Oxoferryl Porphyrin/Hydrogen Peroxide System Whose Behavior is Equivalent to Hydroperoxoferric Porphyrin

Monomeric, Oligomeric, Polymeric, and Supramolecular Cyclodextrins as Catalysts for Green Chemistry

This review comprehensively covers recent developments of cyclodextrin-mediated chemical transformations for green chemistry. These cyclic oligomers of glucose are nontoxic, eco-friendly, and recyclable to accomplish eminent functions in water. Their most important feature is to form inclusion complexes with reactants, intermediates, and/or catalysts. As a result, their cavities serve as sterically restricted and apolar reaction fields to promote the efficiency and selectivity of reactions. Furthermore, unstable reagents and intermediates are protected from undesired side reactions. The scope of their applications has been further widened through covalent or noncovalent modifications. Combinations of them with metal catalysis are especially successful. In terms of these effects, various chemical reactions are achieved with high selectivity and yield so that valuable chemicals are synthesized from multiple components in one-pot reactions. Furthermore, cyclodextrin units are orderly assembled in oligomers and polymers to show their cooperation for advanced properties. Recently, cyclodextrin-based metal-organic frameworks and polyoxometalate-cyclodextrin frameworks have been fabricated and employed for unique applications. Cyclodextrins fulfill many requirements for green chemistry and should make enormous contributions to this growing field.

Histidine-Bound Dinitrosyl Iron Complexes: Antioxidant and Antiradical Properties

Dinitrosyl iron complexes (DNICs) are important physiological derivatives of nitric oxide. These complexes have a wide range of biological activities, with antioxidant and antiradical ones being of particular interest and importance. We studied the interaction between DNICs associated with the dipeptide L-carnosine or serum albumin and prooxidants under conditions mimicking oxidative stress. The ligands of these DNICs were histidine residues of carnosine or His39 and Cys34 in bovine serum albumin. Carnosine-bound DNICs reduced the level of piperazine free radicals in the reaction system containing tert-butyl hydroperoxide (t-BOOH), bivalent iron ions, a nitroxyl anion donor (Angeli's salt), and HEPES buffer. The ability of carnosine DNICs to intercept organic free radicals produced from t-BOOH decay could lead to this effect. In addition, carnosine DNICs reacted with the superoxide anion radical (O2•-) formed in the xanthine/xanthine oxidase enzymatic system. They also reduced the oxoferryl form of the heme group formed in the reaction of myoglobin with t-BOOH. DNICs associated with serum albumin were found to be rapidly destroyed in a model system containing metmyoglobin and t-BOOH. At the same time, these protein DNICs inhibited the t-BOOH-induced oxidative degradation of coenzymes Q9 and Q10 in rat myocardial homogenate. The possible mechanisms of the antioxidant and antiradical action of the DNICs studied and their role in the metabolism of reactive oxygen and nitrogen species are discussed.

Mechanistic Studies on the Epoxidation of Alkenes by Macrocyclic Manganese Porphyrin Catalysts

Macrocyclic metal porphyrin complexes can act as shape-selective catalysts mimicking the action of enzymes. To achieve enzyme-like reactivity, a mechanistic understanding of the reaction at the molecular level is needed. We report a mechanistic study of alkene epoxidation by the oxidant iodosylbenzene, mediated by an achiral and a chiral manganese(V)oxo porphyrin cage complex. Both complexes convert a great variety of alkenes into epoxides in yields varying between 20-88 %. We monitored the process of the formation of the manganese(V)oxo complexes by oxygen transfer from iodosylbenzene to manganese(III) complexes and their reactivity by ion mobility mass spectrometry. The results show that in the case of the achiral cage complex the initial iodosylbenzene adduct is formed on the inside of the cage and in the case of the chiral one on the outside of the cage. Its decomposition leads to a manganese complex with the oxo ligand on either the inside or outside of the cage. These experimental results are confirmed by DFT calculations. The oxo ligand on the outside of the cage reacts faster with a substrate molecule than the oxo ligand on the inside. The results indicate how the catalytic activity of the macrocyclic porphyrin complex can be tuned and explain why the chiral porphyrin complex does not catalyze the enantioselective epoxidation of alkenes.

