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.
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.
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.
