Chiral ligand exchange capillary electrophoresis with L-dipeptides as chiral ligands for separation of Dns-D,L-amino acids
Herein, D- and L-oligopeptides were explored for the first time as chiral ligands in a chiral ligand exchange capillary electrophoresis (CLE-CE) protocol with Zn(II) as central ion for separation of derived d , l -amino acid enantiomers (Dns-D,L-AAs). The effect of four D- and L-oligopeptides, including glycine- l -lysine (Gly-L-Lys), glycine- d -lysine (Gly-D-Lys), l -lysine-lysine-OH and l -lysine-lysine-lysine-lysine-OH on the CLE-CE separation efficiency were evaluated. Thermodynamic calculations and circular dichroism spectra properties showed that the ternary species [(Gly-L-Lys)Zn(II)(D-AAs)] and [(Gly-L-Lys)Zn(II)(L-AAs)] presented the best stereoselectivity , possibly due to entropic effects. Notably, the migration order of Dns-D-AAs and Dns-L-AAs could be tuned by using Gly-D-Lys as the ligand for the CLE-CE system. To obtain satisfactory CLE-CE performance, the concentration ratio of Zn(II) to Gly-L-Lys, the pH of the buffer solution, and the concentration of Zn(II) were investigated. Under the optimized CLE-CE conditions using 100.0 mM H 3 BO 3 , 10.0 mM NH 4 Ac, 3.0 mM Zn (II) and 12.0 mM Gly-L-Lys as the buffer solution at pH 8.30, nine pairs of Dns-D,L-AAs achieved baseline separation, with the partial separation of another five pairs. Furthermore, the proposed CLE-CE protocol, which presented a good linear relationship with the concentration of the test analytes in the range of 75.0–625.0 μM (r 2 ≥ 0.994) and the limit of detection of the method was 5.0 μM, was successfully applied in the kinetics study of l -asparaginase using l -asparagine as the substrate. Our strategy shows the great potential of L-dipeptides in the CLE-CE separation of D,L-AAs enantiomers and bio-applications.
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