Oxidation of an indole substrate by porphyrin iron(iii) superoxide: relevance to indoleamine and tryptophan 2,3-dioxygenases

Revealing Significant Electronic Effects on the Oxygen Reduction Reaction with Iron Porphyrins

Understanding electronic effects on catalysis from a mechanism point of view is of fundamental significance but is also challenging. We herein report on electronic effects on the oxygen reduction reaction (ORR) with Fe porphyrins. By using FeIII tetraphenylporphyrin (TPP-Fe) and FeIII tetra(pentafluorophenyl)porphyrin (TPFP-Fe), we showed their different electrochemical and chemical behaviors for ORR. Mechanism studies revealed that the FeIII-superoxo species of TPP-Fe can undergo smooth protonation with trifluoroacetic acid (TFA) but the electron-deficient FeIII-superoxo species of TPFP-Fe cannot be protonated with TFA. The FeIII-superoxo reactivity difference between TPP-Fe and TPFP-Fe is the origin of their different catalytic ORR behaviors.

Nonheme iron catalyst mimics heme-dependent haloperoxidase for efficient bromination and oxidation

The [Fe]/H2O2 oxidation system has found wide applications in chemistry and biology. Halogenation with this [Fe]/H2O2 oxidation protocol and halide (X-) in the biological system is well established with the identification of heme-iron-dependent haloperoxidases. However, mimicking such halogenation process is rarely explored for practical use in organic synthesis. Here, we report the development of a nonheme iron catalyst that mimics the heme-iron-dependent haloperoxidases to catalyze the generation of HOBr from H2O2/Br- with high efficiency. We discovered that a tridentate terpyridine (TPY) ligand designed for Fenton chemistry was optimal for FeBr3 to form a stable nonheme iron catalyst [Fe(TPY)Br3], which catalyzed arene bromination, Hunsdiecker-type decarboxylative bromination, bromolactonization, and oxidation of sulfides and thiols. Mechanistic studies revealed that Fenton chemistry ([Fe]/H2O2) might operate to generate hydroxyl radical (HO•), which oxidize bromide ion [Br-] into reactive HOBr. This nonheme iron catalyst represents a biomimetic model for heme-iron-dependent haloperoxidases with potential applications in organic synthesis, drug discovery, and biology.

Secondary Sphere Lewis Acid Activated Heme Superoxo Adducts Mimic Crucial Non-Covalent Interactions in IDO/TDO Heme Dioxygenases

Heme enzymes play a central role in a medley of reactivities within a wide variety of crucial biological systems. Their active sites are highly decorated with pivotal evolutionarily optimized non-covalent interactions that precisely choreograph their biological functionalities with specific regio-, stereo-, and chemo-selectivities. Gaining a clear comprehension of how such weak interactions within the active sites control reactivity offers powerful information to be implemented into the design of future therapeutic agents that target these heme enzymes. To shed light on such critical details pertaining to tryptophan dioxygenating heme enzymes, this study investigates the indole dioxygenation reactivities of Lewis acid-activated heme superoxo model systems, wherein an unprecedented kinetic behavior is revealed. In that, the activated heme superoxo adduct is observed to undergo indole dioxygenation with the intermediacy of a non-covalently organized precursor complex, which forms prior to the rate-limiting step of the overall reaction landscape. Spectroscopic and theoretical characterization of this precursor complex draws close parallels to the ternary complex of heme dioxygenases, which has been postulated to be of crucial importance for successful 2,3-dioxygenative cleavage of indole moieties.

Iron Dioxygen Adduct Formed during Electrochemical Oxygen Reduction by Iron Porphyrins Shows Catalytic Heme Dioxygenase Reactivity

Heme dioxygenases oxidize the indole ring of tryptophan to kynurenine which is the first step in the biosynthesis of several important biomolecules like NAD, xanthurenic acid, and picolinic acid. A ferrous heme dioxygen adduct (or FeIII-O2•-) is the oxidant, and both the atoms of O2 are inserted in the product and its catalytic function has been difficult to emulate as it is complicated by competing rapid reactions like auto-oxidation and/or formation of the μ-oxo dimer. In situ resonance Raman spectroscopy technique, SERRS-RDE, is used to probe the species accumulated during electrochemical ORR catalyzed by site-isolated imidazole-bound iron porphyrin installed on a self-assembled monolayer covered electrode. These in situ SERRS-RDE data using labeled O2 show that indeed a FeIII-O2•- species accumulate on the electrode during ORR between -0.05 and -0.30 V versus Ag/AgCl (satd. KCl) and is reduced by proton coupled electron transfer to a FeIII-OOH species which, on the other hand, builds up on the electrode between -0.20 and -0.40 V versus Ag/AgCl (satd. KCl). This FeIII-OOH species then gives way to a FeIV═O species, which accumulates at -0.50 V versus Ag/AgCl (satd. KCl). When 2,3-dimethylindole is present in the solution and the applied potential is held in the range where FeIII-O2•- species accumulate, it gets oxidized to N-(2-acetylphenyl)acetamide retaining both the oxygens from O2 mimicking the reaction of heme dioxygenases. Turnover numbers more than 104 are recorded, establishing this imidazole-bound ferrous porphyrin as a functional model of heme dioxygenases.

