Comparison of liquid and supercritical fluid chromatography mobile phases for enantioselective separations on polysaccharide stationary phases

Insights Into the Enantioseparation of Polyhalogenated 4,4'-Bipyridines With a Cellulose Tris(3,5-Dimethylphenylcarbamate)-Based Chiral Column by Using Supercritical Fluid Chromatography

In the last decade, by integrating experimental and computational analyses, it was demonstrated that halogen bond (HaB) may contribute to binding and enantiorecognition mechanisms underlying the HPLC enantioseparation of halogenated chiral analytes by using cellulose tris(3,5-dimethylphenylcarbamate) (CDMPC)-based chiral columns and n-hexane-based mixtures as mobile phases. When used as a pivotal component of the mobile phase in supercritical fluid chromatography (SFC), carbon dioxide is often considered as an n-hexane-like nonpolar solvent because of its low dielectric constant and zero molecular dipole moment. On the other hand, carbon dioxide may also serve as hydrogen bond (HB) and HaB acceptor due to the presence of nonbonding electrons on the two oxygen atoms, interacting with analyte enantiomers, chiral selectors, and co-solvents. On this basis, we report herein the results of a study aiming at evaluating the impact of using carbon dioxide in SFC in place of n-hexane in HPLC on halogen-dependent enantioseparations by using atropisomeric halogenated 4,4'-bipyridines as analytes and Lux Cellulose-1 as CDMPC-based chiral column. The experimental investigation was complemented by a computational study performed using (a) quantum mechanics (QM) calculations to map and quantify noncovalent interactions possibly underlying the contact of the analytes with carbon dioxide and with the distinctive pendant groups of the CDMPC and (b) molecular dynamics (MD) simulations to visualize noncovalent interactions acting in the analyte 1/CDMPC chromatographic system in different media. The use of MD simulations to model enantioseparations performed in carbon dioxide-based media was not reported in the literature so far.

Simultaneous separation of the enantiomers of six anticoagulant rodenticides using chiral supercritical fluid chromatography-mass spectrometry

The simultaneous separation of the enantiomers of six anticoagulant rodenticides, derived from 4-hydroxycoumarin, has been studied in this work. Ten different stationary phases (zwitterionic, Pirkle-type, polysaccharides and macrocyclic antibiotics derivatives) were evaluated by using supercritical fluid chromatography coupled to two different detectors (circular dichroism and mass spectrometry-single quadrupole). The effect of the type of organic modifier and temperature on the chiral separation was investigated, and the best results were obtained with the column Regis S,S-Whelk-O1 at 25 °C when using a gradient elution program with methanol as organic modifier. Considering detection, the highest sensitivity was obtained with the single quadrupole mass spectrometry detector. Under these conditions, eighteen stereoisomers were baseline resolved within 18 min. It should be also mentioned that the columns Chiralpak AS-H, Lux Cellulose-2, Chiralpak AD and Chiralpak Zwix (+) provided also good results for the enantiomeric separation of some individual compounds.

Automated multifactorial design of experiment and Bayesian optimisation algorithm approaches to method development for the green analysis by supercritical fluid chromatography of a pharmaceutical ingredient

During drug development, chromatography is frequently used for purity and stability testing of both drug substance and drug product. Reversed phase liquid chromatography (RPLC) is one of the most widely used methodologies due to its wide scope of application. In the later stages of drug development, the specified impurities and degradation products that define the critical quality attribute of the final API, also known as Key Predictive Sample Set (KPSS), are usually well defined and controlled. At this point, a method review enables selecting the most appropriate technique which should be the one providing optimal robustness (ICH-Q14[1]), with the support of Quality by Design (QbD) approaches. Supercritical Fluid Chromatography (SFC) is a preferred technique for its proven diversity in selectivity. The adoption of a technique which presents the most favourable environmental impact, such as, but not limited to, SFC, is also becoming increasingly important as laboratories strive to reduce carbon footprint. Re-developing a method requires high resource-demands in terms of staff, materials, and time. Any step of the process that can be automated can facilitate this approach, speeding up the delivery of the method whilst preserving robustness. In this article we describe how an SFC method was developed for the purity profiling of a late-stage oncology candidate, taking advantage of the superior selectivity of SFC towards structurally similar analytes, owed to the high orthogonality with R2 as low as 0.014 towards the KPSS. We describe two approaches to automate the method development. Firstly, a multifactorial design of experiments (DoE) and secondly, an optimization via a Bayesian algorithm, which was completed in one night, highlighting the potential and limitations, with an insight into the robustness. Both methods achieved baseline separation with varying levels of automation embedded into the process and a large reduction of the resource demands when compared to traditional optimisation methods. Finally, we describe the beneficial environmental impact that implementing SFC methods can yield, with a calculated green score reduced to a value between 17 and 30 % compared to RPLC, depending on the number of runs per sequence.

