Models of the cytochromes b. 2. Effect of unsymmetrical phenyl substitution on pyrrole proton shifts of a series of low-spin (tetraphenylporphinato)iron(III) bis(N-methylimidazole) complexes

The temperature dependence of longitudinal and transverse relaxation times (T 1 and T 2) has been studied for the pyrrole protons of [(p-Cl)3(p-NEt2)TPPFe(III)(N-MeIm)2]Cl (1), [(p-Cl)(p-NEt2)3TPPFe(III)(N-MeIm)2]Cl (2), and [TMPFe(III)(2-MeImH)2]Cl (3), where TMP = tetramesitylporphyrin and TPP = tetraphenylporphyrin, in the temperature range 190−310 K. All three complexes are paramagnetic and have electron spin S = 1/2. Up to 273 K, all complexes exhibit four distinct pyrrole proton signals, with the asymmetry caused by unsymmetrical substitution in complexes 1 and 2 and by axial ligands fixed in a definite orientation in complex 3. Above 273 K the four-peak pattern in complex 3 collapses into a single peak due to fast synchronous rotation of axial ligands. At low temperatures, T 1s and T 2s in all complexes increase as temperature increases. At higher temperatures, T 1s continue to increase and equalize in complex 3, but decrease in complexes 1 and 2. T 2s in complexes 1 and 2 mimic the T 1s at all temperatures. In complex 3, T 2s decrease as the four-peak pyrrole proton pattern collapses and increase again when the collapse is complete. This behavior has been attributed to chemical exchange induced by the rotation of 2-methylimidazole ligands. In complexes 1 and 2, the decrease in both T 1s and T 2s at high temperatures is attributed to equilibrium between low-spin and high-spin complexes induced by dissociation of imidazole ligands in the TPP complexes. In all complexes, T 2s are considerably shorter than T 1s. Relaxation times in the TMP complex are generally larger than the corresponding values for the TPP complexes. The temperature dependence of the chemical shift follows the Curie law in complex 3 and is close to Curie behavior in complexes 1 and 2, with slight deviations at high temperatures in the two latter complexes attributed to the low spin−high spin equilibrium. The NOE buildup curve for the pair of NOE-exhibiting pyrrole protons of complex 3 has been measured; the rate of NOE buildup has been found to be consistent with theoretical prediction based on the Stokes-estimated rotational correlation time and interproton distance measured from the MM2-minimized structure. A method has been proposed to predict the detectability of the NOE between a pair of structurally rigid protons in similar complexes, as well as to predict optimum detection conditions. Contrary to previous studies, no NOE is detected between pyrrole protons of 1 and 2, and this fact is justified and discussed in light of our findings for complex 3.

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“…Materials. Synthesis of unsymmetrically substituted iron(III) tetraphenylporphyrinates has been described elsewhere …”

Section: Methodsmentioning

confidence: 99%

Proton NMR Relaxation in Six-Coordinate Low-Spin Iron(III) Tetraphenylporphyrinates: Temperature Dependence of Proton Relaxation Rates and Interpretation of NOESY Experiments

Momot

Walker

1997

J. Phys. Chem. A

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“…Co(II)TPP, specialty chemicals (pyrrole, benzaldehyde, o -nitrobenzaldehyde, tin chloride, acetyl chloride, pivaloyl chloride, phenyl isocyanate, 1-methylimidazole), and solvents (proprionic acid, pyridine, benzene, methanol, toluene, dichloromethane) were obtained from Aldrich Chemical Company. Substituted TPPH 2 were prepared by first synthesizing a mixture of TPP and nitrophenylporphyrins using a modified procedure , of the Alder synthesis …”

