Iron Porphyrin Complexes Research Articles

Harnessing Noninnocent Porphyrin Ligand to Circumvent Fe-Hydride Formation in the Selective Fe-Catalyzed CO2 Reduction in Aqueous Solution

The iron–porphyrin complex with four positively charged N,N,N-trimethyl-4-ammoniumphenyl substituents (called WSCAT) is an efficient catalyst for the reduction of CO2 to CO in aqueous solution with...

Utilizing Essential Symmetry Breaking in Auxiliary-Field Quantum Monte Carlo: Application to the Spin Gaps of the C36 Fullerene and an Iron Porphyrin Model Complex.

We present three distinct examples where phaseless auxiliary-field quantum Monte Carlo (ph-AFQMC) can be reliably performed with a single-determinant trial wave function with essential symmetry breaking. Essential symmetry breaking was first introduced by Lee and Head-Gordon [ Phys. Chem. Chem. Phys. 2019, 21, 4763-4778, 10.1039/C8CP07613H]. We utilized essential complex and time-reversal symmetry breaking with ph-AFQMC to compute the triplet-singlet energy gap in the TS12 set. We found statistically better performance of ph-AFQMC with complex-restricted orbitals than with spin-unrestricted orbitals. We then showed the utilization of essential spin symmetry breaking when computing the singlet-triplet gap of a known biradicaloid, C36. ph-AFQMC with spin-unrestricted Hartree-Fock (ph-AFQMC+UHF) fails catastrophically even with spin-projection and predicts no biradicaloid character. With approximate Brueckner orbitals obtained from regularized orbital-optimized second-order Møller-Plesset perturbation theory (κ-OOMP2), ph-AFQMC quantitatively captures strong biradicaloid character of C36. Lastly, we applied ph-AFQMC to the computation of the quintet-triplet gap in a model iron porphyrin complex where brute-force methods with a small active space fail to capture the triplet ground state. We show unambiguously that neither triplet nor quintet is strongly correlated using UHF, κ-OOMP2, and coupled-cluster with singles and doubles (CCSD) performed on UHF and κ-OOMP2 orbitals. There is no essential symmetry breaking in this problem. By virtue of this, we were able to perform UHF+ph-AFQMC reliably with a cc-pVTZ basis set and predicted a triplet ground state for this model geometry. The largest ph-AFQMC in this work correlated 186 electrons in 956 orbitals. Our work highlights the utility, scalability, and accuracy of ph-AFQMC with a single-determinant trial wave function with essential symmetry breaking for systems mainly dominated by dynamical correlation with little static correlation.

Efficient and convenient pretreatment to enhance waste activated sludge anaerobic hydrolysis by iron porphyrin biomimetic enzyme

A novel biomimetic enzyme was applied in the pretreatment of waste activated sludge (WAS) for enhancing the hydrolysis properties. An iron porphyrin complex was efficiently synthesized and purified. In comparison with the blank test, iron porphyrin complex used as biomimetic enzyme could significantly improve the sludge disintegration. Hydrogen peroxide was not the main influencing factor. During the pretreatment process, concentrations of the ammonia nitrogen (NH4 +-N) and reducing sugar (RS) both almost increased, which indicated that the protein and polysaccharide in sludge were effectively decomposed. Results demonstrated that the optimum treatment time and temperature of this biomimetic enzyme was at 7 hours and 40 °C, respectively. The efficient and economical dosage of the biomimetic enzyme used in treatment sludge digestion was 1%. Under the optimal treatment condition, the SCOD/TCOD ratio, the rate of VSS reduction, the concentrations of NH4 +-N and RS reached the values of 65.3%, 38.4%, 215.9 mg/L and 65.9 mg/L, respectively. And the possible mechanism of iron porphyrin biomimetic enzyme to promote anaerobic hydrolysis of WAS had been proposed. Therefore, the application of biomimetic enzyme was a desirable method in the WAS pretreatment.

Spectroscopic Evidence for Acid-Catalyzed Disproportionation Reaction of Oxoiron(IV) Porphyrin to Oxoiron(IV) Porphyrin π-Cation Radical and Iron(III) Porphyrin.