Intelligent Recommendation Systems Powered by Consensus Neural Networks: The Ultimate Solution for Finding Suitable Chiral Chromatographic Systems?

The selection of suitable combinations of chiral stationary phases (CSPs) and mobile phases (MPs) for the enantioresolution of chiral compounds is a complex issue that often requires considerable experimental effort and can lead to significant waste. Linking the structure of a chiral compound to a CSP/MP system suitable for its enantioseparation can be an effective solution to this problem. In this study, we evaluate algorithmic tools for this purpose. Our proposed consensus model, which uses multiple optimized artificial neural networks (ANNs), shows potential as an intelligent recommendation system (IRS) for ranking chromatographic systems suitable for the enantioresolution of chiral compounds with different molecular structures. To evaluate the IRS potential in a proof-of-concept stage, 56 structural descriptors for 56 structurally unrelated chiral compounds across 14 different families are considered. Chromatographic systems under study comprise 7 cellulose and amylose derivative CSPs and acetonitrile or methanol aqueous MPs (14 chromatographic systems in all). The ANNs are optimized using a fit-for-purpose version of the chaotic neural network algorithm with competitive learning (CCLNNA), a novel approach not previously applied in the chemical domain. CCLNNA is adapted to define the inner ANN complexity and perform feature selection of the structural descriptors. A customized target function evaluates the correctness of recommending the appropriate CSP/MP system. The ANN-consensus model exhibits no advisory failures and requires only an experimental attempt to verify the IRS recommendation for complete enantioresolution. This outstanding performance highlights its potential to effectively resolve this problem.

Supercritical fluid chromatography is not (only) normal-phase chromatography

Supercritical fluid chromatography (SFC), now using carbon dioxide as a major component of the mobile phase, has been known for over 60 years but still some misunderstandings remain about its capabilities. Amongst them, SFC is often described as a normal-phase chromatographic technique, based on different considerations: polarity of the stationary phase, elution order of the analytes, relative non-polarity of the mobile phase, non-linear retention behaviour, or adsorption retention mechanisms. All of these assumptions are true to a certain extent, and in certain circumstances. But also, all of these assumptions are wrong in different circumstances. In this paper, the criteria to categorize SFC as a normal-phase chromatographic method will be examined individually, considering all knowledge acquired from the early years of its development. Finally, it will appear that the "normal-phase" glass lens is greatly reducing the true extent of SFC's possibilities.

Recognition in the Domain of Molecular Chirality: From Noncovalent Interactions to Separation of Enantiomers

It is not a coincidence that both chirality and noncovalent interactions are ubiquitous in nature and synthetic molecular systems. Noncovalent interactivity between chiral molecules underlies enantioselective recognition as a fundamental phenomenon regulating life and human activities. Thus, noncovalent interactions represent the narrative thread of a fascinating story which goes across several disciplines of medical, chemical, physical, biological, and other natural sciences. This review has been conceived with the awareness that a modern attitude toward molecular chirality and its consequences needs to be founded on multidisciplinary approaches to disclose the molecular basis of essential enantioselective phenomena in the domain of chemical, physical, and life sciences. With the primary aim of discussing this topic in an integrated way, a comprehensive pool of rational and systematic multidisciplinary information is provided, which concerns the fundamentals of chirality, a description of noncovalent interactions, and their implications in enantioselective processes occurring in different contexts. A specific focus is devoted to enantioselection in chromatography and electromigration techniques because of their unique feature as "multistep" processes. A second motivation for writing this review is to make a clear statement about the state of the art, the tools we have at our disposal, and what is still missing to fully understand the mechanisms underlying enantioselective recognition.

Stereoselective Processes Based on σ-Hole Interactions

The σ-hole interaction represents a noncovalent interaction between atoms with σ-hole(s) on their surface (such as halogens and chalcogens) and negative sites. Over the last decade, significant developments have emerged in applications where the σ-hole interaction was demonstrated to play a key role in the control over chirality. The aim of this review is to give a comprehensive overview of the current advancements in the use of σ-hole interactions in stereoselective processes, such as formation of chiral supramolecular assemblies, separation of enantiomers, enantioselective complexation and asymmetric catalysis.

