (sodium tosylate), with salt concentrations ranging from 0 to 2 molal. The pH of the saturated solutions was registered, and solid-phase studies were also performed. All the thiocyanate salts and sodium tosylate induced a salting-in effect, except in the systems composed of glycine in aqueous sodium tosylate solutions at 0.5 and 1 molal. For L-leucine, L-phenylalanine, and L-aspartic acid the salting effect of anions followed the order tosylate(-)> SCN-> NO3- > Cl-, in good agreement with the behavior predicted by the Hofmeister series. Differently, the relative solubility of glycine in aqueous nitrate solutions was higher than in those containing thiocyanate, followed by the chloride and, closing the rank, the solutions containing the tosylate anion, suggesting that the solubility change in this case is achieved by a different mechanism.