Physico-chemical Approach to Biopharmaceutical Phenomena. II. Hydrophobic Hydration of Tryptophan in Aqueous Solution

Abstract

In order to discuss the entropy increase on the adsorption process of tryptophan by carbon black from aqueous solution, the water-tryptphan interaction was investigated on the basis of solubility properties of tryptophan in water and in aqueous urea solution. Calculating the thermodynamic functions of the dissolution of tryptophan in water, the entropy change was negative, meaning that the water structure changed to more ordered one by dissolving tryptophan in water, that is, an iceberg might be formed in the neighborhood of hydrophobic moiety of tryptophan, i. e., indole group. Calculating the thermodynamic functions of the transfer of tryptophan from aqueous solution to aqueous urea solution, the entropy change increased with the concentration of urea and the enthalpy change was negligible, as were contrary to those of a general complex formation. It was reasonable that the solubility of tryptophan increased with the entropy increase through the break-down of iceberg with the addition of urea in aqueous solution of tryptophan, i. e., through the decrease of hydrophobic bonding. Finally it was concluded that the entropy increase overcame the enthalpy increase in the adsorption of tryptophan by carbon black from aqueous solution. In other words, the adsorption was forced with the entropy increase caused by the change of water structure.