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.

Bio-inspired nitrogen oxide (NOx) interconversion reactivities of synthetic heme Compound-I and Compound-II intermediates

Dioxygen activating heme enzymes have long predicted to be powerhouses for nitrogen oxide interconversion, especially for nitric oxide (NO) oxidation which has far-reaching biological and/or environmental impacts. Lending credence, reactivity of NO with high-valent heme‑oxygen intermediates of globin proteins has recently been implicated in the regulation of a variety of pivotal physiological events such as modulating catalytic activities of various heme enzymes, enhancing antioxidant activity to inhibit oxidative damage, controlling inflammatory and infectious properties within the local heme environments, and NO scavenging. To reveal insights into such crucial biological processes, we have investigated low temperature NO reactivities of two classes of synthetic high-valent heme intermediates, Compound-II and Compound-I. In that, Compound-II rapidly reacts with NO yielding the six-coordinate (NO bound) heme ferric nitrite complex, which upon warming to room temperature converts into the five-coordinate heme ferric nitrite species. These ferric nitrite complexes mediate efficient substrate oxidation reactions liberating NO; i.e., shuttling NO₂^- back to NO. In contrast, Compound-I and NO proceed through an oxygen-atom transfer process generating the strong nitrating agent NO₂, along with the corresponding ferric nitrosyl species that converts to the naked heme ferric parent complex upon warmup. All reaction components have been fully characterized by UV-vis, ²H NMR and EPR spectroscopic methods, mass spectrometry, elemental analyses, and semi-quantitative determination of NO₂^- anions. The clean, efficient, potentially catalytic NO_x interconversions driven by high-valent heme species presented herein illustrate the strong prospects of a heme enzyme/O₂/NO_x dependent unexplored territory that is central to human physiology, pathology, and therapeutics.

A New Potent Antimicrobial Metalloporphyrin

A series of bis-acryl functionalized porphyrins and their corresponding metalloporphyrins (M=Co, Mn) were synthesized and investigated for their antimicrobial properties through MIC screening and bacteria time-kill kinetic studies. The Mn(III) 4-(bis)methylphenyl-substituted-porphyrins showed superior batericidal activities even in the dark with low hemotoxicity and good cytotoxicity profile.

Water-soluble manganese porphyrins as good catalysts for cipro- and levofloxacin degradation: Solvent effect, degradation products and DFT insights

Synthetic manganese porphyrins (MnPs), in the presence of oxidants, were employed for the degradation of fluoroquinolone antibiotics. Ciprofloxacin (CIP) and levofloxacin (LEV) degradation by iodosylbenzene, iodobenzene diacetate, H₂O₂ and meta-chloroperbenzoic acid using water-soluble MnP catalysts yielded thirteen and nine products, respectively, seven of which have been proposed for the first time. The MnP catalysts have demonstrated the ability to degrade these antibiotics to a high degree (up to 100% degradation). The structures of the degradation products were proposed based on mass spectrometry analysis, and density functional theory calculations could confirm how the substituent moieties attached to the basic chemical structure of the fluoroquinolones influence the degradation reactions. CIP has been shown to be a more reactive substrate towards the porphyrinic catalysts tested because of its three-membered ring. However, the catalysts could almost completely degrade LEV, highlighting the ability of these porphyrins to act as catalysts to degrade environmental pollutants.

Proton Switch in the Secondary Coordination Sphere to Control Catalytic Events at the Metal Center: Biomimetic Oxo Transfer Chemistry of Nickel Amidate Complex

High-valent metal-oxo species are key intermediates for the oxygen atom transfer step in the catalytic cycles of many metalloenzymes. While the redox-active metal centers of such enzymes are typically supported by anionic amino acid side chains or porphyrin rings, peptide backbones might function as strong electron-donating ligands to stabilize high oxidation states. To test the feasibility of this idea in synthetic settings, we have prepared a nickel(II) complex of new amido multidentate ligand. The mononuclear nickel complex of this N5 ligand catalyzes epoxidation reactions of a wide range of olefins by using mCPBA as a terminal oxidant. Notably, a remarkably high catalytic efficiency and selectivity were observed for terminal olefin substrates. We found that protonation of the secondary coordination sphere serves as the entry point to the catalytic cycle, in which high-valent nickel species is subsequently formed to carry out oxo-transfer reactions. A conceptually parallel process might allow metalloenzymes to control the catalytic cycle in the primary coordination sphere by using proton switch in the secondary coordination sphere.

