Porphyrin Cation Radical Research Articles

Factors controlling the reactivity of synthetic compound-I Analogs.

A high-valent iron(IV)-oxo porphyrin radical cation (FeIV(O)(porph+•) serves as a key, reactive intermediate for a range of heme enzymes, including cytochrome P450 (CYP), horseradish peroxidase (HRP), and catalase (CAT). Synthetic analogs of this intermediate, known as Compound-I (Cpd-I) in the heme enzyme literature, have been generated with different tetrapyrrolic, macrocyclic ligands, including porphyrin derivatives, and the closely related ring-contracted macrocycles, corroles and corrolazines. These synthetic analogs have been useful for assigning and understanding structural and spectroscopic features and examining the reactivity of Cpd-I-like species in controlled and well-defined environments. This review focuses on summarizing recent developments in the synthesis and reactivity of high-valent iron-oxo porphyrinoid complexes in two main classes of reactions, proton-coupled electron transfer (PCET) and oxygen atom transfer (OAT). The relationship between the structure of the complexes and their reactivity is emphasized, including the influence of axial ligation and peripheral macrocyclic substitution, as well as the effects of solvent and secondary coordination spheres on the reactivity of the Cpd-I analogs. In bringing together the latest findings on Cpd-I analogs, this review intends to broaden our current understanding of the factors that control the stability and reactivity of Cpd-I species. This new knowledge should, in turn, point toward new synthetic strategies for constructing catalysts that rely on Cpd-I-like reactive intermediates.

Generation of high-valent iron-oxo porphyrin cation radicals on hemin loaded carbon nanotubes for efficient degradation of sulfathiazole.

Hemin has attracted considerable interest as an efficient catalyst recently, however, its direct application is inefficient due to severe molecular aggregation. Immobilizing hemin on various supports is a feasible approach to address this issue. In this work, a CNTs-hemin catalyst was prepared by loading hemin onto multiwalled carbon nanotubes (CNTs) through ball milling. Compared with hemin, CNTs-hemin demonstrates remarkably enhanced performance in the peroxymonosulfate system, with a 650-fold improvement of apparent rate constant, reaching 97.8% degradation of sulfathiazole in 5min. High-valent iron-oxo porphyrin cation ((Porp)+•FeIV=O) radicals are proposed as the dominant reactive species in the CNTs-hemin/peroxymonosulfate system instead of sulfate radicals (SO4•-), hydroxyl radicals (•OH), superoxide radicals (O2•-) and singlet oxygen (1O2). More in-depth mechanisms reveal that the strong electron transfer between CNTs and hemin promotes the generation of (Porp)+•FeIV=O radicals through a heterolysis pathway. This research enriches the understanding of the catalytic mechanism of supported biomimetic catalysts for PMS activation and provides a perspective on the role of support materials for catalytic activity.

Caught in the act: Monitoring O[sbnd]O bond cleavage in Acylperoxoferric cytochrome P450cam to form compound I in real time

While monitoring the reaction of ferric cytochrome P450cam (Cyp101) with substituted peroxybenzoic acids using rapid-scanning, stopped-flow (RSSF) spectroscopy, an intermediate appears en route to formation of the high-valent moiety known as Compound I [Fe(IV)=O/porphyrin radical cation] that is thought to be the key catalytic species for O-atom transfer to substrate. We have previously suggested (Spolitak, T., Dawson, J.H., Ballou, D.P., J. Biol. Chem.2005, 280, 20,300–20,309) that this species is an acylperoxo-ferric heme adduct that subsequently undergoes OO bond cleavage to generate Compound I. Singular value decomposition analysis of the RSSF data for formation of this intermediate shows that the energy of its Soret absorption peak is sensitive to the electron donor properties of the aryl substituents on the peracid. A linear Hammett correlation plot is seen for the energy of the Soret absorption peak vs. the Hammett σ constant. This correlation requires that the aryl substituents remain as part of the ligand bound to the heme iron, providing direct evidence that the adduct is indeed a ferric acylperoxo derivative. Linear Hammett correlation plots are also seen for both the rate of formation of the intermediate as well as for its conversion to Compound I. It is proposed that the electron donating/withdrawing properties of the aryl-bound substituents affect the electrophilic nature for binding substrate, changing the observed rate of formation for the acylperoxo intermediate, as well as the propensity and stability of the substituted benzoic acid to serve as the leaving group during OO bond cleavage yielding Compound I.

