Enantiomeric Separation of Sulfur- and Selenium-Containing Amino Acids by Capillary Electrophoresis Using Vancomycin as a Chiral Selector

Abstract

Vancomycin is a highly efficient chiral selector for the enantioseparation of methionine, ethionine, cystine, and their selenium analogues, derivatized with 6-aminoquinolyl-N-hydroxysuccinimidoyl carbamate. Concentrations of vancomycin as low as 0.3 mM in 20 mM MOPS/Tris buffer pH 7.0 produce differences in mobility that lead to a resolution far exceeding baseline separation, at a pH where the amino acids are fully ionized. Under these conditions, meso-cystine and meso-selenocystine are also baseline separated from the corresponding D-isomers. At these low vancomycin concentrations, the migration of the L-enantiomers is hardly decelerated. The differences in mobility observed when the concentration of vancomycin is raised to 5 mM are higher than 3.2 x 10 -9 m 2 V -1 s -1 for singly-charged species, such as the derivatives of methionine, selenomethionine, ethionine, and selenoet-hionine, and they increase to 7.6 x 10 -9 and 8.5 x 10 -9 m 2 V -1 s -1 for cystine and selenocystine, respectively. For a capillary with an internal diameter of 50 μm, the separation efficiency is 250 000 theoretical plates/m.