Synthesis and circular dichroism studies of N,N-bis(2-quinolylmethyl)amino acid Cu(II) complexes: determination of absolute configuration and enantiomeric excess by the exciton coupling method

We report a method to determine the absolute configuration of alpha-amino acids by exciton coupled circular dichroism (ECCD). Naturally occurring amino acids were successfully derivatized with 2-bromomethylquinoline. Complexation of these conformationally flexible ligands with Cu(II) salts yielded defined propeller-like structures. The direction of the twist (i.e., the relative orientation of the chromophores to each other) is governed by the asymmetric amino acid carbon center. The transition moments of the chromophores couple and yield a bisignate circular dichroism spectrum, the sign of which corresponds to the absolute configuration of the chiral center of the amino acid. Enantiomeric excess (e.e.) of amino acid derivatives is linearly related to the differential extinction coefficient Delta(epsilon) and can be assessed easily utilizing a standard curve. This efficient, sensitive technique requires low analyte concentrations, offers several advantages over established methods, and could be applied in medicinal, pharmaceutical, or chemical retail and manufacturing industry.