Structure elucidation and absolute configuration of phomopsin a, a hexapeptide mycotoxin produced by phomopsis leptostromiformis

Molecularly imprinted polymer coupled to UHPLC-MS/MS for the analysis of phomopsins in lupin samples

The demand for plant-based protein sources in the food industry has significantly increased in recent years, leading to the introduction of legume-based products as meat substitutes. However, concerns regarding food quality have emerged, particularly related to the presence of mycotoxins. This study addresses the need for the sensitive detection of phomopsins (PHOs), a class of peptide-based toxins. A selective extraction method using molecularly imprinted polymer (MIP) coupled with ultra-high performance liquid chromatography and tandem mass spectrometry (UHPLC-MS/MS) was focused on the most toxic Phomopsin A (PHO-A). A rapid ultrasonochemical synthesis of MIP (5 min) was proposed and its performance was optimized in response to various factors, including the choice of dummy template and the selection of the monomer. The methacrylic acid-vinyl pyridine (MAA-VP) MIP exhibited high selectivity and affinity for PHO-A. The method was tested in lupin samples and the validation, according to SANTE/11312/2021 international guidelines, gave excellent recovery (80-90 %), low matrix effects, and high accuracy and precision. Real samples analysis confirmed the presence of PHO-A in artificially fungal inoculated lupins, with levels ranging from 0.377 to 0.576 mg kg-1. In order to identify further PHOs, a semi-untargeted approach using multiple reaction monitoring-information dependent acquisition-enhanced product ion (MRM-IDA-EPI) was developed. PHO-B, PHO-D, PHO-E and PHO-P, rarely previously reported in lupin matrix, were tentatively identified. This study accounts for the effectiveness of MIP-based extraction coupled with UHPLC-triple quadrupole with linear ionic trap-MS/MS (UHPLC-QqQ-LIT-MS/MS) for quantification of PHO-A and putative detection of other PHOs, offering a promising method for investigating this class of toxins in food.

Proline Analogues

Within the canonical repertoire of the amino acid involved in protein biogenesis, proline plays a unique role as an amino acid presenting a modified backbone rather than a side-chain. Chemical structures that mimic proline but introduce changes into its specific molecular features are defined as proline analogues. This review article summarizes the existing chemical, physicochemical, and biochemical knowledge about this peculiar family of structures. We group proline analogues from the following compounds: substituted prolines, unsaturated and fused structures, ring size homologues, heterocyclic, e.g., pseudoproline, and bridged proline-resembling structures. We overview (1) the occurrence of proline analogues in nature and their chemical synthesis, (2) physicochemical properties including ring conformation and cis/trans amide isomerization, (3) use in commercial drugs such as nirmatrelvir recently approved against COVID-19, (4) peptide and protein synthesis involving proline analogues, (5) specific opportunities created in peptide engineering, and (6) cases of protein engineering with the analogues. The review aims to provide a summary to anyone interested in using proline analogues in systems ranging from specific biochemical setups to complex biological systems.

Characterization of the Biosynthetic Gene Cluster for the Ribosomally Synthesized Cyclic Peptide Epichloëcyclins in Epichloë festucae

The various grass-induced epichloëcyclins of the Epichloë spp. are ribosomally synthesized and post-translationally modified peptides (RiPPs), produced as small, secreted cyclopeptides from a single gene, gigA. Here, four clustered and coregulated genes (gigA, gigB, gigC, and kexB) with predicted roles in epichloëcyclin production in Epichloë festucae were evaluated through gene disruption. Subsequent chemical analysis indicates that GigB is a DUF3328 domain-containing protein associated with cyclization of epichloëcyclins; GigC is a methyltransferase enzyme responsible for N-methylation of desmethylepichloëcyclins; and KexB is a subtilisin-like enzyme, partly responsible for the propeptide cleavage of epichloëcyclin intermediates. Symbiotic effects on the host phenotype were not observed for gigA, gigC, or kexB mutants, although ΔgigB infection correlated with increased host tiller height and biomass, while only ΔkexB exhibited an effect on endophyte morphology. Disrupting epichloëcyclin biosynthesis showed negligible influence on the biosynthesis of E. festucae-associated alkaloids. Epichloëcyclins may perform other secondary metabolism functions in Epichloë and other fungi.

