Elansolid A, a unique macrolide antibiotic from Chitinophaga sancti isolated as two stable atropisomers

Biocontrol potential of Chitinophaga flava HK235 producing antifungal-related peptide chitinocin

Botrytis cinerea is a necrotrophic fungal pathogen with an extremely broad host range, causing significant economic losses in agricultural production. In this study, we discovered a culture filtrate of bacterial strain HK235, which was identified as Chitinophaga flava, exhibiting high levels of antifungal activity against B. cinerea. From the HK235 culture filtrate, we isolated a new antimicrobial peptide molecule designated as chitinocin based on activity-guided fractionation followed by characterization of the amino acid composition and spectroscopic analyses. The HK235 culture filtrate and chitinocin completely inhibited both conidial germination and mycelial growth of B. cinerea at a concentration of 20% and 200 μg/mL, respectively. In addition to antibiosis against B. cinerea, the active compound chitinocin had a broad antifungal and antibacterial activity in vitro. When tomato plants were treated with the culture filtrate and chitinocin, the treatment strongly reduced the development of gray mold disease in a concentration-dependent manner compared to the untreated control. Here, considering the potent antifungal property in vitro and in vivo, we present the biocontrol potential of C. flava HK235 for the first time.

Nonenzymatic Reactions in Natural Product Formation

Biosynthetic mechanisms of natural products primarily depend on systems of protein catalysts. However, within the field of biosynthesis, there are cases in which the inherent chemical reactivity of metabolic intermediates and substrates evades the involvement of enzymes. These reactions are difficult to characterize based on their reactivity and occlusion within the milieu of the cellular environment. As we continue to build a strong foundation for how microbes and higher organisms produce natural products, therein lies a need for understanding how protein independent or nonenzymatic biosynthetic steps can occur. We have classified such reactions into four categories: intramolecular, multicomponent, tailoring, and light-induced reactions. Intramolecular reactions is one of the most well studied in the context of biomimetic synthesis, consisting of cyclizations and cycloadditions due to the innate reactivity of the intermediates. There are two subclasses that make up multicomponent reactions, one being homologous multicomponent reactions which results in dimeric and pseudodimeric natural products, and the other being heterologous multicomponent reactions, where two or more precursors from independent biosynthetic pathways undergo a variety of reactions to produce the mature natural product. The third type of reaction discussed are tailoring reactions, where postmodifications occur on the natural products after the biosynthetic machinery is completed. The last category consists of light-induced reactions involving ecologically relevant UV light rather than high intensity UV irradiation that is traditionally used in synthetic chemistry. This review will cover recent nonenzymatic biosynthetic mechanisms and include sources for those reviewed previously.

Genomic and Chemical Decryption of the Bacteroidetes Phylum for Its Potential to Biosynthesize Natural Products

With progress in genome sequencing and data sharing, 1,000s of bacterial genomes are publicly available. Genome mining-using bioinformatics tools in terms of biosynthetic gene cluster (BGC) identification, analysis, and rating-has become a key technology to explore the capabilities for natural product (NP) biosynthesis. Comprehensively, analyzing the genetic potential of the phylum Bacteroidetes revealed Chitinophaga as the most talented genus in terms of BGC abundance and diversity. Guided by the computational predictions, we conducted a metabolomics and bioactivity driven NP discovery program on 25 Chitinophaga strains. High numbers of strain-specific metabolite buckets confirmed the upfront predicted biosynthetic potential and revealed a tremendous uncharted chemical space. Mining this data set, we isolated the new iron chelating nonribosomally synthesized cyclic tetradeca- and pentadecalipodepsipeptide antibiotics chitinopeptins with activity against Candida, produced by C. eiseniae DSM 22224 and C. flava KCTC 62435, respectively. IMPORTANCE The development of pipelines for anti-infectives to be applied in plant, animal, and human health management are dried up. However, the resistance development against compounds in use calls for new lead structures. To fill this gap and to enhance the probability of success for the discovery of new bioactive natural products, microbial taxa currently underinvestigated must be mined. This study investigates the potential within the bacterial phylum Bacteroidetes. A combination of omics-technologies revealed taxonomical hot spots for specialized metabolites. Genome- and metabolome-based analyses showed that the phylum covers a new chemical space compared with classic natural product producers. Members of the Bacteroidetes may thus present a promising bioresource for future screening and isolation campaigns.