The disproportionation reaction of oxoiron(IV) porphyrin complex (II) to oxoiron(IV) porphyrin π-cation radical complex (I) and iron(III) porphyrin complex (III) have been proposed in various reactions. However, there have been no report that clarifies the disproportionation reaction spectroscopically. Here, we show direct evidence for the disproportionation reaction of II with absorption, 2H NMR, and EPR spectroscopy. Kinetic study of the disproportionation reaction with stopped flow technique can be analyzed as the second-order reaction for the concentration of proton and the first-order for the concentration of II. These results allow us to propose the mechanism of the disproportionation reaction, involving the sequential addition of two protons to the oxo ligand of II, to give an iron(III) porphyrin π-cation radical species, which reacts with another II to afford I and III.

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 Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O Por˙+] with geff⊥ = 3.80 and geff‖ = 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.

Iron Porphyrin Complexes Catalyzed Cyclopropanation Reactions and C-S Bond Cleavage Reactions for Phenyl Vinyl Sulfides and Diazoreagents

Iron Porphyrin Complexes Catalyzed Cyclopropanation Reactions and C-S Bond Cleavage Reactions for Phenyl Vinyl Sulfides and Diazoreagents

Influence of Mesityl and Thiophene Peripheral Substituents on Surface Attachment, Redox Chemistry, and ORR Activity of Molecular Iron Porphyrin Catalysts on Electrodes.

Two iron porphyrin complexes with either mesityl (FeTMP) or thiophene (FeT3ThP) peripheral substituents were attached to basal pyrolytic graphite and Ag electrodes via different immobilization methods. By combining cyclic voltammetry and in-operando surface-enhanced Raman spectroscopy along with MD simulations and DFT calculations, their respective surface attachment, redox chemistry and activity toward electrocatalytic oxygen reduction was investigated. For both porphyrin complexes, it could be shown that catalytic activity is restricted to the first (few) molecular layer(s), although electrodes covered with thiophene-substituted complexes showed a better capability to consume the oxygen at a given overpotential even in thicker films. The spectroscopic data and simulations suggest that both porphyrin complexes attach to a Ag electrode surface in a way that maximum planarity and minimum distance between the catalytic iron site and the electrode is achieved. However, due to the distinctive design of the FeT3ThP complex, the thiophene rings are capable of occupying a conformation, via rotation around the bonding axis to the porphyrin, in which all four sulfur atoms can coordinate to the Ag surface. This effect creates a dense and planar surface coverage of the porphyrin on the electrode facilitating a fast (multi) electron transfer via several covalent Ag-S bonds. In contrast, bulky mesityl groups as peripheral substituents, which have been initially introduced to prevent aggregation and improve catalytic behavior in solution, exert a negative effect on the overall electrocatalytic performance in the immobilized state as a less dense coverageand less stable interactions with the surface are formed. Our results underline the importance of rationally designed heterogenized molecular catalysts to achieve optimal turnover, which not onlystrictly applies to the here discussed oxygen reduction reaction but eventually holds also true for other energy conversion reactions such as carbon dioxide reduction.

(Invited) Electrochemical Analogues of Dioxygenase and Monooxygenase Activity in Synthetic Iron Porphyrins

