SYSTEMATIC STUDY ON THE RESOLUTION OF DERIVATIZED AMINO-ACIDS ENANTIOMERS ON DIFFERENT CYCLODEXTRIN-BONDED STATIONARY PHASES

The chiral separation of amino acids by liquid chromatography using cyclodextrin-bonded stationary phases was studied systematically. Six types of native and chemically modified beta-cyclodextrin-bonded stationary phases have been used to separate enantiomers of some derivatized amino acids in the reversed-phase high-performance liquid chromatography mode. Chiral separations with (R)- and (S)-naphthyl-ethylcarbamate-beta-cyclodextrin (NEC-beta-CD) bonded phases were compared with similar separations with the native beta-CD stationary phase. Racemic dansyl amino acids were separated best on #-CD column while 3,5-dinitro-2-pyridyl-, dabsyl-, and 3,5-dinitrobenzoyl-amino acids were resolved best on the (R)- or (S)NEC-beta-CD column. The role of the mobile phase was studied. Effects of organic modifiers, ionic strength, and pH of the mobile phase on retention and enantioselectivity of the analytes were investigated on three stationary phases. The enantiomer elution order of all dansyl amino acids on native and NEC-beta-CD bonded stationary phases was L before D enantiomer, while a reverse elution order for dabsyl amino acids was observed. Hence, elution order can be controlled by choosing the appropriate functional group. This provides an alternative to the current method of changing retention order by changing the stationary phase (i.e., obtaining a different column). It appears that the NEC-beta-CD bonded phases are highly effective multimodal chiral stationary phases. The chiral recognition model involving inclusion complexation and pi-pi-interactions is discussed.