Section: Methodsmentioning

confidence: 99%

Electron Spin Echo Envelope Modulation and Extended X-ray Absorption Fine Structure Studies of Active Site Models of Oxygenated Cobalt-Substituted Hemoproteins: Correlating Electron-Nuclear Couplings and Metal−Ligand Bond Lengths

et al. 1997

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Electron spin echo envelope modulation (ESEEM) and extended X-ray absorption fine structure (EXAFS) spectroscopic studies of oxygenated cobalt (oxyCo) [tetraphenylporphyrin(TPP)][1-methylimidazole (1-MeIm)], an active site model of oxyCo-substituted globins (functional and EPR-active [S = 1/2] analogues of oxygen carrying hemoproteins), are carried out in order to examine the correlation of oxygen affinity with electron-nuclear coupling parameters and metal−ligand bond lengths. ESEEM demonstrates that the magnitude of the electron-nuclear hyperfine and nuclear quadrupole couplings to the directly-coordinated 14N of 1-MeIm decrease (A iso, from 3.54 to 3.04 MHz; e 2 qQ, from 2.39 to 2.08 MHz) as the solvent compositon is varied from 0 to 50% (v/v) dichloromethane in toluene. For oxyCo[(o-R)1TPP][1-MeIm] (where R = −H, −NHCOC(CH3)3, −NHCOCH3, or −NHCONHC6H5, an ortho substitutent on one of the four meso phenyls of TPP), couplings to the axial nitrogen decrease (A iso, from 3.54 to 3.07 MHz; e 2 qQ, from 2.39 to 2.09 MHz) with increased electron-withdrawing strength of R, i.e., with increased acidity of the amide proton of R that may interact with the bound dioxygen. EXAFS measurements, and analysis using ab initio EXAFS codes and global mapping, find that the cobalt−axial nitrogen (Nax) bond of oxyCoTPP-1-MeIm shortens by 0.18 ± 0.06 Å when the solvent is changed from 100% toluene (Co−Nax = 2.12 Å) to 50% toluene/50% dichloromethane (Co−Nax = 1.94 Å). The average cobalt−equatorial nitrogens (1.94−1.96 Å) and cobalt−oxygen (1.95−1.98 Å) distances are unchanged within the error. Similar results were obtained when oxyCoTPP-1-MeIm was compared (in 100% toluene) with its (o-NHCONHC6H5)1TPP counterpart, where the cobalt−ligand bond lengths are indistinguishable from those of oxyCoTPP-1-MeIm in 50% toluene/50% dichloromethane. Increasing the polarity of the solvent and of the vicinity of the bound dioxygen increases oxygen affinity of the metal due to an increase in the ionicity of the cobalt−dioxygen bond that is manifested in reduction in electron-nuclear couplings to the axial nitrogen [Lee et al. Biochemistry 1994, 33, 7609] and shortening of the cobalt−axial nitrogen bond. These ESEEM and EXAFS characterizations of metal−ligand interactions demonstrate the correlation of electron-nuclear coupling and metal−ligand bond lengths with oxygen affinity of hemoprotein model complexes.

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“…Iron was inserted and the iron porphyrins were purified as described previously. 71,72 Covalently linked imidazole TPPs were prepared from the desired ((o-NH2)"TPP)FeCl, phosgene, and 2-(iV-imidazolyl)ethanol:73 For the monoimidazole derivative, [H+ ImCH2CH2OCONHTPP)FeCl]Cr, iron was first inserted71 into 50 mg (0.08 mmol) of o-NH2TPP, prepared as before. 74 It was chromatographed on silica gel and eluted with 10% MeOH/CHCl3.…”

Section: Methodsmentioning

confidence: 99%

Models of the cytochromes b. 5. EPR studies of low-spin iron(III) tetraphenylporphyrins

Walker¹,

Reis²,

Balke³

1984

J. Am. Chem. Soc.

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148

137

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Models of the cytochromes b. 2. Effect of unsymmetrical phenyl substitution on pyrrole proton shifts of a series of low-spin (tetraphenylporphinato)iron(III) bis(N-methylimidazole) complexes

Cited by 23 publications

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Proton NMR Relaxation in Six-Coordinate Low-Spin Iron(III) Tetraphenylporphyrinates: Temperature Dependence of Proton Relaxation Rates and Interpretation of NOESY Experiments

Proton NMR Relaxation in Six-Coordinate Low-Spin Iron(III) Tetraphenylporphyrinates: Temperature Dependence of Proton Relaxation Rates and Interpretation of NOESY Experiments

Electron Spin Echo Envelope Modulation and Extended X-ray Absorption Fine Structure Studies of Active Site Models of Oxygenated Cobalt-Substituted Hemoproteins: Correlating Electron-Nuclear Couplings and Metal−Ligand Bond Lengths

Models of the cytochromes b. 5. EPR studies of low-spin iron(III) tetraphenylporphyrins

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