25442-52-8Relevant articles and documents
Resonant inelastic X-ray scattering on ferrous and ferric bis-imidazole porphyrin and cytochrome c: Nature and role of the axial methionine-fe bond
Kroll, Thomas,Hadt, Ryan G.,Wilson, Samuel A.,Lundberg, Marcus,Yan, James J.,Weng, Tsu-Chien,Sokaras, Dimosthenis,Alonso-Mori, Roberto,Casa, Diego,Upton, Mary H.,Hedman, Britt,Hodgson, Keith O.,Solomon, Edward I.
, p. 18087 - 18099 (2014)
Axial Cu-S(Met) bonds in electron transfer (ET) active sites are generally found to lower their reduction potentials. An axial S(Met) bond is also present in cytochrome c (cyt c) and is generally thought to increase the reduction potential. The highly covalent nature of the porphyrin environment in heme proteins precludes using many spectroscopic approaches to directly study the Fe site to experimentally quantify this bond. Alternatively, L-edge X-ray absorption spectroscopy (XAS) enables one to directly focus on the 3d-orbitals in a highly covalent environment and has previously been successfully applied to porphyrin model complexes. However, this technique cannot be extended to metalloproteins in solution. Here, we use metal K-edge XAS to obtain L-edge like data through 1s2p resonance inelastic X-ray scattering (RIXS). It has been applied here to a bis-imidazole porphyrin model complex and cyt c. The RIXS data on the model complex are directly correlated to L-edge XAS data to develop the complementary nature of these two spectroscopic methods. Comparison between the bis-imidazole model complex and cyt c in ferrous and ferric oxidation states show quantitative differences that reflect differences in axial ligand covalency. The data reveal an increased covalency for the S(Met) relative to N(His) axial ligand and a higher degree of covalency for the ferric states relative to the ferrous states. These results are reproduced by DFT calculations, which are used to evaluate the thermodynamics of the Fe-S(Met) bond and its dependence on redox state. These results provide insight into a number of previous chemical and physical results on cyt c.
Spectroscopic Evidence of Pore Geometry Effect on Axial Coordination of Guest Molecules in Metalloporphyrin-Based Metal Organic Frameworks
Kucheryavy, Pavel,Lahanas, Nicole,Lockard, Jenny V.
supporting information, p. 3339 - 3347 (2018/03/25)
A systematic comparison of host-guest interactions in two iron porphyrin-based metal-organic frameworks (MOFs), FeCl-PCN222 and FeCl-PCN224, with drastically different pore sizes and geometries is reported in this fundamental spectroscopy study. Guest molecules (acetone, imidazole, and piperidine) of different sizes, axial binding strengths, and reactivity with the iron porphyrin centers are employed to demonstrate the range of possible interactions that occur at the porphyrin sites inside the pores of the MOF. Binding patterns of these guest species under the constraints of the pore geometries in the two frameworks are established using multiple spectroscopy methods, including UV-vis diffuse reflectance, Raman, X-ray absorption, and X-ray emission spectroscopy. Line shape analysis applied to the latter method provides quantitative information on axial ligation through its spin state sensitivity. The observed coordination behaviors derived from the spectroscopic analyses of the two MOF systems are compared to those predicted using space-filling models and relevant iron porphyrin molecular analogues. While the space-filling models show the ideal axial coordination behavior associated with these systems, the spectroscopic results provide powerful insight into the actual binding interactions that occur in practice. Evidence for potential side reactions occurring within the pores that may be responsible for the observed deviation from model coordination behavior in one of the MOF/guest molecule combinations is presented and discussed in the context of literature precedent.
Kinetic study of halide ionization from cobalt(III) porphyrin complexes. Rate enhancements produced by hydrogen bonding to the halide and by steric congestion from coordinated 2-substituted imidazoles
Mahmood, Ashfaq,Liu, Hsiang-Lan,Jones, John G.,Edwards, John O.,Sweigart, Dwight A.