Dioxygen adducts of heme proteins are ubiquitous in all O2 activating/reducing heme proteins in nature. While artificial dioxygen adducts of heme are known for six decades, till date these 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 a 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=O) and resulting in the 2e-/2H+ oxidation of the pendant hydroquinol to quinone. All the intermediate species are trapped at cryogenic temperatures and spectroscopically characterized using EPR and resonance Raman spectroscopy. DFT calculations reproduce the experimental kinetic parameters and provide insight into the nature of TS involved. These results demonstrate that artificial analogues of both heme ferric superoxide and ferric hydroperoxide species can perform HAT from an organic substrate when appropriately pre-organized to hydrogen bond to the bound superoxide and peroxide species. Siilarly, catalytic oxidation of organic substrates, using a green oxidant like O2, has been a long-term goal of the scientific community. In nature, these oxidations are performed by metalloenzymes which generate highly oxidizing species from O2 which, in turn, can oxidize inert organic substrates e.g. mono/di oxygenases. The same oxidants are produced during O2 reduction during respiration in the mitochondria but are reduced by electron transfer i.e. reductases. Iron porphyrin mimics of the active site of Cytochrome P450 (Cyt P450) are created atop self-assembled monolayer covered electrode. The rate of electron transfer from the electrode to the iron porphyrin site is attenuated to derive monooxygenase reactivity from these constructs which generally show reductase activity and catalytic hydroxylation of inert C-H bonds to alcohol and epoxidation alkenes, using molecular O2, is demonstrated with turnover numbers > 104. Mechanistic investigations suggest that a compound I analogue, formed during O2 reduction, is the primary oxidant and the selectivity is determined by the shape of the distal environment of the catalyst.

A five-coordinate iron(III) porphyrin complex including a neutral axial pyridine N-oxide ligand.

While six-coordinate iron(III) porphyrin complexes with pyridine N-oxides as axial ligands have been studied as they exhibit rare spin-crossover behavior, studies of five-coordinate iron(III) porphyrin complexes including neutral axial ligands are rare. A five-coordinate pyridine N-oxide-5,10,15,20-tetraphenylporphyrinate-iron(III) complex, namely (pyridine N-oxide-κO)(5,10,15,20-tetraphenylporphinato-κ4N,N',N'',N''')iron(III) hexafluoroantimonate(V) dichloromethane disolvate, [Fe(C44H28N4)(C5H5NO)][SbF6]·2CH2Cl2, was isolated and its crystal structure determined in the space group P-1. The porphyrin core is moderately saddled and the Fe-O-N bond angle is 122.08 (13)°. The average Fe-N bond length is 2.03 Å and the Fe-ONC5H5 bond length is 1.9500 (14) Å. This complex provides a rare example of a five-coordinate iron(III) porphyrin complex that is coordinated to a neutral organic ligand through an O-monodentate binding mode.

Reduction of CO2 to CO by an Iron Porphyrin Catalyst in the Presence of Oxygen

Reduction of CO2 to value-added chemicals is a logical way of fixing the rising levels of CO2. Activation and reduction of CO2 requires low-valent transition metals as catalysts. A major challenge in this chemistry is sensitivity of these low-valent metal sites to more abundant O2. Since O2 is a stronger oxidant than CO2 and isolated from the obvious competitive inhibition of CO2, partial reduction of O2 leads to formation of reactive oxygen species like O2– and H2O2, which are deleterious to the catalyst itself. An iron porphyrin complex appended with four ferrocene groups in its distal site is demonstrated to reduce CO2 unabated in the presence of O2 as it can reduce O2 to benign H2O under the same conditions. Further investigations reveal that iron porphyrins, in general, reduce CO2 selectively in the presence of O2. The aforementioned selectivity is derived from a 500 times faster rate of reaction of CO2 with Fe(0) porphyrin relative to O2 despite a higher driving force for the latter.

Crystallographic identification of a series of manganese porphyrin complexes with nitrogenous bases.

