Modern screening techniques for plant extracts

In order to discover new bioactive compounds from plant sources which could become new leads or new drugs, extracts should be simultaneously evaluated by chemical screening and by various biological or pharmacological targets. Chemical screening using hyphenated techniques such as LC/UV and LC/MS, and more recently LC/NMR, quickly provides ample structural information, leading in many cases to the identification of compounds. This allows researchers to distinguish between known compounds (dereplication) and new molecules directly from crude plant extracts. Thus, the tedious isolation of known compounds can be avoided, and a targeted isolation of constituents presenting novel or unusual spectroscopic features can be undertaken. In parallel, extracts are also subjected to various bioassays that should be simple, reproducible, and rapid. This approach will be illustrated by the search for new molluscicidal, antioxidant, and antifungal compounds from tropical plants.     

色谱及其联用技术在海洋药用生物核苷类化合物筛选排重方面的应用

为了解决天然产物化学研究中核苷类化合物重复提取、分离、纯化和表征等问题,避免人力物力的大量消耗并提高新化合物的发现效率,因此对已知核苷类化合物的快速鉴别、排重显得尤为重要。本文以笔者所在课题组取得的研究成果为例,从高效液相色谱、高效液相色谱-质谱联用、毛细管电泳和毛细管电泳-质谱联用等4种色谱及其联用技术,对海洋生物中核苷类化合物快速鉴别、排重方法进行了系统介绍。上述方法可以实现对海洋药用生物粗提物中常见核苷类化合物的快速筛选、排重,避免对已知成分繁杂的分离纯化、结构鉴定过程,为海洋天然产物化学研究提供了一种新思路。     

Comparison of LC-NMR and conventional NMR for structure elucidation in drug metabolism studies

Liquid chromatography-nuclear magnetic resonance (LC-NMR) has proven to be a useful technique for the structure elucidation of novel metabolites from pharmaceutical compounds. Proponents of LC-NMR tout the advantage of eliminating the step of a separate chromatographic isolation. However, the advantages of directly coupling NMR and HPLC instrumentation must be weighed against compromises in performance made to each technique to achieve a hyphenated system. While significant advances have been made in LC-NMR technology, a strong case can be made that HPLC purification of metabolites followed by conventional tube NMR is equally useful. It is relatively rare that one approach will be successful and the other not. The fundamental consideration is whether there is sufficient chromatographic expertise in the NMR laboratory to adequately design and execute appropriate experiments such that a pure chromatographic peak will be produced in the hyphenated system. Due to speed and sensitivity differences between NMR spectroscopy and mass spectrometry, liquid chromatography/mass spectrometry (LC/MS) continues to be the front-line approach for the structure elucidation of metabolites.     

Hyphenated NMR methods in natural products research, Part 2: HPLC-SPE-NMR and other new trends in NMR hyphenation

This review describes the principles and performance of a novel and highly promising hyphenated technique, HPLC-SPE-NMR, which is based on post-column analyte trapping by solid-phase extraction. The analytes are subsequently eluted from the SPE cartridges using deuterated solvents. This indirect HPLC-NMR hyphenation offers numerous advantages compared to direct HPLC-NMR methods. Multiple trapping leads to a dramatic increase of analyte amounts available for NMR, enabling acquisition of high-quality 2D NMR data within a short time. Other new developments, including combination of solenoidal coil capillary flow-probes with microflow HPLC, are also discussed. Fast extract dereplication using these techniques enables focusing of isolation efforts on truly novel and promising natural products, based on precise structural data obtained with crude extracts or fractions.     

Analysis and screening of combinatorial libraries using mass spectrometry

Mass spectrometry is a highly selective and high throughput analytical technique that is ideally suited for the identification and purity determination of large numbers of compounds prepared using combinatorial chemistry or for the dereplication of natural products. Compounds may be characterized based on molecular weight, elemental composition and structural features based on fragmentation patterns. When coupled to a separation technique such as high-performance liquid chromatography (HPLC) or capillary electrophoresis, mass spectrometric applications may be expanded to include analysis of complex mixtures. However, the slower speed of the separation step reduces the throughput of the analysis. This review concerns the application of mass spectrometry to the characterization of combinatorial libraries and the screening of library and natural product mixtures. Strategies to enhance the throughput of LC-MS are discussed including fast HPLC and parallel LC-MS. Also, mass spectrometry-based screening methods are described including frontal affinity chromatography-mass spectrometry, gel permeation chromatography LC-MS, direct electrospray mass spectrometry of receptor-ligand complexes, affinity chromatography-mass spectrometry, and pulsed ultrafiltration mass spectrometry.     

