Light- and singlet oxygen-mediated antifungal activity of phenylphenalenone phytoalexins

Singlet oxygen signalling and its potential roles in plant biotic interactions

Singlet oxygen (SO) is among the most potent reactive oxygen species, and readily oxidizes proteins, lipids and DNA. It can be generated at the plant surface by phototoxins in the epidermis, acting as a direct defense against pathogens and herbivores (including humans). SO can also accumulate within mitochondria, peroxisomes, cytosol and the nucleus through multiple enzymatic and nonenzymatic processes. However, the majority of research on intracellular SO generation in plants has focused on transfer of light energy to triplet oxygen by photopigments from the chloroplast. SO accumulates in response to diverse stresses that perturb chloroplast metabolism, and while its high reactivity limits diffusion distances, it participates in retrograde signalling through the EXECUTER1 sensor, generation of carotenoid metabolites and possibly other unknown pathways. SO thereby reprogrammes nuclear gene expression and modulates hormone signalling and programmed cell death. While SO signalling has long been known to regulate plant responses to high-light stress, recent literature also suggests a role in plant interactions with insects, bacteria and fungi. The goals of this review are to provide a brief overview of SO, summarize evidence for its involvement in biotic stress responses and discuss future directions for the study of SO in defense signalling.

Overview of methods and considerations for the photodynamic inactivation of microorganisms for agricultural applications

Antimicrobial resistance in agriculture is a global concern and carries huge financial consequences. Despite that, practical solutions for growers that are sustainable, low cost and environmentally friendly have been sparse. This has created opportunities for the agrochemical industry to develop pesticides with novel modes of action. Recently the use of photodynamic inactivation (PDI), classically used in cancer treatments, has been explored in agriculture as an alternative to traditional chemistries, mainly as a promising new approach for the eradication of pesticide resistant strains. However, applications in the field pose unique challenges and call for new methods of evaluation to adequately address issues specific to PDI applications in plants and challenges faced in the field. The aim of this review is to summarize in vitro, ex vivo, and in vivo/in planta experimental strategies and methods used to test and evaluate photodynamic agents as photo-responsive pesticides for applications in agriculture. The review highlights some of the strategies that have been explored to overcome challenges in the field.

Photoantimicrobials in agriculture

Classical approaches for controlling plant pathogens may be impaired by the development of pathogen resistance to chemical pesticides and by limited availability of effective antimicrobial agents. Recent increases in consumer awareness of and/or legislation regarding environmental and human health, and the urgent need to improve food security, are driving increased demand for safer antimicrobial strategies. Therefore, there is a need for a step change in the approaches used for controlling pre- and post-harvest diseases and foodborne human pathogens. The use of light-activated antimicrobial substances for the so-called antimicrobial photodynamic treatment is known to be effective not only in a clinical context, but also for use in agriculture to control plant-pathogenic fungi and bacteria, and to eliminate foodborne human pathogens from seeds, sprouted seeds, fruits, and vegetables. Here, we take a holistic approach to review and re-evaluate recent findings on: (i) the ecology of naturally-occurring photoantimicrobials, (ii) photodynamic processes including the light-activated antimicrobial activities of some plant metabolites, and (iii) fungus-induced photosensitization of plants. The inhibitory mechanisms of both natural and synthetic light-activated substances, known as photosensitizers, are discussed in the contexts of microbial stress biology and agricultural biotechnology. Their modes-of-antimicrobial action make them neither stressors nor toxins/toxicants (with specific modes of poisonous activity), but a hybrid/combination of both. We highlight the use of photoantimicrobials for the control of plant-pathogenic fungi and quantify their potential contribution to global food security.

Protoporphyrin IX-Chitosan Oligosaccharide Conjugate with Potent Antifungal Photodynamic Activity

A new chemical conjugate between protoporphyrin IX (PPIX) and chitosan oligosaccharides (CH) was prepared and evaluated in vitro as an antifungal agent against Penicillium digitatum. Chemical characterization and photophysical/photochemical studies were conducted. The antifungal effect of the CH-PPIX conjugate was compared to its components (PPIX and CH) and a physical mixture of both, under dark and illuminated conditions. The CH-PPIX conjugate was photostable and inhibited fungal growth with 100% efficiency at a dose of 0.005% w/v under visible light irradiation, while no antifungal activity was observed in the dark. Under the same conditions, CH and PPIX did not display any fungicidal activity, demonstrating the improved properties of the conjugate. Insights into the mechanism of fungal inactivation revealed an efficient spore uptake and photoinduced membrane damage through singlet oxygen generation. This new bioconjugate, which is based on natural components, represents a promising agent for fungicidal formulations based on antimicrobial photodynamic therapy.