Studying the axial ligation behavior of metalloporphyrins with nitrogenous bases helps to better understand not only the biological function of heme-based protein systems, but also the catalytic properties of porphyrin-based reaction sites in other biomimetic synthetic support environments. Unlike iron porphyrin complexes, little is known about the axial ligation behavior of Mn porphyrins, particularly in the solid state with Mn in the +3 oxidation state. Here, we present the syntheses and crystal and molecular structures of three new high-spin manganese(III) porphyrin complexes with the different amine-based axial ligands imidazole (im), piperidine (pip), and 1,4-diazabicyclo[2.2.2]octane (DABCO), namely bis(imidazole)(5,10,15,20-tetraphenylporphyrinato)manganese(III) chloride chloroform disolvate, [Mn(C44H28N4)(C3H4N2)2]Cl·2CHCl3 or [Mn(TPP)(im)2]Cl·2CHCl3 (TPP = 5,10,15,20-tetraphenylporphyrin), (I), bis(piperidine)(5,10,15,20-tetraphenylporphyrinato)manganese(III) chloride, [Mn(C44H28N4)(C5H11N)2]Cl or [Mn(TPP)(pip)2]Cl, (II), and chlorido(1,4-diazabicyclo[2.2.2]octane)(5,10,15,20-tetraphenylporphyrin)manganese(III)-1,4-diazabicyclo[2.2.2]octane-toluene-water (4/4/4/1), [Mn(C44H28N4)Cl(C6H12N2)]·C6H12N2·C7H8·0.25H2O or [Mn(TPP)Cl(DABCO)]·(DABCO)·(toluene)·0.25H2O, (IV). A fourth complex, chlorido(pyridine)(5,10,15,20-tetraphenylporphryinato)manganese(III) pyridine disolvate, [Mn(C44H28N4)Cl(C5H5N)]·2C5H5N or [Mn(TPP)Cl(py)]·2(py), (III), acquired using different crystallization methods from published data, is also reported and compared to the previous structures.

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

Emulating enzymatic reactivity using small molecules has been a long-time challenging pursuit of the scientific community. Peroxidases, ubiquitous heme enzymes that are involved in hormone synthesis and the immune system, have been a prime target of such efforts due to their tremendous potential in the chemical industry as well as in wastewater treatment. Here it is demonstrated that inclusion of a second sphere guanidine moiety in an iron porphyrin not only makes this small molecule a veritable peroxidase catalyst but also offers an auxiliary binding site for organic substrates, facilitating their rapid oxidation with a green oxidant like H2O2. This small molecule analogue exhibits a "ping-pong" mechanism and Michaelis-Menten type kinetics, which is generally typical of metallo-enzymes and follows a mechanism of the natural enzyme in its entirety, including the formation of compound I as the primary oxidant.

Role of 2nd sphere H-bonding residues in tuning the kinetics of CO2 reduction to CO by iron porphyrin complexes.

Iron porphyrins are potential catalysts for the electrocatalytic and photocatalytic reduction of CO2. It has been recently established that the reduction of CO2 by an iron porphyrin complex with a hydrogen bonding distal pocket involves at least two intermediates: a Fe(ii)-CO22- and a Fe(ii)-COOH species. A distal hydrogen bonding interaction was found to be key in determining the stability of these intermediates and affecting both the selectivity and rate of CO2 reduction. In this report, a series of iron porphyrins that vary only in the distal H-bonding network are further investigated and these exhibit turnover frequencies (TOFs) ranging from 1.0 s-1 to 103 s-1. The experimental TOFs correlate with the H-bonding ability of the distal superstructure of these iron porphyrin complexes and analysis suggests that H-bonding alone can tune the rate of CO2 reduction by as much as 1000 fold. DFT calculations provide a detailed insight into how the, apparently weak, 2nd sphere interactions lead to efficient CO2 activation for reduction. The ability to tune CO2 reduction rates by changing the H-bonding residue instead of the acid source is a convenient way to tune CO2 reduction electrocatalysis without compromising selectivity by introducing competitive hydrogen evolution reaction or formate generation.

Imidazole-modified G-quadruplex DNA as metal-triggered peroxidase.

Four imidazoles, serving as metalloprotein-inspired ligands for complexing a range of transition metal cations, were incorporated into tetramolecular G-quadruplex DNA structures. Modified quadruplexes were found to complex Cu(ii), Ni(ii), Zn(ii) and Co(ii) in a 1 : 1 ratio with unprecedented strong thermal stabilizations of up to ΔT 1/2 = +51 °C. Furthermore, addition of Cu(ii) was found to lead to extraordinarily fast G-quadruplex association rates with k on values being ∼100 times higher compared to unmodified G-quadruplexes. This is ascribed to a template effect of Cu(ii), preorganizing the four single strands via coordination, followed by rapid formation of hydrogen-bonded G-quartets. Native electrospray ionization mass spectrometry (ESI), coupled with trapped ion-mobility spectrometry (timsTOF), supports the proposed 1 : 1 G-quadruplex-metal complexes and could further disclose their ability to bind the iron-porphyrin complex hemin in a 1 : 1 stoichiometry. DNA sequence design allowed us to equip this G-quadruplex-hemin complex, known to function as a horseradish peroxidase mimic, with a metal-dependent trigger. A competitive screen of transition metals revealed a high selectivity for Cu(ii), even in mixtures of several divalent metal cations. Once formed, the Cu(ii)-carrying DNAzyme was shown to be preserved in the presence of EDTA, attributed to its remarkable kinetic stability. Stimuli-responsive G-quadruplexes promise application in DNAzymes with switchable activity, adaptive sensors and dynamic DNA origami constructs.

