Discrimination of α‑Amino Acids Using Green Tea Flavonoid (-)-Epigallocatechin Gallate as a Chiral Solvating Agent Divya Kumari, † Prasun Bandyopadhyay,* ,‡ and N. Suryaprakash* ,†,§ † NMR Research Centre, Indian Institute of Science, Bangalore 560012, India ‡ Unilever R & D Bangalore, 64 Main Road, Whitefield, Bangalore 560066, India § Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore, India * S Supporting Information ABSTRACT: We report a special, hitherto-unexplored property of (-)-epigallocatechin gallate (EGCG) as a chiral solvating agent for enantiodiscrimination of α-amino acids in the polar solvent DMSO. This phenomenon has been investigated by 1 H NMR spectroscopy. The mechanism of the interaction property of EGCG with α-amino acids has been understood as arising out of hydrogen-bonded noncovalent interactions, where the -OH groups of two phenyl rings of EGCG play dominant roles. The conversion of the enantiomeric mixture into diastereomers yielded well- resolved peaks for D and L amino acids permitting the precise measurement of enantiomeric composition. Often one encounters complex situations when the spectra are severely overlapped or partially resolved hampering the testing of enantiopurity and the precise measurement of enantiomeric excess (ee). Though higher concentration of EGCG yielded better discrimination, the use of lower concentration being economical, we have exploited an appropriate 2D NMR experiment in overcoming such problems. Thus, in the present study we have successfully demonstrated the utility of the bioflavonoid (-)-EGCG, a natural product as a chiral solvating agent for the discrimination of large number of α-amino acids in a polar solvent DMSO. Another significant advantage of this new chiral sensing agent is that it is a natural product and does not require tedious multistep synthesis unlike many other chiral auxiliaries. ■ INTRODUCTION (-)-Epigallocatechin gallate (EGCG), a polyphenolic bioflavo- noid abundantly present in green tea, 1,2 is well-known for its biological activities, including anticancer, antidiabetic, anti- bacterial, and anti-inflammatory, and also as an antioxidant that inhibits cellular oxidation of low density lipo protein in the body. There is also a report on the binding of EGCG to the T- cell receptor CD4 at the gp120 site, establishing it as a potential therapeutic treatment for HIV-1 infection. 3 Because of its numerous health benefits and it is a natural product, EGCG has drawn the attention of many researchers. Though the binding of EGCG to proline-rich proteins like human serum albumin (HAS), pepsin, etc. is well established, 4 there is a paucity of information on its direct interaction with individual amino acids. In this regard, the present study reports the new utility of (-)-EGCG as a chiral solvating agent for discrimination of amino acids and the measurement of their ee in polar solvent DMSO. The determination of ee is of profound importance in drug design, a consequence of the fact that different enantiomers of a chiral drug may have diverse biological properties. 5 For testing the enantiopurity of a chiral compound various analytical techniques are available, such as capillary electrophoresis, crystallization, chiral HPLC, NMR spectrosco- py, etc. 6,7 A serious limitation of NMR spectroscopy is that the spectra of enantiopure molecules are identical in the commonly employed achiral solvents, thereby hampering its utility for chiral analysis. In the classical approach, such problems are circumvented by converting enantiomers to diastereomers by using any of the chiral auxiliaries, such as chiral lanthanide shift reagents, chiral solvating agents, and chiral derivatizing agents. 7 These are specific to functional groups present in the molecules, 8 and the recent book and a review gives the account of the latest developments in the field. 7a,9 A number of studies has been reported on the discrimination of α-amino acids and their derivatives and many solvating agents and derivatizing agents have been reported. 10 The studies have also been reported on β-amino acids. 11 The reported chiral auxiliaries used for the discrimination of α- and β-amino acids are required to be synthesized in the laboratory, which often may involve tedious multiple steps. In this work, we demonstrate the biologically important natural product EGCG as another auxiliary, where a detailed study on the interaction of EGCG with α-amino acids in the DMSO solvent has been carried out. The results clearly provide unambiguous evidence for the utility of EGCG as a chiral solvating agent for enantiodiscrimination of α-amino acids in the polar DMSO solvent. Received: November 15, 2012 Published: February 11, 2013 Article pubs.acs.org/joc © 2013 American Chemical Society 2373 dx.doi.org/10.1021/jo3025016 | J. Org. Chem. 2013, 78, 2373-2378