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.

Circular Dichroism Sensing: Strategies and Applications

The analysis of the absolute configuration, enantiomeric composition, and concentration of chiral compounds are frequently encountered tasks across the chemical and health sciences. Chiroptical sensing methods can streamline this work and allow high-throughput screening with remarkable reduction of operational time and cost. During the last few years, significant methodological advances with innovative chirality sensing systems, the use of computer-generated calibration curves, machine learning assistance, and chemometric data processing, to name a few, have emerged and are now matched with commercially available multi-well plate CD readers. These developments have reframed the chirality sensing space and provide new opportunities that are of interest to a large group of chemists. This review will discuss chirality sensing strategies and applications with representative small-molecule CD sensors. Emphasis will be given to important milestones and recent advances that accelerate chiral compound analysis by outperforming traditional methods, conquer new directions, and pioneering efforts that lie at the forefront of chiroptical high-throughput screening developments. The goal is to provide the reader with a thorough understanding of the current state and a perspective of future directions of this rapidly emerging field.

Enhanced Structure-Based Prediction of Chiral Stationary Phases for Chromatographic Enantioseparation from 3D Molecular Conformations

The accurate prediction of suitable chiral stationary phases (CSPs) for resolving the enantiomers of a given compound poses a significant challenge in chiral chromatography. Previous attempts at developing machine learning models for structure-based CSP prediction have primarily relied on 1D SMILES strings [the simplified molecular-input line-entry system (SMILES) is a specification in the form of a line notation for describing the structure of chemical species using short ASCII strings] or 2D graphical representations of molecular structures and have met with only limited success. In this study, we apply the recently developed 3D molecular conformation representation learning algorithm, which uses rapid conformational analysis and point clouds of atom positions in the 3D space, enabling efficient chemical structure-based machine learning. By harnessing the power of the rapid 3D molecular representation learning and a data set comprising over 300,000 chromatographic enantioseparation records sourced from the literature, our models afford notable improvements for the chemical structure-based choice of appropriate CSP for enantioseparation, paving the way for more efficient and informed decision-making in the field of chiral chromatography.

Chemometric sensing of stereoisomeric compound mixtures with a redox-responsive optical probe

The analysis of mixtures of chiral compounds is a common task in academic and industrial laboratories typically achieved by laborious and time-consuming physical separation of the individual stereoisomers to allow interference-free quantification, for example using chiral chromatography coupled with UV detection. Current practice thus impedes high-throughput and slows down progress in countless chiral compound development projects. Here we describe a chemometric solution to this problem using a redox-responsive naphthoquinone that enables chromatography-free click chemistry sensing of challenging mixtures. The achiral probe covalently binds amino alcohols within a few minutes at room temperature and generates characteristic UVA and CDA spectra that are intentionally altered via sodium borohydride reduction to provide a second, strikingly different chiroptical data set (UVB and CDB). Chemometric partial least squares processing of the chiroptical outputs then enables spectral deconvolution and accurate determination of individual analyte concentrations. The success of this approach is demonstrated with 35 samples covering considerably varied total analyte amounts and stereoisomeric ratios. All chemicals and machine learning algorithms are readily available and can be immediately adapted by any laboratory.

Recent developments in the high-throughput separation of biologically active chiral compounds via high performance liquid chromatography

Bioactive compounds, including active pharmaceutical ingredients (APIs), are often chiral molecules where stereoisomers have different biological and therapeutic activity. Nevertheless, the preparation of these molecules can lead to racemic or scalemic mixtures (it is not trivial to produce just the optically pure compound). The evaluation of the enantiomeric purity of bioactive compounds, and therefore quality, is indeed of fundamental importance for regulatory scopes. Chiral high performance liquid chromatography (HPLC) is the gold standard technique to separate and to purify enantiomers. This comes from the wide availability of commercial chiral stationary phases (CSPs) and operational modes, which makes the technique extremely versatile. In recent years, the most relevant trend in the field of chiral analytical HPLC has been the development of CSPs suitable for fast or even ultrafast separations, thus favoring the high throughput screening of biologically active chiral compounds. This process has somehow lagged behind compared to achiral HPLC, due to a series of practical and fundamental issues. The experience has shown how in chiral chromatography even very basic concepts, such as the supposed kinetic superiority of core-shell (pellicular) particles over fully porous ones to improve the chromatographic efficiency, cannot be taken for granted. In this review, the most relevant fundamental and practical features that must be taken into consideration to design successful high-throughput, fast enantioseparations will be discussed. Afterwards, the main classes of CSPs and the most relevant, recent (last five-year) high-throughput applications in the field of the separation of chiral bioactive compounds (for pharmaceutical, forensic, food, and omics applications) will be considered.