Screen, Design and Enzymatic Activity Determination of Artificial Microperoxidases

Peroxidase activity greatly impacts the maintenance of free radical homeostasis, and can prevent or treat diseases related to free radicals. Microperoxidase-11(MP-11) is created via hydrolysis of cytochrome c iron-porphyrin complexes. In these complexes, the heme iron is penta-coordinate with histidine and exhibits excellent antioxidant activity when decomposing hydrogen peroxide. In this study, we screened the Ph.D.-7 and Ph.D.-12 phage display peptide libraries and obtained ten small peptide ligands of deuterohemin(the vinyl groups of oxidized heme). Among these polypeptides, DhHP-7P1, 12P1, 12P2 and 12P6 have good enzymatic activity compared with MP-11, and exhibit activities up to 50% of MP-11. Based on the screened sequences, we designed a series of artificial microperoxidases and determined that a higher peroxidase activity could be achieved with an enzymatic active site containing a second site of histidine residue spaced between two arginine residues with an interval of two amino acids(Dh-XHRXXR).

Rational Design of Mononuclear Iron Porphyrins for Facile and Selective 4e-/4H+ O2 Reduction: Activation of O-O Bond by 2nd Sphere Hydrogen Bonding.

Facile and selective 4e-/4H+ electrochemical reduction of O2 to H2O in aqueous medium has been a sought-after goal for several decades. Elegant but synthetically demanding cytochrome c oxidase mimics have demonstrated selective 4e-/4H+ electrochemical O2 reduction to H2O is possible with rate constants as fast as 105 M-1 s-1 under heterogeneous conditions in aqueous media. Over the past few years, in situ mechanistic investigations on iron porphyrin complexes adsorbed on electrodes have revealed that the rate and selectivity of this multielectron and multiproton process is governed by the reactivity of a ferric hydroperoxide intermediate. The barrier of O-O bond cleavage determines the overall rate of O2 reduction and the site of protonation determines the selectivity. In this report, a series of mononuclear iron porphyrin complexes are rationally designed to achieve efficient O-O bond activation and site-selective proton transfer to effect facile and selective electrochemical reduction of O2 to water. Indeed, these crystallographically characterized complexes accomplish facile and selective reduction of O2 with rate constants >107 M-1 s-1 while retaining >95% selectivity when adsorbed on electrode surfaces (EPG) in water. These oxygen reduction reaction rate constants are 2 orders of magnitude faster than all known heme/Cu complexes and these complexes retain >90% selectivity even under rate determining electron transfer conditions that generally can only be achieved by installing additional redox active groups in the catalyst.

Pressure Effect on Zero-Field Splitting Parameter of Hemin: Model Case of Hemoproteins under Pressure.

We experimentally studied the pressure dependence of the zero-field splitting (ZFS) parameter of hemin (iron(III) protoporphyrin IX chloride), which is a model complex of hemoproteins, via high-frequency and high-field electron paramagnetic resonance (HFEPR) under pressure. Owing to the large ZFS, the pressure effect on the electronic structure of iron-porphyrin complexes has not yet been explored using EPR. Therefore, we systematically studied this effect using our newly developed sub-terahertz EPR spectroscopy system in the frequency range of 80-515 GHz, under magnetic fields up to 10 T and pressure up to 2 GPa. We observed a systematic shift of the resonance fields of hemin upon pressure application, from which the axial component of the ZFS parameter was found to increase from D = 6.9 to 7.9 cm-1 at 2 GPa. In contrast to the previous methods used to study proteins under pressure, which mainly focused on conformational changes, our HFEPR technique can obtain more microscopic insights into the electronic structures of metal ions under pressure. In this sense, our technique provides novel opportunities to study the pressure effects on biofunctional active centers of versatile metalloproteins.

