A New Type of Glycosidic Linkage: An Open‐Chain Acetal‐Linked N‐Acetylgalactosamine in the Core Part of the Lipopolysaccharides from Proteus Microorganisms

Chemical stability similar to that of a normal glycosidic linkage is displayed by the novel acetal linkage between monosaccharides through the structural element shown. This survives hydrolysis with dilute acetic acid, but is cleaved under the hydrolytic conditions normally used in monosaccharide analysis (2 M HCl)—a possible reason why the structural element has only now been identified.     

Discovery of New Natural Products by Intact‐Cell Mass Spectrometry and LC‐SPE‐NMR: Malbranpyrroles,Novel Polyketides from Thermophilic Fungus Malbranchea sulfurea

Six photosensitive polyketides, malbranpyrroles A–F, were discovered from the thermophilic fungus Malbranchea sulfurea by using intact‐cell desorption/ionization on silicon mass (ICD‐MS) and LC‐SPE‐NMR. These two strategies facilitate the searching and structural determination of unstable natural products. The ICD‐MS indicated that only brown hyphae of M. sulfurea can produce malbranpyrroles. The biosynthetic pathway of malbranpyrroles was evidenced by ¹³C isotope precursors and amino acid feeding experiments. The cytotoxicity data revealed that the conformation of the conjugated system in malbranpyrroles does not affect cytotoxic potency against cancer cell lines. In addition, the chlorine atom was shown to be the pharmacophore for cytotoxicity.     

NMR Fingerprints of the Drug‐like Natural‐Product Space Identify Iotrochotazine A: A Chemical Probe to Study Parkinson’s Disease

The NMR spectrum of a mixture of small molecules is a fingerprint of all of its components. Herein, we present an NMR fingerprint method that takes advantage of the fact that fractions contain simplified NMR profiles, with minimal signal overlap, to allow the identification of unique spectral patterns. The approach is exemplified in the identification of a novel natural product, iotrochotazine A ( 1 ), sourced from an Australian marine sponge Iotrochota sp. Compound 1 was used as a chemical probe in a phenotypic assay panel based on human olfactory neurosphere‐derived cells (hONS) from idiopathic Parkinson’s disease patients. Compound 1 at 1 μM was not cytotoxic but specifically affected the morphology and cellular distribution of lysosomes and early endosomes.     

Probing the Motional Behavior of Eumelanin and Pheomelanin with Solid‐State NMR Spectroscopy: New Insights into the Pigment Properties

Melanin is the most widespread pigment in the animal kingdom. Despite its importance, its detailed structure and overall molecular architecture remain elusive. Both eumelanin (black) and pheomelanin (red) occur in the human body. These two melanin compounds show very different responses to UV‐radiation exposure, which could relate to their microscopic features. Herein, the structural properties and motional behavior of natural eu‐ and pheomelanin extracted from black and red human hair are investigated by means of solid‐state NMR spectroscopy. Several 1D and 2D NMR spectroscopic techniques were combined to highlight the differences between the two forms of the pigment. The quantitative analysis of the ¹H NMR wide‐line spectra extracted from 2D ¹H–¹³C LG‐WISE experiments revealed the presence of two dynamically distinguishable components in both forms. Remarkably, the more mobile fraction of the pigment showed a higher mobility with respect to the proteinaceous components that coexist in the melanosome, which is particularly evident for the red pigment. An explanation of the observed effects takes into account the different architecture of the proteinaceous matrix that constitutes the physical substrate onto which melanin polymerizes within the eu‐ and pheomelanosomes. Further insight into the molecular structure of the more mobile fraction of pheomelanin was also obtained by means of the analysis of 2D ¹H–¹³C INEPT experiments. Our view is that not only structural features inherent in the pure pigment, but also the role of the matrix structure in defining the overall melanin supramolecular arrangement and the resulting dynamic behavior of the two melanin compounds should be taken into account to explain their functions. The reported results could pave a new way toward the explanation of the molecular origin of the differences in the photoprotection activity displayed by black and red melanin pigments.     

Forazoline A: Marine‐Derived Polyketide with Antifungal In Vivo Efficacy

Forazoline A, a novel antifungal polyketide with in vivo efficacy against Candida albicans, was discovered using LCMS‐based metabolomics to investigate marine‐invertebrate‐associated bacteria. Forazoline A had a highly unusual and unprecedented skeleton. Acquisition of ¹³C–¹³C gCOSY and ¹³C–¹⁵N HMQC NMR data provided the direct carbon–carbon and carbon–nitrogen connectivity, respectively. This approach represents the first example of determining direct ¹³C–¹⁵N connectivity for a natural product. Using yeast chemical genomics, we propose that forazoline A operated through a new mechanism of action with a phenotypic outcome of disrupting membrane integrity.     

