Pi-cation Radical Research Articles

Natural alternative heme-environments allow efficient peroxygenase activity by cytochrome P450 monooxygenases

Mild oxyfunctionalization reactions driven by hydrogen peroxide using cytochrome P450 monooxygenases with alternative heme environments.

Probing the Rate of Hole Transfer in Oxidized Porphyrin Dyads Using Thallium Hyperfine Clocks

Understanding hole/electron-transfer processes among interacting constituents of multicomponent molecular architectures is central to the fields of artificial photosynthesis and molecular electronics. One strategy for examining ground-state hole/electron transfer in oxidized tetrapyrrolic arrays entails analysis of the hyperfine interactions observed in the electron paramagnetic resonance (EPR) spectrum of the pi-cation radical. Herein, it is demonstrated that (203)Tl/(205)Tl hyperfine "clocks" are greatly superior to those provided by (1)H, (14)N, or (13)C owing to the fact that the (203)Tl/(205)Tl hyperfine couplings are much larger (15-25 G) than those of the (1)H, (14)N, or (13)C nuclei (1-6 G). The large (203)Tl/(205)Tl hyperfine interactions permit accurate simulations of the EPR spectra and the extraction of specific rates of hole/electron transfer. The (203)Tl/(205)Tl hyperfine clock strategy is applied to a series of seven porphyrin dyads. All of the dyads are joined at a meso position of the porphyrin macrocycle via linkers of a range of lengths and composition (diphenylethyne, diphenylbutadiyne, and (p-phenylene)(n), where n = 1-4); substituents such as mesityl at the nonlinking meso positions are employed to provide organic solubility. The hole/electron-transfer time constants are in the hundreds of picoseconds to sub-10 ns regime, depending on the specific porphyrin and/or linker. Density functional theory calculations on the constituents of the dyads are consistent with the view that the relative energies of the porphyrin versus linker highest occupied molecular orbitals strongly influence the hole/electron-transfer rates. Variable-temperature EPR studies further demonstrate that the hole/electron-transfer process is at best weakly activated (12-15 kJ mol(-1)) at room temperature and somewhat below. At lower temperatures, the process is essentially activationless. The weak activation is attributed to restricted torsional motions of the phenyl rings of the linker. Collectively, the studies provide the physical basis for the rational design of multicomponent architectures for efficient hole/electron transfer.

Structural and Magnetic Effects of meso-Substitution in Alkyl-Substituted Metalloporphyrinate π-Cation Radicals: Characterization of [Fe(TalkylP•)(Cl)]SbCl6 (alkyl = ethyl and n-propyl)

We report the preparation and characterization of two meso-alkyl substituted porphyrin pi-cation radical derivatives, [Fe(TalkylP(*))(Cl)]SbCl(6) (alkyl = ethyl or propyl). Both complexes have been characterized by UV/vis/near-IR, IR, and Mossbauer spectroscopy, temperature-dependent solid-state magnetic susceptibility measurements, and X-ray structure determinations. All data for both oxidized species are consistent with the formulation of the complexes as ring-oxidized iron(III) porphyrin species. The molecular structures of the two five-coordinate species have the typical square-pyramidal coordination group of high-spin iron(III) derivatives. The crystal structures also reveal that the species form cofacial pi-pi dimers with lateral shifts of 1.44 A and 3.22 A, respectively, for the propyl and ethyl radical derivatives. Both radicals exhibit porphyrin cores with alternating bond distance patterns in the inner 16-membered ring. In addition, [Fe(TEtP(*))(Cl)]SbCl(6) and [Fe(TPrP(*))(Cl)]SbCl(6) have been characterized by temperature-dependent (6-300 K) magnetic susceptibility studies, the best fitting of the temperature-dependent moments reveal strong coupling between iron spins and porphyrin radical, and a smaller magnitude of antiferromagnetic coupling between ring radicals, which are opposite to those found in the five-coordinate iron(III) OEP radicals. The differences in structure and properties of the cation radical meso-alkyl and beta-alkyl derivatives possibly reflect differences in properties of a(1u)- and a(2u)-forming radicals.