Endophytic Diaporthe as Promising Leads for the Development of Biopesticides and Biofertilizers for a Sustainable Agriculture

Plant pathogens are responsible for causing economic and production losses in several crops worldwide, thus reducing the quality and quantity of agricultural supplies. To reduce the usage of chemically synthesized pesticides, strategies and approaches using microorganisms are being used in plant disease management. Most of the studies concerning plant-growth promotion and biological agents to control plant diseases are mainly focused on bacteria. In addition, a great portion of registered and commercialized biopesticides are bacterial-based products. Despite fungal endophytes having been identified as promising candidates for their use in biological control, it is of the utmost importance to develop and improve the existing knowledge on this research field. The genus Diaporthe, encompasses plant pathogens, saprobes and endophytes that have been screened for secondary metabolite, mainly due to their production of polyketides and a variety of unique bioactive metabolites with agronomic importance. Some of these metabolites exhibit antifungal and antibacterial activity for controlling plant pathogens, and phytotoxic activity for the development of potential mycoherbicides. Moreover, species of Diaporthe are reported as promising agents in the development of biofertilizers. For this reason, in this review we summarize the potential of Diaporthe species to produce natural products with application in agriculture and describe the benefits of these fungi to promote their host plant's growth.

Out for a RiPP: challenges and advances in genome mining of ribosomal peptides from fungi

Covering up to June 2021Ribosomally synthesized and post-translationally modified peptides (RiPPs) from fungi are an underexplored class of natural products, despite their propensity for diverse bioactivities and unique structural features. Surveys of fungal genomes for biosynthetic gene clusters encoding RiPPs have been limited in their scope due to our incomplete understanding of fungal RiPP biosynthesis. Through recent discoveries, along with earlier research, a clearer picture has been emerging of the biosynthetic principles that underpin fungal RiPP pathways. In this Highlight, we trace the approaches that have been used for discovering currently known fungal RiPPs and show that all of them can be assigned to one of three distinct families based on hallmarks of their biosynthesis, which are in turn imprinted on their corresponding gene clusters. We hope that our systematic exposition of fungal RiPP structural and gene cluster features will facilitate more comprehensive approaches to genome mining efforts in the future.

Discovery and biosynthesis of cyclic plant peptides via autocatalytic cyclases

Many bioactive plant cyclic peptides form side-chain-derived macrocycles. Lyciumins, cyclic plant peptides with tryptophan macrocyclizations, are ribosomal peptides (RiPPs) originating from repetitive core peptide motifs in precursor peptides with plant-specific BURP (BNM2, USP, RD22 and PG1beta) domains, but the biosynthetic mechanism for their formation has remained unknown. Here, we characterize precursor-peptide BURP domains as copper-dependent autocatalytic peptide cyclases and use a combination of tandem mass spectrometry-based metabolomics and plant genomics to systematically discover five BURP-domain-derived plant RiPP classes, with mono- and bicyclic structures formed via tryptophans and tyrosines, from botanical collections. As BURP-domain cyclases are scaffold-generating enzymes in plant specialized metabolism that are physically connected to their substrates in the same polypeptide, we introduce a bioinformatic method to mine plant genomes for precursor-peptide-encoding genes by detection of repetitive substrate domains and known core peptide features. Our study sets the stage for chemical, biosynthetic and biological exploration of plant RiPP natural products from BURP-domain cyclases.

Biosynthetic Studies of Phomopsins Unveil Posttranslational Installation of Dehydroamino Acids by UstYa Family Proteins

UstYa family proteins (DUF3328) are widely and specifically distributed in fungi. They are known to be involved in the biosynthesis of ribosomally synthesized and posttranslationally modified peptides (RiPPs) and nonribosomal peptides, and possibly catalyze various reactions, including oxidative cyclization and chlorination. In this study, we focused on phomopsin A, a fungal RiPP consisting of unique nonproteinogenic amino acids. Gene knockout experiments demonstrated that three UstYa homologues, phomYc, phomYd, and phomYe, are essential for the desaturation of amino acid moieties, showing unprecedented function among UstYa family proteins. Sequence similarity network analysis indicated that their amino acid sequences are highly diverged and that most remain uncharacterized, paving the way for genome mining of fungal metabolites with unique modifications.

Shipworm symbiosis ecology-guided discovery of an antibiotic that kills colistin-resistant Acinetobacter

Teredinibacter turnerae is an intracellular bacterial symbiont in the gills of wood-eating shipworms, where it is proposed to use antibiotics to defend itself and its animal host. Several biosynthetic gene clusters are conserved in T. turnerae and their host shipworms around the world, implying that they encode defensive compounds. Here, we describe turnercyclamycins, lipopeptide antibiotics encoded in the genomes of all sequenced T. turnerae strains. Turnercyclamycins are bactericidal against challenging Gram-negative pathogens, including colistin-resistant Acinetobacter baumannii. Phenotypic screening identified the outer membrane as the likely target. Turnercyclamycins and colistin operate by similar cellular, although not necessarily molecular, mechanisms, but turnercyclamycins kill colistin-resistant A. baumannii, potentially filling an urgent clinical need. Thus, by exploring environments that select for the properties we require, we harvested the fruits of evolution to discover compounds with potential to target unmet health needs. Investigating the symbionts of shipworms is a powerful example of this principle.