Asymmetric Total Synthesis of Antibiotic Elansolid A

Elansolid A is a structurally complex polyketide macrolactone natural product that exhibits promising antibacterial properties. Its challenging asymmetric total synthesis was achieved by a convergent strategy, in which the tetrahydroindane core of the molecule and an eastern vinyl iodide moiety were combined as the main fragments. The central tetrahydroindane motif was constructed with high stereoselectivity by a bioinspired intramolecular Diels-Alder cycloaddition, generating four stereogenic centers in a single step. The stereocontrol of this key step could be achieved by virtue of a 1,3-allylic strain generated by the temporary introduction of a steric-directing iodine substituent on the substrate. The formation of the macrolactone motif that completes the synthesis was achieved via two different retrosynthetic disconnections, namely, a Suzuki-Miyaura cross-coupling or an alternative Mukaiyama esterification reaction.

Bioactive natural products from Bacteroidetes

Covering: up to end of January 2022Bacteria representing the phylum Bacteroidetes produce a diverse range of natural products, including polyketides, peptides and lactams. Here, we discuss unique aspects of the bioactive compounds discovered thus far, and the corresponding biosynthetic pathways if known, providing a comprehensive overview of the Bacteroidetes as a natural product reservoir.

Application of Allylzinc Reagents as Nucleophiles in Matteson Homologations

Allylzinc reagents are versatile nucleophiles that can be used in Matteson homologations. The linear substitution products are formed almost exclusively, and excellent E selectivities are observed in reactions of reagents with sterically demanding or aryl substituents on the double bond. The allylated boronic esters obtained can be converted into trifluoroborates or subjected to further homologations. Ozonolysis of the double bond provides aldehydes or ketones, and therefore, allylzinc reagents are useful acetaldehyde or ketone enolate equivalents.

A Leap Forward Towards Unraveling Newer Anti-infective Agents from an Unconventional Source: a Draft Genome Sequence Illuminating the Future Promise of Marine Heterotrophic Bacillus sp. Against Drug-Resistant Pathogens

During the previous decade, genome-built researches on marine heterotrophic microorganisms displayed the chemical heterogeneity of natural product resources coupled with the efficacies of harnessing the genetic divergence in various strains. Herein, we describe the whole genome data of heterotrophic Bacillus amyloliquefaciens MB6 (MTCC 12,716), isolated from a marine macroalga Hypnea valentiae, a 4,107,511-bp circular chromosome comprising 186 contigs, with 4154 protein-coding DNA sequences and a coding ratio of 86%. Simultaneously, bioactivity-guided purification of the bacterial extract resulted in six polyketide classes of compounds with promising antibacterial activity. Draft genome sequence of B. amyloliquefaciens MB6 unveiled biosynthetic gene clusters (BGCs) engaged in the biosynthesis of polyketide-originated macrolactones with prospective antagonistic activity (MIC ≤ 5 µg/mL) against nosocomial pathogens. Genome analysis manifested 34 putative BGCs necessitated to synthesize biologically active polyketide-originated frameworks or their derivatives. These results provide insights into the genetic basis of heterotrophic B. amyloliquefaciens MTCC 12,716 as a prospective lead for biotechnological and pharmaceutical applications.

Total synthesis of apratoxin A and B using Matteson's homologation approach

Apratoxin A and B, two members of an interesting class of marine cyclodepsipeptides are synthesized in a straightforward manner via Matteson homologation. Starting from a chiral boronic ester, the polyketide fragment of the apratoxins was obtained via five successive homologation steps in an overall yield of 27% and very good diastereoselectivity. This approach is highly flexible and should allow modification also of this part of the natural products, while previous modifications have been carried out mainly in the peptide fragment.

Soil microbiome manipulation triggers direct and possible indirect suppression against Ralstonia solanacearum and Fusarium oxysporum

Soil microbiome manipulation can potentially reduce the use of pesticides by improving the ability of soils to resist or recover from pathogen infestation, thus generating natural suppressiveness. We simulated disturbance through soil fumigation and investigated how the subsequent application of bio-organic and organic amendments reshapes the taxonomic and functional potential of the soil microbiome to suppress the pathogens Ralstonia solanacearum and Fusarium oxysporum in tomato monocultures. The use of organic amendment alone generated smaller shifts in bacterial and fungal community composition and no suppressiveness. Fumigation directly decreased F. oxysporum and induced drastic changes in the soil microbiome. This was further converted from a disease conducive to a suppressive soil microbiome due to the application of organic amendment, which affected the way the bacterial and fungal communities were reassembled. These direct and possibly indirect effects resulted in a highly efficient disease control rate, providing a promising strategy for the control of the diseases caused by multiple pathogens.