Metabolism of 8-aminoquinoline (8AQ) primaquine via aromatic hydroxylation step mediated by cytochrome P450 enzyme using density functional theory

The 8-aminoquinoline (8AQ) drug Primaquine (PQ) is a prime anti-malarial drug, used in the treatment of malaria due to plasmodium vivax and plasmodium ovale. The hydroxylated metabolite of Primaquine is also responsible for many essential sexual transmission stages of Plasmodium falciparum. Present work reported the hydroxylation of Primaquine at ortho (2PQ) and para (4PQ) position by Cytochrome P450 enzyme. Density functional theory (DFT) is used to investigate the underlying pathway for aromatic hydroxylation at ortho (2PQ) and para (4PQ) position to produce 2-hydroxylated and 4-hydroxylated Primaquine respectively. Truncated model of putative active oxidant i.e. ferryl oxo porphyrin cation radical [FeIV(O)(heme+·)], which is referred as Cpd I in Cytochrome P450 enzymes has been used to mimic the behavior of enzyme. Substrate was modelled and reaction mechanism for two degenerate spin states namely doublet and quartet were performed to dwell the overall potential energy landscape, along with electronic structure and properties of reactant complex (RC), transition states (TS), intermediates (IM) and product complex (PC). The reaction was stepwise with electrophillic addition as the rate determining step, spin selectivity product formation was observed for hydroxylated product formation on high spin (HS) surface. All calculations are done for isolated reaction coordinate.

Interplay between two degenerate spin state determines the hydroxylation of 4-nitrophenol catalyzed via Cytochrome P450

4-Nitrophenol is formed during the synthesis of paracetamol. It is used in various xenobiotic metabolism processes and other essential biochemical processes. It is metabolized via cytochrome P450 enzyme. The present work reported the hydroxylations of 4-nitrophenol at the ortho position by Cytochrome P450 metalloenzyme. Truncated model of putative active oxidant i.e. ferryl oxo porphyrin cation radical [FeIV(O)(heme+)], referred as Cpd I in cytochrome P450 enzymes has been used to mimic the behavior of enzyme. In the current investigations, 4-nitrophenol (Substrate) is modeled with Cpd I and reaction mechanism for two degenerate spin states named as doublet (LS) and quartet (HS) is performed to dwell the overall potential energy landscape, along with electronic structures and properties of reactant complex (RC), transition states (TS), intermediates (IM) and product complex (PC). The reaction is stepwise with electrophillic addition as the rate determining step, spin selectivity product formation is observed only at high spin (HS) surface. So, the present reaction pathway is single state reactivity (SSR) by forming the suicidal complex at low spin state (LS).

Tyrosyl Radical-Mediated Sequential Oxidative Decarboxylation of Coproporphyrinogen III through PCET: Theoretical Insights into the Mechanism of Coproheme Decarboxylase ChdC.

The peroxide-dependent coproheme decarboxylase ChdC from Geobacillus stearothermophilus catalyzes two key steps in the synthesis of heme b, i.e., two sequential oxidative decarboxylations of coproporphyrinogen III (coproheme III) at propionate groups P2 and P4. In the binding site of coproheme III, P2 and P4 are anchored by different residues (Tyr144, Arg217, and Ser222 for P2 and Tyr113, Lys148, and Trp156 for P4); however, strong experimental evidence supports that the generated Tyr144 radical acts as an unique intermediary for hydrogen atom transfer (HAT) from both reactive propionates. So far, the reaction details are still unclear. Herein, we carried out quantum mechanics/molecular mechanics calculations to explore the decarboxylation mechanism of coproheme III. In our calculations, the coproheme Cpd I, Fe(IV) = O coupled to a porphyrin radical cation (por•+) with four propionate groups, was used as a reactant model. Our calculations reveal that Tyr144 is directly involved in the decarboxylation of propionate group P2. First, the proton-coupled electron transfer (PCET) occurs from Tyr144 to P2, generating a Tyr144 radical, which then abstracts a hydrogen atom from the Cβ of P2. The β-H extraction was calculated to be the rate-limiting step of decarboxylation. It is the porphyrin radical cation (por•+) that makes the PCET from Tyr144 to P2 to be quite easy to initiate the decarboxylation. Finally, the electron transfers from the Cβ• through the porphyrin to the iron center, leading to the decarboxylation of P2. Importantly, the decarboxylation of P4 mediated by Lys148 was calculated to be very difficult, which suggests that after the P2 decarboxylation, the generated harderoheme III intermediate should rebind or rotate in the active site so that the propionate P4 occupies the binding site of P2, and Tyr144 again mediates the decarboxylation of P4. Thus, our calculations support the fact that Tyr144 is responsible for the decarboxylation of both P2 and P4.

