Retention and selectivity of teicoplanin stationary phases after copper complexation and isotopic exchange

Teicoplanin is a macrocyclic glycopeptide that is highly effective as a chi ral selector for LC enantiomeric separations. Two possible interaction path s were investigated and related to solute retention and selectivity: (1) in teractions with the only teicoplanin amine group and (2) role of hydrogen b onding interactions. Mobile phases containing 0.5 and 5 mM copper ions were used to try to block the amine group. In the presence of copper ions, it w as found that the teicoplanin stationary phase has a decreased ability to s eparate most underivatized racemic amino acids. However, it maintained its ability to separate enantiomers that were not alpha -amino acids. It is est ablished that there is little copper-teicoplanin complex formation. The eff ect of Cu2+ on the enantioseparation of some a-amino acids appears to be du e to the fact that these solutes are good bidentate ligands and form comple xes with copper ions in the mobile phase. Isotopic exchange with deuterium oxide was performed using acetonitrile-heavy water mobile phases. It was fo und that the retention times of all amino acids were lower with deuterated mobile phases. The retention times of polar or apolar molecules without ami ne groups were higher with deuterated mobiles phases. In all cases, the ena ntioselectivity factors were unaffected by the deuterium exchange. It is pr oposed that the electrostatic interactions are decreased in the deuterated mobile phases and the solute-accessible stationary-phase volume is somewhat swollen by deuterium oxide. The balance of these effects is a decrease in the amino acid retention times and an increase in the apolar solute retenti on time. The enantioselectivity factors of all of the molecules remain unch anged because all of the interactions are changed equally. We propose a new global quality criterion (the E factor) for comparing and evaluating enant iomeric separations.