Sensitive Efficiency of Photoinduced Electron Transfer to Band Gaps of Semiconductive Single-Walled Carbon Nanotubes with Supramolecularly Attached Zinc Porphyrin Bearing Pyrene Glues

Photoinduced electron transfer in self-assembled single-walled carbon nanotube (SWNT)/zinc porphyrin (ZnP) hybrids utilizing (7,6)- and (6,5)-enriched SWNTs has been investigated. Toward this, first, zinc porphyrin was covalently functionalized to possess four pyrene entities (ZnP(pyr)(4)). Exfoliation of the semiconducting nanotube bundles occurred due to pi-pi-type interactions with the pyrene and porphyrin entities in organic solvents. The nanohybrids thus formed were isolated and characterized by TEM, UV-visible-near-IR, and Raman spectroscopy. Free-energy calculations suggested the possibility of electron transfer in both the (7,6)- and (6,5)-possessing ZnP(pyr)(4)/SWNT nanohybrids. Accordingly, fluorescence studies revealed efficient quenching of the singlet excited state of ZnP in the nanohybrids, originating from the charge separation, as confirmed by observation of a ZnP pi-cation radical in transient absorption spectra. The rates of charge separation were found to be slightly higher for (7,6)-SWNT-derived hybrids compared to the (6,5)-SWNT-derived hybrids. Charge recombination revealed an opposite effect, indicating that the (7,6)-SWNTs are slightly better for charge stabilization compared to the (6,5)-SWNTs. The present nanohybrids were further utilized to photochemically reduce the hexyl viologen dication in the presence of a sacrificial electron donor in an electron-pooling experiment, offering additional proof for the occurrence of photoinduced charge separation and potential utilization of these materials in light-energy-harvesting applications. Finally, solar cells constructed using the ZnP/SWNT hybrids revealed higher efficiency for the ZnP(pyr)(4)/(7,6)-SWNT hybrid with narrower nanotube band gap compared with the ZnP(pyr)(4)/(6,5)-SWNT having a relatively wider band gap.

Synthesis and characterization of free-base, copper, and nickel isocorroles.

A series of free-base and metalated isocorroles represented as (TT-n-iso-Cor)H(2) and (TT-n-iso-Cor)M(II), where n = 5 or 10 and M = Ni or Cu, were synthesized and characterized by electrochemistry and spectroelectrochemistry in CH(2)Cl(2) containing 0.1 M tetra-n-butylammonium perchlorate. The metalation of the free-base macrocycles with Co(II), Mn(III), or Zn(II) was also attempted but was unsuccessful. Six isocorroles were isolated and shown to undergo two stepwise oxidations to give pi-cation radicals and dications in CH(2)Cl(2), with the most stable products being obtained in the case of the 10-substituted derivatives. The same isocorroles could also be reduced by one or two electrons, but the initial one-electron addition products are unstable and undergo a rapid chemical reaction giving a reduced corrole or corrole-like product, which could be reoxidized to the corresponding (TTCor)M at a controlled positive potential. This series of reactions effectively illustrates an isocorrole to corrole conversion upon reduction and reoxidation and was monitored by both electrochemistry and thin-layer spectroelectrochemistry.

Probing the Rate of Hole Transfer in Oxidized Synthetic Chlorin Dyads via Site-Specific 13C-Labeling

Understanding electronic communication among interacting constituents of multicomponent molecular architectures is important for rational design in diverse fields including artificial photosynthesis and molecular electronics. One strategy for examining ground-state hole/electron transfer in an oxidized tetrapyrrolic array relies on analysis of the hyperfine interactions observed in the EPR spectrum of the pi-cation radical. This strategy has been previously employed to probe the hole/electron-transfer process in oxidized multiporphyrin arrays of normal isotopic composition, wherein (1)H and (14)N serve as the hyperfine "clocks", and in arrays containing site-specific (13)C-labels, which serve as additional hyperfine clocks. Herein, the hyperfine-clock strategy is applied to dyads of dihydroporphyrins (chlorins). Chlorins are more closely related structurally to chlorophylls than are porphyrins. A de novo synthetic strategy has been employed to introduce a (13)C label at the 19-position of the chlorin macrocycle, which is a site of large electron/hole density and is accessible synthetically beginning with (13)C-nitromethane. The resulting singly (13)C-labeled chlorin was coupled with an unlabeled chlorin to give a dyad wherein a diphenylethyne linker spans the 10-positions of the two zinc chlorins. EPR studies of the monocations of both the natural abundance and (13)C-labeled zinc chlorin dyads and benchmark zinc chlorin monomers reveal that the time scale for hole/electron transfer is in the 4-7 ns range, which is 5-10-fold longer than that in analogous porphyrin arrays. The slower hole/electron transfer rate observed for the chlorin versus porphyrin dyads is attributed to the fact that the HOMO is a(1u)-like for the chlorins versus a(2u)-like for the porphyrins; the a(1u)-like orbital exhibits little (or no) electron/hole density at the site of linker attachment whereas the a(2u)-like orbital exhibits significant electron/hole density at this site. Collectively, the studies of the chlorin and porphyrin dyads provide insights into the structural features that influence the hole/electron-transfer process.

