Enantioseparation of β-substituted tryptophan analogues with modified cyclodextrins by capillary zone electrophoresis

Direct capillary zone electrophoretic methods were developed for the separation of the enantiomers of unnatural β-substituted tryptophan analogues such as erythro- and threo-β-methyl-, β-2-propyl-, β-3-pentyl-, β-phenyl- and β-2,5-dimethoxyphenyltryptophan. Cyclodextrins (CDs) were chosen as chiral selectors because of their favorable properties (stability, commercial availability, low cost, UV transparency, inertness, etc.). 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. With this technique all compounds investigated are baseline resolved using different background electrolytes and chiral additives. The elution sequence was determined in all cases.