, p. 2149 - 2154 (2008/10/08)
The reaction of Co(TPP)Cl and various imidazoles (RIm) has been investigated in acetone and dichloromethane solvents. Kinetic measurements at room temperature as well as low-temperature spectroscopic, conductivity, and kinetic studies show that Co-(TPP)(RIm)Cl is rapidly formed and then slowly converts to Co(TPP)(RIm)2+Cl- in the presence of excess RIm. When the imidazole has a methyl or phenyl group at the N-1 position, the Co(TPP)(RIm)Cl intermediate reacts via rate-determining chloride ionization. With imidazoles bearing a hydrogen at the N-1 position, the chloride ionization is accelerated due to a hydrogen-bonding interaction with the N-H group. Although the cobalt porphyrin complexes react much more slowly than analogous iron porphyrins, the two metalloporphyrins follow the same detailed mechanism and display similar sensitivities to hydrogen-bonding interactions. The reaction of Co(TPP)Cl with imidazoles containing a 2-substituent was also studied in order to probe the effect of steric interactions in the intermediate Co(TPP)(RIm)Cl. When a 2-methyl or 2-phenyl substituent is present, the steric strain with the porphyrin ligand increases the lability of the trans chloride by a factor of at least 10; this may be related to proximal strain present in certain hemoproteins.
Kinetic study of the reaction of ferric porphyrin fluorides and imidazole
Jones,Tondreau,Edwards,Sweigart
, p. 296 - 300 (2008/10/08)
A kinetic study is reported for the reaction of Fe(porphyrin)F (porphyrin = dianion of tetraphenylporphyrin (TPP) and protoporphyrin IX dimethyl ester (PPIXDME)) with imidazole (HIm) in acetone at 25°C to give Fe(porphyrin)(HIm)2+F-. The reaction rate shows a complex dependence on the HIm concentration, being second order in HIm at low concentrations and progressing to zero order at high concentrations of HIm. With Fe(TPP)F, optical spectra at 25 and -78°C show the existence of two reaction intermediates. The first intermediate to form is the six-coordinate high-spin Fe(TPP)(HIm)F and the second is Fe(TPP)(HIm)F?HIm, in which an external imidazole is hydrogen bonded to the fluoride. Thus, hydrogen bonding is shown to play a major role in assisting ionization of the fluoride. A comparison to analogous Fe(porphyrin)Cl complexes shows that hydrogen bonding is much more important with the fluoride, as expected for the stronger Br?nsted base (F- compared to Cl-). A complete analysis of the data is provided that yields rate constants as well as equilibrium constants for the formation of both intermediates. The relevance of these reactions to hydrogen bonding to superoxide in oxymetalloporphyrins is noted.
Hydrogen bonding in metalloporphyrin reactions. Reaction of (tetraphenylporphinato)iron(III) chloride and imidazole
Tondreau,Sweigart
, p. 1060 - 1065 (2008/10/08)
A detailed kinetic study is reported for the reaction Fe(TPP)Cl + 2HIm → Fe(TPP)(HIm)2+Cl-. In acetone the rapid reversible formation of the mono(imidazole) intermediate Fe(TPP)(HIm)Cl is followed by rate-determining chloride ionization and further HIm addition to give the low-spin bis(imidazole) product. Hydrogen bonding from free imidazole to the chloride in the intermediate greatly accelerates the ionization and causes the rate law for the overall reaction to be second order in imidazole. This behavior contrasts with the analogous reaction with N-methylimidazole, for which the chloride ionization is unassisted and the rate law first order in N-MeIm at low concentrations of N-MeIm. The relationship between these results and hydrogen bonding from a distal histidine to a coordinated anion (e.g., superoxide) in hemoproteins is discussed. The intermediate Fe(TPP)(HIm)Cl was prepared in solution at -78°C and characterized by electronic spectral and conductivity measurements. It is a rare example of a six-coordinate high-spin iron(III) porphyrin with nonidentical axial ligands.