Dispersion and stability mechanism of Pt nanoparticles on transition-metal oxides

The heterogeneous catalysts of Pt/transition-metal oxides are typically synthesized through calcination at 500 °C, and Pt nanoparticles are uniformly and highly dispersed when hydrogen peroxide (H₂O₂) is applied before calcination. The influence of H₂O₂ on the dispersion and the stability of Pt nanoparticles on titania-incorporated fumed silica (Pt/Ti-FS) supports was examined using X-ray absorption fine structure (XAFS) measurements at the Pt L₃ and Ti K edges as well as density functional theory (DFT) calculations. The local structural and chemical properties around Pt and Ti atoms of Pt/Ti-FS with and without H₂O₂ treatment were monitored using in-situ XAFS during heating from room temperature to 500 °C. XAFS revealed that the Pt nanoparticles of H₂O₂-Pt/Ti-FS are highly stable and that the Ti atoms of H₂O₂-Pt/Ti-FS support form into a distorted-anatase TiO₂. DFT calculations showed that Pt atoms bond more stably to oxidized-TiO₂ surfaces than they do to bare- and reduced-TiO₂ surfaces. XAFS measurements and DFT calculations clarified that the presence of extra oxygen atoms due to the H₂O₂ treatment plays a critical role in the strong bonding of Pt atoms to TiO₂ surfaces.

Sensitive quantification of carbon monoxide in vivo reveals a protective role of circulating hemoglobin in CO intoxication

Carbon monoxide (CO) is a gaseous molecule known as the silent killer. It is widely believed that an increase in blood carboxyhemoglobin (CO-Hb) is the best biomarker to define CO intoxication, while the fact that CO accumulation in tissues is the most likely direct cause of mortality is less investigated. There is no reliable method other than gas chromatography to accurately determine CO content in tissues. Here we report the properties and usage of hemoCD1, a synthetic supramolecular compound composed of an iron(II)porphyrin and a cyclodextrin dimer, as an accessible reagent for a simple colorimetric assay to quantify CO in biological samples. The assay was validated in various organ tissues collected from rats under normal conditions and after exposure to CO. The kinetic profile of CO in blood and tissues after CO treatment suggested that CO accumulation in tissues is prevented by circulating Hb, revealing a protective role of Hb in CO intoxication. Furthermore, hemoCD1 was used in vivo as a CO removal agent, showing that it acts as an effective adjuvant to O₂ ventilation to eliminate residual CO accumulated in organs, including the brain. These findings open new therapeutic perspectives to counteract the toxicity associated with CO poisoning.

Mao et al. report highly sensitive quantification of carbon monoxide with a simple colorimetric assay, exploiting a synthetic supramolecular compound, hemoCD1. It can reveal distribution of CO in organs including the brain and can also serve as a CO scavenger for residual CO accumulated in organs. Finally, the authors showed circulating hemoglobin plays a protective role in CO intoxication.

Synthetic heme protein models that function in aqueous solution

Supramolecular porphyrin–cyclodextrin complexes act as biomimetic heme protein models in aqueous solution.

Oxygen Reduction by Iron Porphyrins with Covalently Attached Pendent Phenol and Quinol

Phenols and quinols participate in both proton transfer and electron transfer processes in nature either in distinct elementary steps or in a concerted fashion. Recent investigations using synthetic heme/Cu models and iron porphyrins have indicated that phenols/quinols can react with both ferric superoxide and ferric peroxide intermediates formed during O₂ reduction through a proton coupled electron transfer (PCET) process as well as via hydrogen atom transfer (HAT). Oxygen reduction by iron porphyrins bearing covalently attached pendant phenol and quinol groups is investigated. The data show that both of these can electrochemically reduce O₂ selectively by 4e^-/4H⁺ to H₂O with very similar rates. However, the mechanism of the reaction, investigated both using heterogeneous electrochemistry and by trapping intermediates in organic solutions, can be either PCET or HAT and is governed by the thermodynamics of these intermediates involved. The results suggest that, while the reduction of the Fe^III-O₂̇^- species to Fe^III-OOH proceeds via PCET when a pendant phenol is present, it follows a HAT pathway with a pendant quinol. In the absence of the hydroxyl group the O₂ reduction proceeds via an electron transfer followed by proton transfer to the Fe^III-O₂̇^- species. The hydrogen bonding from the pendant phenol group to Fe^III-O₂̇^- and Fe^III-OOH species provides a unique advantage to the PCET process by lowering the inner-sphere reorganization energy by limiting the elongation of the O-O bond upon reduction.