Mechanistic Insights into the P450 TleB-Catalyzed Unusual Intramolecular C-N Bond Formation Involved in the Biosynthesis of Indolactam V.

Indolactam V, a known biosynthetic precursor of indolactam alkaloids, is the main pharmacophore of this family and exhibits potential protein kinase C activation. A key step in the biosynthesis of indolactam V is the formation of an indole-fused nine-membered lactam core by intramolecular C-N bond formation. In this work, we report a computational study of the unique cytochrome P450 TleB enzyme-catalyzed direct and selective C-H bond amination reaction that can generate indolactam V from the dipeptide N-methylvalyl-tryptophanol. By performing molecular dynamics simulations and quantum-mechanical/molecular-mechanical calculations, we revealed that the C-H bond amination involves one step of proton transfer from N1-H of the indole ring to the FeIV═O unit, one step of hydrogen abstraction of N13-H in the side chain of the substrate by the FeIV-OH unit, and diradical coupling, in which two conformational changes of the side chain of the substrate are necessary. In the enzyme-substrate complex of TleB, the N-H bond of the indole ring of the substrate forms a strong hydrogen bond with the FeIV═O unit in compound I, and the porphyrin radical cation accepts an electron from the substrate to form the closed-shell electronic configuration. Thus, compound I in the enzyme-substrate complex cannot be described as FeIV═O coupled to a porphyrin radical cation, which is different from those of other P450 enzymes. Besides, two stages of conformational changes of the side chains of the substrate may increase the relative energies of reaction intermediates by 10-12 kcal/mol. From the structure point of view, it is the rotatable long side chain of the substrate and the large flexible active pocket of TleB that make the intramolecular diradical coupling feasible. Our findings may provide useful information to further understand the Tleb-catalyzed intramolecular C-H bond amination and the other bio-catalyzed intramolecular diradical coupling.

A 3D electropolymerized thin film based on an isoporphyrin and on a pyridine end-decorated molybdenum(II) halide cluster: Photoelectrochemical and impedance properties

Herein, the first example of formation of a 3D hybrid organic-inorganic copolymer based on an isoporphyrin and on a pyridine end-decorated molybdenum(II) halide cluster. The isoporphyrin copolymer thin film was obtained by electropolymerization of zinc-5,15-bis(p-tolyl)porphyrin in the presence of polyiodometalate with 6 pendant pyridyl ligands (((Bu4N)2[{Mo6I8}(OOC-C5H4N)6]). Electrogenerated porphyrin radical cation is a powerful electrophile which can rapidly react with the pyridine end-decorated Mo(II) halide cluster as nucleophile to form 3D copolymer containing stable isoporphyrin.The electropolymerization was monitored by electrochemical quartz crystal microbalance (EQCM). The copolymer was characterized by UV-Vis-NIR spectroscopy, X-ray photoelectron spectroscopy (XPS), electrochemistry, and atomic force microscopy (AFM). The existence of the radical of the isoporphyrin copolymer was proved by electron spin resonance spectroscopy (EPR).Interestingly, the UV-Vis-NIR spectra of the copolymer exhibited broadening and splitting of the Soret band as well as one additional band in the NIR between 750 nm and 1000 nm.The photoelectrochemical properties have been studied by electrochemical impedance spectroscopy and by photocurrent transient measurements under visible-NIR light irradiation.The 3D copolymer containing stable isoporphyrin radical exhibits interesting photocurrent response even under only NIR illumination.In the solid state, the 3D copolymer could be stored for over several months and even one year without any degradation under ambient conditions in air.

