Bioautography with TLC-MS/NMR for Rapid Discovery of Anti-tuberculosis Lead Compounds from Natural Sources

Eicosane: An antifungal compound derived from Streptomyces sp. KF15 exhibits inhibitory potential against major phytopathogenic fungi of crops

In the present scenario, food security is of major concern due to exponentially increasing population and depleted crop production. The fungal diseases have contributed majorly to the scarcity of staple food products and economic loss worldwide. This problem could be tackled by preventing the crop loss during both pre and post-harvest seasons. During the current investigation, the bioactive compound eicosane was extracted from Streptomyces sp. KF15, subjected to purification and identified based on mass spectrometry and NMR analysis. The evaluation of in-vitro antifungal activity was done by poisoned food method, SEM analysis and growth pattern analysis. The bioactive compound eicosane with molecular weight of 282.5475 g/mol was purified by column chromatography and the straight chain hydrocarbon structure of CH3CH2(18)CH3 was elucidated by NMR analysis. In poisoned food assay, eicosane effectively inhibited the radial growth of all tested fungal pathogens; F. oxysporum was found to be the most sensitive with 24.2%, 33.3%, 42.4%, and 63.6% inhibition at 25-100 μg/ml concentrations. The SEM micrograph established clear differences in the morphology of eicosane treated fungi with damaged hyphae, flaccid mycelium and collapsed spores as compared to the tubular, turgid and entire fungi in control sample. Finally, the growth curve assay depicted the right side shift in the pattern of eicosane treated fungi indicating the delay in adapting to the conditions of growth and multiplication. The findings of this study encourage further research and development towards the novel antifungal drugs that can act against major phytopathogens.

TLC-MS-Bioautographic Identification of Antityrosinase Compounds and Preparation of a Topical Gel Formulation from a Bioactive Fraction of an RSM-Optimized Alcoholic Extract of Rubia Cordifolia L. stem

BACKGROUND

Rubia cordifolia L., Rubiaceae, is globally reported to treat skin-related problems. The study aimed to assess the antityrosinase potential of Rubia cordifolia (ARC) and the development of gel formulation.

METHODS

The AutoDock Vina (version V.1.2.0) program package was used for molecular docking to check for the binding affinity of ligands with protein. Response surface methodology (RSM) software was used to optimize extraction parameters for an alcoholic extract of Rubia cordifolia (ARC). The developed HPTLC method for the quantification of purpurin in ARC was validated as per the International Conference on Harmonization (ICH) guidelines. A bioautographic study for the evaluation of antityrosinase effects was performed; an anthraquinone-enriched fraction (AEF)-loaded gel formulation developed and evaluated physicochemically which could be used to reduce skin pigmentation.

RESULTS

Purpurin showed optimum binding affinity (-7.4 kcal/mol) with the molecular target (tyrosinase) when compared to that of standard kojic acid (-5.3 kcal/mol). Quantification of purpurin in ARC, optimized by RSM software, was validated and physiologically significant results were observed for the antityrosinase potential of an AEF, along with TLC-MS-bioautographic identification for antityrosinase compounds: purpurin (m/z 256.21) and ellagic acid (m/z 302.19). Evaluation of an AEF-loaded gel formulation by in vitro and ex vivo permeation studies was performed.

CONCLUSION

ARC extraction parameters optimized by RSM, and a bioautographic study helped identify antityrosinase compounds. The development of a gel formulation could be a cost-effective option for the treatment of depigmentation in the future.

HIGHLIGHTS

A TLC-MS-Bioautography-based Identification of Antityrosinase Compounds and development of AEF-loaded Topical Gel formulation from a Bioactive Fraction of an RSM-Optimized Alcoholic Extract of Rubia Cordifolia L. stem, which could help with promising results in reducing skin pigmentation and maintaining even tone.

Comprehensive Study of Si-Based Compounds in Selected Plants (Pisum sativum L., Medicago sativa L., Triticum aestivum L.)