The oxidative reactivity of three manganese(III) porphyrin complexes with hydrogen peroxide and nitrite toward catalytic nitration of protein tyrosine

Understanding the toxicological properties of MnIII-porphyrins (MnTPPS, MnTMPyP, or MnTBAP) can provide important biochemical rationales in developing them as the therapeutic drugs against protein tyrosine nitration-induced inflammation diseases. Here, we present a comprehensive understanding of the pH-dependent redox behaviors of these MnIII-porphyrins and their structural effects on catalyzing bovine serum albumin (BSA) nitration in the presence of H2O2 and NO2-. It was found that both MnTPPS and MnTBAP stand out in catalyzing BSA nitration at physiologically close condition (pH 8), yet they are less effective at pH 6 and 10. MnTMPyP was shown to have no ability to catalyze BSA nitration under all tested pHs (pH 6, 8, and 10). The kinetics and active intermediate determination through electrochemistry method revealed that both the pH-dependent redox behavior of the central metal cation and the antioxidant capability of porphin derivative contribute to the catalytic activities of three MnIII-porphyrins in BSA nitration in the presence of H2O2/NO2-. These comprehensive studies on the oxidative reactivity of MnIII-porphyrins toward BSA nitration may provide new clues for searching the manganese-based therapeutic drugs against the inflammation-related diseases.

Structure relationship of metalloporphyrins in inhibiting the aggregation of hIAPP

Metalloporphyrins (FeTBAP, MnTBAP, FeTMPyP and MnTMPyP) have been proposed as effective therapeutic agents in ONOO^--related disease including type 2 diabetes (T2D). As these metalloporphyrins share the structural similarities of the planar aromatic conjugation with a valuable class of inhibitors against amyloids fibrillation, they might be effective inhibitors via aromatic π-π stacking interactions with amyloid peptides. Here, we found that the anionic metalloporphyrins (FeTBAP and MnTBAP) are effective inhibitors against hIAPP fibrillation, while, the cationic metalloporphyrins (FeTMPyP and MnTMPyP) only have limited inhibitory effects. Besides, the porphyrin with iron center is more effective than the one with manganese center. Our results favor the electrostatic attraction contributes the main reason to the inhibitory effect between the anionic porphyrins and hIAPP, followed by the π-π stacking interactions between aromatic ring of porphyrins and hIAPP and the stronger coordination ability of iron center to hIAPP. Additionally, by comparison with FeTBAP, which can completely inhibit cytotoxicity induced by hIAPP via stabilizing hIAPP monomers, MnTBAP fails to reverse the cytotoxicity due to that it can only delay the transition of hIAPP from α-helix to β-sheet rich oligomers. Our results provide theoretical significance for further designing or screening of metalloporphyrins as bifunctional antidiabetic drugs.

First-row transition metal porphyrins for electrocatalytic hydrogen evolution — a SPP/JPP Young Investigator Award paper

