Surface-anchored counterions on weak chiral anion-exchangers accelerate separations and improve their compatibility for mass-spectrometry-hyphenation

Cross-linked polysiloxane-coated stable bond O-9-(2,6-diisopropylphenylcarbamoyl)quinine and quinidine chiral stationary phases as well as application in enantioselective cryo-HPLC

In this work, brush-type chiral stationary phases (CSPs) with O-9-(2,6-diisopropylphenylcarbamoyl)-modified quinidine (DIPPCQD-brush/-SH) and O-9-(2,6-diisopropylphenylcarbamoyl)-modified quinine (DIPPCQN-brush/-SH) were prepared as benchmarks for comparison with new corresponding polymeric CSPs with more stable bonding chemistry. These polymeric CSPs were prepared by coating a thin poly(3-mercaptopropyl)-methylsiloxane film together with the chiral selector onto vinyl-modified silica. In a second step, immobilization of the quinine/quinidine derivatives as well as cross-linking of the polysiloxane film to the vinyl-silica is achieved by a double thiol-ene click reaction. The polymeric CSPs exhibited similar enantioselectivity as the corresponding brush phases, but showed lower chromatographic efficiencies. Chiral acidic substances were separated into enantiomers (e.g., N-protected amino acids, herbicides like dichlorprop) in accordance with an enantioselective anion-exchange process. Oxidation of residual thiol groups of the polymer DIPPCQN-CSP introduced sulfonic acid co-ligands on the silica surface, which resulted in greatly reduced retention times. Acting as immobilized counterions, they allowed to reduce the concentration of counterions in the mobile phase, which is favorable for liquid chromatography (LC)-electrospray ionization-mass spectrometry application. Ibuprofen showed a single peak under ambient column temperature. However, application of cryogenic cooling of the column enabled to achieve baseline separation at -20°C column temperature. It can be explained by an enthalpically dominated separation, which leads to an increase in separation factors when the temperature is reduced. While it is quite uncommon to work at subzero degree column temperature, this work illustrates the potential to exploit such temperature regime for optimization of LC enantiomer separations.

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

The aim of this research study was to provide a more thorough understanding of the underlying mechanism and to broaden the application field of the recently introduced racemization method employing the amino acid derivatization tag 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate (AQC, AccQ) for heat-induced stereoisomerization of common amino acids as well as uniformly isotopically labeled [U-¹³C¹⁵N]-amino acids. The influence of different buffer types such as sodium borate buffer and sodium carbonate buffer as well as their pH and molarity on the racemization and deamidation of amino acids were investigated. It was found that a 0.4 M borate buffer with a pH of 8.0 +/- 0.2 was the most suitable derivatization as well as racemization buffer to ensure degradation free racemization of deamidation prone compounds such as glutamine. Hereby essential was the in-solution pH measurement before and after derivatization with AQC as well as after heat-induced racemization. This strategy provided further insight at which pH an actual racemization event was observed and when an unwanted deamidation of glutamine to glutamic acid occurred. In addition also the influence of the presence of oxygen during racemization was studied. In this context it was possible to determine ideal oxidation and racemization conditions for the production of scalemic mixtures of chiral isotopically labeled methionine AQC-DL-[U-¹³C¹⁵N]-Met as well as its oxidation products, AQC-DL-[U-¹³C¹⁵N]-Met-O and AQC-DL-[U-¹³C¹⁵N]-Met-O₂. All stereoselective separations were performed on the zwitterionic Chiralpak ZWIX(+) column combined with HPLC-ESI-QTOF-MS analysis in positive ionization mode.

In-situ photopolymerized polyhedral oligomeric silsesquioxane-derived monolithic capillary columns with quinidine functionality for enantioseparation by nano-liquid chromatography

The successful fabrication of monolithic capillary columns for enantiomer separations was achieved within vinylized fused silica capillaries via fast "one-pot" photo-initiated free radical polymerization reaction. A mixture consisting of polyhedral oligomeric silsesquioxane, O-[2-(methacryloyloxy)ethylcarbamoyl]-10,11-dihydroquinidine was copolymerized in the presence of n-butanol, ethylene glycol and photo-initiator 2,2-dimethoxy-2-phenylacetophenone. The morphology of the resultant polymeric hybrid inorganic-organic material and its permeability as well as porosity can be controlled by adjusting the composition of the monomers and binary porogenic solvent. The chromatographic characteristics of the columns have been investigated. Separation factors of N-acetyl-phenylalanine (Ac-Phe) and dichlorprop dropped with decrease of chiral functional monomer. Permeability was better when the macroporogen ethyleneglycol was present at higher concentrations during the polymerization. In general, the chiral compounds were well separated (dichlorprop: α = 1.53, R_s up to 4.14; Ac-Phe: α = 1.36, R_s up to 2.69) by nano-HPLC with an optimized enantioselective monolithic capillary column which can be prepared within a few minutes.