Characterization by liquid chromatography-nuclear magnetic resonance spectroscopy and liquid chromatography-mass spectrometry of two coupled oxidative-conjugative metabolic pathways for 7-ethoxycoumarin in human liver microsomes treated with alamethicin.

The microsomal metabolism of 7-ethoxycoumarin (7-EC) was investigated using liquid chromatography (LC)-NMR and liquid chromatography-mass spectrometry (LC-MS) to characterize the coupling of oxidative-conjugative metabolism events. Within microsomes, cytochromes P450 (P450s) and UDP-glucuronosyltransferases (UGTs) are spatially disparate, each having surface and luminal localization, respectively. To optimize cofactor and substrate transit to UGT without compromising P450 activity, the pore-forming peptide alamethicin was used for microsomal perforation. Aqueous extracts of microsomal incubations containing NADPH and UDP-glucuronic acid were injected for LC-NMR and LC-MS analysis. The analytical complementarity of LC-NMR and LC-MS permitted the identification of four metabolites (M1 to M4). The metabolites M1 and M2 are novel microsomal metabolites for 7-EC, consistent with 3-hydroxylation and subsequent glucuronidation, respectively. Metabolites M3 and M4 were 7-hydroxycoumarin (7-HC) and 7-HC glucuronide, respectively. Viewed collectively, these results illustrate the utility of alamethicin in the examination of coupled oxidative-conjugative metabolism and the synergy of LC-NMR and LC-MS in metabolite identification.     

Development of a two-dimensional liquid chromatography system for isolation of drug metabolites

The separation, isolation and identification of drug metabolites from complex endogenous matrices like urine, plasma and tissue extracts are challenging tasks. Metabolites are usually first identified by mass spectrometry and tentative structures proposed from product ion spectra. In many cases mass spectrometry cannot be used to determine positional isomers and metabolites have to be fractionated in microgram amounts for analysis by NMR. To overcome the difficulties associated with separation and isolation of drug metabolites from biological matrices, a new two-dimensional liquid chromatography system has been developed.     

Recent advances in NMR: expanding its role in rational drug design

The technology of nuclear magnetic resonance spectroscopy continues to advance at a rapid pace. Recent improvements in gradient technology and the coupling of NMR to various chromatography methods will provide new opportunities in drug discovery. These opportunities include rapid high throughput screening to determine the receptor-bound conformations of small organic ligands. NMR coupled to liquid chromatography has opened a new door to the quantitative and qualitative analysis of complex mixtures including metabolites extracted from body fluids and extracts containing various natural products. This review will focus on the following four advances in NMR technology: 1) pulse-field gradient (PFG) NMR, 2) SAR (structure activity relationship) by NMR, 3) LC-NMR (liquid chromatography), and 4) application of membrane models for the study of neuropeptides. The information content available to medicinal chemists from each experiment will be discussed.     

Characterization of steroidal saponins in crude extracts from Dioscorea zingiberensis C. H. Wright by ultra-performance liquid chromatography/electrospray ionization quadrupole time-of-flight tandem mass spectrometry

Steroidal saponins are the major bioactive constituents of Dioscorea zingiberensis C. H. Wright (D. zingiberensis). In this work, ultra-performance liquid chromatography/electrospray ionization quadrupole time-of-flight tandem mass spectrometry (UPLC/Q-TOF-MS/MS) was applied to the separation and characterization of steroidal saponins in crude extracts from D. zingiberensis. The results showed that fragment ions from glycosidic and cross-ring cleavages gave a wealth of structural information related to aglycone skeletons, sugar types and the sequence of sugar units. According to the summarized fragmentation patterns, identification of steroidal saponins from D. zingiberensis could be fulfilled, even when reference standards were unavailable. As a result, a total of thirty-one saponins with five aglycone skeletons, including fourteen new trace saponins, were identified or tentatively elucidated in crude extracts from D. zingiberensis based on their retention times, the mass spectrometric fragmentation patterns, and MS and MS/MS data.     

