Effect of Third Component on Water Structure around Tryptophan in Aqueous Solution

Abstract

The effect of 3rd component (additive) on the water structure around tryptophan molecule was discussed on the basis of solubility properties of tryptophan in aqueous urea solution and of the adsorption of tryptophan by carbon black from aqueous EtOH solution. Considering that the iceberg formation around non-polar molecules is an enthalpy effect at room temperature on the basis of Shinoda and Fujihira's new view on the hydrophobic bonding, the following mechanism seemed reasonable for the increase in solubility of tryptophan upon addition of urea : urea comes in contact with hydrophobic moiety of tryptophan to result in a simultaneous structural change (a kind of destruction) of the iceberg around that moiety, accompanying an increase of its affinity to water to result in a predominantly large decrease in enthalpy of mixing. The decrease in adsorption of tryptophan from aqueous solution with the addition of EtOH also might be due to the contact between EtOH and the carbon black surface or the tryptophan molecule containing a simultaneous structural change of iceberg, in the same way as described above. Higher solubility of L-tryptophan than DL-isomer was considered to come from its stronger molecular interaction, as was suggested by a differential scanning calorimetry. Solubility properties of tryptophan in 1, 3-dimethyl urea solution and EtOH solution suggested that the effects of these additive on the hydrophilic moiety were not negligible