Supercritical fluid chromatography for the enantioseparation of pharmaceuticals

A novel switchable water stationary phase for supercritical fluid chromatography

A novel pH switchable water stationary phase is presented for use in supercritical fluid chromatography (SFC). By adding NH4OH to the water coating and system hydration, changes in CO2 pressure and temperature allow a wide range of stationary phase pH conditions (∼3-9) to be achieved, which impact analyte retention properties. For example, 100 atm and 50 °C produces an acidic water stationary phase (pH near 4.0) where octanoic acid readily elutes while the base caffeine does not. Conversely, at 80 atm and 120 °C a basic water stationary phase (pH near 8.0) is obtained and the opposite occurs. Further, under constant pressure and temperature conditions, simply adding or removing NH4OH from the system is also found to readily allow switching between the basic and acidic water stationary phase modes and demonstrates control over ionizable analyte elution. For instance, hexanoic acid elution is near 40 times more delayed on a basic water stationary phase and, as such, it can be eluted at later points in time as desired by removing the NH4OH and switching to an acidic stationary phase. Experiments indicate that stationary phase pH switching occurs uniformly across the 15 m column length within about 18 s and that analyte retention times are very reproducible upon performing a switch (1.4% RSD; n = 3). Results demonstrate the selectivity factor between acidic and neutral analytes can be reversed and increased about 35 times, while in other trials resolution also similarly increased near 40-fold. By rapidly switching the stationary phase pH back and forth between acidic and basic modes, the selectivity between ionizable analytes could also be increased as desired. Various applications with the system show that it can vastly increase the separation between target analytes and matrix components as required by the dynamics of a particular separation.

Identification of chemical constituents in pomegranate seeds based on ultra-high-performance supercritical fluid chromatography coupled with quadrupole time-of-flight mass spectrometry

Pomegranate seeds are a potential source of bioactive compounds. Nonetheless, most pomegranate seeds are discarded in the food processing industry, likely due to the lack of convincing data on their component analysis.

METHODS

To reveal the main chemical constituents of pomegranate seeds, a reliable and sensitive method based on ultra-high-performance supercritical fluid chromatography coupled with electrospray ionization and quadrupole time-of-flight mass spectrometry (MS) was developed. A time-dependent MSE data acquisition mode was applied to acquire the mass spectrometric data. The chemical constituents were identified by an automatic retrieval of a traditional Chinese medicine library and relevant literature.

RESULTS

A total number of 59 compounds, including fatty acids, sterols, vitamins, cerebrosides, phospholipids, flavonoids, phenylpropanoids, and others, were tentatively identified. Their possible fragmentation pathways and characteristic ions were proposed and elucidated.

CONCLUSIONS

The findings of this study, along with the developed methodology, could provide a reference for basic research on the pharmacodynamic substances of pomegranate seeds and shed light on their potential nutritional and therapeutic applications in the future.

Exploitation of the enantioselectivity space of coated amylose tris(3,5-dimethylphenylcarbamate) in mixtures of 2-propanol and acetonitrile

Chiral stationary phases (CSPs) with coated amylose tris(3,5-dimethylphenylcarbamate) (ADMPC) selector have long been recognized for their excellent chiral recognition ability in liquid chromatography. The conformational versatility behind this feature is the source of their known hysteretic behavior, which has been previously observed in polar organic (PO) mode eluents containing 2-propanol (IPA). Mixtures of IPA and acetonitrile (MeCN), a typical PO mode eluent system, have not been examined in this aspect yet, even though hysteresis is promising for finding unique unexplored enantioselectivities. Not only was the hysteresis detectable on ADMPC using mixtures of IPA and MeCN, but it was the typical behavior in a diverse set of test compounds. The difference in the retention time of the same analyte under conditions which only differed in the eluent history on the column can go up to 20-fold. The assumed hindered conformational changes of the selector were reflected in retention drift at certain eluent compositions. On the two sides of the transitions, distinct, useful states of the selector were detected. A series of IPA - MeCN compositions with defined pretreatment was selected and recommended as an extension of the preliminary, first choice method screening set that used only alcohols. The incorporation of a solvent possessing substantially different characteristics enhances the potential in practical applications, while keeping the technical simplicity. Stability and robustness of the additional states of the CSP were characterized. The examined columns of different brands shared the observed behavior. Kinetic stability of a column state is adequate for successful application. The evaluated states of ADMPC provide multiple enantiorecognition potential by using mixtures of IPA and MeCN also considering the pretreatment of the column. Unprecedented double and triple elution order reversals along the composition range supported the versatility of the available states. Our findings further enhance the usefulness of ADMPC-containing CSPs. We provide instructions for the application of the widespread chiral selector in common eluent mixtures to avoid pitfalls regarding reproducibility and robustness.

