Resolution of optical isomers of Dns-amino acids by high-performance liquid chromatography with l-histidine and its derivatives in the mobile phase

Abstract

This paper describes our continuing work in resolving the optical isomers of Dns-amino acids by high-performance liquid chromatography with mixed chelate complexation. Addition of copper(II) complexes of l-histidine to the mobile phase resulted in resolution of many d- and l-Dns-amino acids, including those with aliphatic, polar and aromatic substituents. With polar substituents, highly selective incorporation of the l-enantiomer into the ternary complex with increased retention on the column was observed. The reverse occurred with amino acids with aliphatic side chains; the d-isomers were incorporated preferentially. With aromatic substituted amino acids, the resolution was pH dependent. Substitution of copper(II) complexes of l-histidine methyl ester in the mobile phase dramatically reduced the stereoselectivity, although the isomers were still resolved. Copper(II) complexes of N-acetyl- l-histidine used in the mobile phase gave no stereoselectivity. Excellent separations of isomers were achieved with several of these systems. Many pairs of amino acids could be separated in the same chromatographic analysis.