Exploring the redox reactivity of magnesium porphine. Insight into the origins of electropolymerisation

Magnesium(II) porphine, MgP (1), was synthesised according to the Lindsey procedure allowing to isolate and crystallise 1-formyldipyrromethane (2) as a synthetic intermediate. Unprecedented X-ray diffraction studies revealed multiple intermolecular associations in the crystal between neighbouring units of 2, namely hydrogen bonds and CH...pi. The electrochemical behaviour of 1 was examined by means of cyclic voltammetry. In oxidation, two well-defined and distinct steps are assigned to macrocycle concerned electron transfers yielding initially the pi-cation radical and pi-dication, respectively. The highly reactive dication condenses neutral magnesium porphine to form a diprotonated di-isoporphine species, which is assumed to be a key intermediate at the origin of the electropolymerisation. Electrolyses were performed at the potential of the pi-cation radical generation. Investigation of the electrolysed solution by UV-vis spectroscopy and MALDI-TOF spectrometry revealed the presence of several oligoporphyrins, with diporphine as an important product. In all the oligomers series, extensive demetallation is evidenced, due to the increasing acidity of the medium as oligomerisation progresses. This demetallation could be prevented by addition of 2,6-lutidine as a base prior to electrolysis. In the oligomer series, the porphine units seem to be connected through meso-meso bonds, an argument supported by certain features of the UV-vis spectrum. Finally, the mechanism of oxidative oligomerisation is discussed, from the point of view of the initial steps of the electropolymerisation.

Spectroscopic studies of the oxidation of ferric CYP153A6 by peracids: Insights into P450 higher oxidation states

Our previous rapid-scanning stopped-flow studies of the reaction of substrate-free cytochrome P450cam with peracids [T. Spolitak, J.H. Dawson, D.P. Ballou, J. Biol. Chem. 280 (2005) 20300-20309; J. Inorg. Biochem. 100 (2006) 2034-2044; J. Biol. Inorg. Chem. 13 (2008) 599-611] spectrally characterized compound I (ferryl iron plus a porphyrin pi-cation radical (Fe(IV)O/Por(.+))), Cpd ES, and Cpd II (Fe(IV)O/Tyr() or Fe(IV)O). We now report that reactions of CYP153A6 with peracids yield all these intermediates, with kinetic profiles allowing better resolution of all forms at pH 8.0 compared to similar reactions with WT P450cam. Properties of the reactions of these higher oxidation state intermediates were determined in double-mixing experiments in which intermediates are pre-formed and ascorbate is then added. Reactions of heptane-bound CYP153A6 (pH 7.4) with mCPBA resulted in conversion of P450 to the low-spin ferric form, presumably as heptanol was formed, suggesting that CYP 153A6 is a potential biocatalyst that can use peracids with no added NAD(P)H or reducing systems for bioremediation and other industrial applications.