Enantioseparation of syn- and anti-3,5-Disubstituted Hydantoins by HPLC and SFC on Immobilized Polysaccharides-Based Chiral Stationary Phases

The enantioseparation of syn- and anti-3,5-disubstituted hydantoins 5a–i was investigated on three immobilized polysaccharide-based columns (CHIRAL ART Amylose-SA, CHIRAL ART Cellulose-SB, CHIRAL ART Cellulose-SC) by high performance liquid chromatography (HPLC) using n-hexane/2-PrOH (90/10, v/v) or 100% dimethyl carbonate (DMC) as mobile phases, respectively, and by supercritical fluid chromatography (SFC) using CO2/alcohol (MeOH, EtOH, 2-PrOH; 80/20, v/v) as a mobile phase. The chromatographic parameters, such as separation and resolution factors, have indicated that Amylose-SA is more suitable for enantioseparation of the most analyzed syn- and anti-3,5-disubstituted hydantoins than Celullose-SB and Cellulose-SC in both HPLC and SFC modalities. All three tested columns showed better enantiorecognition ability toward anti-hydantoins compared to syn-hydantoins, both in HPLC and SFC modes. We have demonstrated that environmentally friendly solvent DMC can be efficiently used as the mobile phase in HPLC mode for enantioseparation of hydantoins on the immobilized polysaccharide-based chiral stationary phases.

Advances and applications of chiral resolution in pharmaceutical field

The attention to chiral drugs has been raised to an unprecedented level as drug discovery and development strategies grow rapidly. However, separation of enantiomers is still a huge task, which leads to an increasing significance to equip a wider range of expertise in chiral separation science to meet the current and future challenges. In the last few decades, remarkable progress of chiral resolution has been achieved. This review summarizes and classifies chiral resolution methods in analytical scale and preparative scale systematically and comprehensively, including crystallization-based method, inclusion complexation, chromatographic separation, capillary electrophoresis, kinetic resolution, liquid-liquid extraction, membrane-based separation, and especially one bold new progress based on chiral-induced spin selectivity theory. The advances and recent applications will be presented in detail, in which the contents may bring more thinking to wide-ranging readers in various professional fields, from analytical chemistry, pharmaceutical chemistry, natural medicinal chemistry, to manufacturing of drug production.

Supercritical Fluid Chromatography for Chiral Analysis, Part 1: Theoretical Background

The quantification of the enantiomers of racemic substances is of great importance in the development and regulation of pharmaceutical compounds. Active ingredients are often chiral; typically, only one of the stereoisomers has the desired pharmacokinetic and/or pharmacodynamic properties. Therefore, the stereoisomer distribution of chiral drug products must be characterized and evaluated during the drug discovery and development pipeline. Moreover, various chiral drugs present a stereoselective metabolism, highlighting the need for appropriate analytical strategies for the stereoselective analysis of metabolites, for example, in clinical and environmental studies. Due to its ease of use, robustness, and transferability, chiral liquid chromatography (LC) is the most common approach used in pharmaceutical analysis. Compared with LC, supercritical fluid chromatography (SFC) allows higher linear flow velocities while maintaining high chromatographic efficiency, often enabling the reduction of analysis time. In addition, SFC provides enhanced or complementary chiral selectivity and avoids or reduces toxic solvents, such as those used in normal-phase LC. In the first part of this review article the theoretical advantages, technological developments, and common practices in chiral SFC are discussed. This will be followed by a contribution discussing recent applications in pharmaceutical, clinical, forensic, and environmental analysis.

