Induction of Enzyme-like Peroxidase Activity in an Iron Porphyrin Complex Using Second Sphere Interactions

Peroxymonosulfate activation by trace iron(III) porphyrin for facile degradation of organic pollutants via nonradical oxidation

Nonradical species with great resistance to interference have shown great advantages in complex wastewater treatment. Herein, a novel system constructed by biodegradable tetrakis-(4-carboxyphenyl)-porphyrinatoiron(III) (FeIII-TCPP) and peroxymonosulfate (PMS) was proposed for facile decontamination. Nonradical pathway is observed in FeIII-TCPP/PMS, where 1O2 and high-valent iron-oxo species play dominant roles. The genres and valence of high-valent iron-oxo species, including iron(IV)-oxo porphyrin radical-cationic species [OFeIV-TCPP•+] and iron(IV)-hydroxide species [FeIV-TCPP(OH)], are ascertained, along with their generation mechanism. The axial ligand on the iron axial site affects the ground spin state of FeIII-TCPP, further influencing the thermodynamic reaction pathway of active species. With trace catalyst in micromoles, FeIII-TCPP exhibits high efficiency by degrading bisphenol S (BPS) completely within 5 min, while Co2+/PMS can only achieve a maximum of 26.2% under identical condition. Beneficial from nonradical pathways, FeIII-TCPP/PMS demonstrates a wide pH range of 3-10 and exhibits minimal sensitivity to interference of concomitant materials. BPS is primarily eliminated through β-scission and hydroxylation. Specifically, 1O2 electrophilically attacks the C-S bond of BPS, while high-valent iron-oxo species interacts with BPS through an oxygen-bound mechanism. This study provides novel insights into efficient activation of PMS by iron porphyrin, enabling the removal of refractory pollutants through nonradical pathway.

Role of distal arginine residue in the mechanism of heme nitrite reductases

Heme nitrite reductases reduce NO₂^- by 1e^-/2H⁺ to NO or by 6e^-/8H⁺ to NH₄⁺ which are key steps in the global nitrogen cycle. Second-sphere residues, such as arginine (with a guanidine head group), are proposed to play a key role in the reaction by assisting substrate binding and hydrogen bonding and by providing protons to the active site for the reaction. The reactivity of an iron porphyrin with a NO₂^- covalently attached to a guanidinium arm in its 2nd sphere was investigated to understand the role of arginine residues in the 2nd sphere of heme nitrite reductases. The presence of the guanidinium residue allows the synthetic ferrous porphyrin to reduce NO₂^- and produce a ferrous nitrosyl species ({FeNO}⁷), where the required protons are provided by the guanidinium group in the 2nd sphere. However, in the presence of additional proton sources in solution, the reaction of ferrous porphyrin with NO₂^- results in the formation of ferric porphyrin and the release of NO. Spectroscopic and kinetic data indicated that re-protonation of the guanidine group in the 2nd sphere by an external proton source causes NO to dissociate from a ferric nitrosyl species ({FeNO}⁶) at rates similar to those observed for enzymatic sites. This re-protonation of the guanidine group mimics the proton recharge mechanism in the active site of NiR. DFT calculations indicated that the lability of the Fe-NO bond in the {FeNO}⁶ species is derived from the greater binding affinity of anions (e.g. NO₂^-) to the ferric center relative to neutral NO due to hydrogen bonding and electrostatic interaction of these bound anions with the protonated guanidium group in the 2nd sphere. The reduced {FeNO}⁷ species, once formed, is not affected significantly by the re-protonation of the guanidine residue. These results provide direct insight into the role of the 2nd sphere arginine residue present in the active sites of heme-based NiRs in determining the fate of NO₂^- reduction. Specifically, the findings using the synthetic model suggest that rapid re-protonation of these arginine residues may trigger the dissociation of NO from the {FeNO}⁶, which may also be the case in the protein active site.

Peroxymonosulfate activated by natural porphyrin derivatives for rapid degradation of organic pollutants via singlet oxygen and high-valent iron-oxo species