A pH‐Responsive Carrier System that Generates NO Bubbles to Trigger Drug Release and Reverse P‐Glycoprotein‐Mediated Multidrug Resistance

Multidrug resistance (MDR) resulting from the overexpression of drug transporters such as P‐glycoprotein (Pgp) increases the efflux of drugs and thereby limits the effectiveness of chemotherapy. To address this issue, this work develops an injectable hollow microsphere (HM) system that carries the anticancer agent irinotecan (CPT‐11) and a NO‐releasing donor (NONOate). Upon injection of this system into acidic tumor tissue, environmental protons infiltrate the shell of the HMs and react with their encapsulated NONOate to form NO bubbles that trigger localized drug release and serve as a Pgp‐mediated MDR reversal agent. The site‐specific drug release and the NO‐reduced Pgp‐mediated transport can cause the intracellular accumulation of the drug at a concentration that exceeds the cell‐killing threshold, eventually inducing its antitumor activity. These results reveal that this pH‐responsive HM carrier system provides a potentially effective method for treating cancers that develop MDR.     

Mannose‐Binding Geometry of Pradimicin A

Pradimicins (PRMs) and benanomicins are the only family of non‐peptidic natural products with lectin‐like properties, that is, they recognize D ‐mannopyranoside (Man) in the presence of Ca²⁺ ions. Coupled with their unique Man binding ability, they exhibit antifungal and anti‐HIV activities through binding to Man‐containing glycans of pathogens. Notwithstanding the great potential of PRMs as the lectin mimics and therapeutic leads, their molecular basis of Man recognition has yet to be established. Their aggregate‐forming propensity has impeded conventional interaction analysis in solution, and the analytical difficulty is exacerbated by the existence of two Man binding sites in PRMs. In this work, we investigated the geometry of the primary Man binding of PRM‐A, an original member of PRMs, by the recently developed analytical strategy using the solid aggregate composed of the 1:1 complex of PRM‐A and Man. Evaluation of intermolecular distances by solid‐state NMR spectroscopy revealed that the C2–C4 region of Man is in close contact with the primary binding site of PRM‐A, while the C1 and C6 positions of Man are relatively distant. The binding geometry was further validated by co‐precipitation experiments using deoxy‐Man derivatives, leading to the proposal that PRM‐A binds not only to terminal Man residues at the non‐reducing end of glycans, but also to internal 6‐substituted Man residues. The present study provides new insights into the molecular basis of Man recognition and glycan specificity of PRM‐A.     

Determination of the Absolute Configuration of Super‐Carbon‐Chain Compounds by a Combined Chemical,Spectroscopic, and Computational Approach: Gibbosols A and B

Marine dinoflagellates produce remarkable organic molecules, particularly those with polyoxygenated long‐carbon‐chain backbones, namely super‐carbon‐chain compounds (SCCCs), characterized by the presence of numerous stereogenic carbon centers on acyclic polyol carbon chains. Even today, it is a challenge to determine the absolute configurations of these compounds. In this work, the planar structures and absolute configurations of two highly flexible SCCCs, featuring either a C₆₉‐ or C₇₁‐linear carbon backbone, gibbosols A and B, respectively, each containing thirty‐seven stereogenic carbon centers, were unambiguously established by a combined chemical, spectroscopic, and computational approach. The discovery of gibbosols A and B with two hydrophilic acyclic polyol chains represents an unprecedented class of SCCCs. A reasonable convergent strategy for the biosynthesis of these SCCCs was proposed.     

Lessons To Be Learned: The Molecular Basis of Kinase‐Targeted Therapies and Drug Resistance in Non‐Small Cell Lung Cancer

The treatment of non‐small cell lung cancer (NSCLC) is currently experiencing a revolution. Over the last decade, the knowledge gained about the biochemical features of biomarkers and their predictive abilities has led to the development of targeted small‐molecule inhibitors that present an alternative to harsh chemotherapy. The use of these new therapies has improved the quality of life and increased the survival of patients. The occurrence of inevitable drug resistance requires the constant development of precision medicine. The detailed understanding of the target biology and the search for innovative chemical approaches has encouraged investigations in this field. Herein, we review selected aspects of the molecular targets and present an overview of current topics and challenges in the rational development of small molecules to target NSCLC.     