Hyphenated NMR methods in natural products research, part 1: direct hyphenation

This review describes the current status of the development of techniques combining liquid chromatography and nuclear magnetic resonance and their use in the context of natural products. HPLC-NMR methods have a rapidly growing impact on natural products research by enabling structure determination of natural products directly from small amounts of extracts, i. e., prior to the investment in an often lengthy and laborious preparative-scale isolation process. This speeds up extract dereplication and helps to avoid re-isolation of already known extract constituents. Direct HPLC-NMR hyphenation techniques, defined as methods in which NMR spectra are recorded using the HPLC eluate, are described. The recently developed indirect HPLC-NMR method employing an automated solid-phase extraction interface between HPLC and NMR, which replaces the chromatographic solvent with a different solvent for NMR data acquisition, is described in the second part of this review.     

Cell-based bioreporter assay coupled to HPLC micro-fractionation in the evaluation of antimicrobial properties of the basidiomycete fungus Pycnoporus cinnabarinus

Context Identification of bioactive components from complex natural product extracts can be a tedious process that aggravates the use of natural products in drug discovery campaigns.

Objective This study presents a new approach for screening antimicrobial potential of natural product extracts by employing a bioreporter assay amenable to HPLC-based activity profiling.

Materials and methods A library of 116 crude extracts was prepared from fungal culture filtrates by liquid–liquid extraction with ethyl acetate, lyophilised, and screened against Escherichia coli using TLC bioautography. Active extracts were studied further with a broth microdilution assay, which was, however, too insensitive for identifying the active microfractions after HPLC separation. Therefore, an assay based on bioluminescent E. coli K-12 (pTetLux1) strain was coupled with HPLC micro-fractionation.

Results Preliminary screening yielded six fungal extracts with potential antimicrobial activity. A crude extract from a culture filtrate of the wood-rotting fungus, Pycnoporus cinnabarinus (Jacq.) P. Karst. (Polyporaceae), was selected for evaluating the functionality of the bioreporter assay in HPLC-based activity profiling. In the bioreporter assay, the IC₅₀ value for the crude extract was 0.10?mg/mL. By integrating the bioreporter assay with HPLC micro-fractionation, the antimicrobial activity was linked to LC-UV peak of a compound in the chromatogram of the extract. This compound was isolated and identified as a fungal pigment phlebiarubrone.

Discussion and conclusion HPLC-based activity profiling using the bioreporter-based approach is a valuable tool for identifying antimicrobial compound(s) from complex crude extracts, and offers improved sensitivity and speed compared with traditional antimicrobial assays, such as the turbidimetric measurement.     

液相色谱-核磁共振联用技术在药物分析中的应用

液相色谱(LC)是分离复杂混合物的较好方法,核磁共振(NMR)是结构分析的强有力工具.液相色谱-核磁共振(LC-NMR)联用技术是快速分离、确定结构的一种分析手段,具有广阔的应用前景.主要综述了LC-NMR联用技术在化学药物分析(包括未知杂质的结构分析、混合物中已知成分的定量分析)、中药及天然药物分析(包括异构体分析、新化合物的结构鉴定)、海洋生物及生化大分子、代谢产物分析中的应用.     

LC-SPE-NMR-MS: a total analysis system for bioanalysis

Liquid chromatography (LC)-solid-phase extraction (SPE)-nuclear magnetic resonance (NMR)-mass spectrometry (MS) coupling is a key technology for fast and thorough structure elucidation of valuable mass-limited samples. Laborious serial isolation and purification procedures of metabolites, byproducts or impurities from complex biomatrices, natural product extracts or other mixtures of several components can be circumvented by the use of this integrated modular system. This combination of high-end analytical technology significantly accelerates the structure-elucidation process for valuable samples present in minute quantities in mixtures. The information depth is significantly increased by the concurrent availability of NMR and MS data of one chromatographic peak. Thus, this flexible technique is well on its way to becoming the gold standard in analytical chemistry of mixtures. LC-SPE-NMR-MS overcomes the limitations of directly coupled LC-NMR. Full flexibility regarding chromatographic conditions and NMR acquisition is gained by this modular technique. LC-SPE-NMR-MS allows for a rapid structure-elucidation process that would not be possible on the basis of MS or NMR data alone.     