Separation of chiral and achiral impurities in paroxetine hydrochloride in a single run using supercritical fluid chromatography with a polysaccharide stationary phase

Separating paroxetine hydrochloride and its impurities using conventional reversed-phase liquid chromatography (RPLC) is challenging due to their highly similar structures. In the present study, a rapid, simple, sensitive and environmentally friendly method was developed for the determination of chiral and achiral impurities in raw materials of paroxetine hydrochloride using chiral supercritical fluid chromatography (SFC). The impacts of chiral stationary phases (CSPs), mobile phases, column temperature and back pressure on the retention and separation of analytes were comprehensively evaluated. After method optimization, a satisfying result was obtained on a cellulose tris-(3-chloro-4-methylphenylcarbamate) stationary phase in 4.0 min using 70% CO₂ and 20 mM ammonium acetate in 30% methanol as the mobile phase. Molecular docking was further performed to understand the interactions between the analytes and CSP. The results suggested that hydrogen bonding and π-π interactions were the dominant interactions. The affinity given by the software was in good agreement with the elution order and free energy (△G) values obtained from van't Hoff equations. The results of molecular docking also provide insights into the different retentions of N-methylparoxetine at different temperatures. The results of method validation revealed that the method was sensitive with a limit of detection of approximately 0.05 μg·mL^-1 (corresponding to approximately 0.005% paroxetine hydrochloride in the sample solution). The relative standard deviations (RSDs) of precision and intra-assay precision were all less than 2.0%, and the recoveries of the method were 93.8~105.3% with RSDs less than 3.0%. The chiral and achiral RPLC methods included in the Chinese pharmacopoeia and the SFC method proposed in this study were simultaneously used to determine the impurity content in the raw materials of paroxetine hydrochloride. The results showed that impurities that cannot be detected by the reference method can be accurately quantified using the SFC method. In addition, the SFC method has advantages in terms of throughput, analysis cost and simplicity. This study can provide a reference for further research of impurities in paroxetine hydrochloride and promote the application of chiral SFC in the rapid separation of structurally similar compounds.

Isolation of achiral aliphatic acid derivatives from Piper kadsura using preparative two-dimensional chiral supercritical fluid chromatography

The separation of structural analogues in natural products has always been one of the challenges in separation science, where supercritical fluid chromatography (SFC) with chiral stationary phases (CSPs) is an unconventional but potential solution. In this study, a preparative two-dimensional chiral SFC (2D cSFC) method that was established with two kinds of CSPs was applied in the isolation of the aliphatic acid derivatives in Piper kadsura (P. kadsura). The RPLC unseparated peaks of two samples A and B of P. kadsura were evenly scattered on the CSP-1 column while they clustered into two groups on the CSP-2 column by SFC. There was impressively complementary selectivity between CSP-1 and CSP-2, which were used for construction of 2D cSFC. The first dimension (¹D) separation with CSP-1 fractionated the sample A into six parts by a heart-cutting method and the sample B into nine parts for a comprehensive 2D analysis; then 29 and 71 peaks were respectively found in these parts in the second dimension (²D) separation with CSP-2. Further through ²D preparative separation, 19 high purity components were obtained, and the chemical structures of two of them were confirmed, including a novel unsaturated aliphatic acid compound (8Z,10Z)-12-methoxyheptadeca-8,10-dienoic acid and a known octadecadienoic acid lactone Lactariolide. The 2D cSFC method presented the superiority of separating the achiral compounds of complex samples.

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.

