Enantiomeric separation of glycyl dipeptides by capillary electrophoresis with cyclodextrins as chiral selectors

Uncharged cyclodextrins were tested as chiral selectors for the enantiomeric separation of 13 glycyl dipeptides with capillary electrophoresis. Initial experiments were performed on 10 mmol/L of a cyclodextrin in 0.1 mol/L phosphoric acid -0.088 mol/L triethanolamine. Some of the resolved dipeptides were nonaromatic, which is noteworthy since, to our knowledge, no examples of the separation of small, nonaromatic molecules have been published. Mobility difference plots for Gly-DL-Leu and Gly-DL-Phe with heptakis(2,6-di-O-methyl)-beta-cyclodextrin showed relatively flat profiles in a large concentration range, which is an advantage for the development of robust quantitative analytical methods. The use of a background electrolyte (BGE) solution with pH 3.0 gave irreproducible results for two of the dipeptides, the acidic Gly-DL-Asp and Gly-DL-Glu; this pH is not advisable for the development of robust methods for these two peptides. The need for purer chiral selectors was demonstrated by comparing different batches of heptakis(2,6-di-Omethyl)-beta-cyclodextrin from the same supplier. A BGE consisting of malonic acid and triethanolamine was introduced to give better buffer capacity than the original BGE at pH 3.0.