Application of mass spectrometry for metabolite identification

Metabolism studies play a pivotal role in drug discovery and development. Characterization of metabolic "hot-spots" as well as reactive and pharmacologically active metabolites is critical to designing new drug candidates with improved metabolic stability, toxicological profile and efficacy. Metabolite identification in the preclinical species used for safety evaluation is required in order to determine whether human metabolites have been adequately tested during non-clinical safety assessment. From an instrumental standpoint, high performance liquid chromatography (HPLC) coupled with mass spectrometry (MS) dominates all analytical tools used for metabolite identification. The general strategies employed for metabolite identification in both drug discovery and drug development settings together with sample preparation techniques are reviewed herein. These include a discussion of the various ionization methods, mass analyzers, and tandem mass spectrometry (MS/MS) techniques that are used for structural characterization in a modern drug metabolism laboratory. Mass spectrometry-based techniques, such as stable isotope labeling, on-line H/D exchange, accurate mass measurement to enhance metabolite identification and recent improvements in data acquisition and processing for accelerating metabolite identification are also described. Rounding out this review, we offer additional thoughts about the potential of alternative and less frequently used techniques such as LC-NMR/MS, CRIMS and ICPMS.     

Direct analysis in real time ‐ high resolution mass spectrometry (DART‐HRMS): a high throughput strategy for identification and quantification of anabolic steroid esters

High throughput screening is essential for doping, forensic, and food safety laboratories. While hyphenated chromatography‐mass spectrometry (MS) remains the approach of choice, recent ambient MS techniques, such as direct analysis in real time (DART), offer more rapid and more versatile strategies and thus gain in popularity. In this study, the potential of DART hyphenated with Orbitrap‐MS for fast identification and quantification of 21 anabolic steroid esters has been evaluated. Direct analysis in high resolution scan mode allowed steroid esters screening by accurate mass measurement (Resolution = 60 000 and mass error < 3 ppm). Steroid esters identification was further supported by collision‐induced dissociation (CID) experiments through the generation of two additional ions. Moreover, the use of labelled internal standards allowed quantitative data to be recovered based on isotopic dilution approach. Linearity (R² > 0.99), dynamic range (from 1 to 1000 ng mL^‐1), bias (<10%), sensitivity (1 ng mL^‐1), repeatability and reproducibility (RSD < 20%) were evaluated as similar to those obtained with hyphenated chromatography‐mass spectrometry techniques. This innovative high throughput approach was successfully applied for the characterization of oily commercial preparations, and thus fits the needs of the competent authorities in the fight against forbidden or counterfeited substances. Copyright © 2014 John Wiley & Sons, Ltd.     

Rapid detection and subsequent isolation of bioactive constituents of crude plant extracts

Rapid detection of biologically active natural products plays a strategic role in the phytochemical investigation of crude plant extracts. In order to perform an efficient screening of the extracts, both biological assays and HPLC analysis with various detection methods are used. Combined techniques such as HPLC coupled to UV photodiode array detection (LC/UV) and to mass spectrometry (LC/MS or LC/MS/MS) provide useful structural information on the metabolites on-line prior to isolation. The recent introduction of HPLC coupled to nuclear magnetic resonance (LC/NMR) represents a powerful complement to the LC/UV/MS screening. Various plants belonging to the Gentianaceae and Leguminosae families have been analysed by LC/UV, LC/MS, LC/MS/MS, and LC/NMR. The use of all these coupled techniques allows the structural determination of known plant constituents rapidly and with only a minute amount of plant material. With such an approach, the time-consuming isolation of common natural products is avoided and an efficient targeted isolation of compounds presenting interesting chemical or biological features can be performed.     