The activation of peroxymonosulfate (PMS) by sodium ferric chlorophyllin (SFC), a natural porphyrin derivative extracted from chlorophyll-rich substances, was systematically investigated for facile degradation of bisphenol A (BPA). SFC/PMS is capable of degrading 97.5% of BPA in the first 10 min with the initial BPA concentration of 20 mg/L and pH = 3, whereas conventional Fe²⁺/PMS could only remove 22.6% of BPA under identical conditions. It demonstrates a prominent flexibility to a broad pH range of 3-11 with complete pollutant degradation. A remarkable tolerance toward concomitant high concentration of inorganic anions (100 mM) was also observed, among which (bi)carbonates can even accelerate the degradation. The nonradical oxidation species, including high-valent iron-oxo porphyrin species and ¹O₂, are identified as dominant species. Particularly, the generation and participation of ¹O₂ in the reaction is evidenced by experimental and theoretical methods, which is vastly different from the previous study. The specific activation mechanism is unveiled by density functional theory (DFT) calculations and ab initio molecular dynamics (AIMD) simulations. The results shed light on effective PMS activation by iron (III) porphyrin and the proposed natural porphyrin derivative would be a promising candidate for efficient abatement of recalcitrant pollutants toward complicated aqueous media in wastewater treatment.

Amine Groups in the Second Sphere of Iron Porphyrins Allow for Higher and Selective 4e-/4H+ Oxygen Reduction Rates at Lower Overpotentials

Iron porphyrins with one or four tertiary amine groups in their second sphere are used to investigate the electrochemical O₂ reduction reaction (ORR) in organic (homogeneous) and aqueous (heterogeneous) conditions. Both of these complexes show selective 4e^-/4H⁺ reduction of oxygen to water at rates that are 2-3 orders of magnitude higher than those of iron tetraphenylporphyrin lacking these amines in the second sphere. In organic solvents, these amines get protonated, which leads to the lowering of overpotentials, and the rate of the ORR is enhanced almost 75,000 times relative to rates expected from the established scaling relationship for the ORR by iron porphyrins. In the aqueous medium, the same trend of higher ORR rates at a lower overpotential is observed. In situ resonance Raman data under heterogeneous aqueous conditions show that the presence of one amine group in the second sphere leads to a cleavage of the O-O bond in a Fe^III-OOH intermediate as the rate-determining step (rds). The presence of four such amine groups enhances the rate of O-O bond cleavage such that this intermediate is no longer observed during the ORR; rather, the proton-coupled reduction of the Fe^III-O₂^- intermediate with a H/D isotope effect of 10.6 is the rds. These data clearly demonstrate changes in the rds of the electrochemical ORR depending on the nature of second-sphere residues and explain their deviation from linear scaling relationships.

Spin state dependent peroxidase activity of heme bound amyloid β peptides relevant to Alzheimer's disease

The colocalization of heme rich deposits in the senile plaque of Aβ in the cerebral cortex of the Alzheimer's disease (AD) brain along with altered heme homeostasis and heme deficiency symptoms in AD patients has invoked the association of heme in AD pathology. Heme bound Aβ complexes, depending on the concentration of the complex or peptide to heme ratio, exhibit an equilibrium between a high-spin mono-His bound peroxidase-type active site and a low-spin bis-His bound cytochrome b type active site. The high-spin heme-Aβ complex shows higher peroxidase activity than free heme, where compound I is the reactive oxidant. It is also capable of oxidizing neurotransmitters like serotonin in the presence of peroxide, owing to the formation of compound I. The low-spin bis-His heme-Aβ complex on the other hand shows enhanced peroxidase activity relative to high-spin heme-Aβ. It reacts with H2O2 to produce two stable intermediates, compound 0 and compound I, which are characterized by absorption, EPR and resonance Raman spectroscopy. The stability of compound I of low-spin heme-Aβ is accountable for its enhanced peroxidase activity and oxidation of the neurotransmitter serotonin. The effect of the second sphere Tyr10 residue of Aβ on the formation and stability of the intermediates of low-spin heme-Aβ has also been investigated. The higher stability of compound I for low-spin heme-Aβ is likely due to H-bonding interactions involving Tyr10 in the distal pocket.

Comparative studies of soluble and immobilized Fe(III) heme-peptide complexes as alternative heterogeneous biocatalysts

Fe(III) heme is known to possess low catalytic activity when exposed to hydrogen peroxide and a reducing substrate. Efficient non-covalently linked Fe(III) heme-peptide complexes may represent suitable alternatives as a new group of green catalysts. Here, we evaluated a set of heme-peptide complexes by determination of their peroxidase-like activity and the kinetics of the catalytic conversion in both, the soluble and the immobilized state. We show the impact of peptide length on binding of the peptides to Fe(III) heme and the catalytic activity. Immobilization of the peptide onto a polymer support maintains the catalytic performance of the Fe(III) heme-peptide complex. This study thus opens up a new perspective with regard to the development of heterogeneous biocatalysts with a peroxidase-like activity.