A Straightforward Synthesis of Polyketides via Ester Dienolate Matteson Homologation

Application of ester dienolates as nucleophiles in Matteson homologations allows for the stereoselective synthesis of highly substituted α,β-unsaturated δ-hydroxy carboxyl acids, structural motifs widespread found in polyketide natural products. The protocol is rather flexible and permits the introduction of substituents and functionalities also at those positions which are not accessible by the commonly used aldol reaction. Therefore, this ester dienolate Matteson approach is an interesting alternative to the "classical" polyketide syntheses.

Oxygenated elansolid-type of polyketide spanned macrolides from a marine heterotrophic Bacillus as prospective antimicrobial agents against multidrug-resistant pathogens

Three homologous oxygenated elansolid-type of polyketide spanned macrolides were isolated from a heterotrophic marine bacterium, Bacillus amyloliquefaciens MTCC 12716, associated with an intertidal red alga Hypnea valentiae. The complete genome of the bacterium was sequenced and all detectable natural product gene clusters were analysed. The B. amyloliquefaciens MTCC 12716 genome features polyketide synthase (pks) systems of every known formally classified family, nonribosomal peptide synthetases and hybrid clusters. Comprehensive spectroscopic studies revealed the compounds to possess isobenzofuranyl benzoate and 1H-furopyrano[2,3-c]oxacyclononadecine-6-carboxylate moieties. The identified compounds displayed broad-spectrum bactericidal activity against methicillin-resistant Staphylococcus aureus, vancomycin-resistant Enterococcus faecalis, and drug-resistant strains of Pseudomonas aeruginosa and Klebsiella pneumoniae with minimum inhibitory concentrations (MICs) of ≤1.0 µg/mL, whereas the standard antibiotics ampicillin and chloramphenicol were active only at concentrations of ≥6.25 µg/mL. The plausible mechanism of elansolid-type macrolide biosynthesis by trans-AT polyketide synthases through the pks starter unit para-hydroxybenzoic acid was hypothesised, and the structures were correlated with the gene organisation, with the predicted gene cluster comprising 16 genes (~81 kb in size). The best binding poses for each compound with the peptide deformylase (PDF) protein of S. aureus revealed docking scores (>11.30 kcal/mol) greater than actinonin (6.96 kcal/mol), a natural PDF inhibitor. The higher electronic values along with optimum lipophilic parameters support the potential anti-infective properties of the studied macrolides. These antibacterial elansolid-type of polyketide spanned macrolides in marine symbiotic B. amyloliquefaciens could be potential leads for biotechnological and pharmaceutical applications against emerging multidrug-resistant pathogens.

Some Biogenetic Considerations Regarding the Marine Natural Product (-)-Mucosin

Recently, the identity of the marine hydrindane natural product (−)-mucosin was revised to the trans-fused structure 6, thereby providing a biogenetic puzzle that remains to be solved. We are now disseminating some of our insights with regard to the possible machinery delivering the established architecture. Aspects with regard to various modes of cyclization in terms of concerted versus stepwise processes are held up against the enzymatic apparatus known to be working on arachidonic acid (8). To provide a contrast to the tentative polyunsaturated fatty acid biogenesis, the structural pattern featured in (−)-mucosin (6) is compared to some marine hydrinane natural products of professed polyketide descent. Our appraisal points to a different origin and strengthens the hypothesis of a polyunsaturated fatty acids (PUFA) as the progenitor of (−)-mucosin (6).

Selecting lipopeptide-producing, Fusarium-suppressing Bacillus spp.: Metabolomic and genomic probing of Bacillus velezensis NWUMFkBS10.5