Prediction of designer drugs: Synthesis and spectroscopic analysis of synthetic cathinone analogs that may appear on the Swedish drug market

The use of hyphenated analytical techniques in forensic drug screening enables simultaneous identification of a wide range of different compounds. However, the appearance of drug seizures containing new substances, mainly new psychoactive substances (NPS), is steadily increasing. These new and other already known substances often possess structural similarities and consequently they exhibit spectral data with slight differences. This situation has made the criteria that ensure indubitable identification of compounds increasingly important. In this work, 6 new synthetic cathinones that have not yet appeared in any Swedish drug seizures were synthesized. Their chemical structures were similar to those of already known cathinone analogs of which 42 were also included in the study. Hence, a total of 48 synthetic cathinones making up sets of homologous and regioisomeric compounds were used to challenge the capabilities of various analytical techniques commonly applied in forensic drug screening, ie, gas chromatography–mass spectrometry (GC–MS), gas chromatography–Fourier transform infrared spectroscopy (GC–FTIR), nuclear magnetic resonance (NMR), and liquid chromatography quadrupole time‐of‐flight mass spectrometry (LC–QTOF–MS). Special attention was paid to the capabilities of GC–MS and GC–FTIR to distinguish between the synthetic cathinones and the results showed that neither GC–MS nor GC–FTIR alone can successfully differentiate between all synthetic cathinones. However, the 2 techniques proved to be complementary and their combined use is therefore beneficial. For example, the structural homologs were better differentiated by GC–MS, while GC–FTIR performed better for the regioisomers. Further, new spectroscopic data of the synthesized cathinone analogs is hereby presented for the forensic community. The synthetic work also showed that cathinone reference compounds can be produced in few reaction steps.     

Bioactivity-guided fractionation and analysis of compounds with anti-influenza virus activity from <Emphasis Type="Italic">Gardenia jasminoides</Emphasis> ellis

Bioassay-guided fractionation of extracts from Fructus Gardeniae led to analysis of its bioactive natural products. After infection by influenza virus strain A/FM/1/47-MA in vivo, antiviral activity of the extracts were investigated. The target fraction was orally administered to rats and blood was collected. High-performance liquid chromatography coupled with photo diode array detector and electrospray ion trap multiple-stage tandem mass spectrometry was applied to screen the compounds absorbed into the blood. A structural characterization based on the retention time, ultraviolet spectra, parent ions and fragmentation ions was performed. Thirteen compounds were confirmed or tentatively identified. This provides an accurate profile of the composition of bioactive compounds responsible for the anti-influenza properties.     

Evaluation of microalgae and cyanobacteria as potential sources of antimicrobial compounds

In recent decades, marine microorganisms have become known for their ability to produce a wide variety of secondary bioactive metabolites. Several compounds have been isolated from marine microorganisms for the development of novel bioactives for the food and pharmaceutical industries. In this study, a number of microalgae were evaluated for their antimicrobial activity against gram-positive and gram-negative bacteria, including food and plant pathogens, using various extraction techniques and antimicrobial assays. Disc diffusion and spot-on-lawn assays were conducted to confirm the antimicrobial activity. To measure the potency of the extracts, minimum inhibition concentrations (MIultCs) were measured. Three microalgae, namely Isochrysis galbana, Scenedesmus sp. NT8c, and Chlorella sp. FN1, showed strong inhibitory activity preferentially against gram-positive bacteria. These microalgal species were then selected for further purification and analysis, leading to compound identification. By using a mixture of different chromatography techniques gas chromatography–mass spectrometry (GC–MS) and high-performance liquid chromatography (HPLC) and ultra-high performance liquid chromatography-quadrupole time-of-flight mass spectrometry (UHPLC-Q-TOF-MS), we were able to separate and identify the dominant compounds that are responsible for the inhibitory activity. Additionally, nuclear magnetic resonance (NMR) was used to confirm the presence of these compounds. The dominant compounds that were identified and purified in the extracts are linoleic acid, oleic acid, docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA). These compounds are the potential candidates that inhibit the growth of gram-positive bacteria. This indicates the potential use of microalgae and their antimicrobial compounds as biocontrol agents against food and plant pathogens.