Recent developments in the synthesis of bio-inspired iron porphyrins for small molecule activation

Nature utilizes a diverse set of tetrapyrrole-based macrocycles (referred to as porphyrinoids) for catalyzing various biological processes. Investigation of the differences in electronic structure and reactivity in these reactions have revealed striking differences that lead to diverse reactivity from, apparently, similar looking active sites. Therefore, the role of the different heme cofactors as well as the distal superstructure in the proteins is important to understand. This article summarizes the role of a few synthetic metallo-porphyrinoids towards catalyzing several small molecule activation reactions, such as the ORR, NiRR, CO₂RR, etc. The major focus of the article is to enlighten the synthetic routes to the well-decorated active-site mimic in a tailor-made fashion pursuing a retrosynthetic approach, learning from the biosynthesis of the cofactors. Techniques and the role of the second-sphere residues on the reaction rate, selectivity, etc. are incorporated emulating the basic amino acid residues fencing the active sites. These bioinspired mimics play an important role towards understanding the role of the prosthetic groups as well as the basic residues towards any reaction occurring in Nature.

Bioinorganic Chemistry on Electrodes: Methods to Functional Modeling

One of the major goals of bioinorganic chemistry has been to mimic the function of elegant metalloenzymes. Such functional modeling has been difficult to attain in solution, in particular, for reactions that require multiple protons and multiple electrons (nH⁺/ne^-). Using a combination of heterogeneous electrochemistry, electrode and molecule design one may control both electron transfer (ET) and proton transfer (PT) of these nH⁺/ne^- reactions. Such control can allow functional modeling of hydrogenases (H⁺ + e^- → 1/2 H₂), cytochrome c oxidase (O₂ + 4 e^- + 4 H⁺ → 2 H₂O), monooxygenases (RR'CH₂ + O₂ + 2 e^- + 2 H⁺ → RR'CHOH + H₂O) and dioxygenases (S + O₂ → SO₂; S = organic substrate) in aqueous medium and at room temperatures. In addition, these heterogeneous constructs allow probing unnatural bioinspired reactions and estimation of the inner- and outer-sphere reorganization energy of small molecules and proteins.

Second Sphere Effects on Oxygen Reduction and Peroxide Activation by Mononuclear Iron Porphyrins and Related Systems

Activation and reduction of O₂ and H₂O₂ by synthetic and biosynthetic iron porphyrin models have proved to be a versatile platform for evaluating second-sphere effects deemed important in naturally occurring heme active sites. Advances in synthetic techniques have made it possible to install different functional groups around the porphyrin ligand, recreating artificial analogues of the proximal and distal sites encountered in the heme proteins. Using judicious choices of these substituents, several of the elegant second-sphere effects that are proposed to be important in the reactivity of key heme proteins have been evaluated under controlled environments, adding fundamental insight into the roles played by these weak interactions in nature. This review presents a detailed description of these efforts and how these have not only demystified these second-sphere effects but also how the knowledge obtained resulted in functional mimics of these heme enzymes.

Pd-Immobilized Schiff Base Double-Layer Macrocycle: Synthesis, Structures, Peroxidase Mimic Activity, and Antibacterial Performance

Di-, tri-, and tetra-aldehydes have been employed to access new [2 + 2] [2 + 3] and [2 + 4] double-layer Schiff base macrocycles. The [2 + 3] compound has been used for the immobilization of Pd and the resulting composite has been employed as a peroxidase-like mimetic using 3,3',5,5'-tetramethylbenzidine (TMB) as the substrate; the optimum conditions together with the catalytic kinetics of the enzyme-like activity is discussed. Based on the peroxidase-like catalytic activity, the Pd@Schiff base composite was found to exhibit excellent bactericidal activity against both Escherichia coli (Gram-negative bacterium) and Staphylococcus aureus (Gram-positive bacterium) in the presence of relatively low concentrations of H₂O₂. Furthermore, cytotoxicity measurements illustrate the biosafety of the Pd composite. The above-mentioned findings have the potential to guide the innovation of new Pd-based composites as enzyme mimetics and antibacterial materials.