Phytotoxic Secondary Metabolites from Fungi

Fungal phytotoxic secondary metabolites are poisonous substances to plants produced by fungi through naturally occurring biochemical reactions. These metabolites exhibit a high level of diversity in their properties, such as structures, phytotoxic activities, and modes of toxicity. They are mainly isolated from phytopathogenic fungal species in the genera of Alternaria, Botrytis, Colletotrichum, Fusarium, Helminthosporium, and Phoma. Phytotoxins are either host specific or non-host specific phytotoxins. Up to now, at least 545 fungal phytotoxic secondary metabolites, including 207 polyketides, 46 phenols and phenolic acids, 135 terpenoids, 146 nitrogen-containing metabolites, and 11 others, have been reported. Among them, aromatic polyketides and sesquiterpenoids are the main phytotoxic compounds. This review summarizes their chemical structures, sources, and phytotoxic activities. We also discuss their phytotoxic mechanisms and structure-activity relationships to lay the foundation for the future development and application of these promising metabolites as herbicides.

The high-resolution X-ray structure of vinca-domain inhibitors of microtubules provides a rational approach for drug design

Tubulin vinca-domain ligands can inhibit microtubule polymerization, causing cell death in mitosis, and their potential against multiple cancer types has been demonstrated. However, due to drug resistance and toxicities, development of novel vinca-domain ligands is still needed. In this study, we determined the high-resolution crystal structures of vinorelbine, YXD, and Phomopsin A in complex with tubulin at 2.5 Å. Additionally, we recapitulated all previously published high-resolution crystal structures of the vinca binding site to reveal critical residues and the molecular mechanism of vinca-domain ligands interacting with tubulin. Furthermore, we designed putatively novel triazolopyrimidine derivatives by introducing secondary amine groups to establish salt-bridge and H-bond interactions with Asp179^β1 and Asn329^α2 . Our studies provided the structural basis for designing novel tubulin vinca-domain ligands.

Recent advances in the biosynthesis of RiPPs from multicore-containing precursor peptides

Ribosomally synthesized and post-translationally modified peptides (RiPPs) compose a large structurally and functionally diverse family of natural products. The biosynthesis system of RiPPs typically involves a precursor peptide comprising of a leader and core motif and nearby processing enzymes that recognize the leader and act on the core for producing modified peptides. Interest in RiPPs has increased substantially in recent years as improvements in genome mining techniques have dramatically improved access to these peptides and biochemical and engineering studies have supported their applications. A less understood, intriguing feature in the RiPPs biosynthesis is the precursor peptides of multiple RiPPs families produced by bacteria, fungi and plants carrying multiple core motifs, which we term "multicore". Herein, we present the prevalence of the multicore systems, their biosynthesis and engineering for applications.

Dehydration reactions in polyfunctional natural products

Here, we review methods for chemical dehydration of alcohols to alkenes and discuss the potential of late-stage functionalization by direct, site- and chemo-selective dehydration of complex molecular substrates.

Impact of Dehydroamino Acids on the Structure and Stability of Incipient 310-Helical Peptides

A comparative study of the impact of small, medium-sized, and bulky α,β-dehydroamino acids (ΔAAs) on the structure and stability of Balaram's incipient 3₁₀-helical peptide (1) is reported. Replacement of the N-terminal Aib residue of 1 with a ΔAA afforded peptides 2a-c that maintained the 3₁₀-helical shape of 1. In contrast, installation of a ΔAA in place of Aib-3 yielded peptides 3a-c that preferred a β-sheet-like conformation. The impact of the ΔAA on peptide structure was independent of size, with small (ΔAla), medium-sized (Z-ΔAbu), and bulky (ΔVal) ΔAAs exerting similar effects. The proteolytic stabilities of 1 and its analogs were determined by incubation with Pronase. Z-ΔAbu and ΔVal increased the resistance of peptides to proteolysis when incorporated at the 3-position and had negligible impact on stability when placed at the 1-position, whereas ΔAla-containing peptides degraded rapidly regardless of position. Exposure of peptides 2a-c and 3a-c to the reactive thiol cysteamine revealed that ΔAla-containing peptides underwent conjugate addition at room temperature, while Z-ΔAbu- and ΔVal-containing peptides were inert even at elevated temperatures. These results suggest that both bulky and more accessible medium-sized ΔAAs should be valuable tools for bestowing rigidity and proteolytic stability on bioactive peptides.