Comprehensive reversed-phase×chiral two-dimensional liquid chromatography coupled to quadrupole-time-of-flight tandem mass spectrometry with post-first dimension flow splitting for untargeted enantioselective amino acid analysis

This work describes a comprehensive achiral × chiral two-dimensional liquid chromatography separation for enantioselective amino acid analysis coupled to electrospray ionization-tandem mass spectrometry detection using data-independent acquisition. Flow splitting after the first and second dimension separation was utilized for volumetric flow reduction and for enabling a multi-detector approach (with ultraviolet, fluorescence, charged aerosol, and MS detection), respectively. Derivatization with 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate provided a chromophore, a fluorophore, and an efficient mass tag for efficient ionization in positive electrospray ionization-mass spectrometry. Chiral columns often have limitations in terms of their chemoselectivity, which may be a problem when complex sample mixtures with structurally related compounds need to be separated. It can be alleviated by a reversed-phase×chiral two-dimensional-liquid chromatography setup, in which the first dimension provides the chemoselectivity and a chiral tandem column constituted of quinine-carbamate derived weak anion-exchanger and zwitterionic ion-exchanger in the second dimension separation of D- and L-amino acid enantiomers. The method was used to control the stereointegrity of the therapeutic peptide octreotide. After hydrolysis, all amino acid constituents were detected with the correct configuration and composition. Some options for flow splitting and integration of destructive detectors in the first dimension separation are outlined.

Rapid Enantiomeric Excess Measurements of Enantioisotopomers by Molecular Rotational Resonance Spectroscopy

Recent work in drug discovery has shown that selectively deuterated small molecules can improve the safety and efficacy for active pharmaceutical ingredients. The advantages derive from changes in metabolism resulting from the kinetic isotope effect when deuterium is substituted for a hydrogen atom at a structural position where rate limiting C-H bond breaking occurs. This application has pushed the development of precision deuteration strategies in synthetic chemistry that can install deuterium atoms with high regioselectivity and with stereocontrol. Copper-catalyzed alkene transfer hydrodeuteration chemistry has recently been shown to have high stereoselectivity for deuteration at the metabolically important benzyl C-H position. In this case, stereocontrol results in the creation of enantioisotopomers-molecules that are chiral solely by virtue of the deuterium substitution-and chiral analysis techniques are needed to assess the reaction selectivity. It was recently shown that chiral tag molecular rotational resonance (MRR) spectroscopy provides a routine way to measure the enantiomeric excess and establish the absolute configuration of enantioisotopomers. High-throughput implementations of chiral tag MRR spectroscopy are needed to support optimization of the chemical synthesis. A measurement methodology for high-throughput chiral analysis is demonstrated in this work. The high-throughput ee measurements are performed using cavity-enhanced MRR spectroscopy, which reduces measurement times and sample consumption by more than an order-of-magnitude compared to the previous enantioisotopomer analysis using a broadband MRR spectrometer. It is also shown that transitions for monitoring the enantiomers can be selected from a broadband rotational spectrum without the need for spectroscopic analysis. The general applicability of chiral tag MRR spectroscopy is illustrated by performing chiral analysis on six enantioisotopomer reaction products using a single molecule as the tag for chiral discrimination.

High-throughput infrared spectroscopy for quantification of peptides in drug discovery

Peptides have gained an increasing importance in drug discovery as potential therapeutics. Discovery efforts toward finding new, efficacious peptide-based therapeutics have increased the throughput of peptide development, allowing the rapid generation of unique and pure peptide samples. However, high-throughput analysis of peptides may be still challenging and can encumber a high-throughput drug discovery campaign. We report herein a fit-for-purpose method to quantify peptide concentrations using high-throughput infrared spectroscopy (HT-IR). Through the development of this method, multiple critical method parameters were optimized including solvent composition, droplet deposition size, plate drying procedures, sample concentration, and internal standard. The relative absorbance of the amide region (1600-1750 cm^-1) to the internal standard, K₃Fe(CN)₆ (2140 cm^-1), was determined to be most effective at providing lowest interference for measuring peptide concentration. The best sample deposition was achieved by dissolving samples in a 50:50 v/v allyl alcohol/water mixture. The developed method was used on 96-well plates and analyzed at a rate of 22 min per plate. Calibration curves to measure sample concentration versus response relationship displayed sufficient linearity (R² > 0.95). The repeatability and scope of detection was demonstrated with eighteen peptide samples that were measured with most values below 20% relative standard deviation. The linear dynamic range of the method was determined to be between 1 and 5 mg/mL. This developed HT-IR methodology could be a useful tool in peptide drug candidate lead identification and optimization processes.