Heme in the process of bacterial survival and infection

Heme (heam) is a kind of iron porphyrin complexes. It is the most presence of iron in human bodies and also the main source of iron for pathogenic bacteria. Heme causes oxidative damage, cell death and organ failure and promotes inflammation in bacterial infection. This article will review the structure and sources of heme and its role in the process of bacterial growth and infection. Key words: Heme; Bacteria; Growth; Pathogenesis

Critical Factors in Determining the Heterolytic versus Homolytic Bond Cleavage of Terminal Oxidants by Iron(III) Porphyrin Complexes

Heterolytic versus homolytic cleavage of the metal-bound terminal oxidant is the key for determining the nature of reactive intermediates in metalloenzymes and metal catalyzed oxygenation reactions. Here, we study the bond cleavage process of hypochlorite by iron(III) porphyrin complexes having 4-methoxy-2,6-dimethylphenyl (1), 2,4,6-trimethylphenyl (2), 4-fluoro-2,6-dimethylphenyl (3), 2-chloro-6-methylphenyl (4), 2,6-dichlorophenyl (5), and 2,4,6-trichlorophenyl (6) groups at the meso position. Oxoiron(IV) porphyrin π-cation radical complexes (CompI) are characterized from the reactions of 1-4 with tetra- n-butylammonium hypochlorite (TBA-OCl) in dichloromethane at -80 °C, while oxoiron(IV) porphyrin complexes (CompII) are characterized for 5 and 6 under the same conditions. For all of 1-6, we find the formation of an epoxidation product in good yields from the catalytic reactions with TBA-OCl, suggesting heterolytic cleavages of the O-Cl bonds. CompI of 5 and 6 are reduced to the corresponding CompII by both chloride and hypochlorite, while CompI of 1-4 are not. The reduction reactions with hypochlorite are much faster than those with chloride. These results provide a mechanism where the O-Cl bond of the iron-bound hypochlorite is cleaved heterolytically to form CompI for all of 1-6, but the subsequent reduction reaction with remaining hypochlorite affords CompII for 5 and 6. The E(OCl•/OCl-) value is the boundary to discriminate the identity of the final product: CompI or CompII. Thermodynamic analysis based on the redox potential is successfully applied for explaining the bond cleavage processes of the hypochlorite, hydroperoxide, and tert-butyl peroxide complexes.

Using porphyrin-amino acid pairs to model the electrochemistry of heme proteins: experimental and theoretical investigations.

Quasi reversibility in electrochemical cycling between different oxidation states of iron is an often seen characteristic of iron containing heme proteins that bind dioxygen. Surprisingly, the system becomes fully reversible in the bare iron-porphyrin complex: hemin. This leads to the speculation that the polypeptide bulk (globin) around the iron-porphyrin active site in these heme proteins is probably responsible for the electrochemical quasi reversibility. To understand the effect of such polypeptide bulk on iron-porphyrin, we study the interaction of specific amino acids with the hemin center in solution. We choose three representative amino acids-histidine (a well-known iron coordinator in bio-inorganic systems), tryptophan (a well-known fluoroprobe for proteins), and cysteine (a redox-active organic molecule). The interactions of these amino acids with hemin are studied using electrochemistry, spectroscopy, and density functional theory. The results indicate that among these three, the interaction of histidine with the iron center is strongest. Further, histidine maintains the electrochemical reversibility of iron. On the other hand, tryptophan and cysteine interact weakly with the iron center but disturb the electrochemical reversibility by contributing their own redox active processes to the system. Put together, this study attempts to understand the molecular interactions that can control electrochemical reversibility in heme proteins. The results obtained here from the three representative amino acids can be scaled up to build a heme-amino acid interaction database that may predict the electrochemical properties of any protein with a defined polypeptide sequence.