Structural Diversity and Biological Activities of Fungal Cyclic Peptides, Excluding Cyclodipeptides

Cyclic peptides are cyclic compounds formed mainly by the amide bonds between either proteinogenic or non-proteinogenic amino acids. This review highlights the occurrence, structures and biological activities of fungal cyclic peptides (excluding cyclodipeptides, and peptides containing ester bonds in the core ring) reported until August 2017. About 293 cyclic peptides belonging to the groups of cyclic tri-, tetra-, penta-, hexa-, hepta-, octa-, nona-, deca-, undeca-, dodeca-, tetradeca-, and octadecapeptides as well as cyclic peptides containing ether bonds in the core ring have been isolated from fungi. They were mainly isolated from the genera Aspergillus, Penicillium, Fusarium, Acremonium and Amanita. Some of them were screened to have antimicrobial, antiviral, cytotoxic, phytotoxic, insecticidal, nematicidal, immunosuppressive and enzyme-inhibitory activities to show their potential applications. Some fungal cyclic peptides such as the echinocandins, pneumocandins and cyclosporin A have been developed as pharmaceuticals.

Bulky Dehydroamino Acids Enhance Proteolytic Stability and Folding in β-Hairpin Peptides

The bulky dehydroamino acids dehydrovaline (ΔVal) and dehydroethylnorvaline (ΔEnv) can be inserted into the turn regions of β-hairpin peptides without altering their secondary structures. These residues increase proteolytic stability, with ΔVal at the (i + 1) position having the most substantial impact. Additionally, a bulky dehydroamino acid can be paired with a d-amino acid (i.e., d-Pro) to synergistically enhance resistance to proteolysis. A link between proteolytic stability and peptide structure is established by the finding that a stabilized ΔVal-containing β-hairpin is more highly folded than its Asn-containing congener.

Detection of a Toxic Methylated Derivative of Phomopsin A Produced by the Legume-Infesting Fungus Diaporthe toxica

Phomopsin A (PHO-A), produced by the fungus Diaporthe toxica, is a mycotoxin known to be responsible for fatal liver disease of lupin-fed sheep. The full spectrum of the toxic secondary metabolites produced by D. toxica is still unknown. PHO-A and the naturally occurring derivatives B-E have been subject to several studies to reveal their structures as well as chemical and toxicological properties. In this work, a methylated derivative (1) of PHO-A isolated from lupin seeds inoculated with D. toxica is described. It was characterized by high-resolution mass and NMR data and shown to be the N-methylated derivative of PHO-A. 1 is cytotoxic against HepG2 cells.

The stereoselective construction of E- and Z-Δ-Ile from E-dehydroamino acid ester: the synthesis of the phomopsin A tripeptide side chain

The synthesis of the phomopsin A tripeptide side chain was achieved by the stereoselective construction of E-Δ-Ile and E-Δ-Asp using α-(diphenylphosphono)glycine.

Development of a SIDA-LC-MS/MS Method for the Determination of Phomopsin A in Legumes

A novel method for the determination of phomopsin A (1) in lupin flour, pea flour, and bean flour as well as whole lupin plants was established based on stable isotope dilution assay (SIDA) LC-MS/MS using (15)N6-1 as an isotopically labeled internal standard. Artificially infected samples were used to develop an optimized extraction procedure and sample pretreatment. The limits of detection were 0.5-1 μg/kg for all matrices. The limits of quantitation were 2-4 μg/kg. The method was used to analyze flour samples generated from selected legume seeds and lupin plant samples that had been inoculated with Diaporthe toxica and two further fungal strains. Finally, growing lupin plants infected with D. toxica were investigated to simulate a naturally in-field mycotoxicosis. Toxin levels of up to 10.1 μg/kg of 1 were found in the pods and 7.2 μg/kg in the stems and leaves.

Development and validation of an LC-MS/MS method for the detection of phomopsin A in lupin and lupin-containing retail food samples from the Netherlands

Phomopsins (PHO) are mycotoxins produced by the fungus Diaporthe toxica (also referred to as Phomopsis leptostromiformis). Lupin is the most important host crop for this fungus and PHO are suspected as cause of lupinosis, a deadly liver disease, in sheep. Lupin is currently in use to replace genetically modified soy in many food products available on the European market. However, a validated method for analysis of PHO is not available until now. In this work, a dilute-and-shoot LC-MS/MS-based method was developed for the quantitative determination and identification of phomopsin A (PHO-A) in lupin and lupin-containing food. The method involved extraction by a mixture of acetonitrile/water/acetic acid (80/20/1 v/v), dilution of the sample in water, and direct injection of the crude extract after centrifugation. The method was validated at 5 and 25 µg PHO-A kg(-1) product. The average recovery and RSD obtained were 79% and 9%, respectively. The LOQ (the lowest level for which adequate recovery and RSD were demonstrated) was 5 µg PHO-A kg(-1). Identification of PHO-A was based on retention time and two transitions (789 > 226 and 789 > 323). Using the average of solvent standards from the sequence as a reference, retention times were all within ± 0.03 min and ion ratios were within ± 12%, which is compliant with European Union requirements. The LOD (S/N = 3 for the least sensitive transition) was 1 µg PHO-A kg(-1) product. Forty-two samples of lupin and lupin-containing food products were collected in 2011-2012 from grocery stores and internet shops in the Netherlands and analysed. In none of the samples was PHO-A detected.