Magnetic Molecularly Imprinted Polymers Based on Dehydroabietylamine as Chiral Monomers for the Enantioseparation of RS-Mandelic Acid

Stereoselective adsorption of the enantiomers shows potential in the resolution of a racemate. In this work, we synthesized novel magnetic surface molecularly imprinted polymers (MIPs) on the surface of the γ-methacryloxypropyltrimethoxysilane (MPS)-modified Fe₃O₄@SiO₂ particles to utilize chiral dehydroabietylamine (DHA) as a functional monomer and R-mandelic acid as a template molecule (DHA-MIPs). We performed the resolution of mandelic acid racemate (RS-MA) via adsorption on the as-prepared MIPs. The results revealed that the MIPs have good affinity and high adsorption capacity for R-MA and show better enantioselective adsorption ability for R-MA than that for S-MA. One-stage adsorption of RS-MA on the MIPs can achieve up to 53.7% enantiomeric excess (ee) for R-MA. These help us to improve the chiral separation ability of the traditional MIPs using a chiral rather than an achiral monomer in MIP preparation. The MIPs can be employed as an economic and efficient adsorbent for chiral separation of MA racemate.

The effect of water on the large-scale supercritical fluid chromatography purification of two factor XIa active pharmaceutical ingredients

BMS-962212, a parenteral Factor XIa inhibitor, was scaled-up for toxicity studies. Two steps of supercritical fluid chromatography (SFC) were developed for the chiral resolution of the penultimate and achiral purification of final active pharmaceutical ingredient (API), BMS-962212. A robust SFC process using Chiralcel OD-H with methanol-acetonitrile as modifier in CO₂ was established to achieve a stable and uninterrupted operation with reduced mobile phase viscosity and system pressure drop. More than 230 g of the racemic penultimate was chirally resolved to reach >99% chiral purity, ready for final tert-butyl ester deprotection to provide the API. There were a significant number of impurities in BMS-962212 generated from the final step that needed to be removed. In contrast to conventional SFC conditions, an SFC method exploiting water and ammonia as additives in both the mobile phase and sample solution was developed to accomplish purification and desalting (i.e. removing TFA) of the zwitterionic API in one step. Water as an additive eliminated salt precipitation and improved the resolution while ammonia contributed to the desalting, details of which will be discussed in this article. A throughput of 2 g/h was achieved, and >80 g of the crude API was purified. The same strategy was applied to another Factor XIa API (compound A) and its penultimate.

Chemical derivatization in combination with supercritical fluid chromatography to improve resolution of stereoisomers

Aim: Quantification of stereoisomers in biological matrices is of pivotal importance for drug development. Supercritical fluid chromatography paired with chiral stationary phases is the gold standard for resolution of enantiomers. However, this technique often proves inadequate for resolution of polar stereoisomers. Materials & methods: A combination of achiral chemical derivatization with supercritical fluid chromatography using chiral stationary columns to improve enantiomeric resolution is described. Results: Separation of four possible stereoisomers of linerixibat was achieved after derivatization with 3N HCl in n-butanol within 12 min (case1). Derivatization with acetic, propionic, butyric, isobutyric, valeric and isovaleric anhydrides significantly improved the separation of stereoisomers (case 2 and 3) within 10 min. The best stereoisomeric resolution was achieved using valeric and isovaleric anhydrides.

The Use of Dual Cyclodextrin Chiral Selector Systems in the Enantioseparation of Pharmaceuticals by Capillary Electrophoresis: An Overview

Cyclodextrin (CD) derivatives are the most efficient and frequently used chiral selectors (CSs) in capillary electrophoresis (CE). There are situations when the use of a single CD as CS is not enough to obtain efficient chiral discrimination of the enantiomers; in these cases, sometimes this problem can be resolved using a dual CD system. The use of dual CD systems can often dramatically enhance enantioseparation selectivity and can be applied for the separation of many analytes of pharmaceutical interest for which enantioseparation by CE with another CS systems can be problematic. Usually in a dual CD system an anionic CD is used together with a neutral one, but there are situations when the use of a cationic CD with a neutral one or the use of two neutral CDs or even two ionized CDs can be an efficient solution. In the current review we present general aspects of the use of dual CD systems in the analysis of pharmaceutical substances. Several examples of applications of the use of dual CD systems in the analysis of pharmaceuticals are selected and discussed. Theoretical aspects regarding the separation of enantiomers through simultaneous interaction with the two CSs are also explained. Finally, advantages, disadvantages, potential and new direction in this chiral analysis field are highlighted.