Characteristic and complementary chiral recognition ability of four recently developed immobilized chiral stationary phases based on amylose and cellulose phenyl carbamates and benzoates

To date, various immobilized chiral stationary phases (CSPs) have been developed. The immobilized CSPs have opened up possibilities not only maintaining the high chiral recognition abilities as well as corresponding coated ones but also affording high durability to various mobile phase. This report directed to investigate enantioseparation of recently launched four immobilized CSPs with cellulose and amylose backbones under normal phase liquid chromatography conditions. Their chiral recognition abilities were compared with previously developed six immobilized CSPs. Particularly, we focused on the complementarity for chiral recognitions. Among them, amylose tris(3‐chloro‐5‐methylphenylcarbamate) CSP, namely, CHIRALPAK IG, showed notable chiral recognition abilities to various racemates. As expected, the investigated immobilized CSPs represented remarkable durability to wide range of mobile phases, whereas the corresponding coated CSPs could not be run due to the irreversible degradation. Taking advantage of unrestricted solvent compatibility, chiral separation selectivities were improved for some racemates.

Enantiomeric methadone quantitation on real post-mortem dried matrix spots samples: Comparison of liquid chromatography and supercritical fluid chromatography coupled to mass spectrometry

This study describes two bioanalytical methods for the quantitation of the two methadone enantiomers in dried matrix spots using high performance liquid chromatography coupled to tandem mass spectrometry (HPLC-MS/MS) and high performance supercritical chromatography tandem mass spectrometry (HPSFC-MS/MS). Dried matrix spots were obtained by spotting 10 µL of each sample fluid on a Whatman paper. Methadone and its main metabolite, EDDP, were extracted with 100 µL methanol and subsequently injected into the LC-MS/MS and SFC-MS/MS systems. Enantiomeric separation was achieved with AGP-column for the LC conditions and with Chiralpak IH-3 in SFC. The two methods were fully validated and 93 post-mortem samples were analysed with both analytical methods. Results from validation parameters and results obtained for all post-mortem samples were compared with a significant spearman correlation of r_s = 0.9978 for R-methadone and r_s = 0.9981 for S-methadone. The LC method provided better results in terms of uncertainty, retention factor and resolution, whereas SFC provides better sensitivity, with lower LOD. Median R-/S-methadone ratio in peripheral blood was found equal to 1.60 (N = 32), varying from 0.79 to 4.23. The reported values were in good agreement with previously published results. Based on the results obtained here, SFC-MS/MS can be considered a reliable alternative to the widely used LC-MS/MS for the quantitation of methadone enantiomers in bioanalysis and should be evaluated for other bioanalytical methods. Both methods can be easily and quickly used in toxicological routine analysis for the methadone quantitation in human fluids matrices, even if considering that the polysaccharide coated column IH-3 used in SFC does not allow the enantiomeric EDDP separation.

Study on the stereoselective behaviors of fosthiazate stereoisomers in legume vegetables by supercritical fluid chromatography-tandem mass spectrometry (SFC-MS/MS)

A separation and analysis method of fosthiazate stereoisomers was established utilizing supercritical fluid chromatography-tandem mass spectrometry (SFC-MS/MS) with a CHIRALPAK AD-3 column. The determination of the four fosthiazate stereoisomers could be completed within 6 min. The environmental behaviors of fosthiazate stereoisomers were studied in legume vegetables. After applying fosthiazate granules to soil, the concentrations of fosthiazate stereoisomers in the legume vegetables increased with time, reached maximum values in 7-10 days, and then decreased gradually in all legumes except for in Glycine max. No obvious dissipation behaviors were observed in Glycine max. Interestingly, the stereoselective behaviors were species-specific. A-(-), B-(-) and B-(±)-fosthiazate were preferentially enriched in Phaseolus vulgaris Linn and Vigna unguiculata, while A-(+) and A-(±)-fosthiazate preferentially accumulated in Vicia faba Linn, Pisum sativum Linn and G. max. The opposite stereoselectivity of B-(±)-fosthiazate was observed in different growth stage of G. max. No stereoselective dissipation occurred in soil.