Screening for generality in asymmetric catalysis

Research in the field of asymmetric catalysis over the past half century has resulted in landmark advances, enabling the efficient synthesis of chiral building blocks, pharmaceuticals and natural products1-3. A small number of asymmetric catalytic reactions have been identified that display high selectivity across a broad scope of substrates; not coincidentally, these are the reactions that have the greatest impact on how enantioenriched compounds are synthesized4-8. We postulate that substrate generality in asymmetric catalysis is rare not simply because it is intrinsically difficult to achieve, but also because of the way chiral catalysts are identified and optimized9. Typical discovery campaigns rely on a single model substrate, and thus select for high performance in a narrow region of chemical space. Here we put forth a practical approach for using multiple model substrates to select simultaneously for both enantioselectivity and generality in asymmetric catalytic reactions from the outset10,11. Multisubstrate screening is achieved by conducting high-throughput chiral analyses by supercritical fluid chromatography-mass spectrometry with pooled samples. When applied to Pictet-Spengler reactions, the multisubstrate screening approach revealed a promising and unexpected lead for the general enantioselective catalysis of this important transformation, which even displayed high enantioselectivity for substrate combinations outside of the screening set.

Deuterated Modifiers in Sub/Supercritical Fluid Chromatography for Streamlined NMR Structure Elucidation

Isolation and chemical characterization of target components in fast-paced pharmaceutical laboratories can often be challenging, especially when dealing with mixtures of closely related, possibly unstable species. Traditionally, this process involves intense labor and manual intervention including chromatographic method development and optimization, fraction collection, and drying processes prior to NMR analyses for unambiguous structure elucidation. To circumvent these challenges, a foundational framework for the proper utilization of supercritical carbon dioxide (scCO₂) and deuterated modifiers (CD₃OD) in sub/supercritical fluid chromatography (SFC) is herein introduced. This facilitates a streamlined multicomponent isolation with minimized protic residues, further enabling immediate NMR analysis. In addition to bypassing tedious drying processes and minimizing analyte degradation, this approach (complementary to traditional reversed-phase liquid chromatography, RPLC) delivers highly efficient separations and automated fraction collection using readily available analytical/midscale SFC instrumentation. A series of diverse analytes across a wide spectrum of chemical properties (acid, basic, and neutral), combined with different stationary-phase columns in SFC are investigated using both a protic organic modifier (CH₃OH) and its deuterated counterpart (CD₃OD). The power of this framework is demonstrated with pharmaceutically relevant applications in the context of target characterization and analysis of complex multicomponent reaction mixtures from modern synthetic chemistry, demonstrating high isolation yields while reducing both the environmental footprint and manual intervention. This workflow enables unambiguous fast-paced structure elucidation on the analytical scale, providing results that are comparable to traditional, but time-consuming, RPLC purification approaches.

Preparation of chiral stationary phase based on a [3+3] chiral polyimine macrocycle by thiol-ene click chemistry for enantioseparation in normal-phase and reversed-phase high performance liquid chromatography

Polyimine macrocycles are a new class of organic macrocycles with cyclic structures, well-defined molecular cavities, and multiple cooperative binding sites, which have recently aroused considerable research interest in molecular recognition and separation. Herein, we report the bonding of a [3+3] chiral polyimine macrocycle (H₃L, C₇₈H₇₈N₆O₃) on thiol-functionalized silica gel using thiol-ene click chemistry to prepare a chiral stationary phase (CSP) for high performance liquid chromatography (HPLC). The fabricated column exhibited excellent chiral separation capability under both normal-phase and reversed-phase conditions. Fourteen and 10 racemates were well resolved on the column in normal-phase mode (using n-hexane/isopropanol as the mobile phase) and reversed-phase mode (using methanol/water as the mobile phase), respectively, including alcohols, esters, ethers, ketones, aldehydes, epoxides and organic acids. Moreover, the column also shows good selectivity toward positional isomers. Six positional isomers (dinitrobenzene, chloroaniline, bromoaniline, iodoaniline, nitrobrobenzene and nitrochlorobenzene) were well separated on the column. In addition, the effects of the injection mass and mobile phase composition on the separation were investigated. The column shows good reproducibility and stability after multiple injections with the relative standard deviation (RSD) (n = 5) of the retention time and resolution being < 0.96 % and 0.65 %, respectively. This study indicates that this type of chiral polyimine macrocycles is a promising chiral selector for HPLC enantioseparation and will push forward the applications of more novel chiral macrocycles for chiral chromatographic separation.