The results of this study indicate that the maize rhizosphere remains a reservoir for microbial strains with unique beneficial properties. The study sought to provide an indigenous Bacillus strain with a bioprotective potential to alleviate maize fusariosis in South Africa. We selected seven Bacillus isolates (MORWBS1.1, MARBS2.7, VERBS5.5, MOREBS6.3, MOLBS8.5, MOLBS8.6, and NWUMFkBS10.5) with biosuppressive effects against two maize fungal pathogens (Fusarium graminearum and Fusarium culmorum) based on 16S rDNA gene characterization and lipopeptide gene analysis. The PCR analysis revealed that lipopeptide genes encoding the synthesis of iturin, surfactin, and fengycin might be responsible for their antifungal activities. Few of the isolates also showed possible biosurfactant capability, and their susceptibility to known antibiotics is indicative of their eco‐friendly attributes. In addition, in silico genomic analysis of our best isolate (Bacillus velezensis NWUMFkBS10.5) and characterization of its active metabolite with FTIR, NMR, and ESI‐Micro‐Tof MS confirmed the presence of valuable genes clusters and metabolic pathways. The versatile genomic potential of our Bacillus isolate emphasizes the continued relevance of Bacillus spp. in biological management of plant diseases.

Legacy effects of continuous chloropicrin-fumigation for 3-years on soil microbial community composition and metabolic activity

Chloropicrin is widely used to control ginger wilt in China, which have an enormous impact on soil microbial diversity. However, little is known on the possible legacy effects on soil microbial community composition with continuous fumigation over different years. In this report, we used high throughput Illumina sequencing and Biolog ECO microplates to determine the bacterial community and microbial metabolic activity in ginger harvest fields of non-fumigation (NF), chloropicrin-fumigation for 1 year (F_1) and continuous chloropicrin-fumigation for 3 years (F_3). The results showed that microbial richness and diversity in F_3 were the lowest, while the metabolic activity had no significant difference. With the increase of fumigation years, the incidence of bacterial wilt was decreased, the relative abundance of Actinobacteria and Saccharibacteria were gradually increased. Using LEfSe analyses, we found that Saccharibacteria was the most prominent biomarker in F_3. Eight genera associated with antibiotic production in F_3 were screened out, of which seven belonged to Actinobacteria, and one belonged to Bacteroidetes. The study indicated that with the increase of fumigation years, soil antibacterial capacity may be increased (possible reason for reduced the incidence of bacterial wilt), and Saccharibacteria played a potential role in evaluating the biological effects of continuous fumigation.

Optimization of the biotechnological production of a novel class of anti-MRSA antibiotics from Chitinophaga sancti

BACKGROUND

Recently, the discovery of the elansolids, a group of macrolides, was reported. The molecules show activity against methicillin-resistant Staphylococcus aureus as well as other gram-positive organisms. This fact renders those substances a promising starting point for future chemical development. The active atropisomers A1/A2 are formed by macrolactonization of the biosynthesis product A3 but are prone to ring opening and subsequent formation of several unwanted side products. Recently it could be shown that addition of different nucleophiles to culture extracts of Chitinophaga sancti enable the formation of new stable elansolid derivatives. Furthermore, addition of such a nucleophile directly into the culture led exclusively to formation of a single active elansolid derivative. Due to low product yields, methods for production of gram amounts of these molecules have to be established to enable further development of this promising compound class.

RESULTS

Production of elansolid A2 by C. sancti was enabled using a synthetic medium with sucrose as carbon source to a final concentration of 18.9 mg L-1. A fed-batch fermentation was ensued that resulted in an elansolid A2 concentration of 55.3 mg L-1. When using glucose as carbon source in a fed-batch fermentation only 34.4 mg L-1 elansolid A2 but 223.1 mg L-1 elansolid C1 were produced. This finding was not unexpected since elansolids A1/A2 and A3 have been reported to easily react with nucleophiles like anthranilic acid, a precursor of tryptophan biosynthesis. Due to the fact that nucleophiles can be incorporated in vivo, a fed-batch cultivation under identical conditions, with addition of anthranilic acid was carried out and lead to almost exclusive formation of elansolid C1 (257.5 mg L-1).

CONCLUSION

Reproducible elansolid A2 and C1 production is feasible in different synthetic media at relatively high concentrations that will allow further investigation and semi-synthetic optimization. The feeding of anthranilic acid enables the exclusive production of the stable elansolid derivative C1, which reduces product loss by unspecific reactions and eases downstream processing. This derivative shows activity in the same range as the elansolids A1/A2. Hence, the method can possibly serve as a model-process for incorporation of other nucleophiles and biotechnological production of specifically designed molecules.

