Over or Under: Hydride Attack at the Metal versus the Coordinated Nitrosyl Ligand in Ferric Nitrosyl Porphyrins.

Hydride attack at a ferric heme–NO to give an Fe–HNO intermediate is a key step in the global N--‐cycle. We demonstrate differential reactivity when six--‐ and five--‐coordinate ferric heme--‐NO models react with hydride. Although Fe–HNO formation is thermodynamically favored from this reaction, Fe–H formation is kinetically favored for the 5C case

Electrochemical investigation of the kinetics of chloride substitution upon reduction of [Ru(porphyrin)(NO)Cl] complexes in THF.

The electrochemistry of several ruthenium porphyrin nitrosyl chloride complexes [Ru(por)(NO)Cl] have been examined in tetrahydrofuran. The complexes undergo 1-electron irreversible reductions which result in the diffusion-limited substitutions of the chloride ligands for THF. This chloride metathesis is reversible in the presence of added NBu4Cl, and equilibrium constants and rate constants for chloride loss have been estimated. These parameters correlate with the NO stretching frequencies of the parent complexes, with more electron-donating porphyrin ligands favouring chloride loss from the reduced complexes. The [Ru(por)(NO)(THF)] products of the reductions can be detected by IR, EPR and visible spectroscopies. These species undergo three further reductions, with good reversibility at scan rates \u3e0.40 V s-1. The [Ru(por)(NO)(THF)]+/0 couples have also been determined, and the rate constants and equilibrium constants for recombination with chloride have been estimated. One-electron reductions of the [Ru(por)(NO)Cl] complexes result in ~1018 enhancement of the rates of chloride loss

Endomyocardial Fibrosis: Still a Mystery after 60 Years

The pathologist Jack N. P. Davies identified endomyocardial fibrosis in Uganda in 1947. Since that time, reports of this restrictive cardiomyopathy have come from other parts of tropical Africa, South Asia, and South America. In Kampala, the disease accounts for 20% of heart disease patients referred for echocardiography. We conducted a systematic review of research on the epidemiology and etiology of endomyocardial fibrosis. We relied primarily on articles in the MEDLINE database with either “endomyocardial fibrosis” or “endomyocardial sclerosis” in the title. The volume of publications on endomyocardial fibrosis has declined since the 1980s. Despite several hypotheses regarding cause, no account of the etiology of this disease has yet fully explained its unique geographical distribution

Why Is the Oxidation State of Iron Crucial for the Activity of Heme-Dependent Aldoxime Dehydratase? A QM/MM Study

Aldoxime dehydratase is a heme-containing enzyme that utilizes the ferrous rather than the ferric ion to catalyze the synthesis of nitriles by dehydration of the substrate. We report a theoretical study of this enzyme aimed at elucidating its catalytic mechanism and understanding this oxidation state preference (Fe2+ versus Fe3+). The uncatalyzed dehydration reaction was modeled by including three and four water molecules to assist in the proton transfer, but the computed barriers were very high at both the DFT (B3LYP) and coupled cluster CCSD(T) levels. The enzymatic dehydration of Z-acetaldoxime was explored through QM/MM calculation using two different QM regions and covering all three possible spin states. The reaction starts by substrate coordination to Fe2+ via its nitrogen atom to form a six-coordinated singlet reactant complex. The ferrous heme catalyzes the N–O bond cleavage by transferring one electron to the antibond in the singlet state, while His320 functions as a general acid to deliver a proton to the leaving hydroxide, thus facilitating its departure. The key intermediate is identified as an FeIII(CH3CH═N•) species (triplet or open-shell singlet), with the closed-shell singlet FeII(CH3CH═N+) being about 6 kcal/mol higher. Subsequently, the same His320 residue abstracts the α-proton, coupled with electron transfer back to the iron center. Both steps are calculated to have feasible barriers (14–15 kcal/mol), in agreement with experimental kinetic studies. For the same mode of substrate coordination, the ferric heme does not catalyze the N–O bond cleavage, because the reaction is endothermic by about 40 kcal/mol, mainly due to the energetic penalty for oxidizing the ferric heme. The alternative binding option, in which the anionic aldoxime coordinates to the ferric ion via its oxyanion, also results in a high barrier (around 30 kcal/mol), mainly because of the large endothermicity associated with the generation of a suitable base (neutral His320) for proton abstraction

(1-Methyl-1H-imidazole-κN3)(1-methyl-2-nitrosobenzene-κN)(5,10,15,20-tetraphenylporphyrinato-κ4N)iron(II) dichloromethane monosolvate

The solvated title compound, [Fe(C44H28N4)(C4H6N2)(C7H7NO)]·CH2Cl2, is a porphyrin complex containing an octahedrally coordinated FeII atom with 1-methylimidazole [Fe—N = 2.0651 (17) Å] and o-nitrosotoluene ligands at the axial positions. The o-nitrosotoluene ligand is N-bound to iron(II) [Fe—N = 1.8406 (18)Å and Fe—N—O = 122.54 (14)°]. The axial N—Fe—N linkage is almost linear, with a bond angle of 177.15 (7)°. One phenyl group of the porphyrin ligand is disordered over two orientations in a 0.710 (3):0.290 (3) ratio. The dichloromethane solvent molecule was severely disordered and its contribution to the scattering was removed with the SQUEEZE routine [van der Sluis & Spek (1990). Acta Cryst. A46, 194–201]