Selected Ion Monitoring Using Low-Cost Mass Spectrum Detectors Provides a Rapid, General, and Accurate Method for Enantiomeric Excess Determination in High-Throughput Experimentation

High-Throughput Experimentation (HTE) workflows are efficient means of surveying a broad array of chiral catalysts in the development of catalytic asymmetric reactions. However, use of traditional HPLC-UV/vis methodology to determine enantiomeric excess (ee) from the resulting reactions is often hampered by co-elution of other reaction components, resulting in erroneous ee determination when crude samples are used, and ultimately requiring product isolation prior to ee analysis. In this study, using four published reactions selected as model systems, we demonstrate that the use of LC-MS, SFC-MS, and selected ion monitoring (SIM) mass chromatography provides a highly accurate means to determine ee of products in crude reaction samples using commonplace, low-cost MS detectors. By using ion selection, co-eluting signals can be deconvoluted to provide accurate integrations of the target analytes. We also show that this method is effective for samples lacking UV/vis chromophores, making it ideal for HTE workflows in asymmetric catalysis.

Automated ion exchange chromatography screening combined with in silico multifactorial simulation for efficient method development and purification of biopharmaceutical targets

Bioprocess development of increasingly challenging therapeutics and vaccines requires a commensurate level of analytical innovation to deliver critical assays across functional areas. Chromatography hyphenated to numerous choices of detection has undeniably been the preferred analytical tool in the pharmaceutical industry for decades to analyze and isolate targets (e.g., APIs, intermediates, and byproducts) from multicomponent mixtures. Among many techniques, ion exchange chromatography (IEX) is widely used for the analysis and purification of biopharmaceuticals due to its unique selectivity that delivers distinctive chromatographic profiles compared to other separation modes (e.g., RPLC, HILIC, and SFC) without denaturing protein targets upon isolation process. However, IEX method development is still considered one of the most challenging and laborious approaches due to the many variables involved such as elution mechanism (via salt, pH, or salt-mediated-pH gradients), stationary phase's properties (positively or negatively charged; strong or weak ion exchanger), buffer type and ionic strength as well as pH choices. Herein, we introduce a new framework consisting of a multicolumn IEX screening in conjunction with computer-assisted simulation for efficient method development and purification of biopharmaceuticals. The screening component integrates a total of 12 different columns and 24 mobile phases that are sequentially operated in a straightforward automated fashion for both cation and anion exchange modes (CEX and AEX, respectively). Optimal and robust operating conditions are achieved via computer-assisted simulation using readily available software (ACD Laboratories/LC Simulator), showcasing differences between experimental and simulated retention times of less than 0.5%. In addition, automated fraction collection is also incorporated into this framework, illustrating the practicality and ease of use in the context of separation, analysis, and purification of nucleotides, peptides, and proteins. Finally, we provide examples of the use of this IEX screening as a framework to identify efficient first dimension (¹D) conditions that are combined with MS-friendly RPLC conditions in the second dimension (²D) for two-dimensional liquid chromatography experiments enabling purity analysis and identification of pharmaceutical targets.

Supercritical Fluid Chromatography for Chiral Analysis, Part 2: Applications

In the second part of this review article, the recent progress in supercritical fluid chromatography (SFC) for enantiomeric separations is evaluated. With the substantial developments carried out over the past years in instrumentation, columns, and detector hyphenation, the interest in chiral SFC has been steadily growing in various fields. In combination with novel developments in chiral stationary phase chemistries, the enantioselective analysis range has been significantly extended. Several applications reported on the enantioselective separation of drugs and pharmaceutical compounds using chiral SFC are discussed, including pharmaceutical applications, clinical research, forensic toxicology, and environmental sciences.

