Direct HPLC separation of carnosine enantiomers with two chiral stationary phases based on penicillamine and teicoplanin derivatives

Construction of dual-chiral covalent organic frameworks for enantioselective separation

Developing novel chiral stationary phases (CSPs) with versatility is of great importance in enantiomer separation. This study fabricated a dual-chiral covalent organic framework (PA-CA COF) via successive post-synthetic modifications. The chiral trans-1,2-cyclohexanediamine (CA) and (D)-penicillamine (PA) groups were periodically aligned within nanochannels of the COF, allowing selective recognition of enantiomers through intermolecular interactions. It can be a versatile high-performance liquid chromatography (HPLC) CSP for separating a wide range of enantiomers, including chiral pharmaceutical intermediates and chiral drugs. With separation performance comparable to commercial chiral columns and even greater versatility, the PA-CA COF@SiO2 column held promise for practical applications. Chiral separation results combined with molecular simulation indicated that the mixed mode of PA and CA resulted in the broad separation capability of PA-CA COF. The introduction of the dual-chiral COFs concept opens up a new avenue for chiral recognition and separation, holding great potential for practical enantiomer separation.

Environmentally Sustainable Achiral and Chiral Chromatographic Analysis of Amino Acids in Food Supplements

Two LC methods were developed for the achiral and chiral reversed-phase (RP) analysis of an amino acid (AA) pool in a food supplement, in compliance with the main paradigms of Green Chromatography. A direct achiral ion-pairing RP-HPLC method was optimized under gradient conditions with a water-ethanol (EtOH) eluent containing heptafluorobutyric acid (0.1%, v/v), to quantify the eight essential AAs (Ile, Leu, Lys, Met, Phe, Thr, Trp, and Val) contained in the food supplement. Thus, the usually employed acetonitrile was profitably substituted with the less toxic and more benign EtOH. The method was validated for Leu and Phe. The chiral LC method performed with a teicoplanin chiral stationary phase was developed with a water-EtOH (60:40, v/v) eluent with 0.1%, v/v acetic acid. The enantioselective analysis was carried out without any prior derivatization step. Both developed methods performed highly for all eight AAs and revealed that: (i) the content of six out of eight AAs was consistent with the manufacturer declaration; (ii) only L-AAs were present. Furthermore, it was demonstrated that a two-dimensional achiral-chiral configuration is possible in practice, making it even more environmentally sustainable. A molecular modelling investigation revealed interesting insights into the enantiorecognition mechanism of Lys.

The effect of tandem coupling of NicoShell and TeicoShell columns in sub/supercritical fluid chromatography on enantioresolution

The coupling of columns in sub/supercritical fluid chromatography presents a great opportunity for influencing the separation efficiency and extending the selectivity of the separation system. Combinations of different types of chiral stationary phases could positively affect the enantioresolution if single ones are complementary to each other. In this work, two superficially porous particle (2.7 μm) macrocyclic glycopeptide-based columns, namely TeicoShell and NicoShell, were serially coupled and tested in sub/supercritical fluid chromatography for the first time. The influence of the column arrangement on the enantioseparation of structurally diverse biologically active compounds was examined. The obtained results showed how the column order crucially affected the enantioresolution of compounds tested, but the retention was negligibly affected in most cases. We also demonstrated that single TeicoShell and NicoShell columns are very promising towards the development of highly efficient and fast/ultrafast sub/supercritical fluid chromatography methods for structurally different chiral compounds. The optimized methods for sub-minute enantioselective separation of certain biologically important compounds were proposed.

Enantioselective Liquid Chromatographic Separations Using Macrocyclic Glycopeptide-Based Chiral Selectors

Numerous chemical compounds of high practical importance, such as drugs, fertilizers, and food additives are being commercialized as racemic mixtures, although in most cases only one of the isomers possesses the desirable properties. As our understanding of the biological actions of chiral compounds has improved, the investigation of the pharmacological and toxicological properties has become more and more important. Chirality has become a major issue in the pharmaceutical industry; therefore, there is a continuous demand to extend the available analytical methods for enantiomeric separations and enhance their efficiency. Direct liquid chromatography methods based on the application of chiral stationary phases have become a very sophisticated field of enantiomeric separations by now. Hundreds of chiral stationary phases have been commercialized so far. Among these, macrocyclic glycopeptide-based chiral selectors have proved to be an exceptionally useful class of chiral selectors for the separation of enantiomers of biological and pharmacological importance. This review focuses on direct liquid chromatography-based enantiomer separations, applying macrocyclic glycopeptide-based chiral selectors. Special attention is paid to the characterization of the physico-chemical properties of these macrocyclic glycopeptide antibiotics providing detailed information on their applications published recently.

HPLC Separation of Phenolic Acids on Silica Gels Layer-by-Layer Modified with 6,10-Ionene and Dextran Sulfate

Using phenolic acids as an example, we continue to study the nature of the previously obtained multilayer sorbents for HPLC—silica gels modified up to two times with 6,10-ionene and dextran sulfate (DS). The chromatographic behavior of this class of compounds on the obtained sorbents was studied. A six-component mixture of sorbic, vanillic, sinapic, $p$ -coumaric, caffeic, and ferulic acids was separated on the silica gel twice modified with 6,10-ionene and dextran sulfate in 15 min with a selectivity of up to 1.88 and a column efficiency of up to 26000 theoretical plates per meter. The possibility of separating the two classes of compounds—phenolic acids and basic nitrogen-containing pharmaceuticals—in a single mixture on silica gel, twice modified with 6,10-ionene and dextran sulfate, is shown.

Binding modes identification through molecular dynamic simulations: A case study with carnosine enantiomers and the Teicoplanin A2-2-based chiral stationary phase

In the present study, an in silico methodology able to define the binding modes adopted by carnosine enantiomers in the setting of the chiral recognition process is described. The inter- and intramolecular forces involved in the enantioseparation process with the Teicoplanin A2-2 chiral selector and carnosine as model compound are successfully identified. This approach fully rationalizes, at a molecular level, the (S) < (R) enantiomeric elution order obtained under reversed-phase conditions. Consistent explanations were achieved by managing molecular dynamics results with advanced techniques of data analysis. As a result, the time-dependent identification of all the interactions simultaneously occurring in the chiral selector-enantiomeric analyte binding process was obtained. Accordingly, it was found that only (R)-carnosine is able to engage a stabilizing charge-charge interaction through its ionized imidazole ring with the carboxylate counter-part on the chiral selector. Instead, (S)-carnosine establishes intramolecular contacts between its ionized functional groups, that limit its conformational freedom and impair the association with the chiral selector unit.