Rejigging Electron and Proton Transfer to Transition between Dioxygenase, Monooxygenase, Peroxygenase, and Oxygen Reduction Activity: Insights from Bioinspired Constructs of Heme Enzymes

Nature has employed heme proteins to execute a diverse set of vital life processes. Years of research have been devoted to understanding the factors which bias these heme enzymes, with all having a heme cofactor, toward distinct catalytic activity. Among them, axial ligation, distal super structure, and substrate binding pockets are few very vividly recognized ones. Detailed mechanistic investigation of these heme enzymes suggested that several of these enzymes, while functionally divergent, use similar intermediates. Furthermore, the formation and decay of these intermediates depend on proton and electron transfer processes in the enzyme active site. Over the past decade, work in this group, using in situ surface enhanced resonance Raman spectroscopy of synthetic and biosynthetic analogues of heme enzymes, a general idea of how proton and electron transfer rates relate to the lifetime of different O2 derived intermediates has been developed. These findings suggest that the enzymatic activities of all these heme enzymes can be integrated into one general cycle which can be branched out to different catalytic pathways by regulating the lifetime and population of each of these intermediates. This regulation can further be achieved by tuning the electron and proton transfer steps. By strategically populating one of these intermediates during oxygen reduction, one can navigate through different catalytic processes to a desired direction by altering proton and electron transfer steps.

Proton Relay in Iron Porphyrins for Hydrogen Evolution Reaction

The efficiency of the hydrogen evolution reaction (HER) can be facilitated by the presence of proton-transfer groups in the vicinity of the catalyst. A systematic investigation of the nature of the proton-transfer groups present and their interplay with bulk proton sources is warranted. The HERs electrocatalyzed by a series of iron porphyrins that vary in the nature and number of pendant amine groups are investigated using proton sources whose pK_a values vary from ∼9 to 15 in acetonitrile. Electrochemical data indicate that a simple iron porphyrin (FeTPP) can catalyze the HER at this Fe^I state where the rate-determining step is the intermolecular protonation of a Fe^III-H^- species produced upon protonation of the iron(I) porphyrin and does not need to be reduced to its formal Fe⁰ state. A linear free-energy correlation of the observed rate with pK_a of the acid source used suggests that the rate of the HER becomes almost independent of pK_a of the external acid used in the presence of the protonated distal residues. Protonation to the Fe^III-H^- species during the HER changes from intermolecular in FeTPP to intramolecular in FeTPP derivatives with pendant basic groups. However, the inclusion of too many pendant groups leads to a decrease in HER activity because the higher proton binding affinity of these residues slows proton transfer for the HER. These results enrich the existing understanding of how second-sphere proton-transfer residues alter both the kinetics and thermodynamics of transition-metal-catalyzed HER.

A Single Iron Porphyrin Shows pH Dependent Switch between "Push" and "Pull" Effects in Electrochemical Oxygen Reduction

The "push-pull" effects associated with heme enzymes manifest themselves through highly evolved distal amino acid environments and axial ligands to the heme. These conserved residues enhance their reactivities by orders of magnitude relative to small molecules that mimic the primary coordination. An instance of a mononuclear iron porphyrin with covalently attached pendent phenanthroline groups is reported which exhibit reactivity indicating a pH dependent "push" to "pull" transition in the same molecule. The pendant phenanthroline residues provide proton transfer pathways into the iron site, ensuring selective 4e^-/4H⁺ reduction of O₂ to water. The protonation of these residues at lower pH mimics the pull effect of peroxidases, and a coordination of an axial hydroxide ligand at high pH emulates the push effect of P450 monooxygenases. Both effects enhance the rate of O₂ reduction by orders of magnitude over its value at neutral pH while maintaining exclusive selectivity for 4e^-/4H⁺ oxygen reduction reaction.

A Cofactor-Substrate-Based Supramolecular Fluorescent Probe for the Ultrafast Detection of Nitroreductase under Hypoxic Conditions

Identifying the location and expression levels of enzymes under hypoxic conditions in cancer cells is vital in early-stage cancer diagnosis and monitoring. By encapsulating a fluorescent substrate, L-NO₂ , within the NADH mimic-containing metal-organic capsule Zn-MPB, we developed a cofactor-substrate-based supramolecular luminescent probe for ultrafast detection of hypoxia-related enzymes in solution in vitro and in vivo. The host-guest structure fuses the coenzyme and substrate into one supramolecular probe to avoid control by NADH, switching the catalytic process of nitroreductase from a double-substrate mechanism to a single-substrate one. This probe promotes enzyme efficiency by altering the substrate catalytic process and enhances the electron transfer efficiency through an intra-molecular pathway with increased activity. The enzyme content and fluorescence intensity showed a linear relationship and equilibrium was obtained in seconds, showing potential for early tumor diagnosis, biomimetic catalysis, and prodrug activation.

