A DFT study on the recognition of α-amino acid derivatives by chiral uranyl–salen

Chirality is a fundamental property of matter that plays a main role in several branches of science, from optics to medicine. In particular, chiral molecules containing carboxylate anions (e.g., amino acids) have captured the interest of many researchers due to their large presence in enzymes as substrates, antibodies and cofactors. Here we report on the DFT study of two uranyl–salen receptors, which differ for the presence of two tert-butyl groups on the salen moiety, that have shown excellent enantioselective discrimination toward selected α-amino acid derivatives. Moreover, to clarify the high enantioselectivity values achieved by these uranyl–salen receptors, we combined titration and DOSY NMR data with DFT calculations on these complexes. Our results suggest that the counterion of the amino acid plays a crucial role in the enantioselective recognition: we found that a host–guest system with a high density of atoms, due to the close proximity of the sterically encumbered n-Bu4N+ cation to the uranyl-aminoacid complex, is essential for an efficient enantiodiscrimination.