A New Approach to Sesquiterpene Arenes of the 9,11‐Drimenyl Type (= [(1E,2RS,4aRS,8aRS)‐Octahydro‐2,5,5,8a‐tetramethylnaphthalen‐1(2H)‐ylidene] methyl Type)

A new reaction sequence for the synthesis of the sesquiterpene arenes (±)‐wiedendiol B ((±)‐ 1 ) and the siphonodictyal B derivative (±)‐ 21 consists in the coupling of (±)‐drimanoyl chloride ((±)‐ 3 ) with lithiated and appropriately substituted aromatic synthons to furnish the ketones (±)‐ 7 and (±)‐ 17 which were reduced to the benzyl alcohols (±)‐ 8a,b and (±)‐ 18a,b , respectively (Schemes 5, 4, and 12). The 9,11‐double bond of the drimenes (±)‐ 9 and (±)‐ 19 was formed by elimination of H₂O from the benzyl alcohols (±)‐ 8a,b and (±)‐ 18a,b (Schemes 6 and 12). New alternatives were applied to this elimination reaction involving either the pyridine ? SO₃ complex or chloral as reagents.     

Total Synthesis and Conformational Study of Callyaerin A: Anti‐Tubercular Cyclic Peptide Bearing a Rare Rigidifying (Z)‐2,3‐ Diaminoacrylamide Moiety

The first synthesis of the anti‐TB cyclic peptide callyaerin A ( 1 ), containing a rare (Z)‐2,3‐diaminoacrylamide bridging motif, is reported. Fmoc‐formylglycine‐diethylacetal was used as a masked equivalent of formylglycine in the synthesis of the linear precursor to 1 . Intramolecular cyclization between the formylglycine residue and the N‐terminal amine in the linear peptide precursor afforded the macrocyclic natural product 1 . Synthetic 1 possessed potent anti‐TB activity (MIC₁₀₀=32 μm ) while its all‐amide congener was inactive. Variable‐temperature NMR studies of both the natural product and its all‐amide analogue revealed the extraordinary rigidity imposed by this diaminoacrylamide unit on peptide conformation. The work reported herein pinpoints the intrinsic role that the (Z)‐2,3‐diaminoacrylamide moiety confers on peptide bioactivity.     

Molecular Recognition of Rosmarinic Acid from Salvia sclareoides Extracts by Acetylcholinesterase: A New Binding Site Detected by NMR Spectroscopy

Acetylcholinesterase (AChE) inhibition is one of the most currently available therapies for the management of Alzheimer’s disease (AD) symptoms. In this context, NMR spectroscopy binding studies were accomplished to explain the inhibition of AChE activity by Salvia sclareoides extracts. HPLC‐MS analyses of the acetone, butanol and water extracts eluted with methanol and acidified water showed that rosmarinic acid is present in all the studied samples and is a major constituent of butanol and water extracts. Moreover, luteolin 4′‐O‐glucoside, luteolin 3′,7‐di‐O‐glucoside and luteolin 7‐O‐(6′′‐O‐acetylglucoside) were identified by MS² and MS³ data acquired during the LC‐MSⁿ runs. Quantification of rosmarinic acid by HPLC with diode‐array detection (DAD) showed that the butanol extract is the richest one in this component (134 μg mg^?1 extract). Saturation transfer difference (STD) NMR spectroscopy binding experiments of S. sclareoides crude extracts in the presence of AChE in buffer solution determined rosmarinic acid as the only explicit binder for AChE. Furthermore, the binding epitope and the AChE‐bound conformation of rosmarinic acid were further elucidated by STD and transferred NOE effect (trNOESY) experiments. As a control, NMR spectroscopy binding experiments were also carried out with pure rosmarinic acid, thus confirming the specific interaction and inhibition of this compound against AChE. The binding site of AChE for rosmarinic acid was also investigated by STD‐based competition binding experiments using Donepezil, a drug currently used to treat AD, as a reference. These competition experiments demonstrated that rosmarinic acid does not compete with Donepezil for the same binding site. A 3D model of the molecular complex has been proposed. Therefore, the combination of the NMR spectroscopy based data with molecular modelling has permitted us to detect a new binding site in AChE, which could be used for future drug development.     

A Novel Type of Luciferin from the Siberian Luminous Earthworm Fridericia heliota: Structure Elucidation by Spectral Studies and Total Synthesis

The structure elucidation and synthesis of the luciferin from the recently discovered luminous earthworm Fridericia heliota is reported. This luciferin is a key component of a novel ATP‐dependent bioluminescence system. UV, fluorescence, NMR, and HRMS spectroscopy studies were performed on 0.005 mg of the isolated substance and revealed four isomeric structures that conform to spectral data. These isomers were chemically synthesized and one of them was found to produce light when reacted with a protein extract from F. heliota. The novel luciferin was found to have an unusual extensively modified peptidic nature, thus implying an unprecedented mechanism of action.