Modeling Solubilities for Amino Acids in Water as Functions of Temperature and pH

Experimental results from the literature as well as from
new experimental
results presented here are used to determine parameters to correlate
amino acid solubilities and activities in water. In particular, we
have used a modified Larsen’s UNIQUAC Functional-group Activity
Coefficients (UNIFAC) group-contribution model to illustrate the solubility
behavior of 10 amino acids in water including l-Histidine
and l-Arginine that, to our knowledge, have not been published
previously. New UNIFAC groups have been introduced to obtain the activity
coefficients for the prediction of activities and solubilities as
a function of temperature and pH. Prior models have not accounted
for ionic interactions that affect pH dependent behavior and predictions.
Hence, we examine whether a combination of UNIFAC and a new modified
Debye–Huckel equation by Rapp et al.1 is able to
predict the activity coefficients of ionic species in solution at
the high ionic strengths seen at high and low pH. We measured solubilities
and fitted binary amino acid activity coefficients to estimate the
new UNIFAC interaction parameters. The newly obtained UNIFAC parameters
were used for the prediction of amino acid solubilities when they
were predominantly charge-neutral. Then ionic interactions and pH-dependent
chemical equilibria were added to calculate amino acid solubilities
in aqueous solutions at different values of pH and temperature. The
chemical equilibria required were calculated in a manner similar to
Visual-MINTEQ2 that will be described in a separate publication.
The calculated solubilities were found to be in good agreement with
our experimental measurements and with literature data.