Synthesis and Photophysical Characterizations of Pyrroloquinolone Photosensitizers for Singlet Oxygen Production

A series of pyrroloquinolone photosensitizers bearing different halogen substituents (Cl, Br, I) on the heterocyclic framework was studied. These structures were readily prepared through a multi-step synthetic sequence involving an oxidative protocol as an important step to access the quinolone framework. Spectroscopic characterizations and computational investigations were carried out to study the dyes before and after the oxidative step. Interestingly, the fluorescence emission was significantly reduced upon oxidation. In spite of a low photostability under UV light, the pyrroloquinolone photosensitizers proved effective to produce singlet oxygen. Higher singlet oxygen quantum yields were obtained with photosensitizers bearing halogen atoms with a higher atomic number.

Environmental photochemistry on plants: recent advances and new opportunities for interdisciplinary research

Plants play a central role in the photochemistry of chemicals in the environment. They represent a major atmospheric source of volatile organic compounds (VOCs) but also an important environmental surface for the deposition and photochemical reactions of pesticides, gaseous and particulate pollutants. In this review, we point out the role of plant leaves in these processes, as a support affecting the reactions physically and chemically and as a partner through the release of natural constituents (water, secondary metabolites). We discuss the influence of the chosen support (leaves, needle surfaces or fruit cuticles, extracted cuticular waxes and model surfaces) and other factors (additives, pesticides mixture, and secondary metabolites) on the photochemical degradation kinetics and mechanisms. We also show how plants can be a source of photochemically reactive species which can act as photosensitizers promoting the photodegradation of pesticides or the formation and aging of secondary organic aerosols (SOA) and secondary organic materials (SOM). Understanding the fate of chemicals on plants is a research area located at the interface between photochemistry, analytical chemistry, atmospheric chemistry, microbiology and vegetal physiology. Pluridisciplinary approaches are needed to deeply understand these complex phenomena in a comprehensive way. To overcome this challenge, we summarize future research directions which have been clearly overlooked until now.

Phytochemical Profiling and Biological Testing of the Constituents of the Australian Plant Haemodorum brevisepalum

Phytochemical profiling was undertaken on the crude extracts of the bulbs, stems, and the fruits of Haemodorum brevisepalum, to determine the nature of the chemical constituents present. This represents the first study to investigate the fruits of a species of Haemodorum. In total, 13 new and 17 previously reported compounds were isolated and identified. The new compounds were of the phenylphenalenone-type class, with a representative of a novel structural form, named tentatively "oxabenzochromenone" (1), a compound akin to an intermediate in a recently proposed phenylphenalenone metabolic network (2), seven new phenylphenalenones (4-10), four new phenylbenzoisochromenones (11-14), and a new phenylbenzoisochromenone derivative (18). The previously reported compounds identified were of the following structure classes: oxabenzochrysenone (3, 23-26), flavonol (15, 16), phenylbenzoisochromenone (17, 21, 22, 27-30), and phenylphenalenone (19, 20). Compounds 2-4, 6-9, 15-18, 21, 22, and 26 were subjected to antimicrobial evaluation with moderate activity observed against Staphylococcus aureus MRSA and slight activity against Pseudomonas aeruginosa and Candida albicans. Compounds 4, 6-9, 17, and 21 were also evaluated for anthelminthic activity against larvae of the blood-feeding parasitic nematode Haemonchus contortus.