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.

Effective Separation of Human Milk Glycosides using Carbon Dioxide Supercritical Fluid Chromatography

Carbohydrate purification remains problematic due to the intrinsic diversity of structural isomers present in nature. Although liquid chromatography-based techniques are suitable for analyzing or preparing most glycan structures acquired either from natural sources or through chemical or enzymatic synthesis, the separation of regioisomers or linkage isomers with a clear resolution remains challenging. Herein, a carbon dioxide supercritical fluid chromatography (SFC) method was devised to resolve 18 human milk glycosides: oligomers (disaccharides to hexasaccharides), fucosylated regioisomers (lacto-N-fucopentaose I, III, and V; lacto-N-neofucopentaose V; lacto-N-difucohexaose III; blood group H₁ antigen; and TF-LNnT), and connectivity isomers (lacto-N-tetraose/lacto-N-neotetraose and para-lacto-N-hexaose/para-lacto-N-neohexaose/type-1 hexasaccharide). The analysis of these glycosides represents a major limitation associated with conventional carbohydrate analysis. The unprecedented resolution achieved by the SFC method indicates the suitability of this key technology for revealing complex human milk glycomes.

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.

Enantioseparation using carboxymethyl-6-(4-methoxybenzylamino)-β-cyclodextrin as a chiral selector by capillary electrophoresis and molecular modeling study of the recognition mechanism

In this study, carboxymethyl-6-(4-methoxybenzylamino)-β-cyclodextrin (CMCDPN) was synthesized for the first time and managed to be used as a chiral selector to enantioseparate 13 kinds of chiral drugs by capillary electrophoresis.

Towards eco-friendly secondary plant metabolite quantitation: Ultra high performance supercritical fluid chromatography applied to common vervain (Verbena officinalis L.)

This report presents the first ultra high performance supercritical fluid chromatography diode array detector based assay for simultaneous determination of iridoid glucosides, flavonoid glucuronides, and phenylpropanoid glycosides in Verbena officinalis (Verbenaceae) extracts. Separation of the key metabolites was achieved in less than 7 min on an Acquity UPC² Torus Diol column using a mobile phase gradient comprising subcritical carbon dioxide and methanol with 0.15% phosphoric acid. Method validation for seven selected marker compounds (hastatoside, verbenalin, apigenin‐7‐O‐glucuronide, luteolin‐7‐O‐glucuronide, apigenin‐7‐O‐diglucuronide, verbascoside, and luteolin‐7‐O‐diglucuronide) confirmed the assay to be sensitive, linear, precise, and accurate. Head‐to‐head comparison to an ultra high performance liquid chromatography comparator assay did prove the high orthogonality of the methods. Quantitative result equivalence was evaluated by Passing‐Bablok‐correlation and Bland‐Altman‐plot analysis. This cross‐validation revealed, that one of the investigated marker compound peaks was contaminated in the ultra high performance liquid chromatography assay by a structurally related congener. Taken together, it was proven that the ultra high performance supercritical fluid chromatography instrument setup with its orthogonal selectivity is a true alternative to conventional reversed phase liquid chromatography in quantitative secondary metabolite analysis. For regulatory purposes, assay cross‐validation with highly orthogonal methods seems a viable approach to avoid analyte overestimation due to coeluting, analytically indistinguishable contaminants.