Donor-acceptor conjugates derived from cobalt porphyrin and fullerene via metal-ligand axial coordination: Formation and excited state charge separation

Two types of cobalt porphyrins, viz., meso-tetrakis(tolylporphyrinato)cobalt(II), (TTP)Co (1), and meso-tetrakis(triphenylamino porphyrinato)cobalt(II), [(TPA)4P]Co, (2) were self-assembled via metal-ligand axial coordination of phenyl imidazole functionalized fulleropyrrolidine, ImC[Formula: see text] to form a new series of donor–acceptor constructs. A 1:2 complex formation with ImC[Formula: see text] was established in the case of (TTP)Co while for [(TPA)4P]Co only a 1:1 complex was possible to positively identify. The binding constants [Formula: see text] and [Formula: see text] for step-wise addition of ImC[Formula: see text] to (TTP)Co were found to be 1.07 × 105 and 3.20 × 104 M[Formula: see text], respectively. For [(TPA)4P]Co:ImC[Formula: see text], the measured [Formula: see text] values was found to be 6.48 × 104 M[Formula: see text], slightly smaller than that observed for (TTP)Co. Although both cobalt porphyrins were non-fluorescent, they were able to quench the fluorescence of ImC[Formula: see text] indicating occurrence of excited state events in the supramolecular donor-acceptor complexes. Electrochemistry coupled with spectroelectrochemistry, revealed the formation of cobalt(III) porphyrin cation instead of a cobalt(II) porphyrin radical cation, as the main product, during oxidation of phenyl imidazole coordinated cobalt porphyrin. With the help of computational and electrochemical results, an energy level diagram was constructed to witness excited state photo-events. Competitive energy and electron transfer from excited CoP to coordinated ImC[Formula: see text], and electron transfer from Im1C[Formula: see text]* to cobalt(II) porphyrin resulting into the formation of PCo[Formula: see text]:ImC[Formula: see text] charge separated state was possible to envision from the energy diagram. Finally, using femtosecond transient absorption spectroscopy and data analysis by Glotaran, it was possible to establish sequential occurrence of energy transfer and charge separation processes. The lifetime of the final charge separated state was [Formula: see text] 2 ns. A slightly better charge stabilization was observed in the case of [(TPA)4P]Co:ImC[Formula: see text] due to the presence of electron rich, peripheral triphenylamine substituents on the cobalt porphyrin.

Understanding the Mechanistic Requirements for Efficient and Stereoselective Alkene Epoxidation by a Cytochrome P450 Enzyme

The cytochrome P450 (CYP) family of heme monooxygenase enzymes commonly catalyzes enantioselective hydroxylation and epoxidation reactions. Epoxidation reactions have been hypothesized to proceed via multiple mechanisms involving different reactive intermediates. Here, we use activity, spectroscopic, structural, and molecular dynamics data to investigate the activity and stereoselectivity of 4-vinylbenzoic acid epoxidation by the bacterial enzyme CYP199A4 from Rhodopseudomonas palustris HaA2. The epoxidation of 4-vinylbenzoic acid by CYP199A4 proceeded with high enantioselectivity, giving the (S)-epoxide in 99% ee at an activity of 220 nmol nmol-CYP–1 min–1. Optical and EPR spectroscopy, redox potential measurements, and the crystal structure of 4-vinylbenzoic acid-bound CYP199A4 indicated the partial retention of an aqua ligand at the heme center in the presence of the substrate, providing a justification of the lower activity (∼20%) compared to the oxidative demethylation of 4-methoxybenzoic acid. Mutagenesis at the conserved acid–alcohol pair (D251/T252), which perturbs the generation of the reactive oxygen intermediates, was employed to investigate their role in epoxidation reactions. The T252A mutant increased the rate of turnover of the catalytic cycle, but an elevation in hydrogen peroxide generation via uncoupling resulted in a similar rate of epoxide formation. The activity of epoxidation significantly reduced with the D251N mutant. The chemoselectivity and stereoselectivity of the epoxidation reaction were maintained in the turnovers by these mutants. Overall, there was little evidence that other intermediates, aside from the archetypal reactive ferryl porphyrin cation radical, Compound I, contributed significantly to the epoxidation reaction. The observation of the high selectivity for the (S)-enantiomer was rationalized by molecular dynamics simulations. When the arrangement of the alkene and the active intermediate approached an ideal transition state structure for epoxidation, one face of the alkene was more often exposed to the iron oxo unit.

