Interactions of aminoacids in concentrated aqueous solutions of urea or ethanol. Implications for the mechanism of protein denaturation

Calorimetric measurements were carried out at 298.15 K on binary aqueous solutions of glycine, and on binary and ternary solutions containing the L and D forms of the a-aminoacid leucine at different concentrations of urea or ethanol as cosolvents. The derived pairwise interaction coefficients of the excess enthalpies were rationalized according to the preferential configuration model. The behaviour shown by glycine, when the concentration and the nature of the cosolvent changes, allows important observations on the influence of the medium on hydrophilic interactions. For leucine, differences were found between the values of the homochiral and heterochiral pairwise enthalpic interaction coefficients. This chiral recognition depends on the nature and concentration of the cosolvent, which influences differently hydrophilic and hydrophobic interactions. The results obtained are compared with those found for other model compounds in concentrated aqueous solutions of urea or ethanol: some comments are made on the possible mode of action of ethanol and urea as protein denaturants. (C) 1999 Elsevier Science B.V. All rights reserved.