Production of 3-nitropropionic acid by endophytic fungus Phomopsis longicolla isolated from Trichilia elegans A. JUSS ssp. elegans and evaluation of biological activity

The compound 3-nitropropionic acid is a potent neurotoxic agent in animals and well-known as a potent inhibitor of Mycobacterium tuberculosis. In this research, we were able to extract this compound from the endophytic fungus, Phomopsis longicolla (FJ62759), isolated from Trichilia elegans A. JUSS ssp. elegans. The aim of this study was the isolation of secondary metabolites produced by P. longicolla, the chemical identification of these compounds and evaluation of their antimicrobial and insecticidal activity. To accomplish these goals, the fungus was cultured in BD broth for 25 days without agitation at 28 °C, and then the broth was separated from the mycelium. The supernatant was partitioned with dichloromethane (CH2Cl2), ethyl acetate (EtOAc), and butanol (BuOH) solvents resulting in 3 extracts. However, only the EtOAc extract was used for fractionation and chemical identification because it had the greatest mass. After common chromatographic procedures, the fractions were analyzed by nuclear magnetic resonance to elucidate the chemical components. This procedure resulted in the identification of 3-nitropropionic acid in the D fraction. Evaluation of the insecticidal and antimicrobial activity of this compound has been accomplished, and the results indicate good inhibition of the citrus pathogen Guignardia citricarpa and cocoa pathogen Moniliophthora perniciosa and slight inhibition of the human bacterial pathogens Micrococcus luteus, Salmonella typhi and slight inhibition of phytopathogenic bacteria Xanthomonas axonopodis pv. phaseoli. The evaluation of insecticide activity did not show mortality of the Diatraea saccharalis larvae by the metabolite 3-nitropropionic acid in the D fraction. The results suggest that P. longicolla is a bioactive metabolic producing endophytic fungus with biotechnological properties.

Phomentrioloxin: A phytotoxic pentasubstituted geranylcyclohexentriol produced by Phomopsis sp., a potential mycoherbicide for Carthamus lanatus Biocontrol

A new phytotoxic geranylcyclohexenetriol, named phomentrioloxin, was isolated from the liquid culture of Phomopsis sp., a fungal pathogen proposed for the biological control of Carthamus lanatus, a widespread and troublesome thistle weed belonging to the Asteraceae family causing severe crop and pastures losses in Australia. The structure of phomentrioloxin was established by spectroscopic, X-ray, and chemical methods as (1S,2S,3S,4S)-3-methoxy-6-(7-methyl-3-methylene-oct-6-en-1-ynyl)cyclohex-5-ene-1,2,4-triol. At a concentration of 6.85 mM, the toxin causes the appearance of necrotic spots when applied to leaves of both host and nonhost plants. It also causes growth and chlorophyll content reduction of fronds of Lemna minor and inhibition of tomato rootlet elongation. Finally, in preliminary bioassays, phomentrioloxin did not show any antibacterial, fungicidal, or zootoxic activities.

Phomopsins: an overview of phytopathological and chemical aspects, toxicity, analysis and occurrence

Phomopsis leptostromiformis, and its teleomorph Diaporthe toxica, is a lupin pathogen that causes stem blight in young lupins and, as a saprophyte, has been detected on dead lupine material. Under favourable conditions, the fungus produces phomopsins (PHOs), a family of macrocyclic hexapeptide mycotoxins capable of binding tubulin through the tripeptide side chain. The toxic effects appear largely confined to the liver. In particular, the ingestion of PHO contaminated lupin stubble has been linked to lupinosis, a debilitating disease of sheep (the most sensitive animal) characterised by disorientation, blindness, lethargy, and eventually death. The chemical structures of PHO A, B and D have been identified. Analytical methods to determine PHOs are mainly enzyme-linked immunosorbent assays or chromatographic separations, in combination with ultraviolet and mass spectrometric detection. The data about the PHOs occurrence are limited to Australia, restricted to lupin seed. Only one survey has been carried out on lupin seeds and flours from the Swiss market. Not many strategies have been developed to limit lupin seed contamination. Efforts devoted to control lupinosis in Australia focused on the development of cultivars resistant to Phomopsis infection. There are few examples in literature of decontamination or detoxification of PHOs; moreover, they have been shown to be resistant to extensive processing, including cooking. Australia and New Zealand are the only countries that included PHOs in their mycotoxin regulations, with a limit of 5 µg/kg in lupin seeds and derived products. Phomopsins are poorly studied mycotoxins and risk assessment on PHOs has not been done at the European level. The collection of all available scientific data was requested by EFSA in a specific project and partners involved considered it of general interest preparing this review to highlight the limited available information, which indicate that the assessment of potential risk related to PHOs is currently not feasible.