Enhanced Reactivities of Iron(IV)‐Oxo Porphyrin π‐Cation Radicals in Oxygenation Reactions by Electron‐Donating Axial Ligands

The proximal axial ligand in heme iron enzymes plays an important role in tuning the reactivities of iron(IV)-oxo porphyrin pi-cation radicals in oxidation reactions. The present study reports the effects of axial ligands in olefin epoxidation, aromatic hydroxylation, alcohol oxidation, and alkane hydroxylation, by [(tmp)(+*) Fe(IV)(O)(p-Y-PyO)](+) (1-Y) (tmp = meso-tetramesitylporphyrin, p-Y-PyO = para-substituted pyridine N-oxides, and Y = OCH(3), CH(3), H, Cl). In all of the oxidation reactions, the reactivities of 1-Y are found to follow the order 1-OCH(3) > 1-CH(3) > 1-H > 1-Cl; negative Hammett rho values of -1.4 to -2.7 were obtained by plotting the reaction rates against the sigma(p) values of the substituents of p-Y-PyO. These results, as well as previous ones on the effect of anionic nucleophiles, show that iron(IV)-oxo porphyrin pi-cation radicals bearing electron-donating axial ligands are more reactive in oxo-transfer and hydrogen-atom abstraction reactions. These results are counterintuitive since iron(IV)-oxo porphyrin pi-cation radicals are electrophilic species. Theoretical calculations of anionic and neutral ligands reproduced the counterintuitive experimental findings and elucidated the root cause of the axial ligand effects. Thus, in the case of anionic ligands, as the ligand becomes a better electron donor, it strengthens the FeO-H bond and thereby enhances its H-abstraction activity. In addition, it weakens the Fe=O bond and encourages oxo-transfer reactivity. Both are Bell-Evans-Polanyi effects, however, in a series of neutral ligands like p-Y-PyO, there is a relatively weak trend that appears to originate in two-state reactivity (TSR). This combination of experiment and theory enabled us to elucidate the factors that control the reactivity patterns of iron(IV)-oxo porphyrin pi-cation radicals in oxidation reactions and to resolve an enigmatic and fundamental problem.

A highly reactive p450 model compound I.

The detection and kinetic characterization of a cytochrome P450 model compound I, [OFe(IV)-4-TMPyP](+) (1), in aqueous solution shows extraordinary reaction rates for C-H hydroxylations. Stopped-flow spectrophotometric monitoring of the oxidation of Fe(III)-4-TMPyP with mCPBA revealed the intermediate 1, which displays a weak, blue-shifted Soret band at 402 nm and an absorbance at 673 nm, typical of a porphyrin pi-radical cation. This intermediate was subsequently transformed into the well-characterized OFe(IV)-4-TMPyP. Global analysis afforded a second-order rate constant k(1) = (1.59 +/- 0.06) x 10(7) M(-1) s(-1) for the formation of 1 followed by a first-order decay with k(2) = 8.8 +/- 0.1 s(-1). (1)H and (13)C NMR determined 9-xanthydrol to be the major product (approximately 90% yield) of xanthene oxidation by 1. Electrospray ionization mass spectrometry carried out in 47.5% (18)OH(2) indicated 21% (18)O incorporation, consistent with an oxygen-rebound reaction scenario. Xanthene/xanthene-d(2) revealed a modest kinetic isotope effect, k(H)/k(D) = 2.1. Xanthene hydroxylation by 1 occurred with a very large second-order rate constant k(3) = (3.6 +/- 0.3) x 10(6) M(-1) s(-1). Similar reactions of fluorene-4-carboxylic acid and 4-isopropyl- and 4-ethylbenzoic acid also gave high rates for C-H hydroxylation that correlated well with the scissile C-H bond energy, indicating a homolytic hydrogen abstraction transition state. Mapping the observed rate constants for C-H bond cleavage onto the Brønsted-Evans-Polanyi relationship for similar substrates determined the H-OFe(IV)-4-TMPyP bond dissociation energy to be approximately 100 kcal/mol. The high kinetic reactivity observed for 1 is suggested to result from a high porphyrin redox potential and spin-state-crossing phenomena. More generally, subtle charge modulation at the active site may result in high reactivity of a cytochrome P450 compound I.