A Green-Absorbing, Red-Fluorescent Phenalenone-Based Photosensitizer as a Theranostic Agent for Photodynamic Therapy

Phenalenone is a synthetically accessible, highly efficient photosensitizer with a near-unity singlet oxygen quantum yield. Unfortunately, its UV absorption and lack of fluorescence has made it unsuitable for fluorescence-guided photodynamic therapy against cancer. In this work, we synthesized a series of phenalenone derivatives containing electron-donating groups to red-shift the absorption spectrum and bromine(s) to permit good singlet oxygen production via the heavy-atom effect. Of the derivatives synthesized, the phenalenone containing an amine at the 6-position with bromines at the 2- and 5-positions (OE19) exhibited the longest absorption wavelength (i.e., green) and produced both singlet oxygen and red fluorescence efficiently. OE19 induced photocytotoxicity with nanomolar potency in 2D cultured PANC-1 cancer cells as well as light-induced destruction of PANC-1 spheroids with minimal dark toxicity. Overall, OE19 opens up the possibility of employing phenalenone-based photosensitizers as theranostic agents for photodynamic cancer therapy.

Perinaphthenone and derivatives as control agents of phytopathogenic fungi: fungitoxicity and metabolism

Metabolism and in vitro fungitoxicity of perinaphthenone against three economically important fungi of the citrus, Botryodiplodia spp., Botrytis spp. and Fusarium spp. were investigated. Perinaphthenone exhibited significant antifungal activity at 62.5 μM and above. Even, the inhibitory effect against Fusarium spp. was significantly enhanced by exposure to direct light. In addition, the metabolism of perinaphthenone by the three fungi was studied. Results show that perinaphthenone was transformed almost completely during the first 24 h; two major products, whose concentration increased progressively during the twelve days of the test, were isolated and identified as 2,3-dihydro-1H-phenalen-1-ol and 2,3-dihydro-phenalen-1-one. Although both metabolic products displayed a considerable fungistatic effect, their slightly lower activities in comparison to perinaphthenone indicate that the transformation was a detoxification process. Studies on the relationship between the effect of some substituents in the perinaphthenone core and the mycelial growth inhibition of Botryodiplodia spp. were also carried out. Results show that the α, β-unsaturated carbonyl system is an important structural requirement but not the only to be necessary for the strong antifungal activity of perinaphthenone. In general, the antifungal properties of perinaphthenone may be modulated through the incorporation of substituents in the naphthalene core or in the α, β-unsaturated carbonyl system. It is concluded that perinaphthenone could be used as an antifungal agent or as a structural template for the development of new fungicide compounds.

Novel Chitosan-Riboflavin Conjugate with Visible Light-Enhanced Antifungal Properties against Penicillium digitatum

A novel chemical conjugate between chitosan (CH) and riboflavin (RF) has been synthesized and characterized via Fourier transform infrared, NMR, and other spectroscopic methods. Photophysical and photochemical properties such as absorption spectra, fluorescence emission, fluorescence anisotropy, and singlet oxygen generation were characterized as well. This new biopolymer-based conjugate was designed to have an antifungal effect enhanced through antimicrobial photodynamic therapy. The antifungal effect of this conjugate (CH-RF) was compared with CH and RF against Penicillium digitatum in vitro. The conjugate showed the highest fungal growth inhibition of all systems tested at a dose of 0.5% w/v. This new biopolymer-based compound could be a promising alternative to fungicides used in citrus fruits postharvest.

In vitro activity of 1H-phenalen-1-one derivatives against Leishmania spp. and evidence of programmed cell death

Background

The in vitro activity against Leishmania spp. of a novel group of compounds, phenalenone derivatives, is described in this study. Previous studies have shown that some phenalenones present leishmanicidal activity, and induce a decrease in the mitochondrial membrane potential in L. amazonensis parasites, so in order to elucidate the evidence of programmed cell death occurring inside the promastigote stage, different assays were performed in two different species of Leishmania.

Methods

We focused on the determination of the programmed cell death evidence by detecting the characteristic features of the apoptosis-like process, such as phosphatidylserine exposure, mitochondrial membrane potential, and chromatin condensation among others.

Results

The results showed that four molecules activated the apoptosis-like process in the parasite. All the signals observed were indicative of the death process that the parasites were undergoing.

Conclusions

The present results highlight the potential use of phenalenone derivatives against Leishmania species and further studies should be undertaken to establish them as novel leishmanicidal therapeutic agents.

The photoactivity of natural products - An overlooked potential of phytomedicines?

