Modeling Solubilities\nfor Amino Acids in Water as\nFunctions of Temperature and pH

Experimental results from the literature as well as from\nnew experimental\nresults presented here are used to determine parameters to correlate\namino acid solubilities and activities in water. In particular, we\nhave used a modified Larsen’s UNIQUAC Functional-group Activity\nCoefficients (UNIFAC) group-contribution model to illustrate the solubility\nbehavior of 10 amino acids in water including l-Histidine\nand l-Arginine that, to our knowledge, have not been published\npreviously. New UNIFAC groups have been introduced to obtain the activity\ncoefficients for the prediction of activities and solubilities as\na function of temperature and pH. Prior models have not accounted\nfor ionic interactions that affect pH dependent behavior and predictions.\nHence, we examine whether a combination of UNIFAC and a new modified\nDebye–Huckel equation by Rapp et al.<sup>1</sup> is able to\npredict the activity coefficients of ionic species in solution at\nthe high ionic strengths seen at high and low pH. We measured solubilities\nand fitted binary amino acid activity coefficients to estimate the\nnew UNIFAC interaction parameters. The newly obtained UNIFAC parameters\nwere used for the prediction of amino acid solubilities when they\nwere predominantly charge-neutral. Then ionic interactions and pH-dependent\nchemical equilibria were added to calculate amino acid solubilities\nin aqueous solutions at different values of pH and temperature. The\nchemical equilibria required were calculated in a manner similar to\nVisual-MINTEQ<sup>2</sup> that will be described in a separate publication.\nThe calculated solubilities were found to be in good agreement with\nour experimental measurements and with literature data.