Enantioseparation in high performance liquid chromatography: preparation and evaluation of a vancomycin-based chiral stationary phase via surface-initiated atom transfer radical polymerization

A chromatographic technique based on a chiral stationary phase (CSP) has been explored for enantioseparation. Herein, poly(glycidyl methacrylate) (poly(GMA)) brushes were grafted on the surface of silica gel via surface-initiated atom transfer radical polymerization (SI-ATRP), followed by the introduction of vancomycin as a chiral selector. The as-synthesized material was characterized by elemental analysis, scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) and thermogravimetric analysis (TGA), proving the formation of vancomycin-immobilized brushes. Then the resulting CSP was explored to separate 7 racemic drugs (bicalutamide, 1-benzyl-5-phenylbarbituric acid, chlorpheniramine maleate, fluoxetine hydrochloride, verapamil hydrochloride, benzoxazocine hydrochloride and isoprenaline hydrochloride) in high performance liquid chromatography (HPLC). Several factors affecting the enantioseparation performance of the vancomycin-immobilized CSP, including the triethylamine (TEA) content in the buffer, pH value, content of organic solvent in the mobile phase, flow rate and injection volume, were mainly optimized. Under the optimal conditions, baseline separation of fluoxetine hydrochloride (R_S = 2.52) was achieved, which was better than that on a commercial Chirobiotic V column, while enantioseparation of bicalutamide (R_S = 1.01), chlorpheniramine maleate (R_S = 0.77), 1-benzyl-5-phenylbarbituric acid (R_S = 0.67), isoprenaline hydrochloride (R_S = 0.73), verapamil hydrochloride (R_S = 0.91) and benzoxazocine hydrochloride (R_S = 1.03) was partly achieved. It was concluded that SI-ATRP is a robust way to fabricate vancomycin-based CSPs for enantioseparation.

Wide-Range and Highly Sensitive Chiral Sensing by Discrete 2D Chirality Transfer on Confined Surfaces of Au(I)-Thiolate Nanosheets

Circular dichroism (CD) chiral sensing is very promising to meet the ever-increasing demands for high-throughput chiral analysis in asymmetric synthesis. However, it is still very challenging to sensitively quantify the composition of enantiomers in a wide concentration range because the existing sensing systems show either linear CD response resultant from stoichiometric chiral transfer or nonlinear CD response resultant from amplified chiral transfer and thus have the drawbacks of low sensitivity and narrow quantification range, respectively. Herein, we propose a sensing system of two-dimensional (2D) Au(I)-thiolate nanosheets. The disordered interligand interactions on the confined surfaces of nanosheets enable the formation of discrete amplified chiral domains around the adsorbed chiral analytes, resulting in a linearly amplified chiral transfer behavior, which provides a solution for highly sensitive and wide-range quantification of enantiomer compositions. Taking (1R, 2R)-(-)- and (1S, 2S)-(+)-1,2-diamino cyclohexanes as example analytes, the concentration and full-range enantiomeric excess (ee) values have been quickly determined by adsorbing them on the surface of Au(I)-MPA (MPA: 3-mercaptopropionic acid) nanosheets in the concentration range of 1.0 × 10^-6 to 4.0 × 10^-5 M. By engineering the surface functional groups, Au(I)-thiolate nanosheets can be extended to sense other types of analytes, and several polyols with multiple chiral centers have been sensed by boronic acid functionalized nanosheets at the 10^-7 M level. The high performances, good extendibility, and one-pot high-yield aqueous synthesis ensure these Au(I)-thiolate nanosheets can be developed as a practical and powerful chiral sensing platform.

Enantioselective UHPLC Screening Combined with In Silico Modeling for Streamlined Development of Ultrafast Enantiopurity Assays

Enantioselective chromatography has been the preferred technique for the determination of enantiomeric excess across academia and industry. Although sequential multicolumn enantioselective supercritical fluid chromatography screenings are widespread, access to automated ultra-high-performance liquid chromatography (UHPLC) platforms using state-of-the-art small particle size chiral stationary phases (CSPs) is an underdeveloped area. Herein, we introduce a multicolumn UHPLC screening workflow capable of combining 14 columns (packed with sub-2 μm fully porous and sub-3 μm superficially porous particles) with nine mobile phase eluent choices. This automated setup operates under a vast selection of reversed-phase liquid chromatography, hydrophilic interaction liquid chromatography, polar-organic mode, and polar-ionic mode conditions with minimal manual intervention and high success rate. Examples of highly efficient enantioseparations are illustrated from the integration of chiral screening conditions and computer-assisted modeling. Furthermore, we describe the nuances of in silico method development for chiral separations via second-degree polynomial regression fit using LC simulator (ACD/Labs) software. The retention models were found to be very accurate for chiral resolution of single and multicomponent mixtures of enantiomeric species across different types of CSPs, with differences between experimental and simulated retention times of less than 0.5%. Finally, we illustrate how this approach lays the foundation for a streamlined development of ultrafast enantioseparations applied to high-throughput enantiopurity analysis and its use in the second dimension of two-dimensional liquid chromatography experiments.