Defining a system suitability limit to decide on column deterioration and to facilitate column transfers in chiral supercritical fluid chromatography

The separation of enantiomers is an important requirement during the entire drug life cycle in the pharmaceutical industry. High-performance liquid chromatography and supercritical fluid chromatography (SFC) are the main chromatographic techniques used to separate enantiomers. Since chiral stationary phases are often extensively used once a method has been developed, columns will age and must be replaced after a certain period. However, no practical guidelines exist to determine when a column is deteriorated or to decide whether a transfer to another column (with the same chiral selector) is successful. In this study, a system suitability limit for resolution was defined, based on an intermediate (time-different) precision study in SFC on four immobilized polysaccharide-based columns that only differed in manufacturer or particle size. This system suitability limit could be used to decide on column deterioration or as a requirement to evaluate whether a separation transfer was successful. Some method adaptations may be necessary to obtain successful transfers. An approach was proposed, which helped the analyst to make successful transfers. Graphical abstract.

Doing more with less: Evaluation of the use of high linear velocities in preparative supercritical fluid chromatography

The evaluation of higher than typical linear velocities is discussed for supercritical fluid chromatographic purifications on the preparative scale. SFC separation efficiency suffers far less at high linear velocities than HPLC by the rapid mass transfer of analytes carried by compressed CO₂ through the stationary phase. The technique is discussed using chiral test compounds and columns. In many cases, running at high linear velocities can yield significant time savings and decreased consumption of mobile phase solvent, while also lowering energy consumption. Within the practical limitations of commercial instrumentation, using 20 μm particles can aid in achieving higher linear velocities not attainable with smaller 5 μm particles, particularly when running with high percentages of organic co-solvent. Use of larger particles for the stationary phase also lowers the associated column cost. These benefits can yield an overall purification process that is more productive and environmentally friendly.

Interpretation of in Vitro Metabolic Stability Studies for Racemic Mixtures

In early drug discovery, where chiral syntheses may not yet have been elucidated or enantiomeric separation is not feasible, screening of racemates in metabolic stability assays may offer a pragmatic approach. To assess the risk of incorrectly deprioritizing enantiomers due to misclassification of apparent in vitro intrinsic clearance (CLint_app), we evaluated (1) theoretical simulations; (2) literature on enantiomeric CLint_app differences; (3) historic MSD data; and (4) new data on enantiomers with high eudysmic ratios and low predicted three-dimensional similarity. Overall, the results suggested minimal risk of not progressing an enantiomer due to an appreciably different (>3-fold) racemate CLint_app.

Optical Analysis of Reaction Yield and Enantiomeric Excess: A New Paradigm Ready for Prime Time

This Perspective highlights the advances of optical methods for asymmetric reaction discovery. Optical analysis allows for the determination of absolute configuration, enantiomeric excess and reaction yield that is amenable to high-throughput experimentation. Thus, the synthetic organic community is encouraged to incorporate the methods discussed to expedite the development of high-yielding, enantioselective transformations.

Study of the chromatographic parameters of ultra-high-performance supercritical fluid chromatography and method development using a design of experiments approach for the quantification of pesticides in lettuce

We examine the potential of ultra-high-performance supercritical fluid chromatography for multiresidue quantification of ten pesticides commonly applied to lettuce and compares it to ultra-high-performance liquid chromatography. Initially, a thorough study of the stationary and mobile phase composition and injection solvent was carried out. In a second step, a chemometric approach based on design of experiments was used to simultaneously study the influence of temperature, pressure, and percentage of ethanol on the retention, resolution and symmetry of the peaks. Using this approach, it was possible to obtain the Design Space, a robust region where complete separation of the analytes was achieved, with acceptable peak shape. Both methods were validated according to the figures of merit: selectivity, linearity, quantification limit, accuracy (in terms of recovery), and precision (repeatability and intermediate precision) and used to quantify the pesticides in lettuce samples. Comparing both techniques, it was concluded that the limits of quantification, accuracy, and precision were similar. However, in supercritical fluid chromatography, a reduced volume of organic solvent was used, the method was faster and generated lower amounts of residues.

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.