BACKGROUND

Photoactivity, though known for centuries, is only recently shifting back into focus as a treatment option against cancer and microbial infections. The external factor light is the ingenious key-component of this therapy: Since light activates the drug locally, a high level of selectivity is reached and side effects are avoided. The first reported photoactive medicines were plant extracts. Synthetic entities (so-called photosensitizers PSs), however, paved the route towards the clinical approval of the so-called photodynamic therapy (PDT), and thus natural PSs took a backseat in the past.

HYPOTHESIS

Many isolated bioactive phytochemicals hold a hidden photoactive potential, which is overlooked due to the reduced common awareness of photoactivity.

METHODS

A systematic review of reported natural PSs and their supposed medicinal application was conducted by employing PubMed, Scifinder, and Web of Science. The identified photoactive natural products were compiled including information about their natural sources, their photoyield, and their pharmacological application. Furthermore, the common chemical scaffolds of natural PS are shown to enable the reader to recognize potentially overlooked natural PSs.

RESULTS

The literature review revealed over 100 natural PS, excluding porphyrins. The PSs were classified according to their scaffold. Thereby it was shown that some PS-scaffolds were analyzed in a detailed way, while other classes were only scarcely investigated, which leaves space for future discoveries. In addition, the literature revealed that many PSs are phytoalexins, thus the selection of the starting material significantly matters in order to find new PSs.

CONCLUSION

Photoactive principles are ubiquitous and can be found in various plant extracts. With the increasing availability of light-irradiation setups for the identification of photoactive natural products, we anticipate the discovery of many new natural PSs in the near future. With the accumulation of chemically diverse PSs, PDT itself might finally reach its clinical breakthrough as a promising alternative treatment against multi-resistant microbes and cancer types.

A convenient workflow to spot photosensitizers revealed photo-activity in basidiomycetes

Photodynamic therapy (PDT) is an alternative approach for the treatment of neoplastic diseases employing photosensitizers activated by light. In order to discover new natural photosensitizers, a convenient workflow was established. To validate the workflow, fungi were selected, because we hypothesized that fruiting bodies and mycelia are an overlooked source. The results proved the hypothesis, as exorbitant high photo-cytotoxicity values were detected. For example, the acetone extract of Cortinarius croceus was characterized by an EC50, 9.3 J cm-2 of 1 μg mL-1 against cells of a lung cancer cell-line (A549). In sum, a low-cost workflow for the detection and biological evaluation of photosensitizers is presented and discussed. Furthermore, this paper provides the first experimental evidence for phototoxic metabolites in basidiomycetes. This hints towards a new assignable function of fungal pigments, i.e. photochemical defense.

Distribution, biosynthesis, and biological activity of phenylphenalenone-type compounds derived from the family of plants, Haemodoraceae

This review provides a summary of the current state of research concerning the unique specialised metabolites from Haemodoraceae.

Photodynamic inactivation as an emergent strategy against foodborne pathogenic bacteria in planktonic and sessile states

Foodborne microbial diseases are still considered a growing public health problem worldwide despite the global continuous efforts to ensure food safety. The traditional chemical and thermal-based procedures applied for microbial growth control in the food industry can change the food matrix and lead to antimicrobial resistance. Moreover, currently applied disinfectants have limited efficiency against biofilms. Therefore, antimicrobial photodynamic therapy (aPDT) has become a novel alternative for controlling foodborne pathogenic bacteria in both planktonic and sessile states. The use of aPDT in the food sector is attractive as it is less likely to cause antimicrobial resistance and it does not promote undesirable nutritional and sensory changes in the food matrix. In this review, aspects on the antimicrobial photodynamic technology applied against foodborne pathogenic bacteria and studied in recent years are presented. The application of photodynamic inactivation as an antibiofilm strategy is also reviewed.

Triphenylphosphonium cation: A valuable functional group for antimicrobial photodynamic therapy

Light-mediated killing of pathogens by cationic photosensitisers is a promising antimicrobial approach that avoids the development of resistance inherent to the use of antimicrobials. In this study, we demonstrate that modification of different photosensitisers with the triphenylphosphonium cation yields derivatives with excellent photoantimicrobial activity against Gram-positive bacteria (ie, Staphylococcus aureus and Enterococcus faecalis). Thus, the triphenylphosphonium functional group should be considered for the development of photoantimicrobials for the selective killing of Gram-positive bacteria in the presence of Gram-negative species.