Microscale purification in support of high-throughput medicinal chemistry

In recent years, successful assay miniaturization has enabled the exploration of synthesis scale reduction in pharmaceutical discovery. Miniaturization of pharmaceutical synthesis and purification allows a reduction in material consumption and shortens timelines, which ultimately reduces the cost per experiment without compromising data quality. Isolating and purifying the compounds of interest is a key step in the library synthesis process. In this manuscript we describe a high-throughput purification workflow in support of microscale (1-5 μmol or 0.5-2 mg) library synthesis. The optimized microscale purification system can routinely purify 384-well reaction plates with an analysis time of 4 min per sample. Instrument optimization, critical parameters such as column loading, delay time calibration, ultrafast pre- and post-purification analysis and library purification examples are provided.

Evaluation of a polysaccharide-based chiral reversed-phase liquid chromatography screen strategy in pharmaceutical analysis

Chirality control plays a critical role in developing stereoisomeric drugs. Due to the complexity and lack of predictability in chiral separations, column screening remains the gold standard to initiate chiral method development for active pharmaceutical ingredients (APIs) and synthetic intermediates. Chiral reversed-phase (RP) liquid chromatography (LC) has gained favor over other modes due to its versatility and compatibility in analyzing a wide range of chiral compounds in various matrices. Herein, we established a tier-based chiral RPLC screen strategy by constructing and analyzing a database of 101 chiral screens with a total of 3,401 entries (unique LC runs) for proprietary APIs or intermediates at Bristol Myers Squibb. Up to 17 polysaccharide-based chiral stationary phases (CSPs) and four mobile phases (MPs) have been screened with gradient elution. A selection of ten CSPs with two MPs was found sufficient to achieve successful separation for 82% of the total screens. Two RPLC screen tiers (Tier 1: AZ, OD, ID, and IG) and (Tier 2: AY, OJ, OZ, IA, IC, and IH) were proposed along with two MPs (acidic and neutral) to target ~70% hit rate for Tier 1, and ~80% for the combined set. We also implemented a user-friendly workflow to enable walk-up chiral RPLC screening with automated reports and system suitability tests.

Capillary electrophoresis and molecular modeling of the chiral separation of aromatic amino acids using α/β-cyclodextrin and 18-crown-6

In this work, chiral separation of enantiomers of three amino acids was achieved using capillary electrophoresis technique with α-cyclodextrin (αCD) as a running buffer additive. Only tryptophan has exhibited baseline separation in the presence of αCD, while the enantiomers of the other two amino acids, phenylalanine and tyrosine, were only partially separated. The addition of 18-crown-6 (18C6) as a second additive imparted only slight improvement to the separation of all enantiomers. On the other hand, all three racemic amino acid mixtures demonstrated no indication of separation when the larger cavity cyclodextrin members, β- and γCD, are used as running buffer chiral additives. However, remarkable improvements in the separation of the enantiomers of phenylalanine and tyrosine were obtained when 18C6 is used together with βCD as a running buffer additive. Surprisingly, tryptophan enantiomers were not separated by the dual additive system of cyclodextrin and crown ether. Using electrospray ionization mass spectrometry (ESI-MS), all amino acids were found to form stable binary complexes with individual hosts as well as ternary compounds involving the crown ether and the cyclodextrin. Furthermore, we used molecular dynamics (MD) simulations to build a clear picture about the interaction between the guest and the hosts. Most of these complexes remained stable throughout the simulation times, and the molecular dynamics study allowed better understanding of these supramolecular assemblies.

The 2021 Winners of the Lifetime Achievement and Emerging Leader in Chromatography Awards

Paul Haddad and Erik L. Regalado are the winners of the 14th annual LCGC Lifetime Achievement and Emerging Leader in Chromatography Awards, respectively. We review their achievements.

