Low-Spin Ferriheme Models of the Cytochromes:  Correlation of Molecular Structure with EPR Spectral Type

Abstract

The preparation and characterization of the following bis-imidazole and bis-pyridine complexes of octamethyltetraphenylporphyrinatoiron(III), Fe(III)OMTPP, octaethyltetraphenylporphyrinatoiron(III), Fe(III)OETPP, and tetra-beta,beta'-tetramethylenetetraphenylporphyrinatoiron(III), Fe(III)TC(6)TPP, are reported: paral-[FeOMTPP(1-MeIm)(2)]Cl, perp-[FeOMTPP(1-MeIm)(2)]Cl, [FeOETPP(1-MeIm)(2)]Cl, [FeTC(6)TPP(1-MeIm)(2)]Cl, [FeOMTPP(4-Me(2)NPy)(2)]Cl, and [FeOMTPP(2-MeHIm)(2)]Cl. Crystal structure analysis shows that paral-[FeOMTPP(1-MeIm)(2)]Cl has its axial ligands in close to parallel orientation (the actual dihedral angle between the planes of the imidazole ligands is 19.5 degrees ), while perp-[FeOMTPP(1-MeIm)(2)]Cl has the axial imidazole ligand planes oriented at 90 degrees to each other and 29 degrees away from the closest N(P)-Fe-N(P) axis. [FeOETPP(1-MeIm)(2)]Cl has its axial ligands close to perpendicular orientation (the actual dihedral angle between the planes of the imidazole ligands is 73.1 degrees ). In all three cases the porphyrin core adopts relatively purely saddled geometry. The [FeTC(6)TPP(1-MeIm)(2)]Cl complex is the most planar and has the highest contribution of a ruffled component in the overall saddled structure compared to all other complexes in this study. The estimated numerical contribution of saddled and ruffled components is 0.68:0.32, respectively. Axial ligand planes are perpendicular to each other and 15.3 degrees away from the closest N(P)-Fe-N(P) axis. The Fe-N(P) bond is the longest in the series of octaalkyltetraphenylporphyrinatoiron(III) complexes due to [FeTC(6)TPP(1-MeIm)(2)]Cl having the least distorted porphyrin core. In addition to these three complexes, two crystalline forms each of [FeOMTPP(4-Me(2)NPy)(2)]Cl and [FeOMTPP(2-MeHIm)(2)]Cl were obtained. In all four of these cases the axial planes are in nearly perpendicular planes in spite of quite different geometries of the porphyrin cores (from purely saddled to saddled with 30% ruffling). The EPR spectral type correlates with the geometry of the OMTPP, OETPP and TC(6)TPP complexes. For the paral-[FeOMTPP(1-MeIm)(2)]Cl, a rhombic signal with g(1) = 1.54, g(2) = 2.51, and g(3) = 2.71 is consistent with nearly parallel axial ligand orientation. For all other complexes of this study, "large g(max)" signals are observed (g(max) = 3.61 - 3.27), as are observed for nearly perpendicular ligand plane arrangement. On the basis of this and previous work, the change from "large g(max)" to normal rhombic EPR signal occurs between axial ligand plane dihedral angles of 70 degrees and 30 degrees.

SoxAX Cytochromes, a New Type of Heme Copper Protein Involved in Bacterial Energy Generation from Sulfur Compounds

“…The LS1a,b and LS2 centers thus display "normal rhombic" g values for a low spin ferriheme that have been interpreted as indicative of the axial ligands being in a near parallel rather than perpendicular orientation (37).…”

Nitric oxide interaction with insect nitrophorins and thoughts on the electron configuration of the FeNO complex

“…Formal core diagrams of the nitrophorin complexes of NP4 [101,102] and NP2 [227] have been constructed, and root-mean-square deviations (RMSD) from the mean plane, in units of 0.01 Å , are listed in Table 1, where they are compared to the values obtained for relevant model heme complexes [232][233][234][235][236][237][238][239][240][241][242][243][244][245][246][247][248]. In analyzing these data, several factors must be kept in mind: (1) Among synthetic hemes, it is known that meso-substituted metalloporphyrinates are in general more ruffled than b-pyrrole-only-substituted metalloporphyrinates [229] and (2) Fe(III) porphyrinates are usually much more ruffled than Fe(II) porphyrinates [229,236,242,243].…”

“…There are two possible electron configurations of low-spin iron(III) porphyrinates, the more commonly observed (d xy ) 2 (d xz , d yz ) 3 , or d p , electron configuration, which is usually observed for the elecron-transferring ferricytochromes a, b, c, f and o [243][244][245][246][247], and the ''novel'' (d xz , d yz ) 4 (d xy ) 1 , or d xy , electron configuration, which is often observed in model hemes having axial ligands that are good p-acceptors, such as isocyanides [235,236,248], low-basicity pyridines [234], and under certain conditions, cyanide ions [249][250][251][252]. The d xy ground state is also believed to exist for some reduced hemes in biological systems, including heme d and d 1 and siroheme, but this appears to depend upon the nature of the axial ligands [253][254][255].…”

Ferric species equilibrium of the giant extracellular hemoglobin of Glossoscolex paulistus in alkaline medium: HALS hemichrome as a precursor of pentacoordinate species

“…Walker and co-workers have studied and classified model complexes and hemoproteins regarding the different types of reciprocal orientation among axial ligands and the spatial conformations of the porphyrin ring [12][13][14]. These possible arrangements are decisive to determine the spin state and redox properties of the metallic center in hemoproteins.…”

“…These possible arrangements are decisive to determine the spin state and redox properties of the metallic center in hemoproteins. In the classification proposed by Walker and co-workers [12][13][14] 1 , and independent orientations of the axial ligand planes. This classification is especially important to hemichrome species, since these bis-histidine species clearly present two ligands containing cyclic structures of the imidazole group.…”