Chiral Stationary Phases for Liquid Chromatography: Recent Developments

The planning and development of new chiral stationary phases (CSPs) for liquid chromatography (LC) are considered as continuous and evolutionary issues since the introduction of the first CSP in 1938. The main objectives of the development strategies were to attempt the improvement of the chromatographic enantioresolution performance of the CSPs as well as enlarge their versatility and range of applications. Additionally, the transition to ultra-high-performance LC were underscored. The most recent strategies have comprised the introduction of new chiral selectors, the use of new materials as chromatographic supports or the reduction of its particle size, and the application of different synthetic approaches for preparation of CSPs. This review gathered the most recent developments associated to the different types of CSPs providing an overview of the relevant advances that are arising on LC.

Cinchona Alkaloid-Based Zwitterionic Chiral Stationary Phases Applied for Liquid Chromatographic Enantiomer Separations: An Overview

For the early 2000s, chromatographic methods applying chiral stationary phases (CSPs) became the most effective techniques for the resolution of chiral compounds on both analytical and preparative scales. High-performance liquid chromatography (HPLC) employing various types of chiral selectors covalently bonded to silica-based supports offers a state-of-the-art methodology for "chiral analysis." Although a large number of CSPs are available nowadays, the design and development of new "chiral columns" are still needed since it is obvious that in practice one needs a good portfolio of different columns to face the challenging task of enantiomeric resolutions. The development of the unique chiral anion, cation, and zwitterion exchangers achieved by Lindner and his partners serves as an expansion of the range of the efficiently applicable CSPs.In this context this overview chapter discusses and summarizes direct enantiomer separations of chiral acids and ampholytes applying zwitterionic ion exchangers derived from Cinchona alkaloids. Our aim is to provide comprehensive information on practical solutions with focus on the molecular recognition and methodological variables.

Mechanistic study on the high-selectivity enantioseparation of amino acids using a chiral crown ether-bonded stationary phase and acidic, highly organic mobile phase by liquid chromatography/time-of-flight mass spectrometry

The separation mechanism of amino acid enantiomers using a chiral crown ether-bonded stationary phase, CROWNPAK CR-I(+), and acetonitrile (ACN)-rich mobile phases (MPs) was studied. The retention factors of proteinogenic l-amino acids (except proline) formed U-shaped plots against the ACN content in the MP with a sharp increase at a high ACN content, while d-amino acids showed much smaller increases or monotonous decreases in retention within the same range. The use of an acidic, highly organic MP with trifluoroacetic acid (TFA) provided a high enantioselectivity with a short separation time from the contribution of the increased binding of the ammonium group of the analytes to the crown ether functionality of the stationary phase and electrostatic repulsion counteracting the hydrophilic partition mechanism. Optimizing the sample diluent and MP alleviated the peak distortion caused by a moving water band that accompanied the hydrophilic interaction liquid chromatography-like elution conditions. The liquid chromatography/time-of-flight mass spectrometry method with the optimized MP - ACN/ethanol/water/TFA = 80/15/5/0.5 (v/v/v/v) - enabled the determination of eighteen pairs of proteinogenic amino acid enantiomers within 10 min. The conditions also provided the following advantages: (i) fast and highly reproducible separations under isocratic conditions, (ii) high sensitivity and low backpressure using the MP with a high organic content, and (iii) highly reliable peak identification by combining two columns (CR-I(+) and CR-I(-)), reversing the elution orders of the enantiomers.

Chromatographic behavior of new deazapurine ribonucleosides in hydrophilic interaction liquid chromatography

The chromatographic behavior of new biogenic purine nucleosides in hydrophilic interaction liquid chromatography was examined on three different stationary phases, namely bare silica, and amide- and cyclofructan-based stationary phases. The effects of buffer concentration, pH and acetonitrile-to-aqueous-part ratio in the mobile phase on retention and peak shape were assessed. The retention coefficients and peak symmetry values substantially differed with respect to analytes´ structures, stationary phase properties and mobile phase composition. The bare silica column was unsuitable for these compounds under the chromatographic conditions tested due to very broad and asymmetrical peaks. Furthermore, the cyclofructan-based stationary phase provided almost Gaussian peak shapes of all deazapurine nucleosides under most conditions tested. Therefore, the cyclofructan-based stationary phase is the most suitable choice for the chromatographic analysis of nucleosides.

Liquid chromatographic enantiomer separations applying chiral ion-exchangers based on Cinchona alkaloids

As the understanding of the various biological actions of compounds with different stereochemistry has grown, the necessity to develop methods for the analytical qualification and quantification of chiral products has become particularly important. The last quarter of the century has seen a vast growth of diverse chiral technologies, including stereocontrolled synthesis and enantioselective separation and analysis concepts. By the introduction of covalently bonded silica-based chiral stationary phases (CSPs), the so-called direct liquid chromatographic (LC) methods of enantiomer separation became the state-of-the-art methodology. Although a large number of CSPs is available nowadays, the design and development of new chiral selectors and CSPs are still needed since it is obvious that in practice one needs a good portfolio of different CSPs and focused "chiral columns" to tackle the challenging tasks. This review discusses and summarizes direct enantiomer separations of chiral acids and ampholytes applying anionic and zwitterionic ion-exchangers derived from Cinchona alkaloids with emphasis on literature data published in the last 10 years. Our aim is to provide an overview of practical solutions, while focusing on the integration of molecular recognition and methodological variables.