Chromatographic methods for the separation of enantiomers and epimers of β-alkyl amino acids and peptides containing them

In the past decade, great efforts have been made to obtain receptor-selective ligands with conformationally constrained structures. The β-alkyl-substituted amino acids are important structural features in highly constrained peptide systems. With two chiral centres, these amino acids afford four optical isomers and even their stereospecific syntheses never produce 100% chirally pure product. There is an urgent need for methods of checking the chiral purity of amino acids and of peptides containing them. This article briefly outlines the syntheses of β-alkyl amino acids, and discusses the different Chromatographic methods available for the separation of these amino acids and peptides. For the separation of enantiomers of β-alkyl amino acids mostly high-performance liquid chromatography (HPLC) is applied using precolumn derivatization with a chiral derivatizing reagent, while the application of thin-layer chromatography (TLC) (mostly chiral TLC) is limited because of the lower separation capability. The RP-HPLC methods suitable for the separation of peptide epimers (peptides with enantiomers) containing β-alkyl amino acids are surveyed. The identification procedures after the Chromatographic separation of enantiomers of β-alkyl amino acids and epimers of peptides are also discussed.