Antibiotic Resistance in Mycobacterium tuberculosis: PEROXIDASE INTERMEDIATE BYPASS CAUSES POOR ISONIAZID ACTIVATION BY THE S315G MUTANT OF M. TUBERCULOSIS CATALASE-PEROXIDASE (KatG)

KatG (catalase-peroxidase) in Mycobacterium tuberculosis is responsible for activation of isoniazid (INH), a pro-drug used to treat tuberculosis infections. Resistance to INH is a global health problem most often associated with mutations in the katG gene. The origin of INH resistance caused by the KatG[S315G] mutant enzyme is examined here. Overexpressed KatG[S315G] was characterized by optical, EPR, and resonance Raman spectroscopy and by studies of the INH activation mechanism in vitro. Catalase activity and peroxidase activity with artificial substrates were moderately reduced (50 and 35%, respectively), whereas the rates of formation of oxyferryl heme:porphyrin pi-cation radical and the decay of heme intermediates were approximately 2-fold faster in KatG[S315G] compared with WT enzyme. The INH binding affinity for the resting enzyme was unchanged, whereas INH activation, measured by the rate of formation of an acyl-nicotinamide adenine dinucleotide adduct considered to be a bactericidal molecule, was reduced by 30% compared with WT KatG. INH resistance is suggested to arise from a redirection of catalytic intermediates into nonproductive reactions that interfere with oxidation of INH. In the resting mutant enzyme, a rapid evolution of 5-c heme to 6-c species occurred in contrast with the behavior of WT KatG and KatG[S315T] and consistent with greater flexibility at the heme edge in the absence of the hydroxyl of residue 315. Insights into the effects of mutations at residue 315 on enzyme structure, peroxidation kinetics, and specific interactions with INH are presented.

The Mechanism of Oxidative Halophenol Dehalogenation by Amphitrite ornata Dehaloperoxidase Is Initiated by H2O2 Binding and Involves Two Consecutive One-Electron Steps: Role of Ferryl Intermediates

The enzymatic globin, dehaloperoxidase (DHP), from the terebellid polychaete Amphitrite ornata is designed to catalyze the oxidative dehalogenation of halophenol substrates. In this study, the ability of DHP to catalyze this reaction by a mechanism involving two consecutive one-electron steps via the normal order of addition of the oxidant cosubstrate (H(2)O(2)) before organic substrate [2,4,6-trichlorophenol (TCP)] is demonstrated. Specifically, 1 equiv of H(2)O(2) will fully convert 1 equiv of TCP to 2,6-dichloro-1,4-benzoquinone, implicating the role of multiple ferryl [Fe(IV)O] species. A significant amount of heterolytic cleavage of the O-O bond of cumene hydroperoxide, consistent with transient formation of a Compound I [Fe(IV)O/porphyrin pi-cation radical] species, is observed upon its reaction with ferric DHP. In addition, a more stable high-valent Fe(IV)O-containing DHP intermediate [Compound II (Cpd II) or Compound ES] is characterized by UV-visible absorption and magnetic circular dichroism spectroscopy. Spectral similarities are seen between this intermediate and horse heart myoglobin Cpd II. It is also shown in single-turnover experiments that the DHP Fe(IV)O intermediate is an active oxidant in halophenol oxidative dehalogenation. Furthermore, reaction of DHP with 4-chlorophenol leads to a dimeric product. The results presented herein are consistent with a normal peroxidase order of addition of the oxidant cosubstrate (H(2)O(2)) followed by organic substrate (TCP) and indicate that the enzymatic mechanism of DHP-catalyzed oxidative halophenol dehalogenation involves two consecutive one-electron steps with a dissociable radical intermediate.

Density Functional Studies on Isomerization of Prostaglandin H2 to Prostacyclin Catalyzed by Cytochrome P450

Reaction mechanisms for the isomerization of prostaglandin H(2) to prostacyclin catalyzed by cytochrome P450 are investigated by the unrestricted Becke's three-parameter plus Lee-Yang-Parr density functional level of theory. The results show that the homolytic O-O bond cleavage of endoperoxide in prostaglandin H(2) is the rate-limiting step and that the isomerization proceeds through proton-coupled electron transfer. We located two reaction pathways through an Fe(IV)-porphyrin intermediate and an Fe(III)-porphyrin pi-cation radical intermediate.