Phenalenone-photodynamic therapy induces apoptosis on human tumor cells mediated by caspase-8 and p38-MAPK activation

Photodynamic therapy (PDT) is a rising and hopeful treatment for solid tumors and others malignancies. PDT uses harmless visible light to activate a tumor-associated photosensitizer (PS). The excited PS generates cytotoxic reactive oxygen species (ROS) that induce damage and death of tumor cells. It is known that certain phytoalexins and phytoanticipins derived from plants often display a PS-like activity due to a phenalenone (PN) moiety-an efficient singlet oxygen photosensitizer-in its skeleton. The aim of this study is to explore the phototoxic properties of PN on the human cell line tumor-derived HL60 (acute promyelocytic leukemia) and to identify the cell-specific targets of ROS involved in the tumor cell death. Our results reveal that PN acts as an excellent PS, showing a potent antitumor cell activity in presence of light. PN-PDT generates intracellular ROS, via oxidation reaction mechanisms type I and II, resulting in an induction of apoptosis. Moreover, both extrinsic (through direct activation of caspase-3) and intrinsic (through mitochondrial depolarization) pathways of apoptosis are induced by PN-PDT. Using pharmacologic inhibitors, we also find that PN-PDT activates caspase-8/tBid and p38-MAPK, triggering the activation of the apoptotic pathways. Although, survival pathways are also promoted through PI3 K/Akt and JNK activation, the net result of PN-PDT is the tumor cell death. The present work identifies to PN, for the first time, as a potent photosensitizer in human tumor cell lines and proposes a mechanism by which ROS induces apoptosis of tumor cell.

Precursor-Directed Biosynthesis of Phenylbenzoisoquinolindione Alkaloids and the Discovery of a Phenylphenalenone-Based Plant Defense Mechanism

Phenylbenzoisochromenone glucosides (oxa-phenylphenalenone glucosides) occurring in some phenylphenalenone-producing plants of the Haemodoraceae undergo conversion to phenylbenzoisoquinolindiones (aza-phenylphenalenones) in extracts of Xiphidium caeruleum. Precursor-directed biosynthetic experiments were used to generate a series of new phenylbenzoisoquinolindiones from native phenylbenzoisochromenone glucosides and external amines, amino acids, and peptides. Intermediates of the conversion were isolated, incubated with cell-free extracts, and exposed to reactions under oxidative or inert conditions, respectively, to elucidate the entire pathway from phenylbenzoisochromenones to phenylbenzoisoquinolindiones. An intermediate in this pathway, a reactive hydroxylactone/aldehyde, readily binds not only to amines in vitro but may also bind to the N-terminus of biogenic peptides and proteins of herbivores and pathogens in vivo. The deactivation of biogenic amino compounds by N-terminal modification is discussed as the key reaction of a novel phenylphenalenone-based plant defense mechanism. According to these data, the ecological function of phenylphenalenone-type compounds in the Haemodoraceae, subfamily Haemodoroideae, has been substantiated.

Synthesis of perinaphthenones through rhodium-catalyzed dehydrative annulation of 1-naphthoic acids with alkynes

A convenient method for the synthesis of perinaphthenones via rhodium-catalyzed dehydrative annulation of naphthoic acids with alkynes, which gave good to high yields of perinaphthenones, was developed.

Design, synthesis and evaluation of amino-substituted 1H-phenalen-1-ones as anti-leishmanial agents

Screening of a designed collection of mono-substituted amino-1H-phenalen-1-ones against promastigote forms of L. donovani and L. amazonensis, identified seven compounds with anti-leishmanial activities comparable or better than the commonly prescribed anti-leishmanial drug, miltefosine. Structure-activity analysis revealed that appendages containing a basic tertiary nitrogen were favored, and that the position of the appendage also affected their potency. Like miltefosine, several of these active compounds significantly reduced the mitochondrial membrane potential in promastigotes. Further studies in amastigotes of L. amazonensis revealed that compounds 14, 15 and 33 were more active and more selective than miltefosine, with sub-micromolar potencies and selectivity indices >100.