Cyclophane-Sustained Ultrastable Porphyrins

We report the encapsulation of free-base and zinc porphyrins by a tricyclic cyclophane receptor with subnanomolar binding affinities in water. The high affinities are sustained by the hydrophobic effect and multiple [CH···π] interactions covering large [π···π] stacking surfaces between the substrate porphyrins and the receptor. We discovered two co-conformational isomers of the 1:1 complex, where the porphyrin is orientated differently inside the binding cavity of the receptor on account of its tricyclic nature. The photophysical properties and chemical reactivities of the encapsulated porphyrins are modulated to a considerable extent by the receptor. Improved fluorescence quantum yields, red-shifted absorptions and emissions, and nearly quantitative energy transfer processes highlight the emergent photophysical enhancements. The encapsulated porphyrins enjoy unprecedented chemical stabilities, where their D/H exchange, protonation, and solvolysis under extremely acidic conditions are completely blocked. We anticipate that the ultrahigh stabilities and improved optical properties of these encapsulated porphyrins will find applications in single-molecule materials, artificial photodevices, and biomedical appliances.

Heterocomponent ternary supramolecular complexes of porphyrins: A review

Porphyrins are prominent host molecules which are widely used due to their structural characteristics and directional interaction sites. This review summarizes non-covalently bound ternary complexes of porphyrins, constructed from at least three non-identical species. Progress in supramolecular chemistry allows the creation of complex molecular machinery tools, such as rotors, motors and switches from relatively simple structures in a single self-assembly step. In the current review, we highlight the collection of sophisticated molecular ensembles including sandwich-type complexes, cages, capsules, tweezers, rotaxanes, and supramolecular architectures mediating oxygen-binding and oxidation reactions. These diverse structures have high potential to be applied in sensing, production of new smart materials as well as in medical science.

Modelling haemoproteins: porphyrins and cyclodextrins as sources of inspiration

The association of hydrophobic cavities with porphyrin derivatives has been used to mimic haemoprotein structures. The most employed cavity in this field is β-cyclodextrin (β-CD), and scaffolds combining β-CDs and porphyrins are expected to inspire the combination of porphyrins and cucurbiturils in the near future. Aside from providing water solubility to various porphyrinic structures, the β-CD framework can also modulate and control the reactivity of the metal core of the porphyrin. After a general introduction of the challenges faced in the field of haemoprotein models and the binding behavior of β-CDs, this article will discuss covalent and non-covalent association of porphyrins with β-CDs. In each approach, the role of the CD differs according to the relative position of the concave CD host, either directly controlling the binding and transformation of a substrate on the metalloporphyrin or playing a dual role of controlling the water solubility and selecting the axial ligand of the metal core. The discussion will be of interest to the cucurbituril community as well as to the cavitand community, as the information provided should be useful for the design of haemoprotein mimics using cucurbiturils.

Antimalarial Quinoline Drugs Inhibit β-Hematin and Increase Free Hemin Catalyzing Peroxidative Reactions and Inhibition of Cysteine Proteases