Peroxide-Dependent Formation of a Covalent Link between Trp51 and the Heme in Cytochrome c Peroxidase

Ascorbate peroxidase (APX), cytochrome c peroxidase (CcP), and the catalase-peroxidases (KatG) share very similar active site structures and are distinguished from other peroxidases by the presence of a distal tryptophan residue. In KatG, this distal tryptophan forms a covalent link to an adjacent tyrosine residue, which in turn links to a methionine residue. We have previously shown [ Pipirou, Z. et al. ( 2007 ) Biochemistry 46 , 2174 - 2180 ] that reaction of APX with peroxide leads, over long time scales, to formation of a covalent link with the distal tryptophan (Trp41) in a mechanism that proceeds through initial formation of a compound I species bearing a porphyrin pi-cation radical followed by radical formation on Trp41, as implicated in the KatG enzymes. Formation of such a covalent link in CcP has never been reported, and we proposed that this could be because compound I in CcP uses Trp191 instead of a porphyrin pi-cation radical. To test this, we have examined the reactivity of the W191F variant of CcP with H(2)O(2), in which formation of a porphyrin pi-cation radical occurs. We show, using electronic spectroscopy, HPLC, and mass spectroscopy, that in W191F partial formation of a covalent link from Trp51 to the heme is observed, as in APX. Radical formation on Trp51, as seen for KatG and APX, is implicated; this is supported by QM/MM calculations. Collectively, the data show that all three members of the class I heme peroxidases can support radical formation on the distal tryptophan and that the reactivity of this radical can be controlled either by the protein structure or by the nature of the compound I intermediate.

Mechanistic Insight into Formation of Oxo‐Iron(IV) Porphyrin π‐Cation Radicals from Enzyme Mimics of Cytochrome P450 in Organic Solvents

Two new models for cytochrome P450 in which the thiolate axial ligand is replaced by a RSO(3)(-) group, form oxo-iron(IV) porphyrin pi-cation radicals as sole oxidation products in "peroxo shunt" reactions independent of the nature of the employed solvent (polar or non-polar) and electronic nature of the porphyrin rings. Although the properties of the solvent and push-pull effects from the porphyrin rings do not affect the mode of the O-O bond cleavage (heterolytic or homolytic) in these models, they strongly affect the rate and mechanism of each reaction step leading to the formation of the high-valent iron intermediates. This article reports the results of mechanistic studies involving the measurements of the rate of oxo-iron(IV) porphyrin pi-cation radical formation from the enzyme mimics of P450 for different oxidant concentration, temperature and pressure in selected organic solvents. Extraction of the appropriate rate constants and activation parameters for the reactions studied enable a detailed discussion of the effects of solvent and electronic nature of the porphyrin rings on the position of the first pre-equilibrium involving formation of the acylperoxo-iron(III) porphyrin intermediate, as well as on the rate of heterolytic O-O bond cleavage leading to the formation of the high-valent iron species. Furthermore, an unusual effect of solvent on the kinetics of oxo-iron(IV) porphyrin pi-cation radical formation in methanol is demonstrated and discussed in the present work.

Effect of Imidazole and Phenolate Axial Ligands on the Electronic Structure and Reactivity of Oxoiron(IV) Porphyrin π-Cation Radical Complexes: Drastic Increase in Oxo-Transfer and Hydrogen Abstraction Reactivities

To study the effect of axial ligands on the electronic structure and reactivity of compound I of peroxidases and catalases, oxoiron(IV) porphyrin pi-cation radical complexes with imidazole, 2-methylimidazole, 4(5)-methylimidazole, and 3-fluoro-4-nitrophenolate as the axial ligands were prepared by ozone oxidation of iron(III) complexes of 5,10,15,20-tetramesitylporphyrin (TMP) and 2,7,12,17-tetramethyl-3,8,13,18-tetramesitylporphyrin (TMTMP). These complexes were fully characterized by absorption, (1)H, (2)H, and (19)F NMR, electron paramagnetic resonance (EPR), and electrospray ionization mass spectrometry (ESI-MS) spectroscopy. The characteristic absorption peak of compound I at approximately 650 nm was found to be a good marker for estimation of the electron donor effect from the axial ligand. The axial ligand effect did not change the porphyrin pi-cation radical state, the a(2u) state of the TMP complexes, or the a(1u) radical state of both the TMTMP complexes and compound I. The ferryl iron and porphyrin pi-cation radical spins were effectively transferred into the axial ligands for the a(2u) complexes but not for the a(1u) complexes. Most importantly, the reactivity of the oxoiron(IV) porphyrin pi-cation radical complex was drastically increased by the imidazole and phenolate axial ligands. The reaction rate for cyclooctene epoxidation was increased 100- to 400-fold with axial coordination of imidazoles and phenolate. A similar increase was also observed for the oxidation of 1,4-cyclohexadiene,N,N-dimethyl-p-nitroaniline and hydrogen peroxide. These results suggest extreme enhancement of the reactivity of compound I by the axial ligand in heme enzymes. The functional role of axial ligands on the compound I in heme enzymes is discussed.