Comparison in enantioseparation performance of chiral stationary phases prepared from chitosans of different sources and molecular weights

The aim of the present study was to compare the enantioseparation performance of chiral stationary phases (CSPs) which were derived from chitosans of different sources and molecular weights. Therefore, chitosans of shrimp and crab shells were prepared. The viscosity-average molecular weights of the chitosans both prepared from shrimp and crab shells were 2.8 × 10⁵ and 1.4 × 10⁵. The chitosans were isobutyrylated yielding isopropylcarbonyl chitosans which were then derivatized with 4-methylphenyl isocyanate to provide chitosan 3,6-bis(4-methylphenylcarbamate)-2-(isobutyrylamide)s. The chitosan 3,6-bis(4-methylphenylcarbamate)-2-(isobutyrylamide)s were used as chiral selectors (CSs) with which the corresponding CSPs were prepared. With the same chiral analytes and under the same mobile phase conditions, the enantioseparation capability of the CSPs was evaluated by high-performance liquid chromatography. In two CSs prepared from the same source, the one with higher molecular weight showed better enantioseparation capability; in two CSs prepared with the chitosans of the same molecular weight, the one derived from shrimp shell exhibited better performance. With regard to the two shrimp chitosan CSs, most of chiral analytes interacted more strongly with the one with lower molecular weight, and an opposite trend was found for the two crab chitosan CSs. Based on the results observed in the present study and in previous work, we believe that the influence of molecular weight on CSP enantioseparation performance is related to the substituent introduced in the CS molecule.

Halogen bond in separation science: A critical analysis across experimental and theoretical results

The halogen bond (XB) is a noncovalent interaction involving a halogen acting as electrophile and a Lewis base. In the last decades XB has found practical application in several fields. Nevertheless, despite the pivotal role of noncovalent interactions in separation science, investigations of XB in this field are still in their infancy, and so far a limited number of studies focusing on solid phase extraction, liquid-liquid microextraction, liquid-phase chromatography, and gas chromatography separation have been published. In addition, in the last few years, our groups have been systematically studying the potentiality of XB for HPLC enantioseparations. On this basis, in the present paper up-to-date results emerging from focused experiments and theoretical analyses performed by our laboratories are integrated with a descriptive presentation of XB features and the few studies published until now in separation science. Then, the aim of this article is to provide a comprehensive and critical discussion of the topic, and account for some still open issues in the application of XB to separate chemical mixtures.

Characterization of three macrocyclic glycopeptide stationary phases in supercritical fluid chromatography

Macrocyclic glycopeptides have been used as chromatographic stationary phases for over twenty years, particularly for their ability to separate enantiomers. While they are mostly used with buffered aqueous liquid mobile phases, they can also be used in supercritical fluid chromatography (SFC) with mobile phases comprising pressurized carbon dioxide and a co-solvent (like methanol), possibly comprising acidic or basic additives. In the present study, we compared three macrocyclic glycopeptide stationary phases (Chirobiotic V2, Chirobiotic T and Chirobiotic TAG) in SFC with carbon dioxide - methanol (90:10) containing no additives. First, the interactions contributing to retention are evaluated with a modified version of the solvation parameter model, comprising five Abraham descriptors (E, S, A, B, V) and two additional descriptors to take account of interactions with ionizable species (D^- and D⁺). Linear solvation energy relationships (LSER) are established based on the retention of 145 achiral analytes. Secondly, the contributions of interactions to enantioseparations are discussed, based on the analysis of 67 racemates. The individual success rate on each phase was observed to be moderate, especially as these phases are known to be more efficient when acidic or basic additives are employed. Chirobiotic TAG proved more successful than the other two phases. Discriminant analyses were computed to gain some insight on retention mechanisms, but only Chirobiotic TAG provided interpretable results. Finally, the effects of a small proportion of acidic or basic additive on enantioseparation with Chirobiotic T stationary phase are briefly discussed.

High-performance liquid chromatographic and subcritical fluid chromatographic separation of α-arylated ß-carboline, N-alkylated tetrahydroisoquinolines and their bioisosteres on polysaccharide-based chiral stationary phases

New, pharmacologically interesting chiral amino compounds, namely, stereoisomers of α-hydroxynaphthyl-ß-carboline, benz[d]azepine and benz[c]azepine analogs as well as N-α-hydroxynaphthylbenzyl-substituted isoquinolines were enantioseparated by high-performance liquid chromatographic and subcritical fluid chromatographic methods on polysaccharide-based chiral stationary phases. Separation of the stereoisomers was optimized in both subcritical fluid chromatography and normal phase liquid chromatographic modes by investigating the effects of the composition of the bulk solvent, temperature, and the structures of the analytes and selectors. Both normal phase liquid chromatography and subcritical fluid chromatography exhibited satisfactory performance, albeit with somewhat different effectiveness in the separation of the stereoisomers studied. The optimized methods offer the possibility to apply preparative-scale separations thereby enabling further pharmacological investigations of the enantiomers.

