Bioinspired Photoredox Benzylation of Quinones.

3-Benzylmenadiones were obtained in good yield by using a blue-light-induced photoredox process in the presence of Fe(III), oxygen, and γ-terpinene acting as a hydrogen-atom transfer agent. This methodology is compatible with a wide variety of diversely substituted 1,4-naphthoquinones as well as various cheap, readily available benzyl bromides with excellent functional group tolerance. The benzylation mechanism was investigated and supports a three-step radical cascade with the key involvement of the photogenerated superoxide anion radical.

A Mild and Versatile Friedel-Crafts Methodology for the Diversity-Oriented Synthesis of Redox-Active 3-Benzoylmenadiones with Tunable Redox Potentials.

A series of highly diversified 3-aroylmenadiones was prepared by a new Friedel-Crafts acylation variant/oxidative demethylation strategy. A mild and versatile acylation was performed between 1,4-dimethoxy-2-methylnaphthalene and various activated/deactivated benzoic and heteroaromatic carboxylic acids, in the presence of mixed trifluoroacetic anhydride and triflic acid, at room temperature and in air. The 1,4-dimethoxy-2-methylnaphthalene-derived benzophenones were isolated in high yield, and submitted to oxidative demethylation with cerium ammonium nitrate to produce 3-benzoylmenadiones. All 1,4-naphthoquinone derivatives were investigated as redox-active electrophores by cyclic voltammetry. The electrochemical data recorded for 3-acylated menadiones are characterized by a second redox process, the potentials of which cover a wide range of values (500 mV). These data emphasize the ability of the generated structural diversity at the 3-aroyl chain of these electrophores to fine-tune their corresponding redox potentials. These properties are of significance in the context of antimalarial drug development and understanding of the mechanism of bioactivation/action.

Unprecedented Affinity Labeling of Carbohydrate-Binding Proteins with s-Triazinyl Glycosides.

Carbohydrate-protein interactions trigger a wide range of biological signaling pathways, the mainstays of physiological and pathological processes. However, there are an incredible number of carbohydrate-binding proteins (CBPs) that remain to be identified and characterized. This study reports for the first time the covalent labeling of CBPs by triazinyl glycosides, a new and promising class of affinity-based glycoprobes. Mono- and bis-clickable triazinyl glycosides were efficiently synthesized from unprotected oligosaccharides (chitinpentaose and 2'-fucosyl-lactose) in a single step. These molecules allow the specific covalent labeling of chitin-oligosaccharide-binding proteins (wheat germ agglutinin WGA and Bc ChiA1 D202A, an inactivated chitinase) and fucosyl-binding lectin (UEA-I), respectively.

Exploring the Dispersion and Electrostatic Components in Arene-Arene Interactions between Ligands and G4 DNA to Develop G4-Ligands.

G-Quadruplex (G4) DNA structures are important regulatory elements in central biological processes. Small molecules that selectively bind and stabilize G4 structures have therapeutic potential, and there are currently >1000 known G4 ligands. Despite this, only two G4 ligands ever made it to clinical trials. In this work, we synthesized several heterocyclic G4 ligands and studied their interactions with G4s (e.g., G4s from the c-MYC, c-KIT, and BCL-2 promoters) using biochemical assays. We further studied the effect of selected compounds on cell viability, the effect on the number of G4s in cells, and their pharmacokinetic properties. This identified potent G4 ligands with suitable properties and further revealed that the dispersion component in arene-arene interactions in combination with electron-deficient electrostatics is central for the ligand to bind with the G4 efficiently. The presented design strategy can be applied in the further development of G4-ligands with suitable properties to explore G4s as therapeutic targets.

A Class of Valuable (Pro-)Activity-Based Protein Profiling Probes: Application to the Redox-Active Antiplasmodial Agent, Plasmodione.

Plasmodione (PD) is a potent antimalarial redox-active drug acting at low nM range concentrations on different malaria parasite stages. In this study, in order to determine the precise PD protein interactome in parasites, we developed a class of (pro-)activity-based protein profiling probes (ABPP) as precursors of photoreactive benzophenone-like probes based on the skeleton of PD metabolites (PDO) generated in a cascade of redox reactions. Under UV-photoirradiation, we clearly demonstrate that benzylic oxidation of 3-benzylmenadione 11 produces the 3-benzoylmenadione probe 7, allowing investigation of the proof-of-concept of the ABPP strategy with 3-benzoylmenadiones 7-10. The synthesized 3-benzoylmenadiones, probe 7 with an alkyne group or probe 9 with -NO2 in para position of the benzoyl chain, were found to be the most efficient photoreactive and clickable probes. In the presence of various H-donor partners, the UV-irradiation of the photoreactive ABPP probes generates different adducts, the expected "benzophenone-like" adducts (pathway 1) in addition to "benzoxanthone" adducts (via two other pathways, 2 and 3). Using both human and Plasmodium falciparum glutathione reductases, three protein ligand binding sites were identified following photolabeling with probes 7 or 9. The photoreduction of 3-benzoylmenadiones (PDO and probe 9) promoting the formation of both the corresponding benzoxanthone and the derived enone could be replaced by the glutathione reductase-catalyzed reduction step. In particular, the electrophilic character of the benzoxanthone was evidenced by its ability to alkylate heme, as a relevant event supporting the antimalarial mode of action of PD. This work provides a proof-of-principle that (pro-)ABPP probes can generate benzophenone-like metabolites enabling optimized activity-based protein profiling conditions that will be instrumental to analyze the interactome of early lead antiplasmodial 3-benzylmenadiones displaying an original and innovative mode of action.

Plasmodium falciparum Ferredoxin-NADP+ Reductase-Catalyzed Redox Cycling of Plasmodione Generates Both Predicted Key Drug Metabolites: Implication for Antimalarial Drug Development.

Plasmodione (PD) is a potent antimalarial redox-active 3-benzyl-menadione acting at low nanomolar range concentrations on different malaria parasite stages. The specific bioactivation of PD was proposed to occur via a cascade of redox reactions starting from one-electron reduction and then benzylic oxidation, leading to the generation of several key metabolites including corresponding benzylic alcohol (PD-bzol, for PD benzhydrol) and 3-benzoylmenadione (PDO, for PD oxide). In this study, we showed that the benzylic oxidation of PD is closely related to the formation of a benzylic semiquinone radical, which can be produced under two conditions: UV photoirradiation or catalysis by Plasmodium falciparum apicoplast ferredoxin-NADP+ reductase (PfFNR) redox cycling in the presence of oxygen and the parent PD. Electrochemical properties of both PD metabolites were investigated in DMSO and in water. The single-electron reduction potential values of PD, PD-bzol, PDO, and a series of 3-benzoylmenadiones were determined according to ascorbate oxidation kinetics. These compounds possess enhanced reactivity toward PfFNR as compared with model quinones. Optimal conditions were set up to obtain the best conversion of the starting PD to the corresponding metabolites. UV irradiation of PD in isopropanol under positive oxygen pressure led to an isolated yield of 31% PDO through the transient semiquinone species formed in a cascade of reactions. In the presence of PfFNR, PDO and PD-bzol could be observed during long lasting redox cycling of PD continuously fueled by NADPH regenerated by an enzymatic system. Finally, we observed and quantified the effect of PD on the production of oxidative stress in the apicoplast of transgenic 3D7[Api-roGFP2-hGrx1]P. falciparum parasites by using the described genetically encoded glutathione redox sensor hGrx1-roGFP2 methodology. The observed fast reactive oxygen species (ROS) pulse released in the apicoplast is proposed to be mediated by PD redox cycling catalyzed by PfFNR.