Malaria caused by Plasmodium affects millions people worldwide. Plasmodium consumes hemoglobin during its intraerythrocytic stage leaving toxic heme. Parasite detoxifies free heme through formation of hemozoin (β-hematin) pigment. Proteolysis of hemoglobin and formation of hemozoin are two main targets for antimalarial drugs. Quinoline antimarial drugs and analogs (β-carbolines or nitroindazoles) were studied as inhibitors of β-hematin formation. The most potent inhibitors were quinacrine, chloroquine, and amodiaquine followed by quinidine, mefloquine and quinine whereas 8-hydroxyquinoline and β-carbolines had no effect. Compounds that inhibited β-hematin increased free hemin that promoted peroxidative reactions as determined with TMB and ABTS substrates. Hemin-catalyzed peroxidative reactions were potentiated in presence of proteins (i.e. globin or BSA) while antioxidants and peroxidase inhibitors decreased peroxidation. Free hemin increased by chloroquine action promoted oxidative reactions resulting in inhibition of proteolysis by three cysteine proteases: papain, ficin and cathepsin B. Glutathione reversed inhibition of proteolysis. These results show that active quinolines inhibit hemozoin and increase free hemin which in presence of H2O2 that abounds in parasite digestive vacuole catalyzes peroxidative reactions and inhibition of cysteine proteases. This work suggests a link between the action of quinoline drugs with biochemical processes of peroxidation and inhibition of proteolysis.

Hydrogen atom abstraction by synthetic heme ferric superoxide and hydroperoxide species

To date, artificial dioxygen adducts of heme have not been demonstrated to be able to oxidize organic substrates in sharp contrast to their non-heme analogues and naturally occurring enzymes like heme dioxygenases. To address this apparent anomaly, an iron porphyrin complex is synthesized which includes a pendant quinol group. The corresponding dioxygen bound iron porphyrin species is demonstrated to perform hydrogen atom transfer (HAT) from the quinol group appended to the porphyrin ligand. The resultant ferric peroxide, formed by the first HAT, performs a 2nd HAT generating a ferryl species (FeIV[double bond, length as m-dash]O) and resulting in the 2e-/2H+ oxidation of the pendant hydroquinone to quinone.

Circadian clock disruption by selective removal of endogenous carbon monoxide

Circadian rhythms are regulated by transcription-translation feedback loops (TTFL) of clock genes. Previous studies have demonstrated that core transcriptional factors, NPAS2 and CLOCK, in the TTFL can reversibly bind carbon monoxide (CO) in vitro. However, little is known about whether endogenous CO, which is continuously produced during a heme metabolic process, is involved in the circadian system. Here we show that selective removal of endogenous CO in mice considerably disrupts rhythmic expression of the clock genes. A highly selective CO scavenger, hemoCD1, which is a supramolecular complex of an iron(II)porphyrin with a per-O-methyl-β-cyclodextrin dimer, was used to remove endogenous CO in mice. Intraperitoneal administration of hemoCD1 to mice immediately reduced the amount of internal CO. The removal of CO promoted the bindings of NPAS2 and CLOCK to DNA (E-box) in the murine liver, resulting in up-regulation of the E-box-controlled clock genes (Per1, Per2, Cry1, Cry2, and Rev-erbα). Within 3 h after the administration, most hemoCD1 in mice was excreted in the urine, and heme oxygenase-1 (HO-1) was gradually induced in the liver. Increased endogenous CO production due to the overexpression of HO-1 caused dissociation of NPAS2 and CLOCK from E-box, which in turn induced down-regulation of the clock genes. The down-regulation continued over 12 h even after the internal CO level recovered to normal. The late down-regulation was ascribed to an inflammatory response caused by the endogenous CO reduction. The CO pseudo-knockdown experiments provided the clear evidence that endogenous CO contributes to regulation in the mammalian circadian clock.

Kinetic stabilization of a Ni(ii) bis(dithiobenzoate)-type complex achieved using three-dimensional insulation by a [1]rotaxane structure

We describe herein the synthesis of a Ni(ii) bis(dithiobenzoate)-type complex three-dimensionally insulated by a [1]rotaxane structure to reveal the importance of the insulation. Under cyclic voltammetry conditions, the complex showed a stable and reversible redox behavior in contrast to a non-insulated reference complex, clearly demonstrating the effectiveness of the rotaxane-type insulation as a new method of kinetic metal complex stabilization.

A water-soluble supramolecular complex that mimics the heme/copper hetero-binuclear site of cytochrome c oxidase

The O₂ adduct of an aqueous synthetic heme/copper model system built on a porphyrin/cyclodextrin supramolecular complex has been characterized.