This review describes the role of silicon (Si) in plants. Methods of silicon determination and speciation are also reported. The mechanisms of Si uptake by plants, silicon fractions in the soil, and the participation of flora and fauna in the Si cycle in terrestrial ecosystems have been overviewed. Plants of Fabaceae (especially Pisum sativum L. and Medicago sativa L.) and Poaceae (particularly Triticum aestivum L.) families with different Si accumulation capabilities were taken into consideration to describe the role of Si in the alleviation of the negative effects of biotic and abiotic stresses. The article focuses on sample preparation, which includes extraction methods and analytical techniques. The methods of isolation and the characterization of the Si-based biologically active compounds from plants have been overviewed. The antimicrobial properties and cytotoxic effects of known bioactive compounds obtained from pea, alfalfa, and wheat were also described.

TLC-bioautography-MS-based Identification of Antioxidant, α-Amylase and α-Glucosidase Inhibitory Compounds in a Polyherbal Formulation “Sugreen-120”

Background

Sugreen-120 is one of the famous Indian polyherbal formulations used for the treatment of diabetes. Due to a lack of scientific evidence, the present study is aimed at investigating the phytopharmacology of Sugreen-120 concerning its antioxidant and antidiabetic characteristics.

Materials and Methods

Total phenols and flavonoid content followed by 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radical scavenging activity was estimated in Sugreen-120. α-Amylase and α-glucosidase inhibitory action of Sugreen-120 was estimated to evaluate its antidiabetic potential. Thin layer chromatography (TLC)-bioautography-MS analysis was performed to determine DPPH free radical, α-amylase and α-glucosidase inhibitory phytoconstituents in Sugreen-120. High performance thin layer chromatography (HPTLC)-based quantitative analysis of Sugreen-120 was performed for the simultaneous separation of caffeic acid and kaempferol. In silico docking analysis was performed to determine the effect of Sugreen-120 metabolites against α-amylase and α-glucosidase enzymes.

Results

The results showed that Sugreen-120 is enriched in total phenols and flavonoids and even has good potential to scavenge DPPH free radicals with an inhibitory concentration (IC50) value of 414.59 ± 4.925 µg/mL. In α-amylase and α-glucosidase inhibitory assays, the efficacy of Sugreen-120 was found in a dose-dependent manner and the IC50 values were found as 220.106 ± 1.375 and 441.44 ± 1.992 µg/mL, respectively. TLC-bioautographic analysis showed that 06 constituents were found active against DPPH free radical, 04 constituents active against α-amylase and 03 constituents active against α-glucosidase. A HPTLC quantitative study revealed the content of caffeic acid and kaempferol to be 5.233 ± 0.026 and 16.959 ± 0.036 µg/mg, respectively. In silico docking analysis showed that out of 5 identified metabolites, myricetin, ellagic acid, and kaempferol were found with significant interaction with α-amylase and α-glucosidase proteins. Hence, it can be concluded that Sugreen-120 exhibits not only an antidiabetic effect but also antioxidant potential. It can be a palliative choice and an alternative that can be used for the treatment of diabetes.

Overview on Strategies and Assays for Antibiotic Discovery

The increase in antibiotic resistance poses a major threat to global health. Actinomycetes, the Gram-positive bacteria of the order Actinomycetales, are fertile producers of bioactive secondary metabolites, including antibiotics. Nearly two-thirds of antibiotics that are used for the treatment of bacterial infections were originally isolated from actinomycetes strains belonging to the genus Streptomyces. This emphasizes the importance of actinomycetes in antibiotic discovery. However, the identification of a new antimicrobial compound and the exploration of its mode of action are very challenging tasks. Therefore, different approaches that enable the "detection" of an antibiotic and the characterization of the mechanisms leading to the biological activity are indispensable. Beyond bioinformatics tools facilitating the identification of biosynthetic gene clusters (BGCs), whole cell-screenings-in which cells are exposed to actinomycete-derived compounds-are a common strategy applied at the very early stage in antibiotic drug development. More recently, target-based approaches have been established. In this case, the drug candidates were tested for interactions with usually validated targets. This review focuses on the bioactivity-based screening methods and provides the readers with an overview on the most relevant assays for the identification of antibiotic activity and investigation of mechanisms of action. Moreover, the article includes examples of the successful application of these methods and suggestions for improvement.