Density Functional Studies on Thromboxane Biosynthesis: Mechanism and Role of the Heme‐Thiolate System

Reaction mechanisms for the isomerization of prostaglandin H(2) to thromboxane A(2), and degradation to 12-L-hydroxy-5,8,10-heptadecatrienoic acid (HHT) and malondialdehyde (MDA), catalyzed by thromboxane synthase, were investigated using the unrestricted Becke-three-parameter plus Lee-Yang-Parr (UB3LYP) density functional level theory. In addition to the reaction pathway through Fe(IV)-porphyrin intermediates, a new reaction pathway through Fe(III)-porphyrin pi-cation radical intermediates was found. Both reactions proceed with the homolytic cleavage of endoperoxide O-O to give an alkoxy radical. This intermediate converts into an allyl radical intermediate by a C-C homolytic cleavage, followed by the formation of thromboxane A(2) having a 6-membered ring through a one electron transfer, or the degradation into HHT and MDA. The proposed mechanism shows that an iron(III)-containing system having electron acceptor ability is essential for the 6-membered ring formation leading to thromboxane A(2). Our results suggest that the step of the endoperoxide O-O homolytic bond cleavage has the highest activation energy following the binding of prostaglandin H(2) to thromboxane synthase.

Axial Ligand Coordination in Sterically Strained Vanadyl Porphyrins: Synthesis, Structure, and Properties

A hitherto unknown family of six-coordinate vanadyl porphyrins of the sterically crowded, nonplanar 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetranitroporphyrin incorporating axial ligand L [where L is pyridine, tetrahydrofuran (THF), or methanol (MeOH)] has been isolated as VO(tn-OEP)(L) in the solid phase for the first time and also structurally characterized. The presence of four electron-withdrawing, bulky nitro groups at the meso positions of vanadyl octaethylporphyrins severely distorts the porphyrin macrocycles and significantly enhances the affinity for the axial ligands, where even weak sigma-donating ligands, such as MeOH, bind strongly enough to be isolable in the solid phase and that too under the offset effects of the macrocyclic distortions. Thus, the axial ligand affinity is influenced by both the electronic and conformational effect, which cannot be separated completely in this series. The solid-state magnetic measurements and their typical electron paramagnetic resonance (EPR) spectrum show the presence of a single, unpaired electron, consistent with V(IV) formulation. The VO stretching frequency for VO(tn-OEP) occurs as a sharp, strong peak at 1008 cm(-1), which is consistent with five-coordinate vanadyl porphyrins, while VO(tn-OEP)(L) displays a strong band at lower wavenumbers. The downshift in nu(VO) upon axial coordination increases with increasing donor strength of the axial ligands; for pyridine, the downshift is 30 cm(-1), while for THF and MeOH, the downshifts are nearly 18 cm(-1). X-ray structure determinations authenticate axial coordination in a purely saddle-distorted porphyrin macrocycle for all of the complexes reported here in which V-Np distances are significantly shorter, while the porphyrin cores have been expanded on axial ligand coordination. As a result, vanadium atoms are more inplane than in a five-coordinate species. The binding of L does not change the spin or metal oxidation states (V(IV), d(1)-system) of the complexes; therefore, the changes observed are truly the reflections of axial ligand coordination. Electrochemical data obtained from cyclic voltammetric studies reveal that the complexes are much easier to reduce (by approximately 1200 mV) but more difficult to oxidize (by approximately 500 mV) as compared to nearly planar VO(OEP). The complexes undergo two one-electron oxidations due to pi-cation radical and dication formation and three one-electron reductions. The first two reductions are because of pi-anion radical and dianion formation, while the third quasi-reversible reduction is assigned to a metal-centered process (V(IV) --> V(III)). These results can be useful for identifying the interaction of the vanadyl porphyrins with the biological targets in their reported involvement in potent insulinomimetic activity and in anti-HIV agents.