A series of first-row transition metal complexes of tetrakis(pentafluorophenyl)porphyrin (1), denoted as 1-M (M [Formula: see text] Mn, Fe, Co, Ni, Cu, and Zn), were synthesized and examined as electrocatalysts for the hydrogen evolution reaction (HER). All these transition metal porphyrins were shown to be active for HER in acetonitrile using trifluoroacetic acid (TFA) as the proton source. The molecular nature and the stability of these metal porphyrins when functioning as HER catalysts were confirmed, and all catalysts gave Faradaic efficiency of >97% for H2 generation during bulk electrolysis. Importantly, by using 1-Cu, a remarkably high turnover frequency (TOF) of 48500 s[Formula: see text] 1-Cu the most efficient among this series of metal porphyrin catalysts. This TOF value also represents one of the highest values reported in the literature. In addition, electrochemical analysis demonstrated that catalytic HER mechanisms with these 1-M complexes are different. These results show that with the same porphyrin ligand, the change of metal ions will have significant impact on both catalytic efficiency and mechanism. This work for the first time provides direct comparison of electrocatalytic HER features of transition metal complexes of tetrakis(pentafluorophenyl)porphyrin under identical conditions, and will be valuable for future design and development of more efficient HER electrocatalysts of this series.

Efficient atrazine degradation catalyzed by manganese porphyrins: Determination of atrazine degradation products and their toxicity evaluation by human blood cells test models

Atrazine (ATZ) is an herbicide that has been considered an environmental pollutant worldwide. ATZ contaminates groundwaters and can persist in soils for up to a year causing several environmental and health problems. This study aimed to investigate ATZ degradation catalyzed by manganese porphyrins as biomimetic cytochrome P450 models. We used PhIO, PhI(OAc)₂, H₂O₂, t-BuOOH, m-CPBA, or Oxone^® as oxidant under mild conditions and evaluated a range of manganese porphyrins as catalyst. Concerning oxidant, iodosylbenzene provided the best result-ATZ degradation catalyzed by one of the studied manganese porphyrins in acetonitrile was as high as 47%. We studied the same catalyst/oxidant systems in natural water from a Brazilian river as solvent and obtained up to 100% ATZ degradation when iodobenzene diacetate was the oxidant, regardless of the manganese porphyrin. Besides the already known ATZ degradation products, we also identified unexpected degradation compounds (ring-opening products). Toxicity tests showed that the latter products were capable of proliferate blood cells because they did not show toxicity under the evaluated conditions.

Ciprofloxacin degradation by first-, second-, and third-generation manganese porphyrins

A range of hydrophobic first-, second-, and third-generation manganese porphyrins (MnPs) was investigated as cytochrome P450 models for degradation of the antibiotic ciprofloxacin (CIP). The experiments were carried out under mild conditions; oxidants such as iodosylbenzene (PhIO), H₂O₂, and meta-chloroperbenzoic acid were employed. The PhIO system yielded the best results: CIP degradation ranged between 56% and 76%. CIP degradation was not directly related to MnP generation. The second-generation MnP afforded the best result with the advantage that it required less preparation effort as compared to the third-generation MnP. Some new degradation products in MnP-mediated ciprofloxacin degradation were proposed, and the products of the reaction are not cytotoxic under the conditions evaluated.

Synthesis, Characterization, and Efficient Catalytic Activities of a Nickel(II) Porphyrin: Remarkable Solvent and Substrate Effects on Participation of Multiple Active Oxidants

A new nickel(II) porphyrin complex, [Ni^II (porp)] (1), has been synthesized and characterized by ¹ H NMR, ¹³ C NMR and mass spectrometry analysis. This Ni^II porphyrin complex 1 quantitatively catalyzed the epoxidation reaction of a wide range of olefins with meta-chloroperoxybenzoic acid (m-CPBA) under mild conditions. Reactivity and Hammett studies, H₂¹⁸ O-exchange experiments, and the use of PPAA (peroxyphenylacetic acid) as a mechanistic probe suggested that participation of multiple active oxidants Ni^II -OOC(O)R 2, Ni^IV -Oxo 3, and Ni^III -Oxo 4 within olefin epoxidation reactions by the nickel porphyrin complex is markedly affected by solvent polarity, concentration, and type of substrate. In aprotic solvent systems, such as toluene, CH₂ Cl₂ , and CH₃ CN, multiple oxidants, Ni^II -(O)R 2, Ni^IV -Oxo 3, and Ni^III -Oxo 4, operate simultaneously as the key active intermediates responsible for epoxidation reactions of easy-to-oxidize substrate cyclohexene, whereas Ni^IV -Oxo 3 and Ni^III -Oxo 4 species become the common reactive oxidant for the difficult-to-oxidize substrate 1-octene. In a protic solvent system, a mixture of CH₃ CN and H₂ O (95:5), the Ni^II -OOC(O)R 2 undergoes heterolytic or homolytic O-O bond cleavage to afford Ni^IV -Oxo 3 and Ni^III -Oxo 4 species by general acid catalysis prior to direct interaction between 2 and olefin, regardless of the type of substrate. In this case, only Ni^IV -Oxo 3 and Ni^III -Oxo 4 species were the common reactive oxidant responsible for olefin epoxidation reactions.