In vitro activity of 1H-phenalen-1-one derivatives against Acanthamoeba castellanii Neff and their mechanisms of cell death

Acanthamoeba is an opportunistic pathogen which is the causal agent of a sight-threatening ulceration of the cornea known as Acanthamoeba keratitis (AK) and, more rarely, an infection of the central nervous system called "granulomatous amoebic encephalitis" (GAE). The symptoms of AK are non-specific, and so it can be misdiagnosed as a viral, bacterial, or fungal keratitis. Furthermore, current therapeutic measures against AK are arduous, and show limited efficacy against the cyst stage of Acanthamoeba. 1H-Phenalen-1-one (PH) containing compounds have been isolated from plants and fungi, where they play a crucial role in the defense mechanism of plants. Natural as well as synthetic PHs exhibit a diverse range of biological activities against fungi, protozoan parasites or human cancer cells. New synthetic PHs have been tested in this study and they show a potential activity against this protozoa.

Inactivation of plant-pathogenic fungus Colletotrichum acutatum with natural plant-produced photosensitizers under solar radiation

The increasing tolerance to currently used fungicides and the need for environmentally friendly antimicrobial approaches have stimulated the development of novel strategies to control plant-pathogenic fungi such as antimicrobial phototreatment (APT). We investigated the in vitro APT of the plant-pathogenic fungus Colletotrichum acutatum with furocoumarins and coumarins and solar radiation. The compounds used were: furocoumarins 8-methoxypsoralen (8-MOP) and 5,8-dimethoxypsoralen (isopimpinellin), coumarins 2H-chromen-2-one (coumarin), 7-hydroxycoumarin, 5,7-dimethoxycoumarin (citropten) and a mixture (3:1) of 7-methoxycoumarin and 5,7-dimethoxycoumarin. APT of conidia with crude extracts from 'Tahiti' acid lime, red and white grapefruit were also performed. Pure compounds were tested at 50μM concentration and mixtures and extracts at 12.5mgL(-1). The C. acutatum conidia suspension with or without the compounds was exposed to solar radiation for 1h. In addition, the effects of APT on the leaves of the plant host Citrus sinensis were determined. APT with 8-MOP was the most effective treatment, killing 100% of the conidia followed by the mixture of two coumarins and isopimpinellin that killed 99% and 64% of the conidia, respectively. APT with the extracts killed from 20% to 70% of the conidia, and the extract from 'Tahiti' lime was the most effective. No damage to sweet orange leaves was observed after APT with any of the compounds or extracts.

In vitro photodynamic inactivation of conidia of the phytopathogenic fungus Colletotrichum graminicola with cationic porphyrins

Photodynamic inactivation (PDI) is an efficient approach for the elimination of a series of microorganisms; however, PDI involving phytopathogenic filamentous fungi is scarce in the literature. In the present study, we have demonstrated the photoinactivating properties of five cationic meso-(1-methyl-4-pyridinio)porphyrins on conidia of the phytopathogen Colletotrichum graminicola. For this purpose, photophysical properties (photostability and (1)O2 singlet production) of the porphyrins under study were first evaluated. PDI assays were then performed with a fluence of 30, 60, 90 and 120 J cm(-2) and varying the porphyrin concentration from 1 to 25 μmol L(-1). Considering the lowest concentration that enabled the best photoinactivation, with the respective lowest effective irradiation time, the meso-(1-methyl-4-pyridinio)porphyrins herein studied could be ranked as follows: triple-charged 4 (1 μmol L(-1) with a fluence of 30 J cm(-2)) > double-charged-trans2 (1 μmol L(-1) with 60 J cm(-2)) > tetra-charged 5 (15 μmol L(-1) with 90 J cm(-2)) > mono-charged 1 (25 μmol L(-1) with 120 J cm(-2)). Double-charged-cis-porphyrin 3 inactivated C. graminicola conidia in the absence of light. Evaluation of the porphyrin binding to the conidia and fluorescence microscopic analysis were also performed, which were in agreement with the PDI results. In conclusion, the cationic porphyrins herein studied were considered efficient photosensitizers to inactivate C. graminicola conidia. The amount and position of positive charges are related to the compounds' amphiphilicity and therefore to their photodynamic activity.

