Study of mechanisms of chiral discrimination of amino acids and their derivatives on a teicoplanin-based chiral stationary phase

The behavior of a series of amino acids and some of their methyl ester hydrochloride, N-acetyl and N-tert-butyloxycarbonyl derivatives has been investigated on a teicoplanin-based chiral stationary phase by changing the chromatographic conditions, namely, the type and amount of mobile phase organic modifier and the ionic strength of the solutions. By using species with significantly different characteristics and chemical reactivity, some general conclusions regarding the chiral recognition process on this kind of stationary phase have been formulated. The importance of the carboxylic moiety for the formation of the complex between enantiomers and the aglycone basket of teicoplanin has been demonstrated via chromatography. Additionally, the increased possibility to make an hydrogen bond between the amidic hydrogen of the acetylated compounds and an amidic group on the stationary phase has been proposed to be pivotal for the stability of the complex aglycone d-enantiomer. Phenomena leading to the exclusion from the chiral stationary phase of one or both enantiomers have been rationalized by considering the ionic interactions between stationary phase, molecules to be separated and the surrounding medium and/or steric hindrance effects. The understanding of some of the observed phenomena may be important for optimizing the performance of the separation on aglycone-based media.