Visible light-induced formation of corrole-manganese(V)-oxo complexes: Observation of multiple oxidation pathways

Two manganese(V)-oxo corroles [Mn^V(Cor)O] that differ in their electronic environments were produced by visible light irradiation of highly photo-labile corrole-manganese(IV) bromates. The corrole ligands under study include 5,10,15-tris(pentafluorophenyl)corrole (TPFC), and 5,10,15-triphenylcorrole (TPC). The kinetics of oxygen transfer atom (OAT) reactions with various organic reductants by these photo-generated Mn^V(Cor)O were also studied in CH₃CN and CH₂Cl₂ solutions. Mn^V(Cor)O exhibits remarkable solvent and ligand effect on its reactivity and spectral behavior. In the more electron-deficient TPFC system and in the polar solvent CH₃CN, Mn^V(Cor)O returned Mn^III corrole in the end of oxidation reactions. However, in the less polar solvent CH₂Cl₂ or in the less electron-deficient TPC system, Mn^IV product was formed instead of Mn^III. Furthermore, with the same substrates and in the same solvent, the order of reactivity of Mn^V(Cor)O was TPC>TPFC, which is inverted from that expected based on the electron-demand of corrole ligands. Our spectral and kinetic results in this study provide compelling evidence in favor of multiple oxidation pathways, where Mn^V(Cor)O may serve as direct two-electron oxidant or undergo a disproportionation reaction to form a manganese(VI)-oxo corrole as the true oxidant. The choice of pathways is strongly dependent on the nature of the solvent and the corrole ligand.

A discrete {Co4(μ3-OH)4}(4+) cluster with an oxygen-rich coordination environment as a catalyst for the epoxidation of various olefins

Using the sterically hindered terphenyl-based carboxylate, the tetrameric Co(ii) complex [Co4(μ3-OH)4(μ-O2CAr(4F-Ph))2(μ-OTf)2(Py)4] () with an asymmetric cubane-type core has been synthesized and fully characterized by X-ray diffraction, UV-vis spectroscopy, and electron paramagnetic resonance spectroscopy. Interestingly, the cubane-type cobalt cluster with 3-chloroperoxybenzoic acid as the oxidant was found to be very effective in the epoxidation of a variety of olefins, including terminal olefins which are more challenging targeting substrates. Moreover, this catalytic system showed a fast reaction rate and high epoxide yields under mild conditions. Based on product analysis and Hammett studies, the use of peroxyphenylacetic acid as a mechanistic probe, H2(18)O-exchange experiments, and EPR studies, it has been proposed that multiple reactive cobalt-oxo species Co(V)[double bond, length as m-dash]O and Co(IV)[double bond, length as m-dash]O were involved in the olefin epoxidation.

