NMR and EPR Studies of Chloroiron(III) Tetraphenyl-chlorin and Its Complexes with Imidazoles and Pyridines of Widely Differing Basicities

Cai, Shuqun; Shokhireva, Tatjana Kh.; Lichtenberger, Dennis L.; Walker, F. Ann

doi:10.1021/ic0515352

Cited by 20 publications

(23 citation statements)

References 87 publications

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“…Based on the TDDFT calculation it is difficult to distinguish between FeTPP(Pyr) 2 which in turn suggests that FeTPP(Pyr)Cl is present in neat pyridine solution. This conclusion is in agreement with NMR studies of related adducts of tetraphenylchlorin with a series of basic ligands [40]. Pyridine is a relatively weak base and is therefore unlikely to displace the Cl − ion here.…”

Section: Spectroscopic Characteristics Of the Ligated Forms Of Mtppsupporting

confidence: 90%

Aerobic oxidation of β-isophorone by tetraphenylporphyrin catalysts in pyridine solution

Burns

Huang

Weare

et al. 2015

Journal of Molecular Catalysis A: Chemical

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a b s t r a c tA kinetic study was conducted using five metallo-tetraphenylporphyrins (MTPPs) as catalysts for the aerobic oxidation reaction of ␤-isophorone (␤-IP) to ketoisophorone (KIP). The oxidation reaction catalyzed by MTPPs (M = Cr, Mn, Fe, Co and Cu) was observed over a seven hour period under a range of experimental conditions. Changes in the specificity of conversion of reactant to the product, KIP, and the main side-product, ␣-isophorone (␣-IP), were observed at temperatures ranging from 60 • C to 75 • C, in solvents with varying compositions of pyridine and ␤-IP, with different MTPP catalysts, oxygen gas flow rates, and solution agitation frequencies. Control experiments show that MTPP catalysts modestly increase the rate of KIP formation, but significantly improve specificity through the apparent suppression of ␣-IP formation. Analysis revealed that MnTPP (CH 3 COO − ) produced the highest product specificity ratio [KIP]/ [␣-IP]. Given that all of the metals were in the 3+ oxidation state in this study and there was no observed binding by O 2 to any of the MTPP catalysts, the catalytic mechanism is suggested to involve the binding of ␤-IP to the MTPP metal center. Binding of ␤-IP is most favorable when pyridine is the trans ligand, but ␤-IP is also observed to weakly bind when trans to acetate in the MnTPP adduct. This conclusion is supported by electronic absorption spectroscopy, resonance Raman spectroscopy and density functional theory (DFT) calculations. The role played by the catalyst appears to be the activation of hydrogen abstraction following the substrate's ligation to the metal, rather than the more traditional role of MTPPs as O 2 activation catalysts by ligation of diatomic O 2 . ␤-IP oxidation is an example of a case where the catalyst appears to have a more important role for improving the specificity of the reaction (through decreasing side-product formation) rather than increasing the rate of product formation.

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Section: Spectroscopic Characteristics Of the Ligated Forms Of Mtppsupporting

confidence: 90%

Aerobic oxidation of β-isophorone by tetraphenylporphyrin catalysts in pyridine solution

Burns

Huang

Weare

et al. 2015

Journal of Molecular Catalysis A: Chemical

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“…Spectroscopic analysis and density functional theory calculations of variously substituted ferriporphyrin and ferrichlorin model compounds indicate that the ruffling of a ferriheme stabilizes the d xz and d yz orbitals of the iron and changes the electron configuration of low-spin Fe 3þ from the more common ðd xy Þ 2 ðd xz ;d yz Þ 3 to the less common ðd xz ;d yz Þ 4 ðd xy Þ 1 configuration (48)(49)(50)(51)(52)(53). This change in configuration causes major changes in the 1 H-NMR chemical shifts of the β-pyrrole substituent and meso-H signals of the heme (48)(49)(50)(51)(52). In ruffled heme, the p z orbital from the porphyrin 2a 2u ðπÞ orbital twists away from orthogonality with the porphyrin plane to permit interaction of the 2a 2u ðπÞ orbital with the 3d xy orbital of the iron.…”

Section: Discussionmentioning

confidence: 99%

Electronic properties of the highly ruffled heme bound to the heme degrading enzyme IsdI

Takayama

Ukpabi

Murphy

et al. 2011

Proc. Natl. Acad. Sci. U.S.A.

