Chiral recognition in solution. Interactions of alpha-amino acids in concentrated aqueous solutions of urea or ethanol

Enthalpies of dilution of the L and D forms of glutamine, citrulline, and phenylalanine in concentrated aqueous solutions of urea or ethanol were measured calorimetrically at 298 K. Glycine, urea, formamide, and phenol were also studied under the same experimental conditions, to get information about the behaviour of the zwitterion and of the functional group in the side chain of the cited amino acids when the concentration of the cosolvent changes. The derived pairwise enthalpic interaction coefficients for the three amino acids were rationalized according to the preferential configuration model. Indications are that, in concentrated urea, the coefficients for citrulline and glutamine are determined mainly by the interactions between the cosolvent and the hydrophilic groups in the molecule of the amino acids. For phenylalanine, coefficients are less positive than in water, because the presence of urea, which solvates preferentially the zwitterions, attenuates hydrophobic interactions between the benzene rings. In ethanol, coefficients for the three amino acid become negative or more negative than in water, because in this medium hydrophilic interactions are enhanced. Chiral recognition, namely the difference in the values of homo- and heterochiral interaction coefficients, was detected only for phenylalanine in urea. Hence, the nature of the cosolvent, influencing differently hydrophilic and hydrophobic interactions, can lead to the detection of chiral recognition also for those systems that, as phenylalanine, do not present this effect in pure water.