Effect-directed analysis in food by thin-layer chromatography assays

Thin-layer chromatography (TLC) is widely used for food analysis and quality control. As an open chromatographic system, TLC is compatible with microbial-, biochemical-, and chemical-based derivatization methods. This compatibility makes it possible to run in situ bioassays directly on the plate to obtain activity-profile chromatograms, i.e., the effect-directed analysis of the sample. Many of the properties that can be currently measured using this assay format are related to either desired or undesired features for food related products. The TLC assays can detect compounds related to the stability of foods (antioxidant, antimicrobial, antibrowning, etc.), contaminants (antibiotics, pesticides, estrogenic compounds, etc.), and compounds that affect the absorption, metabolism or excretion of nutrients and metabolites or could improve the consumers health (enzyme inhibitors). In this article, different food related TLC-assays are reviewed. The different detection systems used, the way in which they are applied as well as selected examples are discussed.

Antifungal compound from marine Serratia marcescens BKACT and its potential activity against Fusarium sp

Ecofriendly biocontrol agents to control pathogenic fungi are in demand globally. The present study evaluated the antifungal potentials of marine bacteria Serratia marcescens BKACT against eight different Fusarium species. A highest 75.5 ± 0.80% of mycelial inhibition was observed against Fusarium foetens NCIM 1330. Structural characterization of the purified compound was analyzed by GC-MS and NMR techniques; based on the analysis, it is confirmed as 2, 4-di-tert butyl phenol (2, 4-DTBP) with chemical structure C₁₄H₂₂O. At 0.53 mM concentration, purified compound inhibited complete spore germination of F. foetens NCIM 1330. In vitro assay showed complete inhibition of F. foetens NCIM 1330 on the wheat seeds. Tested concentration does not show any toxic effect on germination of the seeds. By this study, we conclude that, 2, 4-DTBP is a suitable candidate to be used as biocontrol agent against Fusarium infection.

An Evolving Technology That Integrates Classical Methods with Continuous Technological Developments: Thin-Layer Chromatography Bioautography

Thin-layer chromatography (TLC) bioautography is an evolving technology that integrates the separation and analysis technology of TLC with biological activity detection technology, which has shown a steep rise in popularity over the past few decades. It connects TLC with convenient, economic and intuitive features and bioautography with high levels of sensitivity and specificity. In this study, we discuss the research progress of TLC bioautography and then establish a definite timeline to introduce it. This review summarizes known TLC bioautography types and practical applications for determining antibacterial, antifungal, antitumor and antioxidant compounds and for inhibiting glucosidase, pancreatic lipase, tyrosinase and cholinesterase activity constitutes. Nowadays, especially during the COVID-19 pandemic, it is important to identify original, natural products with anti-COVID potential compounds from Chinese traditional medicine and natural medicinal plants. We also give an account of detection techniques, including in situ and ex situ techniques; even in situ ion sources represent a major reform. Considering the current technical innovations, we propose that the technology will make more progress in TLC plates with higher separation and detection technology with a more portable and extensive scope of application. We believe this technology will be diffusely applied in medicine, biology, agriculture, animal husbandry, garden forestry, environmental management and other fields in the future.

