SOLUBILITIES OF THE COMMON L-ALPHA-AMINO ACIDS AS A FUNCTION OF TEMPERATURE AND SOLUTION PH

Combining equations of state for the standard molal thermodynamic prop erties of aqueous amino acids with those for the corresponding propert ies of their crystalline counterparts permits calculation of the solub ilities of these biomolecules at temperatures, pressures, and pHs well beyond those for which experimental data are available. The calculati ons indicate that the solubilities of all the common amino acids in wa ter increase dramatically with increasing temperature and reach (in mo lalities) at 100 degrees C and 1 bar, 0.03 (Tyr), 0.2-0.8 (Trp, Leu, I le, Cys, and Phe), 1-2 (Val, Met, Asn, and Gin), 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 A rg), the solubilities of Asp and Glu increase with increasing pH above similar to 3 and approach a 1:1 dependence of log m on pH at pHs abov e similar to 5, where the solubilities exceed 3 m at 100 degrees C and 1 bar. In contrast, the solubilities of His, Lys, and Arg at 100 degr ees C and 1 bar increase with decreasing pH below similar to pH 6 (His ) and similar to 8 (Lys and Arg) and reach a 1:1 dependence of log m o n -pH at pHs below similar to 5 (His) and similar to 7 (Lys and Arg), where the solubilities are greater than 1.5 and 15 m, respectively. Un like those of Asp and Glu, the solubilities of His, Lys, and Arg minim ize with increasing temperature at P-SAT. (Note a) and constant pH. In unbuffered solutions, the equilibrium pHs for coexisting solid and aq ueous Asp and Glu decrease only slightly with increasing temperature f rom similar to 2.7 at 25 to similar to 2.2 at 100 degrees C. However, the corresponding decrease for His, Lys, and Arg is of the order of a log unit from similar to 9.7 (His) and similar to 11.6 (Lys and Arg) t o similar to 8.6 and similar to 10.3, respectively. The calculated sol ubilities of the amino acids at elevated temperatures are in close agr eement with the bulk of the experimental values reported in the litera ture.