Solubilities of the common L-α-amino acids as a function of temperature and solution pH

Abstract

Combining equations of state for the standard molal thermodynamic properties of aqueous amino acids with those for the corresponding properties of their crystalline counterparts permits calculation of the solubilities of these biomolecules at temperatures, pressures, and pHs well beyond those for which experimental data are available. The calculations indicate that the solubilities of all the common amino acids in water increase dramatically with increasing temperature and reach (in molalities) at 100°C and 1 bar, 0.03 (Tyr), 0.2-0.8 (Trp, Leu, Ile, Cys, and Phe) 1-2 (Val, Met Asn, and Gln), 2-4 (Thr and Ala), 8-10 (Ser and Gly), and >20 (Pro). In the case of the 5 amino acids that dissociate appreciably in the pH range 3-9 (Asp, Glu, His, Lys, and Arg), the solubilities of Asp and Glu increase with increasing pH above ∼3 and abroach a 1:1 dependence of log m on pH at pHs above ∼5, where the solubilities exceed 3 m at 100°C and 1 bar. In contrast, the solubilities of His, Lys, and Arg at 100°C and 1 bar increase with decreasing pH below ∼pH 6 (His) and ∼8 (Lys and Arg) and reach a 1:1 dependence of log m on -pH at pHs below ∼5 (His) and ∼7 (Lys and Arg) where the solubilities are greater than 1.5 and 15 m, respectively. Unlike those of Asp and Glu, the solubilities of His, Lys, and Arg minimize with increasing temperature at PSAT (Note a) and constant pH. In unbuffered solu ions the equilibrium pHs for coexisting solid and aqueous Asp and Glu decrease only slightly with increasing temperature from ∼2.7 at 25 to ∼2.2 at 100°C. However, the corresponding decrease for His, Lys, and Arg is of the order of a log unit from ∼9.7 (His) and ∼11.6 (Lys and Arg) to ∼8.6 and ∼10.3, respectively. The calculated solubilities of the amino acids at elevated temperatures are in close agreement with the bulk of the experimental values reported in the literature.