Rapid screening for natural lipase inhibitors from Alisma orientale combining high-performance thin-layer chromatography-bioautography with mass spectrometry

Lipase inhibitors are an attractive class of hypolipidemic compounds, which inhibit the activity of human pancreatic lipase, thereby preventing the absorption of triglycerides in vivo. As a library of promising lead compounds for drug development, traditional Chinese medicine (TCM) has gained growing attention in quick discovery and identification of enzyme inhibitors of natural-origin. The purpose of this work was to discover unknown lipase inhibitors from Alisma orientale by the activity oriented analysis method thin-layer chromatography-bioautography, then use electrospray ionization mass spectrometry technology via the elution based TLC-MS interface to identify their structures. As a result, eleven natural lipase inhibitors from Alisma orientale extracts were identified based on molecular mass and fragment ions obtained by HPTLC-MS, and further confirmed by a series of complementary means including UV spectra, ¹H NMR characteristic proton signals and polarity of compounds, eleven lipase inhibitors were tentatively assigned as triterpenoids: alisol B (m/z 495.50 [M + Na]⁺), alisol B 23-acetate (m/z 537.58 [M + Na]⁺), 11-deoxy-alisol B (m/z 479.50 [M + Na]⁺), 11-deoxy-alisol B 23-acetate (m/z 521.50 [M + Na]⁺), alisol A/epialisol A (m/z 513.50 [M + Na]⁺), 16-oxo-11-deoxy-alisol A (m/z 511.50 [M + Na]⁺), 16-oxo-alisol A (527.50 [M + Na] ⁺), alisol C (m/z 509.58 [M + Na]⁺), alisol C 23-acetate (m/z 551.50 [M + Na]⁺), alisol M 23-acetate (m/z 567.50 [M + Na]⁺), and alismanol Q/neoalisol (m/z 493.42 [M + Na]⁺). The integrated approach is an efficient method for rapid screening lipase inhibitors from complex plant extracts and provides a reasonable and favorable basis for the identification and separation of other enzymatic system and other important compounds with therapeutic values.

New approaches for antituberculosis leads from Actinobacteria

Bioactive metabolites derived from the phylum Actinobacteria represent many of the existing antimicrobial drugs. Compared with other bacterial pathogens, direct preliminary screening by diffusion assays is a limiting factor against Mycobacterium tuberculosis (Mtb) and different methodologies have been used to improve the search for new molecules. However, the concern remains that most of the previously discovered molecules replicate by conventional procedures. The combination of multidisciplinary approaches with new technologies could advance the discovery of new leads against Mtb like considering the unexplored Actinobacteria jointly with selective and integrative procedures.

Same analytical method for both (bio)assay and zone isolation to identify/quantify bioactive compounds by quantitative nuclear magnetic resonance spectroscopy

Differing sensitivity is the main obstacle for a direct combination of HPTLC with NMR spectroscopy. A sufficient amount of the isolated compound zone must be provided by HPTLC for subsequent offline NMR detection (HPTLC//NMR). To fill the gap, a straightforward procedure was developed using the same analytical HPTLC system for both bioprofiling and isolation of bioactive zones from multicomponent mixtures. The HPTLC-effect-directed analysis (EDA) revealed several bioactive compounds in five botanical extracts, i.e. Salvia officinalis, Thymus vulgaris and Origanum vulgare, all Lamiaceae, and peels of red and green apples (Jonagored and Granny Smith, respectively), both Rosaceae. A tricky case study was designed to show how to deal with potentially coeluting bioactive structural isomers, e.g., ursolic (UA), oleanolic (OA) and betulinic acids (all C30H48O3), which are most difficult to identify and assign. A multipotent bioactive HPTLC zone showed the same hRF value and mass signal in HPTLCHRMS, though containing the coeluting structural isomers UA and OA. After zone isolation from the HPTLC plate, first the 1H NMR spectrum allowed to distinguish distinct allylic H-18 protons, i.e. 2.20 ppm for UA and 2.85 ppm for OA, and at the same time, to quantify the two isomers by using the PUlse Length-based CONcentration methodology (HPTLC//1H qNMR-PULCON). In case of a partial overlap of the diagnostic signal with that of the matrix, results were corroborated with those obtained by using the 1H deconvoluted or 2D 1H-13C Heteronuclear Single Quantum Coherence spectra. The comparison of the quantitative results showed a good correlation (R2 = 0.9718) between the two orthogonal methods HPTLC-Vis and HPTLC//1H qNMR-PULCON. A sufficient zone isolation from the HPTLC plate (mean isolation rate of 82%) for both UA and OA (0.27 - 4.67 mM) was achieved for HPTLC//qNMR, comparing the isolated bioactive compound zone with the respective zone in the botanical extract via HPTLC-Vis densitometry. The HPTLC-EDA-Vis//1H qNMR-PULCON procedure for bioprofiling and quantification/identification/confirmation of bioactive compounds in botanical extracts is considered as straightforward, eco-friendly (only 16 mL solvent required), simple (NMR calibration used over weeks) and reliable new alternative to the status quo of bioactivity-guided fractionation.

