Racemization without deamidation: Effect of racemization conditions on 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate tagged amino acids

Comprehensive Online Reversed-Phase × Chiral Two-Dimensional Liquid Chromatography-Mass Spectrometry with Data-Independent Sequential Window Acquisition of All Theoretical Fragment-Ion Spectra-Acquisition for Untargeted Enantioselective Amino Acid Analysis

This work presents an advanced analytical platform for untargeted enantioselective amino acid analysis (eAAA) by comprehensive achiral × chiral 2D-LC hyphenated to ESI-QTOF-MS/MS utilizing data-independent SWATH (sequential window acquisition of all theoretical fragment-ion spectra) technology. The methodology involves N-terminal pre-column derivatization with 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate (AQC; AccQ) as retention, selectivity, and MS tag, supporting retention and UV detection in RPLC (¹D), chiral recognition, and thus enantioselectivity by the core-shell tandem column composed of a quinine carbamate weak anion exchanger (QN-AX) and a zwitterionic chiral ion-exchanger (ZWIX(+)) (²D) as well as the ionization efficiency during positive electrospray ionization due to a high proton affinity of the AQC label. Furthermore, the urea-type MS tag gives rise to the generation of AQC-tag characteristic signature fragments in MS². The latter allows the chemoselective mass spectrometric filtering of targeted and untargeted N-derivatized amino acids or related labeled species. The chiral core-shell tandem column provides a complete enantioselective amino acid profile of all proteinogenic amino acids within 1 min, with full baseline separation of all enantiomers, but without resolution of isomeric Ile/allo-Ile (aIle)/Leu, which can be resolved by RPLC. The entire LC × LC separation occurs within a total run time of 60 min (¹D), with the chiral ²D operated in gradient elution mode and a cycle time of 60 s. A strategy to mine the 2D-LC-SWATH data is presented and demonstrated for the qualitative eAAA of two peptide hydrolysate samples of therapeutic peptides containing common and uncommon as well as primary and secondary amino acids. Absolute configuration assignment of amino acids using template matching for all proteinogenic amino acids was made feasible due to method robustness and the inclusion of an isotopically labeled L-[U-¹³C¹⁵N]-AA standard. The quantification performance of this LC × LC-MS/MS assay was also evaluated. Accuracies were acceptable for the majority of AAs enabling AA composition determination in peptide hydrolysates simultaneously with configuration assignment, as exemplified by oxytocin. This methodology represents a step toward truly untargeted 2D enantioselective amino acid analysis and metabolomics.

Enantioselective metabolomics by liquid chromatography-mass spectrometry

Metabolomics strives to capture the entirety of the metabolites in a biological system by comprehensive analysis, often by liquid chromatography hyphenated to mass spectrometry. A particular challenge thereby is the differentiation of structural isomers. Common achiral targeted and untargeted assays do not distinguish between enantiomers. This may lead to information loss. An increasing number of publications demonstrate that the enantiomeric ratio of certain metabolites can be meaningful biomarkers of certain diseases emphasizing the importance of introducing enantioselective analytical procedures in metabolomics. In this work, the state-of-the-art in the field of LC-MS based metabolomics is summarized with focus on developments in the recent decade. Methodologies, tagging strategies, workflows and general concepts are outlined. Selected biological applications in which enantioselective metabolomics has documented its usefulness are briefly discussed. In general, targeted enantioselective metabolomics assays are often based on a direct approach using chiral stationary phases (CSP) with polysaccharide derivatives, macrocyclic antibiotics, chiral crown ethers, chiral ion exchangers, donor-acceptor phases as chiral selectors. Rarely, these targeted assays focus on more than 20 analytes and usually are restricted to a certain metabolite class. In a variety of cases, pre-column derivatization of metabolites has been performed, especially for amino acids, to improve separation and detection sensitivity. Triple quadrupole instruments are the detection methods of first choice in targeted assays. Here, issues like matrix effect, absence of blank matrix impair accuracy of results. In selected applications, multiple heart cutting 2D-LC (RP followed by chiral separation) has been pursued to overcome this problem and alleviate bias due to interferences. Non-targeted assays, on the other hand, are based on indirect approach involving tagging with a chiral derivatizing agent (CDA). Besides classical CDAs numerous innovative reagents and workflows have been proposed and are discussed. Thereby, a critical issue for the accuracy is often neglected, viz. the validation of the enantiomeric impurity in the CDA. The majority of applications focus on amino acids, hydroxy acids, oxidized fatty acids and oxylipins. Some potential clinical applications are highlighted.