4-Phenylphenalenones as a template for new photodynamic compounds against Mycosphaerella fijiensis

BACKGROUND

Evaluation of 4-phenylphenalenones and structural analogues against the fungal pathogen Mycosphaerella fijiensis (causal agent of black sigatoka disease in bananas) under light-controlled conditions uncovered some key structural features for the design of photodynamic compounds.

RESULTS

Structure-activity relationship analysis revealed the importance of a chromophoric aryl-ketone and a steroidomimetic structural motif in the activity of the assayed compounds. The results pointed to 1,2-dihydro-3H-naphtho[2',1':3,4]cyclohepta[1,2-b]furan-3-one, which displayed an activity in the range of propiconazole but with photodynamic behaviour.

CONCLUSION

The present work demonstrates that 1,2-dihydro-3H-naphtho[2',1':3,4]cyclohepta[1,2-b]heterocyclic-3-one derivatives can be used as potential lead compounds for the development of fungicides, relying on a dual mode of action. © 2015 Society of Chemical Industry.

Phenylphenalenones protect banana plants from infection by Mycosphaerella fijiensis and are deactivated by metabolic conversion

Phenylphenalenones, polycyclic aromatic natural products from some monocotyledonous plants, are known as phytoalexins in banana (Musa spp.). In this study, (1) H nuclear magnetic resonance (NMR)-based metabolomics along with liquid chromatography and mass spectrometry were used to explore the chemical responses of the susceptible 'Williams' and the resistant 'Khai Thong Ruang' Musa varieties to the ascomycete fungus Mycosphaerella fijiensis, the agent of the black leaf Sigatoka disease. Principal component analysis discriminated strongly between infected and non-infected plant tissue, mainly because of specialized metabolism induced in response to the fungus. Phenylphenalenones are among the major induced compounds, and the resistance level of the plants was correlated with the progress of the disease. However, a virulent strain of M. fijiensis was able to overcome plant resistance by converting phenylphenalenones to sulfate conjugates. Here, we report the first metabolic detoxification of fungitoxic phenylphenalenones to evade the chemical defence of Musa plants.

9-Iodophenalenone and 9-trifluoromethanesulfonyloxyphenalenone: convenient entry points to new phenalenones functionalised at the 9-position. Iodine-carbonyl interaction studies by X-ray crystallography

Triflic anhydride acts like a key to the unreactive, “locked” 9-hydroxyphenalenone, providing access to new phenalenones functionalised in 9-position as well as to a phenalenyl-annelated isoxazole.

β-Phenyl quenching of 9-phenylphenalenones: a novel photocyclisation reaction with biological implications

β-Phenyl quenching of 9-phenylphenalenones leads to the reversible formation of naphthoxanthenes and eventually to stable naphthoxanthenyl radicals or naphthoxanthenium cations.

Co-occurrence of phenylphenalenones and flavonoids in Xiphidium caeruleum Aubl. flowers

A Xiphidium caeruleum flower extract was separated by semi-preparative HPLC into five fractions, from which three flavonoids, two phenylphenalenones and 17 phenylphenalenone-related compounds including five unknown compounds, were isolated and their structures elucidated by Liquid Chromatography-Diode Array Detection-Solid Phase Extraction-Nuclear Magnetic Resonance spectroscopy (LC-DAD-SPE-NMR) and mass spectrometry (MS). This is the first report of the co-occurrence of phenylphenalenones and flavonoids in the Haemodoraceae family. The ecological implications of flavonoids and various phenylphenalenone-type compounds and their putative biosynthesis sites in X. caeruleum are subject to discussion.

Applied biological and physicochemical activity of isoquinoline alkaloids: oxoisoaporphine and boldine

The aim of this study was to determine the electronic influence of substituent groups and annelated rings such as oxazole-oxazinone on the physicochemical and photoprotection, antioxidant capacity, toxicity and singlet oxygen photosensitization biological properties of isoquinoline alkaloid frameworks. Thus, oxoisoaporphine derivatives 1–5 and 3-azaoxoisoaporphine (6), some of them with phenolic structures, did not present any antioxidant capacity, possibly either by formation of keto-enol tautomerism species or the formation of unstable free radicals. Due to the singlet oxygen quantum yields (Φ_Δ) near to unity, and greater photostability than phenalenone, oxoisoaporphines 4–6 may be considered as photosensitizers for singlet oxygen production and can be used as new universal study tools. The biological application as antibacterial agents is an important and possible tool in the study of compounds with low cytotoxicity and high reactivity in antineoplastic chemotherapy. On the other hand, when boldine and its annelated derivatives B1–4 are irradiated, a photoprotector effect is observed (SPF = 2.35), even after 30 minutes of irradiation. They also act as photoprotectors in cell fibroblast cultures. No hemolysis was detected for boldine hydrochloride and its salts without irradiation. In solutions irradiated before incubation (at concentrations over 200 ppm) photoproducts were toxic to the nauplii of Artemia salina.