The TLC-Bioautography as a Tool for Rapid Enzyme Inhibitors detection - A Review

Microorganisms and plants can be important sources of many compounds with potential pharmaceutical applications. Extraction of these matrices is one of the ways of identifying the presence of inhibitory active substances against enzymes whose high activity leads to serious human diseases including cancer, Parkinson's or Crohn's diseases. The isolation and purification of inhibitors are time-consuming and expensive steps in the analysis of the crude extract and therefore, it is necessary to find a fast, efficient, and inexpensive method for screening extracts of interest. TLC-Bioautography combines the separation of the extract on a thin layer with its subsequent biological analysis. TLC-Bioautography methods have been developed for several classes of enzymes including oxidoreductases, hydrolases and isomerases, and there is a potential for developing functional methods for other classes of enzymes. This review summarizes known TLC-Bioautography methods and their applications for determining the presence of enzyme inhibitors in extracts and compares the effectiveness of different methodological approaches. It also indicates the current state and perspective of the development of TLC-Bioautography and its possible future applications.

TLC Bioautography on Screening of Bioactive Natural Products: An Update Review

Background:

TLC bioautography is a hyphenated technique combining planar chromatographic separation and in situ biological activity detection. This coupled method has been receiving much attention in screening bio-active natural products because of its properties of being simple, rapid, inexpensive, and effective.

Methods:

The recent progress in the development of method of TLC bioautography for detecting antimicrobial and enzyme inhibitory activities dating between 2012 and early 2018 has been reviewed. The applications of this method in biological screening of natural products were also presented.

Results:

Some anaerobic and microaerophilic bacteria and a causative bacterium of tuberculosis have been adopted to TLC direct bioautography. Seven types of enzymes including acetylcholinesterase, glucosidase, lipase, xanthine oxidase, tyrosinase, monoamine oxidase, and dipeptidyl peptidase IV have so far been adopted on TLC bioautography. Its new application in screening antiurolithiatic agents was included.

Conclusion:

The standard experimental procedures are required for TLC antioxidant and antimicrobial assays. Some new enzymes should be attempted and adopted on TLC bioautography. The existing TLC methods for enzyme inhibition need more application studies to assess their screening capacity in the discovery of active compounds. The GC-MS or LC-MS approaches have gradually been coupled to TLC bioautography for fast structural characterization of active compounds.

Quantitative inkjet application on self-printed, binder-free HPTLC layers for submicromole-scaled analytical 1H NMR spectroscopy

High-performance thin-layer chromatography (HPTLC) combined with ¹H nuclear magnetic resonance (NMR) spectroscopy has only been demonstrated over a small spectral range so far. The self-printing of chromatographic plates with a modified 3D slurry printer allowed the operator to influence the composition of the adsorbent slurry and thus the purity of the adsorbent layer. The combination of such self-printed, binder-free HPTLC plates with inkjet-driven sample application made possible submicromole-scaled analytical ¹H NMR spectroscopy. This was proven using pure HPTLC adsorbents. For comparison, commercial silica gel HPTLC plates were purified by pre-development with solvents for spectroscopy, whereas commercial silica gel HPTLC particles were self-printed on the glass plate after purification under solvent pressure in a recycled HPLC cartridge. Evaluating the signals from different treatments, seven background signals disappeared in the proton spectra and three were reduced to a minimum by use of pre-developed commercial HPTLC plates. In the case of the self-printed, binder-free HPTLC plates made of purified adsorbent, most of the spectral background signals were reduced to a minimum, thus these spectra showed the highest cleanness and most pure analyte proton spectra. For the first time, the full ¹H NMR spectroscopy range was made available after an HPTLC separation. This proof of principle opens the avenue for submicromole-scaled analytical ¹H NMR spectroscopy.

