Optical Resolution of Racemic A-Amino Acids on a Dynamic Chiral Stationary Phase by Ligand Exchange Chromatography

A dynamic chiral stationary phase for the ligand exchange chiral liquid chromatography was prepared by tentatively loading (lS,2R)-N,N-carboxymethyl dodecylnorephedrine monosodium salt (4) prepared from (1S,2R)-norephedrine onto a commercial reverse phase octadecyl-silica gel column and successfully used for the resolution of various amino acids without derivatization. The retention of the two enantiomers of amino acids on the column was found to be significantly influenced by the organic modifier content, Cu(II) concentration and pH of the mobile phase. However, the enantioselectivity was found to be significantly influenced mainly by the organic modifier content in the mobile phase. Based on the resolution trends of two enantiomers, a chiral recognition model concerning the enantioselective formation of ternary complex from the fixed ligand, amino acids and Cu(II) was proposed.