Comparison of the chiral separation of amino-acid derivatives by a teicoplanin and RN-beta-CD CSPs using waterless mobile phases: Factors that enhance resolution

A variety of compounds containing amines (i.e., amino acids, amino alcohols, etc.) were chemically derivatized with a variety of electrophilic tagging reagents to elucidate the chiral recognition sites on a teicoplanin-bonded chiral stationary phase (CSP) and on R-naphthylethylcarbamate-beta-cyclodextrin (RN-beta-CD)-bonded CSP. Solutes were separated under optimum chromatographic conditions on teicoplanin and RN-beta-CD CSPs for comparison using an acetonitrile-based mobile phase. It was noted that the size of the analyte or tagging reagent exerted a greater influence on compounds separated on teicoplanin than on RN-beta-CD when using the polar organic mode. This suggests that chiral recognition on teicoplanin CSP is more sensitive to size and indicates that the hydrophobic pocket of teicoplanin plays a significant role in chiral recognition in this mode. However, the type of functional groups had a greater impact than the size of analyte on separations obtained from RN-beta-CD phase in the polar-organic mode. Specifically, the pi-pi interaction was enhanced by derivatizing the aromatic ring of the tagging reagent with electron-withdrawing groups and thus altered the resolution substantially. For both phases, chiral recognition is most pronounced when the stereogenic center of the analyte is near the tagging moiety and surrounded by functional groups (e.g., carboxylic, etc.) which are favorable for hydrogen bonding.