Structure-activity relationship in the interaction of substituted perinaphthenones with Mycosphaerella fijiensis

The levels of native fungitoxic perinaphthenone phytoalexins in susceptible Musa varieties (banana), which are commercially grown in large plantations, are too low to provide plants with long-lasting protection against highly pathogenic fungi. Novel strategies for plant protection are necessary to reduce crop losses and to prevent the development of resistant fungal strains. The synthesis of novel fungicides based on the structures of perinaphthenone natural products is considered to be a promising strategy. Thirteen substituted perinaphthenones, among them two known natural products (1, 2) and 11 synthetics (3-13), were evaluated for their activity against Mycosphaerella fijiensis , and their half-maximal inhibitory concentrations (IC(50)) were calculated to establish structure-activity relationships (SAR). A SAR trend was hypothesized, leading to the design of a new compound, 4-methoxy-2-nitro-1H-phenalen-1-one (14); the new compound displayed significantly enhanced in vitro activity against M. fijiensis compared to other perinaphthenone derivatives. The activity of 14 was comparable to that of two commercial fungicides.

Light and singlet oxygen in plant defense against pathogens: phototoxic phenalenone phytoalexins

Plants defend themselves from pathogen infections or mechanical injury by a number of mechanisms, including the induced biosynthesis of antimicrobial secondary metabolites. These compounds, termed phytoalexins, represent a very economical way to counteract hazard, because the carbon and energy resources are diverted to phytoalexin synthesis only at the early period of attack and only at its site. The occurrence of phenalenone chromophores in phytoalexins of plants originally nonphototoxic suggests that these plants respond to pathogen attacks by biosynthesizing singlet oxygen photosensitizers able to use solar energy for defense. This concept may have implications for the development of novel crop protection strategies.

Phototoxic Phytoalexins. Processes that Compete with the Photosensitized Production of Singlet Oxygen by 9-Phenylphenalenones†

Experiments were performed to elucidate the excited-state behavior of 9-phenylphenalenones, which are phototoxic plant secondary metabolites involved in mechanisms of light-mediated plant defense. Using a combination of time-resolved and steady-state UV/visible spectroscopies, time-resolved IR absorption spectroscopy, time-resolved singlet oxygen phosphorescence measurements and cyclic voltammetry, we provide evidence of an intramolecular charge-transfer process in the excited singlet and the triplet states of 9-phenylphenalenones that modulates the photosensitized production of singlet oxygen.

Fungal Elicitor Induces Singlet Oxygen Generation, Ethylene Release and Saponin Synthesis in Cultured Cells of Panax ginseng C. A. Meyer

Singlet oxygen is a high-energy molecular oxygen species. As one of the most active intermediates involved in chemical and biochemical reactions, singlet oxygen plays essential roles in plant responses to UV and strong light. Here, we report that Cle, an elicitor derived from fungal cell walls, induces the generation of singlet oxygen in cell cultures of ginseng, Panax ginseng. Cle treatment also triggers the activation of plasma membrane NADPH oxidase and 1-aminocyclopropane-1-carboxylic acid oxidase (ACO), subsequently leading to ethylene release and increased saponin synthesis, as shown by increased mRNA expression of squalene synthase (SQS) and squalene epoxidase (SQE), and accumulation of beta-amyrin synthase (beta-AS). Suppression of Cle-induced singlet oxygen generation or inhibition of ethylene production blocks saponin synthesis, whereas treatment of ginseng cells with ethylene or singlet oxygen induces the synthesis of saponin. Together, these results indicate that Cle-induced production of both singlet oxygen and ethylene is required for saponin synthesis, and that singlet oxygen may function upstream of ethylene during Cle-induced saponin synthesis.