Ultra-high-throughput SPE-MALDI workflow: Blueprint for efficient purification and screening of peptide libraries

At the forefront of synthetic endeavors in the pharmaceutical industry, including drug discovery and high-throughput screening, timelines are tight and large quantities of pure chemical targets are rarely available. In this regard, the development of novel and increasingly challenging chemistries requires a commensurate level of innovation to develop reliable analytical assays and purification workflows with rapid turnaround that enables accelerated pharmacological evaluation. A small-scale automation platform enabling high-throughput analysis and purification to streamline the selection of candidate leads would be a transformative advance. Herein, we introduce an automation-friendly solid-phase extraction-matrix-assisted laser desorption/ionization (SPE-MALDI) platform applied to the high-throughput purification and analysis of peptide libraries. This advance enabled us to purify peptides from microgram levels in less than a day with results comparable to traditional high-performance liquid chromatography-diode array detection-mass spectrometry (HPLC-DAD-MS).

Chiral chromatography method screening strategies: Past, present and future

Method screening is an integral part of chromatographic method development for the separation of racemates. Due to the highly complex retention mechanism of a chiral stationary-phase, it is often difficult, if not impossible, to device predefined method-development steps that can be successfully applied to a wide group of molecules. The standard approach is to evaluate or screen a series of stationary and mobile-phase combinations to increase the chances of detecting a suitable separation condition. Such a process is often the rate-limiting step for high-throughput analyses and purification workflows. To address the problem, several solutions and strategies have been proposed over the years for reduction of net method-screening time. Some of the strategies have been adopted in practice while others remained confined in the literature. The main objective of this review is to revisit, critically discuss and compile the solutions published over the last two decades. We expect that making the diverse set of solutions available in a single document will help assessing the adequacy of existing screening protocols in laboratories conducting chiral separation.

The theory and practice of ultrafast liquid chromatography: A tutorial

Modern high-throughput experimentation and challenging analytical problems of academic/industrial research have put the responsibility on separation scientists to develop new fast separation approaches. With the availability of high-pressure pumps, small particles with hydrolytically stable surface chemistries, reduced extra-column band broadening, and low volume detectors with fast signal processing, it is now feasible to do sub-minute to sub-second chromatography. Herein, the fundamental theoretical principles of ultrafast chromatography, along with practical solutions, are reviewed. Approaches for rapid separations in packed beds, narrow open tubular columns, and monoliths are demonstrated, along with the challenges that were faced. The instrumentation requirements (pumps, injection systems, detectors, column packing process) for using short columns ranging from 0.5 to 5 cm are examined, followed by real applications. One of the main problems in ultrafast chromatography is partial or complete peak overlap. As per Gidding's statistical overlap theory, peak overlap cannot be avoided for a completely random sample for a column with a given peak capacity. Signal processing techniques based on Fourier transform deconvolution of band broadening, power laws, derivatives, and iterative curve fitting are explained to help improve the chromatographic resolution. An example of ten peaks separated in under a second is shown and discussed. Other ultrafast separations in supercritical fluid chromatography or capillary electrophoresis are briefly mentioned to provide a complete understanding of this emerging field.

Denigrins and Dactylpyrroles, Arylpyrrole Alkaloids from a Dactylia sp. Marine Sponge

Seven new arylpyrrole alkaloids (1-7), along with four known compounds, were isolated from an extract of a Dactylia sp. nov. marine sponge, and their structures were elucidated by interpretation of NMR and MS spectroscopic data. Denigrins D-G (1-4) have highly substituted pyrrole or pyrrolone rings in their core structures, while dactylpyrroles A-C (5-7) have tricyclic phenanthrene cores. Due to the proton-deficient nature of these scaffolds, key heteronuclear correlations from 1H-15N HMBC and LR-HSQMBC NMR experiments were used in the structure assignment of denigrin D (1). Dictyodendrin F (8), a previously described co-metabolite, inhibited transcription driven by the oncogenic PAX3-FOXO1 fusion gene with an IC50 value of 13 μM.

Application of chiral chromatography in radiopharmaceutical fields: A review

Chiral column chromatography (CCC) is a revolutionary analytical methodology for the enantioseparation of novel positron emission tomography (PET) tracers in the primary stages of drug development. Due to the different behaviors of tracer enantiomers (e.g. toxicity, metabolism and side effects) in administrated subjects, their separation and purification is a challenging endeavor. Over the last three decades, different commercial chiral columns have been applied for the enantioseparation of PET-radioligand (PET-RL) or radiotracers (PET-RT), using high-performance liquid chromatography (HPLC). The categorization and reviewing of them is a vital topic. This review presents a brief overview of advances, applications, and future prospectives of CCC in radiopharmaceutical approaches. In addition, the effective chromatographic parameters and degravitation trends to enhance enantioseparation resolution are addressed. Moreover, the application and potential of chiral super fluidical chromatography (CSFC) as an alternative for enantioseparation in the field of radiopharmaceutical is discussed. Finally, the crucial application challenges of CCC are explained and imminent tasks are suggested.

