Enantioseparation of 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate tagged amino acids and other zwitterionic compounds on cinchona-based chiral stationary phases

The fluorescent tag 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate (AQC; AccQ Fluor reagent kit from Waters) is a commercial N-terminal label for proteinogenic amino acids (AAs), designed for reversed-phase separation and quantification of the AA racemates. The applicability of AQC-tagged AAs and AA-type zwitterionic compounds was tested for enantiomer separation on the tert-butyl carbamate modified quinine and quinidine based chiral stationary phases, QN-AX and QD-AX employing polar-organic elution conditions. The investigated test analytes included the enantiomers of the positional isomers of isoleucine (Ile), threonine, homoserine, and 4-hydroxyproline. Furthermore, beta-AAs, cyclic, and heterocyclic AAs including trans-2-amino-cyclohexane carboxylic acid and trans-2-aminocyclohexyl sulfonic acid, phenylalanine derivatives substituted with halides with increasing electronegativity and 3,4-dihydroxyphenylalanine, cysteine-related derivatives including homocysteic acid, methionine sulfone, cysteine-S-acetic acid, and cysteine-S-acetamide as well as a small range of aminophosphonic acids were enantioseparated. A mechanistic interaction study of AQC-AAs in comparison with fluoresceine isothiocyanate-labeled AAs was performed. The chiral and chemoselective recognition processes involved in enantiomer separation and retention was systematically discussed. Special emphasis was set on the influential factors exhibited by the chemistry, branching position, and spatial properties of the investigated zwitterionic analytes. The general interest to separate and distinguish between different types of branched-chained AAs and metabolic side products thereof lies in the toxicity of some of these compounds, which makes for instance allo-Ile an attractive candidate in disease-related biomarker research.