Serial Femtosecond Zero Dose Crystallography Captures a Water‐Free Distal Heme Site in a Dye‐Decolorising Peroxidase to Reveal a Catalytic Role for an Arginine in FeIV=O Formation

AbstractObtaining structures of intact redox states of metal centers derived from zero dose X‐ray crystallography can advance our mechanistic understanding of metalloenzymes. In dye‐decolorising heme peroxidases (DyPs), controversy exists regarding the mechanistic role of the distal heme residues aspartate and arginine in the heterolysis of peroxide to form the catalytic intermediate compound I (FeIV=O and a porphyrin cation radical). Using serial femtosecond X‐ray crystallography (SFX), we have determined the pristine structures of the FeIII and FeIV=O redox states of a B‐type DyP. These structures reveal a water‐free distal heme site that, together with the presence of an asparagine, imply the use of the distal arginine as a catalytic base. A combination of mutagenesis and kinetic studies corroborate such a role. Our SFX approach thus provides unique insight into how the distal heme site of DyPs can be tuned to select aspartate or arginine for the rate enhancement of peroxide heterolysis.

Serial Femtosecond Zero Dose Crystallography Captures a Water-Free Distal Heme Site in a Dye-Decolorising Peroxidase to Reveal a Catalytic Role for an Arginine in FeIV =O Formation.

Obtaining structures of intact redox states of metal centers derived from zero dose X‐ray crystallography can advance our mechanistic understanding of metalloenzymes. In dye‐decolorising heme peroxidases (DyPs), controversy exists regarding the mechanistic role of the distal heme residues aspartate and arginine in the heterolysis of peroxide to form the catalytic intermediate compound I (FeIV=O and a porphyrin cation radical). Using serial femtosecond X‐ray crystallography (SFX), we have determined the pristine structures of the FeIII and FeIV=O redox states of a B‐type DyP. These structures reveal a water‐free distal heme site that, together with the presence of an asparagine, imply the use of the distal arginine as a catalytic base. A combination of mutagenesis and kinetic studies corroborate such a role. Our SFX approach thus provides unique insight into how the distal heme site of DyPs can be tuned to select aspartate or arginine for the rate enhancement of peroxide heterolysis.

Oriented (Local) Electric Fields Drive the Millionfold Enhancement of the H-Abstraction Catalysis Observed for Synthetic Metalloenzyme Analogues.

This contribution follows the recent remarkable catalysis observed by Groves et al. in hydrogen-abstraction reactions by a) an oxoferryl porphyrin radical-cation complex [Por⋅+ FeIV (O)Lax ] and b) a hydroxoiron porphyrazine ferric complex [PyPzFeIII (OH)Lax ], both of which involve positively charged substituents on the outer circumference of the respective macrocyclic ligands. These charge-coronated complexes are analogues of the biologically important Compound I (Cpd I) and synthetic hydroxoferric species, respectively. We demonstrate that the observed enhancement of the H-abstraction catalysis for these systems is a purely electrostatic effect, elicited by the local charges embedded on the peripheries of the respective macrocyclic ligands. Our findings provide new insights into how electrostatics can be employed to tune the catalytic activity of metalloenzymes and can thus contribute to the future design of new and highly efficient hydrogen-abstraction catalysts.

Interaction of light with a non-covalent zinc porphyrin-graphene oxide nanohybrid.

The present study explores the influence of graphene oxide (GO) on deactivation pathways of the excited states of zinc 5,10,15,20-tetrakis(4-(hydroxyphenyl))porphyrin (ZnTPPH). The interaction of light with free ZnTPPH molecules and with ZnTPPH molecules adsorbed on graphene oxide sheets was probed via UV-vis spectroscopy, fluorescence spectroscopy, femtosecond pump-probe technique and nanosecond flash photolysis. Formation of the ground-state ZnTPPH-GO complex in solution was monitored by the red-shift of the porphyrin Soret absorption band. It was found that Stern-Volmer fluorescence quenching can be described in terms of two different quenching regimes depending on the GO concentration. In addition, our comprehensive analysis of the steady-state and time-resolved emission experiments led to the conclusion that the observed quenching was entirely attributable to a static mechanism. Laser flash photolysis showed that the triplet lifetime of the ZnTPPH increased in the presence of GO from 174 μs to 292 μs, which is related to the decrease in the rate constant of a radiationless decay mechanism involving rotation of the peripheral hydroxyphenyl rings of the porphyrin. Femtosecond transient absorption spectroscopy demonstrated the presence of a fast photoinduced electron transfer from the singlet excited state of ZnTPPH to the GO sheets, as indicated by the formation of a porphyrin radical cation. Quantum chemical calculations were used to gain deeper insights into the nature of the electronically excited states in the free ZnTPPH as well as in the ZnTPPH-GO complex.