In mitochondria, cytochrome c oxidase (CcO) catalyses the reduction of oxygen (O₂) to water by using a heme/copper hetero-binuclear active site. Here we report a highly efficient supramolecular approach for the construction of a water-soluble biomimetic model for the active site of CcO. A tridentate copper(ii) complex was fixed onto 5,10,15,20-tetrakis(4-sulfonatophenyl)porphinatoiron(iii) (Fe^IIITPPS) through supramolecular complexation between Fe^IIITPPS and a per-O-methylated β-cyclodextrin dimer linked by a (2,2′:6′,2′′-terpyridyl)copper(ii) complex (Cu^IITerpyCD₂). The reduced Fe^IITPPS/Cu^ITerpyCD₂ complex reacted with O₂ in an aqueous solution at pH 7 and 25 °C to form a superoxo-type Fe^III–O₂^–/Cu^I complex in a manner similar to CcO. The pH-dependent autoxidation of the O₂ complex suggests that water molecules gathered at the distal Cu site are possibly involved in the Fe^III–O₂^–/Cu^I superoxo complex in an aqueous solution. Electrochemical analysis using a rotating disk electrode demonstrated the role of the FeTPPS/CuTerpyCD₂ hetero-binuclear structure in the catalytic O₂ reduction reaction.

Detection and Removal of Endogenous Carbon Monoxide by Selective and Cell-Permeable Hemoprotein Model Complexes

Carbon monoxide (CO) is produced in mammalian cells during heme metabolism and serves as an important signaling messenger. Here we report the bioactive properties of selective CO scavengers, hemoCD1 and its derivative R8-hemoCD1, which have the ability to detect and remove endogenous CO in cells. HemoCD1 is a supramolecular hemoprotein-model complex composed of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphinatoiron(II) and a per-O-methylated β-cyclodextrin dimer having an pyridine linker. We demonstrate that hemoCD1 can be used effectively to quantify endogenous CO in cell lysates by a simple spectrophotometric method. The hemoCD1 assay detected ca. 260 pmol of CO in 10⁶ hepatocytes, which was well-correlated with the amount of intracellular bilirubin, the final breakdown product of heme metabolism. We then covalently attached an octaarginine peptide to a maleimide-appended hemoCD1 to synthesize R8-hemoCD1, a cell-permeable CO scavenger. Indeed, R8-hemoCD1 was taken up by intact cells and captured intracellular CO with high efficiency. Moreover, we revealed that removal of endogenous CO by R8-hemoCD1 in cultured macrophages led to a significant increase (ca. 2.5-fold) in reactive oxygen species production and exacerbation of inflammation after challenge with lipopolysaccharide. Thus, R8-hemoCD1 represents a powerful expedient for exploring specific and still unidentified biological functions of CO in cells.

A cryo-generated ferrous-superoxo porphyrin: EPR, resonance Raman and DFT studies

The resonance Raman analysis of cryo-generated ferrous-superoxy heme has been performed for the first time, and its structure and the reaction mechanism are rationalized by DFT calculations. The presence of another electronic tautomer of ferrous-superoxy heme is predicted computationally.

Differences and comparisons of the properties and reactivities of iron(III)-hydroperoxo complexes with saturated coordination sphere

Heme and nonheme monoxygenases and dioxygenases catalyze important oxygen atom transfer reactions to substrates in the body. It is now well established that the cytochrome P450 enzymes react through the formation of a high-valent iron(IV)-oxo heme cation radical. Its precursor in the catalytic cycle, the iron(III)-hydroperoxo complex, was tested for catalytic activity and found to be a sluggish oxidant of hydroxylation, epoxidation and sulfoxidation reactions. In a recent twist of events, evidence has emerged of several nonheme iron(III)-hydroperoxo complexes that appear to react with substrates via oxygen atom transfer processes. Although it was not clear from these studies whether the iron(III)-hydroperoxo reacted directly with substrates or that an initial O-O bond cleavage preceded the reaction. Clearly, the catalytic activity of heme and nonheme iron(III)-hydroperoxo complexes is substantially different, but the origins of this are still poorly understood and warrant a detailed analysis. In this work, an extensive computational analysis of aromatic hydroxylation by biomimetic nonheme and heme iron systems is presented, starting from an iron(III)-hydroperoxo complex with pentadentate ligand system (L5(2)). Direct C-O bond formation by an iron(III)-hydroperoxo complex is investigated, as well as the initial heterolytic and homolytic bond cleavage of the hydroperoxo group. The calculations show that [(L5(2))Fe(III)(OOH)](2+) should be able to initiate an aromatic hydroxylation process, although a low-energy homolytic cleavage pathway is only slightly higher in energy. A detailed valence bond and thermochemical analysis rationalizes the differences in chemical reactivity of heme and nonheme iron(III)-hydroperoxo and show that the main reason for this particular nonheme complex to be reactive comes from the fact that they homolytically split the O-O bond, whereas a heterolytic O-O bond breaking in heme iron(III)-hydroperoxo is found.

