Iron-57 nuclear magnetic resonance chemical shifts of hindered iron porphyrins. Ruffling as a possible mechanism for d-orbital energy level inversion

Baltzer, Lars; Landergren, Marie

doi:10.1021/ja00163a054

Cited by 30 publications

(13 citation statements)

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“…The chemical shifts they observe are all more shielded than we find experimentally for MbCO in solution. , For example, with increased ruffling, the shifts decrease from 8036 to 7500 ppm (from Fe(CO) 5 ) in a basket handle porphyrin . The computed isotropic chemical shift of 7760 ppm (Table ) is well within the range of 7500−8036 reported by Baltzer and Landegren for their ruffled iron porphyrins containing CO and alkylimidazole ligands . Thus, since the degree of ruffle in the protein is uncertain, while a saddle distortion is seen in the model system, the possibility exists that part of the error seen in the calculations originates from a porphyrin ring distortion.…”

Section: Resultssupporting

confidence: 80%

“…It is also possible that the agreement between theory and experiment may actually be rather better than these results indicate. Specifically, Baltzer and Landegren have reported the results of a series of 57 Fe NMR chemical shift measurements on a range of porphyrins having ruffled structures, due to steric constraints. The chemical shifts they observe are all more shielded than we find experimentally for MbCO in solution. , For example, with increased ruffling, the shifts decrease from 8036 to 7500 ppm (from Fe(CO) 5 ) in a basket handle porphyrin .…”

Section: Resultsmentioning

confidence: 99%

“…Specifically, Baltzer and Landegren have reported the results of a series of 57 Fe NMR chemical shift measurements on a range of porphyrins having ruffled structures, due to steric constraints. The chemical shifts they observe are all more shielded than we find experimentally for MbCO in solution. , For example, with increased ruffling, the shifts decrease from 8036 to 7500 ppm (from Fe(CO) 5 ) in a basket handle porphyrin . The computed isotropic chemical shift of 7760 ppm (Table ) is well within the range of 7500−8036 reported by Baltzer and Landegren for their ruffled iron porphyrins containing CO and alkylimidazole ligands .…”

Section: Resultsmentioning

confidence: 99%

“…The chemical shifts they observe are all more shielded than we find experimentally for MbCO in solution. 5,34 For example, with increased ruffling, the shifts decrease from 8036 to 7500 ppm (from Fe(CO) 5 ) in a basket handle porphyrin. 34 The computed isotropic chemical shift of 7760 ppm a Evaluated using the locally dense basis/B3LYP DFT method described in the text.…”

Section: Resultsmentioning

confidence: 99%

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Iron-57 NMR Chemical Shifts and Mössbauer Quadrupole Splittings in Metalloporphyrins, Ferrocytochrome c, and Myoglobins: A Density Functional Theory Investigation

et al. 1998

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We have evaluated the 57Fe nuclear magnetic resonance chemical shielding and Mössbauer electric field gradient tensors and their orientations for a cytochrome c model compound as well as for isopropyl isocyanide and carbon monoxy−myoglobin model systems and two simple metalloporphyrins containing bis(pyridine) and bis(trimethylphosphine) ligands, using Kohn−Sham density functional theory. For cytochrome c we used a model Fe(II) porphyrin structure together with a 1-methylimidazole base (to represent His-18) and a dimethyl sulfide molecule (to represent Met-80 in the structure of horse heart ferrocytochrome c), both located at the X-ray coordinates for cyt c Fe(II). For the Mb calculations, we used the coordinates of two recently characterized metalloporphyrins: (i-PrNC)(1-methylimidazole)(5,10,15,20-tetraphenylporphinato)Fe(II) and (CO)(1-methylimidazole)(5,10,15,20-tetraphenylporphinato)Fe(II), while literature structures were used for the bis-ligand adducts. We used a “locally dense” basis to evaluate the 57Fe shieldings and electric field gradients at iron and compared them with the measured chemical shifts and Mössbauer quadrupole splittings, respectively. There is moderately good agreement between theory and experiment for the cytochrome c and Mb 57Fe chemical shifts and shielding tensors, and very good (0.10 mm s-1 rmsd) agreement for the 57Fe Mössbauer quadrupole splittings, using the following basis sets and functional: a Wachters' all electron representation for iron, a 6-311++G(2d) basis for all atoms directly attached to iron, 6-31G* for the second shell and 3-21G* bases for the other more distant atoms, together with a B3LYP hybrid exchange-correlation functional. Extensive tests with other functionals and basis set schemes are also reported. The shift and electric field gradient tensor orientations are generally close to obvious molecular symmetry axes, with the skew of the shielding tensor reversing sign on transition from strong to weak ligand fields. The paramagnetic contribution to shielding overwhelmingly dominates overall shielding and the variations seen between weak ligand field (bis(pyridine), cytochrome c) and strong ligand field (CO, PMe3, i-PrNC) systems. Poor accord between theory and experiment is obtained for the 57Fe chemical shifts when MbCO models having highly distorted X-ray geometries are employed, suggesting that the Fe−C−O is close to the porphyrin normal, both in solution and in the solid state.