Strategies in anti-Mycobacterium tuberculosis drug discovery based on phenotypic screening

The rise of multi- and extensively drug-resistant Mycobacterium tuberculosis (M. tb) strains and co-infection with human immunodeficiency virus has escalated the need for new anti-M. tb drugs. Numerous challenges associated with the M. tb, in particular slow growth and pathogenicity level 3, discouraged use of this organism in past primary screening efforts. From current knowledge of the physiology and drug susceptibility of mycobacteria in general and M. tb specifically, it can be assumed that many potentially useful drug leads were missed by failing to screen directly against this pathogen. This review discusses recent high-throughput phenotypic screening strategies for anti-M. tb drug discovery. Emphasis is placed on prioritization of hits, including their extensive biological and chemical profiling, as well as the development status of promising drug candidates discovered with phenotypic screening.

Residual Complexity Does Impact Organic Chemistry and Drug Discovery: The Case of Rufomyazine and Rufomycin

Residual complexity (RC) involves the impact of subtle but critical structural and biological features on drug lead validation, including unexplained effects related to unidentified impurities. RC commonly plagues drug discovery efforts due to the inherent imperfections of chromatographic separation methods. The new diketopiperazine, rufomyazine (6), and the previously known antibiotic, rufomycin (7), represent a prototypical case of RC that (almost) resulted in the misassignment of biological activity. The case exemplifies that impurities well below the natural abundance of ¹³C (1.1%) can be highly relevant and calls for advanced analytical characterization of drug leads with extended molar dynamic ranges of >1:1,000 using qNMR and LC-MS. Isolated from an actinomycete strain, 6 was originally found to be active against Mycobacterium tuberculosis with a minimum inhibitory concentration (MIC) of 2 μg/mL and high selectivity. As a part of lead validation, the dipeptide was synthesized and surprisingly found to be inactive. The initially observed activity was eventually attributed to a very minor contamination (0.24% [m/m]) with a highly active cyclic peptide (MIC ∼ 0.02 μM), subsequently identified as an analogue of 7. This study illustrates the serious implications RC can exert on organic chemistry and drug discovery, and what efforts are vital to improve lead validation and efficiency, especially in NP-related drug discovery programs.

Sweet spot matching: A thin-layer chromatography-based countercurrent solvent system selection strategy

TLC-based strategies were proposed in 1979 (Hostettmann et al.) and 2005 (Friesen & Pauli; GUESS method) to minimize the number of partitioning experiments required for countercurrent separation (CCS) solvent system selection. As semi-empirical approaches, both proposed that the K values defining the sweet spot of optimal CCS corresponded to a matching Rf value range from the silica gel TLC plate developed in the organic phase of a biphasic or a corresponding monophasic solvent system. Despite their simplicity, there has been an absence of theoretical support and a deficiency of reported experimental evidence. The present study explores the theory required to develop correlations between Rf and K. All theoretical models surmise that the optimal Rf value range should be centered at 0.5. In order to validate the feasibility of the concept of matching Rf and K values, 43 natural products and six solvent system families were investigated. Out of 62 correlations, 45 resulted in matched Rf and K values. Based on this study, practical guidelines for the TLC-based prediction strategy are provided. These approaches will equip CCS users with an updated understanding of how to apply the TLC-based solvent system selection strategy to accelerate a targeted selection of CCS conditions.