Antibiotics from Gram-negative bacteria: a comprehensive overview and selected biosynthetic highlights

Covering: up to 2017The overwhelming majority of antibiotics in clinical use originate from Gram-positive Actinobacteria. In recent years, however, Gram-negative bacteria have become increasingly recognised as a rich yet underexplored source of novel antimicrobials, with the potential to combat the looming health threat posed by antibiotic resistance. In this article, we have compiled a comprehensive list of natural products with antimicrobial activity from Gram-negative bacteria, including information on their biosynthetic origin(s) and molecular target(s), where known. We also provide a detailed discussion of several unusual pathways for antibiotic biosynthesis in Gram-negative bacteria, serving to highlight the exceptional biocatalytic repertoire of this group of microorganisms.

Coupled Biosynthesis of Volatiles and Salinosporamide A in Salinispora tropica

Terrestrial bacteria, especially actinomycetes, are known to be prolific producers of volatile compounds. We show here that bacteria from ocean sediments can also release complex bouquets of volatiles. The actinomycete Salinispora tropica produces cyclohexenyl compounds not previously known in nature, such as methyl cyclohex-2-ene-1-carboxylate (9), methyl 2-(cyclohex-2-en-1-yl)acetate (10), methyl (E/Z)-2-(cyclohex-2-en-1-ylidene)acetate (11/12), and related alcohols 8 and 13. These compounds were identified by GC/MS and confirmed by synthesis. In addition, rare spiroacetals, aromatic compounds, short-chain acids and esters, alcohols, and various cyclic compounds were produced by the bacteria. The biosynthesis of the cyclohexenyl compounds is closely coupled to that of cyclohexenylalanine (4), a building block of salinosporamide A, a proteasome inhibitor produced by S. tropica. Analysis of S. tropica strains that harbor knockouts of the salinosporamide biosynthetic genes salX and salD, coupled with feeding experiments, revealed that 3-(cyclohex-2-en-1-yl)-2-oxopropanoic acid (60) and 3-(cyclohex-2-en-1-ylidene)-2-oxopropanoic acid (isomers 61 and 62) are important intermediates in the biosynthesis of salinosporamide A, 4, and 8-13.

Detailed Mechanistic Study of the Non-enzymatic Formation of the Discoipyrrole Family of Natural Products

Discoipyrroles A–D (DPA–DPD) are recently discovered natural products produced by the marine bacterium Bacillus hunanensis that exhibit anticancer properties in vitro. Initial biosynthetic studies demonstrated that DPA is formed in the liquid fermentation medium of B. hunanensis from three secreted metabolites through an unknown but protein-independent mechanism. The increased identification of natural products that depend on non-enzymatic steps creates a significant need to understand how these different reactions can occur. In this work, we utilized ¹⁵N-labeled starting materials and continuous high-sensitivity ¹H–¹⁵N HMBC NMR spectroscopy to resolve scarce reaction intermediates of the non-enzymatic discoipyrrole reaction as they formed in real time. This information guided supplemental experiments using ¹³C- and ¹⁸O-labeled materials to elucidate the details of DPA’s non-enzymatic biosynthesis, which features a highly concerted pyrrole formation and necessary O₂-mediated oxidation. We have illustrated a novel way of using isotopically enhanced two-dimensional NMR spectroscopy to interrogate reaction mechanisms as they occur. In addition, these findings add to our growing knowledge of how multicomponent non-enzymatic reactions can occur through inherently reactive bacterial metabolites.

Discovery, Characterization, and Analogue Synthesis of Bohemamine Dimers Generated by Non-enzymatic Biosynthesis

Dibohemamines A-C (5-7), three new dimeric bohemamine analogues dimerized through a methylene group, were isolated from a marine-derived Streptomyces spinoverrucosus. The structures determined by spectroscopic analysis were confirmed through the semi-synthetic derivatization of monomeric bohemamines and formaldehyde. These reactions, which could occur under mild conditions, together with the detection of formaldehyde in the culture, revealed that this dimerization is a non-enzymatic process. In addition to the unique dimerization of the dibohemamines, dibohemamines B and C were found to have nm cytotoxicity against the non-small cell-lung cancer cell line A549. In view of the potent cytotoxicity of compounds 6 and 7, a small library of bohemamine analogues was generated for biological evaluation by utilizing a series of aryl and alkyl aldehydes.

Biosynthesis of polyketides by trans-AT polyketide synthases

This review discusses the biosynthesis of natural products that are generated bytrans-AT polyketide synthases, a family of catalytically versatile enzymes that represents one of the major group of proteins involved in the production of bioactive polyketides.

