Chiral CE of aromatic amino acids by ligand‐exchange with zinc(II)–L‐lysine complex

Abstract

A novel method of chiral ligand-exchange CE was developed with either L- or D-lysine (Lys) as a chiral ligand and zinc(II) as a central ion. This type of chiral complexes was explored for the first time to efficiently separate either individual pairs of or mixed aromatic amino acid enantiomers. Using a running buffer of 5 mM ammonium acetate, 100 mM boric acid, 3 mM ZnSO(4) x 7H(2)O and 6 mM L-Lys at pH 7.6, unlabeled D,L-tryptophan, D,L-phenylalanine, and D,L-tyrosine were well separated, giving a chiral resolution of up to 7.09. The best separation was obtained at a Lys-to-zinc ratio of 2:1, zinc concentration of 2-4 mM and running buffer pH 7.6. The buffer pH was determined to have a strong influence on resolution, while buffer composition and concentration impacted on both the resolution and peak shape. Boric acid with some ammonium acetate was an adoptable buffer system, and some additives like ethylene diamine tetraacetic acid capable of destroying the complex should be avoided. Fine-tuning of the chiral resolution and elution order was achieved by regulating the ratio of L-Lys to D-Lys; i.e. the resolution increased from zero to its highest value as the ratio ascended from 1:0 to 1:infinitive, and L-isomers eluted before or after D-isomers in excessive D- or L-Lys, respectively.

Enantioselective determination of thyroxine enantiomers by ligand‐exchange CE with UV absorbance and ICP‐MS detection

“…It was also found that no separation occurred without L-proline, while the resolution was enhanced at Cu(II)/L-proline ratiosZ1:4. Our results showed clearly that the ratio of 1:2, which had been reported to be the most adequate for the chiral separation of T 4 enantiomers by LE-HPLC [6] and the enantiomeric separation of underivatized aminoacids using aqueous LE-CE and LE-HPLC [24][25][26][27][28][29], was not optimum in this case. Karbaum and Jira reported that the optimum ratio was 1:3 when using methanol as electrophoretic media for the LE-CE chiral separation of eight unmodified amino acids and concluded that the ratio of 1:2 was not optimal in nonaqueous media [30].…”

“…The ratio of copper ions to L-proline was kept constant at 1:2, which was proved to be optimum for aqueous LE-CE and LE-HPLC according to the existing literature [6,[24][25][26][27][28][29]. Under our working conditions (15 mmol/L borate, pH 9.6), T 4 should be ionic with negative charges, since its pK a values are 2.1, 6.7 and 8.9 [3] and therefore, the analyte peaks should appear later than the EOF.…”

Assay of aromatic amino acid enantiomers in rice‐brewed suspensions by chiral ligand‐exchange CE

“…According to our previous study [28], Zn(II)-based CLE-CE was adopted for the direct analysis of aromatic AAs in Laozao's samples and the previously explored running buffer of 5 mM ammonium acetate, 100 mM boric acid, 3 mM ZnSO 4 ?7H 2 O, and 6 mM L-Lys (lysine) at pH 7.6 [28] was first tried by rechecking the critical parameters including buffering reagent, its concentration, pH, and especially the molar ratio of central ion-to-ligand, aiming at further improving the resolution and shortening the separation time. Unfortunately, the purpose did not achieved by just optimizing the known critical factors.…”

“…Because Cu(II) tends to adsorb onto silica capillary wall and has a fairly high UV absorption background, Mn(II), Co(II), Cd(II), and Ni(II) have been tried to replace the Cu(II) [27]. In our previous study, Zn(II) was used [28] and the related running conditions have been explored. We have then focused our efforts on assaying the AAs in Laozao and other foods.…”

Enantioseparation of Dansylated Amino Acids by Ligand-exchange Capillary Electrophoresis Using L-phenylalaninamide, L-lysine or L-threonine as Chiral Selector

“…The suitability of lysine complexes for the chiral separation of underivatized amino acids was introduced by Lu 24 for Cu(II) as central metal ion and by Qi 25 for Zn(II) complexes. To our knowledge there is no publication that deals with the enantioseparation of derivatized amino acids using this selector-ion system.…”