Comparison of Electron-Transfer Rates for Metal- versus Ring-Centered Redox Processes of Porphyrins in Monolayers on Au(111)

The standard electron-transfer rate constants ( k ( 0 )) are measured for redox processes of Fe versus Zn porphyrins in monolayers on Au(111); the former undergoes a metal-centered redox process (conversion between Fe (III) and Fe (II) oxidation states) whereas the latter undergoes a ring-centered redox process (conversion between the neutral porphyrin and the pi-cation radical). Each porphyrin contains three meso-mesityl groups and a benzyl thiol for surface attachment. Under identical solvent (propylene carbonate)/electrolyte (1.0 M Bu 4NCl) conditions, the Zn (II) center has a coordinated Cl (-) ion when the porphyrin is in either the neutral or oxidized state. In the case of the Fe porphyrin, two species are observed a low-potential form ( E l (0) approximately -0.6 V) wherein the metal center has a coordinated Cl (-) ion when it is in either the Fe (II) or Fe (III) state and a high-potential form ( E h (0) approximately +0.2 V) wherein the metal center undergoes ligand exchange upon conversion from the Fe (III) to Fe (II) states. The k ( 0 ) values observed for all of the porphyrins depend on surface concentration, with higher concentrations resulting in slower rates, consistent with previous studies on porphyrin monolayers. The k ( 0 ) values for the ring-centered redox process (Zn chelate) are 10-40 times larger than those for the metal-centered process (Fe chelate); the k ( 0 ) values for the two forms of the Fe porphyrin differ by a factor of 2-4 (depending on surface concentration), the Cl (-) exchanging form generally exhibiting a faster rate. The faster rates for the ring- versus metal-centered redox process are attributed to the participating molecular orbitals and their proximity to the surface (given that the porphyrins are relatively upright on the surface): a pi molecular orbital that has significant electron density at the meso-carbon atoms (one of which is the site of attachment of the linker to the surface anchoring thiol) versus a d-orbital that is relatively well localized on the metal center.

Androgynous Porphyrins. Silver(II) Quinoxalinoporphyrins Act as Both Good Electron Donors and Acceptors

The metal-centered and macrocycle-centered electron-transfer oxidations and reductions of silver(II) porphyrins were characterized in nonaqueous media by electrochemistry, UV-vis spectroelectrochemistry, EPR spectroscopy, and DFT calculations. The investigated compounds are {5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrinato}silver(II), {5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)quinoxalino[2,3-b']porphyrinato}silver(II), {5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)bisquinoxalino[2,3-b':7,8-b'']porphyrinato}silver(II), and {5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)bisquinoxalino[2,3-b':12,13-b'']porphyrinato}silver(II). The first one-electron oxidation and first one-electron reduction both occur at the metal center to produce stable compounds with Ag(III) or Ag(I) metal oxidation states, irrespective of the type of porphyrin ligand. The electrochemical HOMO-LUMO gap, determined by the difference in the first oxidation and first reduction potentials, decreases by introduction of quinoxaline groups fused to the Ag(II) porphyrin macrocycle. This provides a unique androgynous character to Ag(II) quinoxalinoporphyrins that enables them to act as both good electron donors and good electron acceptors, something not previously observed in other metalloporphyrin complexes. The second one-electron oxidation and second one-electron reduction of the compounds both occur at the porphyrin macrocycle to produce Ag(III) porphyrin pi-radical cations and Ag(I) porphyrin pi-radical anions, respectively. The macrocycle-centered oxidation potentials of each quinoxalinoporphyrin are shifted in a negative direction, while the macrocycle-centered reduction potentials are shifted in a positive direction as compared to the same electrode reactions of the porphyrin without the fused quinoxaline ring(s). Both potential shifts are due to a stabilization of the radical cations and radical anions by pi-extension of the porphyrin macrocycle after fusion of one or two quinoxaline moieties at the beta-pyrrolic positions of the macrocycle. Introduction of quinoxaline groups fused to the Ag(II) porphyrin macrocycle provides a unique androgynous character to Ag(II) quinoxalinoporphyrins that enables them to act as both good electron donors and good electron acceptors.