Hydroxytryptophan biosynthesis by a family of heme-dependent enzymes in bacteria

Hydroxytryptophan serves as a chemical precursor to a variety of bioactive specialized metabolites, including the human neurotransmitter serotonin and the hormone melatonin. Although the human and animal routes to hydroxytryptophan have been known for decades, how bacteria catalyze tryptophan indole hydroxylation remains a mystery. Here we report a class of tryptophan hydroxylases that are involved in various bacterial metabolic pathways. These enzymes utilize a histidine-ligated heme cofactor and molecular oxygen or hydrogen peroxide to catalyze regioselective hydroxylation on the tryptophan indole moiety, which is mechanistically distinct from their animal counterparts from the nonheme iron enzyme family. Through genome mining, we also identify members that can hydroxylate the tryptophan indole ring at alternative positions. Our results not only reveal a conserved way to synthesize hydroxytryptophans in bacteria but also provide a valuable enzyme toolbox for biocatalysis. As proof of concept, we assemble a highly efficient pathway for melatonin in a bacterial host.

Lewis acid improved dioxygen activation by a non-heme iron(II) complex towards tryptophan 2,3-dioxygenase activity for olefin oxygenation

Dioxygen activation and catalysis around ambient temperature is a long-standing challenge in chemistry. Inspired by the significant roles of the hydrogen bond network in dioxygen activation and catalysis by redox enzymes, this work presents a Lewis acid improved dioxygen activation by an Fe^II(BPMEN)(OTf)₂ complex towards tryptophan 2,3-dioxygenase (TDO) activity for 3-methylindole and common olefinic CC  bond oxygenation and cleavage (enzymatic Brønsted acid vs. chemical Lewis acid). It was found that the presence of a Lewis acid such as Sc³⁺ could substantially improve olefinic CC  bond oxygenation and cleavage activity through Fe^II(BPMEN)(OTf)₂ catalyzed dioxygen activation. Notably, a more negative ρ value in the Hammett plot of para-substituted styrene oxygenations was observed in the presence of a stronger Lewis acid, disclosing the enhanced electrophilic oxygenation capability of the putative iron(III) superoxo species through its electrostatic interaction with a stronger Lewis acid. Thereof, this work has demonstrated a new strategy in catalyst design for dioxygen activation and catalysis for olefin oxygenation, a significant process in the chemical industry.

Following Nature's Footprint: Mimicking the High-Valent Heme-Oxo Mediated Indole Monooxygenation Reaction Landscape of Heme Enzymes

Pathways for direct conversion of indoles to oxindoles have accumulated considerable interest in recent years due to their significance in the clear comprehension of various pathogenic processes in humans and the multipotent therapeutic value of oxindole pharmacophores. Heme enzymes are predominantly responsible for this conversion in biology and are thought to proceed with a compound-I active oxidant. These heme-enzyme-mediated indole monooxygenation pathways are rapidly emerging therapeutic targets; however, a clear mechanistic understanding is still lacking. Additionally, such knowledge holds promise in the rational design of highly specific indole monooxygenation synthetic protocols that are also cost-effective and environmentally benign. We herein report the first examples of synthetic compound-I and activated compound-II species that can effectively monooxygenate a diverse array of indoles with varied electronic and steric properties to exclusively produce the corresponding 2-oxindole products in good to excellent yields. Rigorous kinetic, thermodynamic, and mechanistic interrogations clearly illustrate an initial rate-limiting epoxidation step that takes place between the heme oxidant and indole substrate, and the resulting indole epoxide intermediate undergoes rearrangement driven by a 2,3-hydride shift on indole ring to ultimately produce 2-oxindole. The complete elucidation of the indole monooxygenation mechanism of these synthetic heme models will help reveal crucial insights into analogous biological systems, directly reinforcing drug design attempts targeting those heme enzymes. Moreover, these bioinspired model compounds are promising candidates for the future development of better synthetic protocols for the selective, efficient, and sustainable generation of 2-oxindole motifs, which are already known for a plethora of pharmacological benefits.

An Iron(III) Superoxide Corrole from Iron(II) and Dioxygen

A new structurally characterized ferrous corrole [FeII (ttppc)]- (1) binds one equivalent of dioxygen to form [FeIII (O2-. )(ttppc)]- (2). This complex exhibits a 16/18 O2 -isotope sensitive ν(O-O) stretch at 1128 cm-1 concomitantly with a single ν(Fe-O2 ) at 555 cm-1 , indicating it is an η1 -superoxo ("end-on") iron(III) complex. Complex 2 is the first well characterized Fe-O2 corrole, and mediates the following biologically relevant oxidation reactions: dioxygenation of an indole derivative, and H-atom abstraction from an activated O-H bond.