Subcritical Fluid Chromatography at Sub-Ambient Temperatures for the Chiral Resolution of Ketamine Metabolites with Rapid-Onset Antidepressant Effects

Chiral metabolites of ketamine exerting rapid-onset yet sustained antidepressant effects may be marketed directly in the future, but require chemo- and enantio-selective chromatographic methods for quality assurance and control. The chromatographic behavior of S-/R-ketamine, S-/R-norketamine, S-/R-dehydronorketamine, and (2R,6R)-/(2S,6S)-hydroxynorketamine in supercritical fluid chromatography (SFC) was investigated computationally and experimentally with the aim of identifying problematic pairs of enantiomers and parameters for chiral resolution. Retention on three different polysaccharide-based chiral stationary phases (Lux Amylose-2, i-Amylose-3, and i-Cellulose-5) provided new information on the significance of halogen atoms as halogen bond donors and hydrogen bond acceptors for enantioselectivity, which could be corroborated in silico by molecular docking studies. Modifiers inversely affected enantioselectivity and retention. Methanol yielded lower run times but superior chiral resolution compared to 2-propanol. Lower temperatures than those conventionally screened did not impair phase homogeneity but improved enantioresolution, at no cost to reproducibility. Thus, sub-ambient temperature subcritical fluid chromatography (SubFC), essentially low-temperature HPLC with subcritical CO2, was applied. The optimization of the SubFC method facilitated the chiral separation of ketamine and its metabolites, which was applied in combination with direct injection and online supercritical fluid extraction to determine the purity of pharmaceutical ketamine formulations for proof of concept.

Concepts and Methods to Access Novel Antibiotics from Actinomycetes

Actinomycetes have been proven to be an excellent source of secondary metabolites for more than half a century. Exhibiting various bioactivities, they provide valuable approved drugs in clinical use. Most microorganisms are still untapped in terms of their capacity to produce secondary metabolites, since only a small fraction can be cultured in the laboratory. Thus, improving cultivation techniques to extend the range of secondary metabolite producers accessible under laboratory conditions is an important first step in prospecting underexplored sources for the isolation of novel antibiotics. Currently uncultured actinobacteria can be made available by bioprospecting extreme or simply habitats other than soil. Furthermore, bioinformatic analysis of genomes reveals most producers to harbour many more biosynthetic gene clusters than compounds identified from any single strain, which translates into a silent biosynthetic potential of the microbial world for the production of yet unknown natural products. This review covers discovery strategies and innovative methods recently employed to access the untapped reservoir of natural products. The focus is the order of actinomycetes although most approaches are similarly applicable to other microbes. Advanced cultivation methods, genomics- and metagenomics-based approaches, as well as modern metabolomics-inspired methods are highlighted to emphasise the interplay of different disciplines to improve access to novel natural products.

Enantioseparation of novel chiral sulfoxides on chlorinated polysaccharide stationary phases in supercritical fluid chromatography

Asymmetric sulfoxides is a particular case of chirality that may be found in natural as well as synthetic products. Twenty-four original molecules containing a sulfur atom as a centre of chirality were analyzed in supercritical fluid chromatography on seven polysaccharide-based chiral stationary phases (CSP) with carbon dioxide - methanol mobile phases. While all the tested CSP provided enantioseparation for a large part of the racemates, chlorinated cellulosic phases proved to be both highly retentive and highly enantioselective towards these species. Favourable structural features were determined by careful comparison of the enantioseparation of the probe molecules. Molecular modelling studies indicate that U-shaped (folded) conformations were most favorable to achieve high enantioresolution on these CSP, while linear (extended) conformations were not so clearly discriminated. For a subset of these species adopting different conformations, a broad range of mobile phase compositions, ranging from 20 to 100% methanol in carbon dioxide, were investigated. While retention decreased continuously in this range, enantioseparation varied in a non-monotonous fashion. Abrupt changes in the tendency curves of retention and selectivity were observed when methanol proportion reaches about 60%, suggesting that a change in the conformation of the analytes and/or chiral selector is occurring at this point.