Heteroleptic Tetrapyrrole-Fused Dimeric and Trimeric Skeletons with Unusual Non-Frustrated Fluorescence

Phthalocyanine (Pc) and porphyrin (Por) chromophores have been fused through the benzo[α]pyrazine moiety, resulting in unprecedented heteroleptic tetrapyrrole-fused dimers and trimers. The heteroleptic tetrapyrrole nature has been clearly revealed based on single-crystal X-ray diffraction analysis of the zinc dimer. Electrochemical analysis, theoretical calculations, and time-resolved spectroscopic results disclose that the two/three-tetrapyrrole-fused skeletons behave as one totally π-conjugated system as a result of the strong conjugative interaction between/among the tetrapyrrole chromophores. In particular, the effectively extended π-electron system through the fused-bridge induced strong electronic communication between the Pc and Por moieties and large transition dipole moments in the Pc-Por-fused systems, providing high fluorescence quantum yields (>0.13) and relatively long excited state lifetimes (>1.3 ns) in comparison with their homo-tetrapyrrole-fused analogues.

Oxygen atom transfer reactions from sterically encumbered brominated (oxo)manganese(V) corroles to styrene

Seven A3- and trans-A2B manganese(III) corroles (1–7-Mn) differing widely in their electronic and steric features were synthesized and transformed to their corresponding [Formula: see text]-brominated manganese(III) corroles derivatives (1a–7a-Mn). Their corresponding (oxo)manganese(V) corroles 1–7-Mn(oxo) and 1a–7a-Mn(oxo) were further prepapred by treating with iodosylbenzene (PhIO). The reactivity for the oxygen atom transfer from 1–7-Mn(oxo) to styrene followed the order of 1-Mn(oxo) > 2-Mn(oxo) > 7-Mn(oxo) > 4-Mn(oxo) > 3-Mn(oxo) > 6-Mn(oxo) [Formula: see text]5-Mn(oxo). The same pattern was observed for their [Formula: see text]-brominated analogs 1a–7a-Mn(oxo), albeit their reactivity was remarkably higher. The steric protection of [Formula: see text] moiety by ortho–ortho′-dibromophenyl substituents was found to enhance the stability of (oxo)manganese(V) corroles significantly.

Crystal structure, magnetic and catalytic oxidation properties of manganese(iii) tetrakis(ethoxycarbonyl)porphyrin

5,10,15,20-tetrakis(ethoxycarbonyl)porphyrin manganese(iii) chloride (Mn^IIITECPCl) existed as a Mn–O coordinated dimer. Mn^IIITECPCl gave high yield in oxidation of styrene by using TBHP or PhIO oxidant and was found recyclable.

Solvent effects on the catalytic activity of manganese(III) corroles

Four Mn ( III ) corroles that differ in their electronic environments based on meso-substitution by pentafluorophenyl and phenyl groups were synthesized and characterized by spectroscopic techniques. Utilization of these Mn ( III ) corroles for styrene oxidation using iodosylbenzene (PhIO) as oxygen source in toluene, dichloromethane (DCM), DMF, DMAc, THF and DMSO revealed a remarkable effect of solvent on the catalytic activity. Furthermore, the transformation of Mn ( III ) corroles into their corresponding Mn ( V )-oxo corroles, and subsequent treatment with styrene also indicated that more electron-deficient Mn ( V )-oxo corroles exhibit higher reactivity in toluene and DCM, while less electron-deficient Mn ( V )-oxo corroles exhibit higher reactivity in DMF and DMAc. A significant difference in the observed rates of reaction suggest that the catalytic oxidation of styrene by manganese corroles may proceed through different pathways, and is strongly solvent-dependent.

Porphyrins functionalized by quaternary pyridinium units

In this review, free-base and metalloporphyrins, functionalized on meso-positions by quaternary pyridinium units, also referred to as cationic porphyrins, are presented. The article consists of five parts. In the first part free-base porphyrins are described, especially taking account on generation of singlet oxygen; next parts concern metalloporphyrins. The second and third parts deal with zinc and manganese porphyrins, respectively; in the fourth part copper, palladium, and platinum porphyrins are presented. In the fifth part, describing porphyrins with various metal ions an attention is paid to porphyrin metal-organic frameworks (MOFs) and metal-organic materials (MOMs) in which metalloporphyrins are immobilized; syntheses and characterization of obtained products are shown.