The conformational properties of dehydrobutyrine and dehydrovaline: theoretical and solid-state conformational studies

Dehydrobutyrine is the most naturally occurring dehydroamino acid. It is also the simplest dehydroamino acid having the geometrical isomers E/Z. To investigate its conformational properties, a theoretical analysis was performed on N-acetyl-alpha,beta-dehydrobutyrine N'-methylamides, Ac-(E)-DeltaAbu-NHMe and Ac-(Z)-DeltaAbu-NHMe, as well as the dehydrovaline derivative Ac-DeltaVal-NHMe. The phi, psi potential energy surfaces and the localised conformers were calculated at the B3LYP/6-311 + + G(d,p) level of theory both in vacuo and with inclusion of the solvent (chloroform, water) effect (SCRF method). The X-ray crystal structures of Ac-(Z)-DeltaAbu-NHMe and Ac-DeltaVal-NHMe were determined at 85 and 100 K, respectively. The solid-state conformational preferences for the studied residues have been analysed and compared with the other related structures. Despite the limitations imposed by the C(alpha) = C(beta) double bond on the topography of the side chains, the main chains of the studied dehydroamino acids are more flexible than in standard alanine. The studied dehydroamino acids differ in their conformational preferences, which depend on the polarity of the environment. This might be a reason why the nature quite precisely differentiates between DeltaVal and each of the DeltaAbu isomers, and why, particularly so with the latter, they are used as a conformational tool to influence the biological action of usually small, cyclic dehydropeptides.

Development and validation of a liquid chromatography/atmospheric pressure photoionization-tandem mass spectrometric method for the analysis of mycotoxins subjected to commission regulation (EC) No. 1881/2006 in cereals

A sensitive and reliable liquid chromatography/photoionization (APPI) tandem mass spectrometry method has been developed for determining nine selected mycotoxins in wheat and maize samples. The analytes were chosen on the basis of the mycotoxins under EU Commission Regulation (EC) No. 1881/2006, i.e., deoxynivalenol (DON), zearalenone (ZON), aflatoxins (AFs), and ochratoxin A (OTA), and considering the possibility of a near future regulation for T-2 and HT-2 toxins. Mycotoxins were extracted from samples by means of an one-step solvent extraction without any cleanup. The developed multi-mycotoxin method permits simultaneous, simple, and rapid determination of several co-existing toxins separated in a single chromatographic run, in which AFs, T-2 and HT-2 toxin are acquired in positive, while OTA, DON and ZON in negative mode. Although a moderate signal suppression was noticeable, matrix effect did not give significant differences at p=0.05. Then, calibration in standard solution were used for quantitation. Based on the EU Commission Decision 2002/657/EC, the method was in-house validated in terms of ruggedness, specificity, linearity, trueness, within-laboratory reproducibility, decision limit (CCalpha) and detection capability (CCbeta). For all the analytes, the regression coefficient r ranged between 0.8752 (DON in wheat) and 0.9465 (ZON in maize), biases related to mean concentrations were from -13% to +12% of the nominal spiking level, and the overall within-laboratory reproducibility ranged 3-16%; finally, CCalpha values did not differ more than 20% and CCbeta not more than 42% from their respective maximum limit. Method quantification limits ranged from 1/20 (AFG1) to 1/4 (AFG2 and OTA) the maximum limit established by European Union in the Commission Regulation (EC) No. 1881/2006 and its subsequent amendments.

Real and perceived risks for mycotoxin contamination in foods and feeds: challenges for food safety control

Mycotoxins are toxic compounds, produced by the secondary metabolism of toxigenic moulds in the Aspergillus, Alternaria, Claviceps, Fusarium, Penicillium and Stachybotrys genera occurring in food and feed commodities both pre- and post-harvest. Adverse human health effects from the consumption of mycotoxins have occurred for many centuries. When ingested, mycotoxins may cause a mycotoxicosis which can result in an acute or chronic disease episode. Chronic conditions have a much greater impact, numerically, on human health in general, and induce diverse and powerful toxic effects in test systems: some are carcinogenic, mutagenic, teratogenic, estrogenic, hemorrhagic, immunotoxic, nephrotoxic, hepatotoxic, dermotoxic and neurotoxic. Although mycotoxin contamination of agricultural products still occurs in the developed world, the application of modern agricultural practices and the presence of a legislatively regulated food processing and marketing system have greatly reduced mycotoxin exposure in these populations. However, in developing countries, where climatic and crop storage conditions are frequently conducive to fungal growth and mycotoxin production, much of the population relies on subsistence farming or on unregulated local markets. Therefore both producers and governmental control authorities are directing their efforts toward the implementation of a correct and reliable evaluation of the real status of contamination of a lot of food commodity and, consequently, of the impact of mycotoxins on human and animal health.

