Enantiomeric discrimination of chiral organic salts by chiral aza-15-crown-5 ether with C 1 symmetry: experimental and theoretical approaches.

The work involves an experimental ((1)H NMR) and theoretical (MD, MM-PBSA and DFT) investigation of the molecular recognition and discrimination properties of a chiral aza-15-crown-5 against methyl esters of alanine, phenylalanine and valine hydrochloride salts. The results indicate that the receptor binds enantiomers with moderate binding constants (88-1,389 M(-1)), with phenylalanine being more discriminated. The difference in experimental binding free energies (ΔG(R) - ΔG(S)) for alanine, phenylalanine and valine enantiomers were calculated as -0.36, -1.58 and 0.80 kcal mol(-1), respectively. The differences in theoretical binding energies were calculated by MM-PBSA (ΔE(R)PB - ΔE(S)PB=) as -0.30, -1.45 and 0.88, by B3LYP/6-31+G(d) as -1.17, -0.84 and 0.74 and by M06-2X/6-31+G(d) as -1.40, -3.26 and 1.66 kcal mol(-1). The data obtained give valuable information regarding the molecular recognition mode of the organoammonium complexes of chiral aza-crown ether with C 1 symmetry, which may be relevant to biological systems.