Future potential for anti-infectives from bacteria - how to exploit biodiversity and genomic potential

The early stages of antibiotic development include the identification of novel hit compounds. Since actinomycetes and myxobacteria are still the most important natural sources of active metabolites, we provide an overview on these producers and discuss three of the most promising approaches toward finding novel anti-infectives from microorganisms. These are defined as the use of biodiversity to find novel producers, the variation of culture conditions and induction of silent genes, and the exploitation of the genomic potential of producers via "genome mining". Challenges that exist beyond compound discovery are outlined in the last section.

Discoipyrroles A-D: isolation, structure determination, and synthesis of potent migration inhibitors from Bacillus hunanensis

Discoidin domain receptor 2 (DDR2) is a receptor tyrosine kinase involved in a variety of cellular response pathways, including regulation of cell growth, proliferation, and motility. Using a newly developed platform to identify the signaling pathway/molecular target of natural products, we identified a family of alkaloid natural products, discoipyrroles A-D (1-4), from Bacillus hunanensis that inhibit the DDR2 signaling pathway. The structure of 1-4, determined by detailed two-dimensional (2D) NMR methods and confirmed by X-ray crystallographic analysis has an unusual 3H-benzo[d]pyrrolo][1,3]oxazine-3,5-dione core. Discoipyrroles A-D potently inhibit DDR2 dependent migration of BR5 fibroblasts and show selective cytotoxicity to DDR2 mutant lung cancer cell lines (IC50 120-400 nM). Examination of the biosynthesis has led to the conclusion that the discoipyrroles are formed through a nonenzymatic process, leading to a one-pot total synthesis of 1.

Bioinformatic evaluation of the secondary metabolism of antistaphylococcal environmental bacterial isolates

The increasing occurrence of drug-resistant Staphylococcus aureus is exacerbated with a declining rate of antibiotic discovery, particularly those with new mechanisms of action. The decline in antibiotic discovery from traditional sources, such as soil actinobacteria, necessitates examination of lesser studied microbes. Here, we present a strategy to select for organisms that may have a propensity to result in new antistaphylococcal agents by using S. aureus as a bait organism, and selecting organisms that have a natural lytic activity towards it. We have isolated over 80 environmental isolates and typed these organisms using 16S rDNA sequence comparison and deployed bioinformatics to assess the secondary metabolic potential of the isolated antistaphylococcal bacteria using genomic sequences. Bioinformatic analysis highlights the enriched and unique suite of potential antibiotic polyketides and nonribosomal peptides and lantibiotic gene clusters from these organisms. Profiling organic microbial extracts further showed that many of the organisms from the 10 staphylolytic genera secrete agents with antistaphylococcal activity and may serve as new sources for future antistaphylococcal drug discovery.

NHC-Cu-catalyzed protoboration of monosubstituted allenes. Ligand-controlled site selectivity, application to synthesis and mechanism

Two types of NHC-Cu complexes catalyze protoborations of terminal allenes to afford valuable 1,1- or trisubstituted vinylboron species with high site selectivity and stereoselectivity. The scope of the method, application to natural product synthesis, and mechanistic basis for the observed selectivity trends are presented.

Isolation and synthesis of falcitidin, a novel myxobacterial-derived acyltetrapeptide with activity against the malaria target falcipain-2

A 384-well microtitre plate fluorescence cleavage assay was developed to identify inhibitors of the cysteine protease falcipain-2, an important antimalarial drug target. Bioassay-guided isolation of a MeOH extract from a myxobacterium Chitinophaga sp. Y23 isolated from soil collected in Singapore, led to the identification of a new acyltetrapeptide, falcitidin (1), which displayed an IC50 value of 6 μM against falcipain-2. The planar structure of 1 was secured by NMR and MS/MS analysis. Attempts to isolate further material for biological testing were hampered by inconsistent production and by a low yield (<100 μg l(-1)). The absolute configuration of 1 was determined by Marfey's analysis and the structure was confirmed through total synthesis as isovaleric acid-D-His-L-Ile-L-Val-L-Pro-NH2. Falcitidin (1) is the first member of a new class of falcipain-2 inhibitors and, unlike other peptide-based inhibitors, does not contain reactive groups that irreversibly bind to active cysteine sites.

Induction of chirality: experimental evidence of atropisomerism in azapeptides

Methylation of the peptide bond in model azadipeptides leads to the E configuration and hence to atropisomerism due to a restricted rotation around the N-N axis.