Evolution of the total syntheses of ustiloxin natural products and their analogues

Ustiloxins A-F are antimitotic heterodetic cyclopeptides containing a 13-membered cyclic core structure with a synthetically challenging chiral tertiary alkyl-aryl ether linkage. The first total synthesis of ustiloxin D was achieved in 31 linear steps using an S(N)Ar reaction. An NOE study of this synthetic product showed that ustiloxin D existed as a single atropisomer. Subsequently, highly concise and convergent syntheses of ustiloxins D and F were developed by utilizing a newly discovered ethynyl aziridine ring-opening reaction in a longest linear sequence of 15 steps. The approach was further optimized to achieve a better macrolactamization strategy. Ustiloxins D, F, and eight analogues (14-MeO-ustiloxin D, four analogues with different amino acid residues at the C-6 position, and three (9R,10S)-epi-ustiloxin analogues) were prepared via the second-generation route. Evaluation of these compounds as inhibitors of tubulin polymerization demonstrated that variation at the C-6 position is tolerated to a certain extent. In contrast, the S configuration of the C-9 methylamino group and a free phenolic hydroxyl group are essential for inhibition of tubulin polymerization.

Total synthesis and biological evaluation of ustiloxin natural products and two analogs

Synthetic investigations of ustiloxin natural products are described. The first total synthesis of ustiloxin F was completed in 15 steps via ethynyl aziridine ring-opening by a phenol derivative. The results of biological tests of synthetic ustiloxins D and F, and two analogs, O-Me-ustiloxin D and 6-Ile-ustiloxin, demonstrated that the free hydroxyl group ortho to the ether linkage is critical for activity and variations at the Val/Ala site produce changes in the biological activity suggesting the need for further perturbations at this site to more extensively study the tubulin binding.

α-Vinylic Amino Acids: Occurrence, Asymmetric Synthesis and Biochemical Mechanisms

This report presents an overview of the family of naturally occurring 'vinylic' amino acids, namely those that feature a C-C double bond directly attached to the α-carbon, along the side chain. Strategies that have been brought to bear on the stereocontrolled synthesis of these olefinic amino acids are surveyed. The mechanistic diversity by which such 'vinylic triggers' can be actuated in a PLP (pyridoxal phosphate) enzyme active site is then highlighted by discussions of vinylglycine (VG), its substituted congeners, particularly AVG [4E-(2'-aminoethoxy)vinylglycine], and a naturally occurring VG-progenitor, SMM (S-methylmethionine).

Peptide mimotopes of phomopsins: identification, characterization and application in an immunoassay

Peptide mimotopes of plant-associated toxins offer the potential for improving analytical and diagnostic methodologies as well as providing candidates for potential protective vaccines against plant poisoning diseases. Monoclonal antibody (mAb) C3C11, which recognizes the antimicrotubule phomopsin mycotoxins, was used to isolate peptide mimics of phomopsin A from a random 15-mer phage display peptide library. A total of 46 clones were isolated that showed specific reactivity with the mAb. Amino acid sequence analysis revealed four different types of mimotope sequences, all of which contained a common motif V-A-L/V-C. Of the 46 clones isolated, 44 contained the motif V-A-L-C while 2 contained the V-A-V-C motif. All four types of phage clones inhibited the reactivity of the mAb with phomopsin A in a competition ELISA. The clone with the mimotope sequence CTVALCNMYFGAKLD demonstrated the strongest binding. It was further shown that synthetic peptides containing these mimotope amino acid sequences were able to inhibit the mAb-phomopsin A interaction, indicating that the peptide mimotopes were responsible for the specific binding, independent of the phage framework. The results also suggest that the mimotope peptides bind to mAb C3C11 at the same site as phomopsin A. The application of recombinant phage particles carrying phomopsin mimotopes in immunoassay was evaluated and the results demonstrated approximately 100-fold increase in sensitivity in comparison with a conventional immunoassay using a chemically linked phomopsin-horseradish peroxidase conjugate.