Supramolecular control of transition metal complexes in water by a hydrophobic cavity: a bio-inspired strategy

Different strategies for obtaining water-soluble cavity-appended metal complexes are described, and their resulting interlocked assets are discussed in relationship with the very specific properties of water as a solvent.

Biomimetic cavity-based metal complexes

The biomimetic association of a metal ion with a cavity allows selective recognition, unusual redox properties and new reactivity patterns.

Reaction between a haemoglobin model compound and hydrosulphide in aqueous solution

The reaction between hydrosulphide and a haemoglobin model compound, composed of a Fe(iii)-porphyrin and a cyclodextrin dimer possessing a pyridine-linker, was studied.

Synthesis of water-soluble dinuclear mn-porphyrin with multiple antioxidative activities

Superoxide dismutase (SOD) and catalase activities of a drug are of great importance for its effective protection against reactive oxygen species (ROS)-induced injury. Achievement of catalase activity of a synthetic compound remains a challenge. Water-soluble Mn-porphyrins have high SOD and peroxynitrite (ONOO(-)) reducing activities, but not catalase-like activity. Herein, we are able to retain the fair SOD-like activity of a mononuclear Mn-5-(N-methylpyridinium-4-yl)-10,15,20-triphenyl porphyrin (MnM4PyP3P), while gaining in catalase-like activity with its dinuclear complex, 1,3-di[5-(N-methylene-pyridinium-4-yl)-10,15,20-triphenyl porphynato manganese] benzene tetrachloride (MnPD). Mechanistic study indicates that catalase-like activity of MnPD is due to synergism of two Mn active sites, where hydroxo-Mn(IV) complex is formed as an intermediate. The in vivo experiments demonstrate that MnPD significantly restores the treadmill-running ability of SOD-deficient mouse and thus indicates the therapeutic potential of MnPD. Furthermore, MnPD may serve as a mechanistic tool and indicate the new directions in the synthesis of catalase-like mimics.

Molecular assemblies of porphyrins and macrocyclic receptors: recent developments in their synthesis and applications

Metalloporphyrins which form the core of many bioenzymes and natural light harvesting or electron transport systems, exhibit a variety of selective functional properties depending on the state and surroundings with which they exist in biological systems. The specificity and ease with which they function in each of their bio-functions appear to be largely governed by the nature and disposition of the protein globule around the porphyrin reaction center. Synthetic porphyrin frameworks confined within or around a pre-organized molecular entity like the protein network in natural systems have attracted considerable attraction, especially in the field of biomimetic reactions. At the same time a large number of macrocyclic oligomers such as calixarenes, resorcinarenes, spherands, cyclodextrins and crown ethers have been investigated in detail as efficient molecular receptors. These molecular receptors are synthetic host molecules with enclosed interiors, which are designed three dimensionally to ensure strong and precise molecular encapsulation/recognition. Due to their complex structures, enclosed guest molecules reside in an environment isolated from the outside and as a consequence, physical properties and chemical reactions specific to that environment in these guest species can be identified. The facile incorporation of such molecular receptors into the highly photoactive and catalytically efficient porphyrin framework allows for convenient design of useful molecular systems with unique structural and functional properties. Such systems have provided over the years attractive model systems for the study of various biological and chemical processes, and the design of new materials and molecular devices. This review focuses on the recent developments in the synthesis of porphyrin assemblies associated with cyclodextrins, calixarenes and resorcinarenes and their potential applications in the fields of molecular encapsulation/recognition, and chemical catalysis.