Stable isoporphyrin copolymer: Electrochemical mechanism and behavior and photovoltaic properties

Two types of porphyrin copolymer thin films were obtained by electropolymerization of zinc-5,15-bis(p-tolyl)porphyrin. Electrogenerated porphyrin radical cation and dication are powerful electrophiles which can rapidly react with 1,1″-(1,3-propanediyl)bis-4,4′-bipyridinium hexafluorophosphate salt (bpy+–(CH2)3–+bpy·2PF6−) as nucleophiles to form copolymers containing stable isoporphyrin radicals or porphyrins, depending of the applied upper potential limit (1.0 V versus 1.6 V). Electrochemical routes leading to these electroactive copolymers are discussed and the unusual redox properties of copolymers containing stable isoporphyrin radicals are studied. The electropolymerization of the two copolymers was monitored by electrochemical quartz crystal microbalance (EQCM). The copolymers were characterized by UV–Vis–NIR spectroscopy, X-ray photoelectron spectroscopy (XPS), electrochemistry, electron spin resonance (ESR) and atomic force microscopy (AFM). Their electrical properties have been studied by electrochemical impedance spectroscopy and their photovoltaic performances have been investigated by photocurrent transient measurements under visible-NIR light irradiation. The isoporphyrin radical copolymer exhibited considerably better photocurrent generation when compared to the porphyrin copolymer.

Electron Transfer Control of Reductase versus Monooxygenase: Catalytic C-H Bond Hydroxylation and Alkene Epoxidation by Molecular Oxygen.

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 that generate highly oxidizing species from O2, which, in turn, can oxidize very stable organic substrates, e.g., mono-/dioxygenases. The same oxidants are produced during O2 reduction/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 a 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 that otherwise show O2 reductase activity. Catalytic hydroxylation of strong C–H bonds to alcohol and epoxidation of alkenes, using molecular O2 (with 18O2 incorporation), is demonstrated with turnover numbers >104. Uniquely, one of the two iron porphyrin catalysts used shows preferential oxidation of 2° C–H bonds of cycloalkanes to alcohols over 3° C-H bonds without overoxidation to ketones. Mechanistic investigations with labeled substrates indicate that a compound I (FeIV=O bound to a porphyrin cation radical) analogue, formed during O2 reduction, is the primary oxidant. The selectivity is determined by the shape of the distal pocket of the catalyst, which, in turn, is determined by the substituents on the periphery of the porphyrin macrocycle.

Substitution effects on olefin epoxidation catalyzed by Oxoiron(IV) porphyrin π-cation radical complexes: A dft study.

The effects of peripheral fluorine atoms on epoxidation reactions of ethylene by oxoiron(IV) porphyrin cation radical complex in the quartet and sextet spin multiplicities are systematically investigated using the DFT method. The overall reaction routes are determined using a model system of ethylene and Fe(IV)OCl-porphyrin with substituted fluorine atoms. By obtaining the energy diagrams and electron- and spin-density difference contour maps of the transition states and intermediate compounds, we confirm that the electron-withdrawing by peripheral fluorine atoms enhances the reactivity as the number of fluorine atoms increases, as is observed experimentally. The intersystem crossing between the quartet and sextet spin multiplicities is discussed by means of the intrinsic reaction coordinate method. We conclude that the rate-determining step is located at the first transition state (TS1) for the activation of CC and FeO bonds, and the ground electronic state changes from quartet to sextet around the TS1. © 2019 Wiley Periodicals, Inc.

Cationic Porphyrin-Graphene Oxide Hybrid: Donor-Acceptor Composite for Efficient Photoinduced Electron Transfer.

Non-covalent nanohybrids composed of cationic 5,10,15,20-tetra(4-trimethylammoniophenyl)porphyrin tetra(p-toluenesulfonate) (TMAP) and the graphene oxide sheets were prepared under two pH values (6.2 vs. 1.8). The TMAP molecule was positively charged, regardless of the pH value during preparation. However, protonation of the imino nitrogens increased the overall charge of the porphyrin molecule from +4 to +6 (TMAP4+ and TMAP6+ ). It was found that at acidic pH, interaction of TMAP6+ with GO was largely suppressed. On the other hand, results of FTIR, Raman spectroscopy, thermogravimetric analysis, atomic force microscopy (AFM) and elemental analysis confirmed effective non-covalent functionalization of graphene oxide with cationic porphyrin at pH 6.2. The TMAP4+ -GO hybrids exhibited well defined structure with a monolayer of TMAP4+ on the GO sheets as confirmed by AFM. Formation of the ground-state TMAP4+ -GO complex in solution was monitored by the red-shift of the porphyrin Soret absorption band. This ground-state interaction between TMAP4+ and GO is responsible for the static quenching of the porphyrin emission. Fluorescence was not detected for the nanohybrid which indicated that a very fast deactivation process had to take place. Ultrafast time-resolved transient absorption spectroscopy clearly demonstrated the occurrence of electron transfer from the photoexcited TMAP4+ singlet state to GO sheets, as proven by the formation of a porphyrin radical cation.