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Section: Resultssupporting

confidence: 80%

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

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Iron-57 NMR Chemical Shifts and Mössbauer Quadrupole Splittings in Metalloporphyrins, Ferrocytochrome c, and Myoglobins: A Density Functional Theory Investigation

et al. 1998

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“…In the series of hybrid, basket-handle porphyrins, the effect of a decrease in the length of the aliphatic chain spanning one side of the porphyrin is not to increase the degree of the tilt of the bound carbonyl but rather to increase the degree of ruffing of the porphyrin core [ 1093 ]. As a 57 Fe NMR study shows [ 1094 ], in the hybrid, basket-handle porphyrins, the ruffing leads to large changes in the iron- d -orbital energies that may be important in understanding ligand binding in heme proteins and models. Studies on porphyrins include the synthesis of S/N, S/O mixed ligand complexes of iron(III) tetramesityl-porphyrin [ 1095 ], and synthetic and structural studies of a sapphyrin, a 22-π-electron pentapyrrolic “expanded porphyrin” [ 1096 ]; also, insertion of iron into porphyrins [ 1097 ], gable porphyrin metal complex as a double recognition model [ 1098 ], and fixation of amino acids with bifunctional metalloporphyrin receptor [ 1099 ].…”

Section: Addendamentioning

confidence: 99%

New chemical and stereochemical applications of organoiron complexes

Fatiadi

1991

J. RES. NATL. INST. STAN.

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The objective of this review is to provide a current overview of the rapidly developing chemistry of organometallic complexes and particularly organoiron complexes useful in asymmetric and stereoselective reactions. Also covered are stereoselective reactions of α, β-unsaturated acyl ligands bound to the chiral auxiliary [(η5-C5H5) Fe(CO)(PPh3)] and new applications of organoiron complexes in the synthesis of natural products. The mechanistic aspects and stabilizing effects of the Fe(CO)3 group for alkenes or conjugated dienes are discussed. A brief summary of recent work on the special role of iron in biological reactions is also included.

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13C- and57Fe-NMR studies of the FeCO unit of heme proteins and synthetic model compounds in solution: Comparison with IR vibrational frequencies and X-ray structural data

1998

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13C‐ and 57Fe‐NMR spectra of several carbon monoxide hemoprotein models with varying polar and steric effects of the distal organic superstructure, constraints of the proximal side, and solvent polarity are reported. The 13C shieldings of heme models cover a 4.0 ppm range that is extended to 7.0 ppm when several hemoglobin CO and myoglobin CO species at different pHs are included. Both heme models and heme proteins obey a similar excellent linear δ(13C) versus ν(CO) relationship that is primarily due to modulation of π backbonding from Fe dπ to the CO π* orbital by the distal pocket polar interactions. There is no direct correlation between δ(13C) and FeCO geometry. The poor monotonic relation between δ(13C) and ν(FeC) indicates that the iron‐carbon π bonding is not a primary factor influencing δ(13C) and δ(57Fe). The δ(57Fe) was found to be extremely sensitive to deformation of the porphyrin geometry, and increased shielding by more than 600 ppm with increased ruffling was observed for various heme models of known X‐ray structures. © 1998 John Wiley & Sons, Inc. Biospectroscopy 4: S57–S69, 1998

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Iron-57 nuclear magnetic resonance chemical shifts of hindered iron porphyrins. Ruffling as a possible mechanism for d-orbital energy level inversion

Cited by 30 publications

References 2 publications

Iron-57 NMR Chemical Shifts and Mössbauer Quadrupole Splittings in Metalloporphyrins, Ferrocytochrome c, and Myoglobins: A Density Functional Theory Investigation

Iron-57 NMR Chemical Shifts and Mössbauer Quadrupole Splittings in Metalloporphyrins, Ferrocytochrome c, and Myoglobins: A Density Functional Theory Investigation

New chemical and stereochemical applications of organoiron complexes

13C- and57Fe-NMR studies of the FeCO unit of heme proteins and synthetic model compounds in solution: Comparison with IR vibrational frequencies and X-ray structural data

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