A designed second-sphere hydrogen-bond interaction that critically influences the O-O bond activation for heterolytic cleavage in ferric iron-porphyrin complexes

Heme hydroperoxidases catalyze the oxidation of substrates by H2O2. The catalytic cycle involves the formation of a highly oxidizing species known as Compound I, resulting from the two-electron oxidation of the ferric heme in the active site of the resting enzyme. This high-valent intermediate is formed upon facile heterolysis of the O-O bond in the initial FeIII-OOH complex. Heterolysis is assisted by the histidine and arginine residues present in the heme distal cavity. This chemistry has not been successfully modeled in synthetic systems up to now. In this work, we have used a series of iron(iii) porphyrin complexes (FeIIIL2(Br), FeIIIL3(Br) and FeIIIMPh(Br)) with covalently attached pendent basic groups (pyridine and primary amine) mimicking the histidine and arginine residues in the distal-pocket of natural heme enzymes. The presence of pendent basic groups, capable of 2nd sphere hydrogen bonding interactions, leads to almost 1000-fold enhancement in the rate of Compound I formation from peracids relative to analogous complexes without these residues. The short-lived Compound I intermediate formed at cryogenic temperatures could be detected using UV-vis electronic absorption spectroscopy and also trapped to be unequivocally identified by 9 GHz EPR spectroscopy at 4 K. The broad (2000 G) and axial EPR spectrum of an exchange-coupled oxoferryl-porphyrin radical species, [FeIV[double bond, length as m-dash]O Por˙+] with g eff ⊥ = 3.80 and g eff ‖ = 1.99, was observed upon a reaction of the FeIIIL3(Br) porphyrin complex with m-CPBA. The characterization of the reactivity of the FeIII porphyrin complexes with a substrate in the presence of an oxidant like m-CPBA by UV-vis electronic absorption spectroscopy showed that they are capable of oxidizing two equivalents of inorganic and organic substrate(s) like ferrocene, 2,4,6-tritertiary butyl phenol and o-phenylenediamine. These oxidations are catalytic with a turnover number (TON) as high as 350. Density Functional Theory (DFT) calculations show that the mechanism of O-O bond activation by 2nd sphere hydrogen bonding interaction from these pendent basic groups, which are protonated by a peracid, involves polarization of the O-O σ-bond, leading to lowering of the O-O σ*-orbital allowing enhanced back bonding from the iron center. These results demonstrate how inclusion of 2nd sphere hydrogen bonding interaction can play a critical role in O-O bond heterolysis.

Influence of the distal guanidine group on the rate and selectivity of O2 reduction by iron porphyrin

The O2 reduction reaction (ORR) catalysed by iron porphyrins with covalently attached pendant guanidine groups is reported. The results show a clear enhancement in the rate and selectivity for the 4e-/4H+ ORR. In situ resonance Raman investigations show that the rate determining step (rds) is O2 binding to ferrous porphyrins in contrast to the case of mononuclear iron porphyrins and heme/Cu analogues where the O-O bond cleavage of a heme peroxide is the rds. The selectivity is further enhanced when an axial imidazole ligand is introduced. Thus, the combination of the axial imidazole ligand and pendant guanidine ligand, analogous to the active site of peroxidases, is determined to be very effective in enabling a facile and selective 4e-/4H+ ORR.

Simultaneous Sensing of Seven Pathogenic Bacteria by Guanidine-Functionalized Upconversion Fluorescent Nanoparticles

The method capable of simultaneously detecting multiple target bacterial pathogens is necessary and of great interest. In this research, we demonstrated our initial effort to simultaneously detect seven common foodborne bacteria by developing a straightforward upconversion fluorescence sensing approach. The fluorescent nanosensor was constructed from a designed guanidine-functionalized upconversion fluorescent nanoparticles (UCNPs@GDN), tannic acid, and hydrogen peroxide (HP) and could quantify pathogenic bacteria in a nonspecific manner because the luminescence of the upconversion fluorescent nanoparticle was effectively strengthened in the presence of bacteria. When the developed nanosensor was applied to quantify multiple bacteria including Escherichia coli, Salmonella, Cronobacter sakazakii, Shigella flexneri, Vibrio parahaemolyticus, Staphylococcus aureus, and Listeria monocytogenes, a linear range of 10³ to 10⁸ cfu mL^-1 and a detection limit of 1.30 × 10² cfu mL^-1 have been obtained for the seven model mixture bacteria. In addition, the similar linear range and detection limit were also obtained for the detection of single bacteria. The present approach also exhibited acceptable recovery values ranging from 70.0 to 118.2% for bacteria in real samples (water, milk, and beef). All these results suggested that the guanidine-functionalized upconversion fluorescent nanosensor could be considered as a promising candidate for the rapid detection and surveillance of microbial pollutants in food and water.