Electrochemical Evidence of the Catalytic Activity of Meso-Tetraphenylporphyrin Diacid in the Proton Reduction in Benzonitrile

Our work shows the electrochemical behavior of meso-tetraphenylporphyrin diacid (H4TPP2+) in tetrabutylammonium perchlorate Bu4NClO4 0.1 M / benzonitrile. On a cathodic sweep and scan rates between 0.1 and 3.0 V/s, H4TPP2+ undergoes two closely spaced consecutive reversible one-electron transfers (Ic’/Ia’ and IIc’/IIa’; Figure 1), which can be assigned to formation of diprotonated porphyrin radical cation (H4TPP•+), and isophlorin H4TPP species, respectively1. In presence of an extra equivalent of HClO4, a new reduction peak, IIIc’, is seen, suggesting that isophlorin H4TPP is being protonated. It is believed that the protonation takes place at one of the meso-positions of the porphyn ring leading to phlorin cation (H4TPFH+). The phlorin cation is electroactive and can be either oxidized (peaks IVa’ and Va’) or reduced (IIIc’)2. As seen in Figure 1, the addition of extra equivalents of H+ to solutions of H4TPP2+ provokes an increase in the current and loss of reversibility of the two reduction processes, Ic’/Ia’ and IIc’/IIa’. This behavior was explained as due to the reaction between the free protons in the medium and the hydrogen atom of the meso-position of the H4TPFH+, in a reaction that produces molecular hydrogen and regenerates the porphyrin diacid. Digital simulation of the experimental voltammograms of H4TPP2+ indicates that the H4TPP•+ and H4TPP species are stable on the cyclic voltammetry time scale. However, coulometry experiments at the potential of the second reduction process show that at longer time scale, the isophlorin H4TPP undergoes coupled chemical reactions. Voltammograms of the electrolyzed solution exhibit oxidation and reduction signals that are characteristics of H4TPFH+ species, also the presence of the bands at 460 nm and 773 nm in the UV-visible spectrum is consistent with the presence of H4TPFH+ species3,4. It is also demonstrated that the H4TPFH+ species can be reoxidized back to the original H4TPP2+ in a two-electron process. References. 1. H. M. Castro-Cruz, thesis, UNAM (2017). http://132.248.9.195/ptd2017/junio/410088896/Index.html 2. Y. Cui, L. Zeng, Y. Fang, J. Zhu, C. H. Devillers, D. Lucas, N. Desvbois, C. P. Gros and K. M. Kadish, ChemElectroChem, 3, 228–241 (2016). 3. G. Peychal-Heiling and G. S. Wilson, Anal. Chem., 43, 550–556 (1971). 4. G. L. Closs and L. E. Closs, J. Am. Chem. Soc., 85, 818–819 (1963). Figure 1

Water Molecules Gating a Photoinduced One-Electron Two-Protons Transfer in a Tyrosine/Histidine (Tyr/His) Model of Photosystem II.

We investigate a biomimetic model of a TyrZ /His190 pair, a hydrogen-bonded phenol/imidazole covalently attached to a porphyrin sensitizer. Laser flash photolysis in the presence of an external electron acceptor reveals the need for water molecules to unlock the light-induced oxidation of the phenol through an intramolecular pathway. Kinetics monitoring encompasses two fast phases with distinct spectral properties. The first phase is related to a one-electron transfer from the phenol to the porphyrin radical cation coupled with a domino two-proton transfer leading to the ejection of a proton from the imidazole-phenol pair. The second phase concerns conveying the released proton to the porphyrin N4 coordinating cavity. Our study provides an unprecedented example of a light-induced electron-transfer process in a TyrZ /His190 model of photosystem II, evidencing the movement of both the phenol and imidazole protons along an isoenergetic pathway.