Conformational investigation of alpha,beta-dehydropeptides. IX. N-Acetyl-(E)-alpha,beta-methylamide: stereoelectronic properties from infrared and theoretical studies

The Fourier transform infrared spectra of Ac-(E)-deltaAbu-NHMe were analyzed to determine the predominant solution conformation(s) of this (E)-alpha,beta-dehydropeptide-related compound and the electron density perturbation in its amide groups. The measurements were performed in dichloromethane and acetonitrile in the region of mode vs (N-H), amide I, amide II and vs (C(alpha)=Cbeta). The equilibrium geometrical parameters, calculated by a method based on the density functional theory with the B3LYP functional and the 6-31G* basis set, were used to support spectroscopic interpretation and gain some deeper insight into the molecule. The experimental and theoretical data were compared with those of three previously described molecules: isomeric Ac-(Z)-deltaAbu-NHMe, Ac-deltaAla-NHMe, which is deprived of any beta-substituent, and saturated species Ac-Abu-NHMe. The titled compound assumes two conformational states in equilibrium in the DCM solution. One conformer is extended almost fully and like Ac-deltaAla-NHMe is C5 hydrogen-bonded. The other adopts a warped C5 structure similar to that of Ac-(Z)-deltaAbu-NHMe. The C5 hydrogen bond, unlike the H-bond in Ac-deltaAla-NHMe, is disrupted by acetonitrile. The resonance within the N-terminal amide groups in either of the (E)-deltaAbu conformers is not as well developed as the resonance in Ac-Abu-NHMe. However, these N-terminal groups, compared with the other unsaturated compounds, constitute better resonance systems in each conformationally related couple: the C5 hydrogen-bonded Ac-(E)-deltaAbu-NHMe/Ac-deltaAla-NHMe and the warped C5 Ac-(E)-deltaAbu-NHMe/Ac-(Z)-deltaAbu-NHMe. The resonance within the C-terminal groups of the latter couple apparently is similar, but less developed than the resonance in Ac-Abu-NHMe. The electron distribution within the C-terminal group of the hydrogen-bonded C5 (E)-deltaAbu conformer apparently is determined mainly by the electron influx from the C(alpha)=Cbeta double bond.

Mycotoxins, general view, chemistry and structure

Mycotoxins induce diverse and powerful biological effects in test systems; some are carcinogenic, mutagenic, teratogenic, estrogenic, hemorrhagic, immunotoxic, nephrotoxic, hepatotoxic, dermotoxic, and neurotoxic. Mycotoxins have been unambiguously linked to the etiology of several diseases in animals. The discovery of aflatoxins in the early 1960s led to the resurgence of interest in human mycotoxicoses; mycotoxins are now recognized as causal factors of primary liver cancer, ergotism and alimentary toxic aleukia. The fumonisins and ochratoxins are suspected of playing a role in the etiology of esophageal cancer and Balkan endemic nephrotoxicity, respectively.

Conformational investigation of alpha, beta-dehydropeptides. Part VI. Molecular and crystal structure of benzyloxycarbonylglycyl-(Z)-dehydrophenylalanine

The structure of a peptide containing C-terminal dehydrophenylalanine, Z-Gly-(Z)-delta Phe (C19H18N2O5, MW = 354) was determined from single-crystal X-ray diffraction data. Needle-shaped crystals were grown from a 1:1 mixture of methanol-acetone in the monoclinic space group P2(1) with a = 14.717(4), b = 4.941(2), c = 12.073(4) A, beta = 103.72(4) degrees; V = 852.86(8) A3, Z = 2 and Dc = 1.32 g cm-3. The structure was solved by direct methods using SHELXS-86 and refined to a final R-index of 0.032 for 1714 observed reflections. The peptide adopts a conformation folded at the glycine residue, and principal torsion angles are omega 0 = -167.6(2) degrees, phi 1 = -71.8(3) degrees, psi 1 = -31.6(4) degrees, omega 1 = -165.7(3) degrees, phi 2 = 65.6(4) degrees, psi 1(2) = -174.4(3) degrees and psi 2(2) = 5.2(4) degrees. Two intermolecular hydrogen bonds, N1-H...O0' and O2-H...O1', join the folded molecules into columns and link columns to each other, respectively. FTIR spectroscopy shows the presence of three hydrogen bonds. This third one has been interpreted as an intramolecular hydrogen bond of the N2-H...N1 type.

X-ray crystallography of peptides: the contributions of the Italian laboratories

The review article summarizes the most relevant solid state structural and conformational results obtained in the laboratories involved in Italy in the studies of synthetic and natural peptides by x-ray diffraction analyses. Some of the topics will include research studies carried out in other European countries, whereas in other cases studies carried out in Italy will be included in other review articles included in this volume. The review deals with peptides containing symmetrically achiral and unsymmetrically chiral C alpha,alpha-dialkylated glycine residues, peptides containing beta-alanine residues, alpha,beta-dehydroamino acid residues, and aminosuccinyl residues, peptides containing the thioamide surrogate, heterochiral peptides and several bioactive peptides systems with the proposed relationships between function and structure.