Chiral recognition in aqueous solutions: on the role of urea in hydrophilic and hydrophobic interactions of unsubstituted a-amino acids.

Calorimetric and nuclear relaxation time measurements were carried out at 25°C on concd. aq. solns. of urea contg. the L and D forms of the following a-amino acids: alanine, a-aminobutyric acid, norvaline, and norleucine. Glycine was also studied under the same exptl. conditions. The enthalpic interaction coeffs. were rationalized according to the preferential configuration model. The trends of the homochiral coeffs. at increasing concn. 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 concns. of urea because the attenuated electrostatic interactions cannot impose preferential configurations.