Models of the Cytochromes b. 5. EPR Studies of Low-Spin Iron(III) Tetraphenylporphyrins
Walker, F. Ann,Reis, David,Balke, Virginia L.
, p. 6888 - 6898 (2007/10/02)
The EPR spectra of a wide range of tetraphenylporphyrin complexes of Fe(III) have been investigated as a function of solvent, ligand type, ligand basicity, porphyrin substituents, covalent attachment of axial ligands, and mixed axial ligand coordination.The results show the following: (1) EPR parameters of low-spin bis-axial ligand complexes of Fe(III) porphyrins depend not only on ligand basicity but also on ligand type.Of the three major classes studied, bis(imidazole) and -(aminopyridine) complexes all have similar values of gx, gy, and gz which are nearly independent of ligand basicity, while bis(pyrazole) (and bis(indazole)) complexes have gx, gy, and gz values which tend to converge as ligand basicity increases. (2) The effect of the electron-donating or -withdrawing nature of phenyl substituents on the EPR parameters of a large series of phenyl-substituted (TPP)Fe(N-MeIm)2+ derivatives is very small: The rhombicity V/Δ=0.64+/-0.01 for all complexes, while the tetragonality Δ/λ ranges from 2.97 for electron-donating substituents to 3.33 for electron-withdrawing substituents, the opposite trend from that expected for increasing axial ligand donor strength.No difference was observed in the EPR parameters of unsymmetrically as compared to symmetrically substituted TPP derivatives. (3) Covalent attachment of axial ligands or steric crowding of externally supplied axial ligands in the hope of seeing variation in the EPR parameters with relative axial ligand plane orientation (parallel vs. perpendicular) was not successful in producing pure isomers, and thus no effects on EPR parameters with axial ligand plane orientation were detected. (4) A covalently attached (N-alkylimidazole-TPP)Fe(III) derivative was utilized to allow formation of mixed-ligand low-spin Fe(III) complexes.The alkylimidazole-imidazolate ligand combination was only very slightly more tetragonal than its protonated imidazole counterpart, while the alkylimidazole-pyrazole, 3-aminopyrazole, 1,2,4-triazole, and 2-methylimidazole mixed ligand complexes all had EPR parameters uniquely different from those of the parent bis-ligand complexes.Discussion of these results in light of the g values of membrane-bound cytochromes b, c, and a3 bound to cyanide is also included.
New Five- and Six-Coordinate Imidazole and Imidazolate Complexes of Ferric Tetraphenylporphyrine
Quinn, Robert,Nappa, Mario,Valentine, Joan S.
, p. 2588 - 2595 (2007/10/02)
The synthesis of several new imidazole and imidazolate complexes of ferric tetraphenylporphyrine (TPP) are reported.The following coplexes have been made with L = imidazole (ImH) or 4-methylimidazole (4MeImH) and L- = imidazolate (Im-) or 4-methylimidazolate (4MeIm-): FeTPP(L)(L-), -2>-, and (SbF6).Addition of Im- to FeTPPCl resulted in the formation first of the imidazolate-bridged complex + and then of -.Similar addition of 4MeIm- to FeTPPCl resulted in the formation first of the high-spin mononuclear complex FeTPP(4MeIm) and then of -.The affinity of (SbF6) for a second 4MeImH was found to be very high, with K >/= 107 M-1 in toluene, 25 deg C.By contrast, 4MeImH was found preferentially to hydrogen bond to Fe(TPP)(4MeIm), with K ca. 5 * 104 M-1 in THF, 25 deg C.Coordination of 4MeImH was only observed at high concentration of ligand (K ca. 50 +/- 10 M-1).The monoimidazole complex (SbF6) was concluded to be high spin after a comparison of the visible spectral properties of a large number of ferric TPP complexes.