159

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IsdI, a heme-degrading protein from Staphylococcus aureus , binds heme in a manner that distorts the normally planar heme prosthetic group to an extent greater than that observed so far for any other heme-binding protein. To understand better the relationship between this distinct structural characteristic and the functional properties of IsdI, spectroscopic, electrochemical, and crystallographic results are reported that provide evidence that this heme ruffling is essential to the catalytic activity of the protein and eliminates the need for the water cluster in the distal heme pocket that is essential for the activity of classical heme oxygenases. The lack of heme orientational disorder in 1 H-NMR spectra of the protein argues that the catalytic formation of β- and δ-biliverdin in nearly equal yield results from the ability of the protein to attack opposite sides of the heme ring rather than from binding of the heme substrate in two alternative orientations.

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“…This reorganization of the spin density in highly ruffled model complexes is believed to be a consequence of a ground state electron configuration change from ( d xy ) 2 ( d xz,yz ) 3 to ( d xz,yz ) 4 ( d xy ) 1 caused by preferential stabilization of the Fe 3 d xz - and 3 d yz -based molecular orbitals (MOs) by strong π-acceptor axial ligands based upon ligand field theory (LFT) arguments. However, in biological systems when there has been a comparison of more planar and more ruffled six-coordinate hemes, the NMR results have not been consistent with a ground state electron configuration change 5,10,11,35,36. Instead, these studies have observed a decrease of the average heme methyl 1 H shift for increased heme ruffling.…”

Section: Introductionmentioning

confidence: 88%

NMR and DFT Investigation of Heme Ruffling: Functional Implications for Cytochrome c

2010

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Out-of-plane (OOP) deformations of the heme cofactor are found in numerous heme-containing proteins and the type of deformation tends to be conserved within functionally-related classes of heme proteins. We demonstrate correlations between the heme ruffling OOP deformation and the 13 C and 1 H nuclear magnetic resonance (NMR) hyperfine shifts of heme aided by density functional theory (DFT) calculations. The degree of ruffling in the heme cofactor of Hydrogenobacter thermophilus cytochrome c 552 has been modified by a single amino acid mutation in the second coordination sphere of the cofactor. The 13 C and 1 H resonances of the cofactor have been assigned using one-and two-dimensional NMR spectroscopy aided by selective 13 C-enrichment of the heme. DFT has been used to predict the NMR hyperfine shifts and electron paramagnetic resonance (EPR) g-tensor at several points along the ruffling deformation coordinate. The DFT-predicted NMR and EPR parameters agree with the experimental observations, confirming that an accurate theoretical model of the electronic structure and its response to ruffling has been established. As the degree of ruffling increases, the heme methyl 1 H resonances move upfield while the heme methyl and meso 13 C resonances move downfield. These changes are a consequence of altered overlap of the Fe 3d and porphyrin π orbitals, which destabilizes all three occupied Fe 3d-based molecular orbitals and decreases the positive and negative spin density on the β-pyrrole and meso carbons, respectively. Consequently, the heme ruffling deformation decreases the electronic coupling of the cofactor with external redox partners and lowers the reduction potential of heme.

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NMR and EPR Studies of Chloroiron(III) Tetraphenyl-chlorin and Its Complexes with Imidazoles and Pyridines of Widely Differing Basicities

Abstract: The NMR and EPR spectra of two bis-imidazole and three bis-pyridine complexes of tetraphenylchlorinatoiron(III) ,

Cited by 20 publications

References 87 publications

Aerobic oxidation of β-isophorone by tetraphenylporphyrin catalysts in pyridine solution

Aerobic oxidation of β-isophorone by tetraphenylporphyrin catalysts in pyridine solution

Electronic properties of the highly ruffled heme bound to the heme degrading enzyme IsdI

NMR and DFT Investigation of Heme Ruffling: Functional Implications for Cytochrome c

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