Quantitative Chirality and Concentration Sensing of Alcohols, Diols, Hydroxy Acids, Amines and Amino Alcohols using Chlorophosphite Sensors in a Relay Assay

Analytical methods that allow simultaneous determination of the concentration and enantiomeric composition of small sample amounts and are also compatible with high-throughput multi-well plate technology have received increasing attention in recent years. We now introduce a new class of broadly useful small-molecule probes and a relay sensing strategy that together accomplish these tasks with five classes of compounds including the challenging group of mono-alcohols-a scope that stands out among previously reported UV, fluorescence, and CD assays. Several chlorophosphite probes and aniline indicators have been evaluated and used for on-the-fly CD/UV sensing following a continuous workflow. The wide application range of the readily available sensors is highlighted with almost 30 alcohols, diols, hydroxy acids, amines and amino alcohols, and the accuracy of the stereochemical analysis is showcased with samples covering a wide range of concentrations and enantiomeric ratios.

High-throughput enantioseparation of Nα-fluorenylmethoxycarbonyl proteinogenic amino acids through fast chiral chromatography on zwitterionic-teicoplanin stationary phases

In this study, 31 racemates of Nα-FMOC (fluorenylmethoxycarbonyl) amino acids (AAs) with different chemico-physical characteristics (neutral nonpolar, neutral polar, acidic and basic) have been successfully resolved in fast enantioselective chromatography on recently-developed zwitterionic-teicoplanin chiral stationary phases (CSPs). The CSPs were prepared by covalently bonding the teicoplanin selector on fully-porous particles of narrow dispersion particle-size distribution (particle diameter 1.9 µm) and superficially-porous particles (2.0 µm). Both the zwitterionic-teicoplanin CSPs have proved to be ideal media for the separation of this important class of compounds. In particular, the zwitterionic CSP prepared on superficially-porous particles exhibited superior enantioselectivity and resolution, compared to that made of fully porous particles, in virtue of more favorable thermodynamics. The zwitterionic nature of these CSPs allowed avoiding the annoying effect of Donnan's exclusion of enantiomers from the stationary phase. This effect, on the opposite, was frequently observed on a commercial teicoplanin CSP (Teicoshell) employed for comparative purposes. Noticeably, on the zwitterionic-teicoplanin CSPs, by using either acetonitrile- or methanol-rich mobile phases (MPs), it was possible to favor speed over enantioresolution and vice versa. This work gives further replies to the request for rapid determination of enantiomeric excess of Nα-FMOC proteinogenic (and non-proteinogenic) AAs, typically used as preferred chiral synthons in the solid-phase synthesis of therapeutic peptides.

Chiral separation of underivatized amino acids in supercritical fluid chromatography with chiral crown ether derived column

The supercritical fluid chromatographic separation of underivatized amino acids was explored using immobilized chiral crown ether column CROWNPAK CR-I (+) and mass spectrometric detection. The type of modifier, acidic additives, and the role of water were investigated. Enantioseparation was achieved for all 18 amino acids investigated with short retention times (less than 3 minutes) and average resolution of greater than 5.0. Analysis of enantiomerically pure standards demonstrated the D enantiomer eluted first for all amino acids using a CROWNPAK CR-I (+) column.

Stereochemical analysis of chiral amines, diamines, and amino alcohols: Practical chiroptical sensing based on dynamic covalent chemistry

Practical chiroptical sensing with a small group of commercially available aromatic aldehydes is demonstrated. Schiff base formation between the electron-deficient 2,4-dinitrobenzaldehyde probe and either primary amines, diamines, or amino alcohols proceeds smoothly in chloroform at room temperature and is completed in the presence of molecular sieves within 2.5 hours. The substrate binding coincides with a distinct circular dichroism signal induction at approximately 330 nm, which can be correlated to the absolute configuration and enantiomeric composition of the analyte. The usefulness of this sensing method is highlighted with the successful sensing of 18 aliphatic and aromatic amines and amino alcohols and five examples showing quantitative %ee determination with good accuracy.