Enantioseparation of β-methyl-substituted amino acids with cyclodextrins by capillary zone electrophoresis

Direct capillary zone electrophoretic methods were developed for the separation of the enantiomers of unnatural β-methyl-amino acids such as erythro- and threo-β-methylphenylalanine, β-methyltyrosine, β-methyltryptophan and β-methyl-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid. Capillary zone electrophoresis was carried out using sulfopropylated-α-CD (SP2-α-CD), sulfopropylated-β-CD (SP2-β-CD) both with a degree of substitution of 2 moles/mole cyclodextrin, and sulfopropylated-β-CD (SP4-β-CD) with a degree of substitution of 4 moles/mole β-cyclodextrin. The effects of selector and buffer concentrations, electrolyte pH and applied voltage were studied on the separation efficiency. Varying the electrophoretic conditions with application of 20 kV, hydrodynamic injection, unmodified silica capillary, three different buffers (borate, phosphate and acetate) and modified cyclodextrins as chiral selectors all compounds investigated are nearly baseline resolved. The elution sequence was determined in most cases.