Chiral recognition in aqueous solutions: On the role of urea in hydrophilic and hydrophobic interactions of unsubstituted alpha-amino acids

Calorimetric and nuclear relaxation time measurements were carried our at 25 degrees C on concentrated aqueous solutions of urea containing the L. and D forms of the following alpha-aminoacids: alanine, alpha-aminobutyric acid, norvaline, and norleucine. Glycine was also studied under the same experimental conditions. The enthalpic interaction coefficients were rationalized according to the preferential configuration model. The trends of the homochiral coefficients at increasing concentration of urea underline an interaction mechanism not different from that occurring in water. Chiral recognition is the quantity more affected by the nature of the solvent Among the amino acids studied, only norleucine presents this effect, which vanishes at the highest concentrations of urea because the attenuated electrostatic interactions cannot impose preferential configurations.