Chiral amino acids play an indispensable role in living organisms. Diffusion-ordered NMR spectroscopy is an effective NMR tool and a noninvasive analytical method for the analyses of mixture without the need for physical separation of the analytes. However, conventional diffusion-ordered NMR spectroscopy method usually fails to resolve the mixtures of chiral amino acids because of their same molecular masses, sizes, and shapes. Microcrystalline cellulose has been gradually gained more and more interest owing to its wide compatibility, surface area, excellent separation efficiency, non-toxicity, cost-effective, and mechanical stability in many fields. Herein we provide a fast, simple, specific and sensitive method to resolve some D and L-amino acids mixtures by using microcrystalline cellulose as a matrix. This work recognized some D and L- proteinogenic amino acid enantiomers with diffusion-ordered NMR for the first time.
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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