Stabilization of a Heme-HNO Model Complex Using a Bulky Bis-Picket Fence Porphyrin and Reactivity Studies with NO

Crystal Structure and Electron Configuration of {MNO}8 Heme Complexes

Although research on nitrosyl (NO) heme complexes and their one-electron reduced form, nitroxyl (or nitroxyl anion, NO-) derivatives, has been going on for decades, there are still disagreements about the electrical configuration of nitroxyl complexes, and the majority of the work on this topic is based on theoretical calculations. Following the initial nitroxyl iron porphyrin crystal structure, we present two further polymorphic forms of [CoCp2][Fe(TFPPBr8)(NO)]. Using the same completely halogenated porphyrin ligand, we also present two polymorphic forms of nitrosyl cobalt(II) complexes, which are another sort of {MNO}8 structure. In addition to the EXANES and EPR studies of these {FeNO}7 and {CoNO}8 complexes, the {FeNO}8 [CoCp2][Fe(TFPPBr8)(NO)] complex is also investigated by temperature-dependent Mössbauer experiments for the first time with the {FeNO}7 precursor as a control sample. The analysis of the Mössbauer and crystal structural parameters between these two types of {MNO}8 (M = Fe or Co) species and previously reported analogous ones allow us to conclude that the electronic configuration of [Fe(TFPPBr8)(NO)]- is best described as an intermediate between low-spin Fe(II)-NO- and Fe(I)-NO•.

Vibrational properties of heme-nitrosoalkane complexes in comparison with those of their HNO analogs, and reactivity studies towards nitric oxide and Lewis acids

C-Nitroso compounds (RNO, R = alkyl and aryl) are byproducts of drug metabolism and bind to heme proteins, and their heme-RNO adducts are isoelectronic to ferrous nitroxyl (NO-/HNO) complexes. Importantly, heme-HNO compounds are key intermediates in the reduction of NO to N2O and nitrite to ammonium in the nitrogen cycle. Ferrous heme-RNO complexes act as stable analogs of these species, potentially allowing for the investigation of the vibrational and electronic properties of unstable heme-HNO intermediates. In this paper, a series of six-coordinate ferrous heme-RNO complexes (where R = iPr and Ph) were prepared using the TPP2- and 3,5-Me-BAFP2- co-ligands, and tetrahydrofuran, pyridine, and 1-methylimidazole as the axial ligands (bound trans to RNO). These complexes were characterized using different spectroscopic methods and X-ray crystallography. The complex [Fe(TPP)(THF)(iPrNO)] was further utilized for nuclear resonance vibrational spectroscopy (NRVS), allowing for the detailed assignment of the Fe-N(R)O vibrations of a heme-RNO complex for the first time. The vibrational properties of these species were then correlated with those of their HNO analogs, using DFT calculations. Our studies support previous findings that RNO ligands in ferrous heme complexes do not elicit a significant trans effect. In addition, the complexes are air-stable, and do not show any reactivity of their RNO ligands towards NO. So although ferrous heme-RNO complexes are suitable structural and electronic models for their HNO analogs, they are unsuitable to model the reactivity of heme-HNO complexes. We further investigated the reaction of our heme-RNO complexes with different Lewis acids. Here, [Fe(TPP)(THF)(iPrNO)] was found to be unreactive towards Lewis acids. In contrast, [Fe(3,5-Me-BAFP)(iPrNO)2] is reactive towards all of the Lewis acids investigated here, but in most cases the iron center is simply oxidized, resulting in the loss of the iPrNO ligand. In the case of the Lewis acid B2(pin)2, the reduced product [Fe(3,5-Me-BAFP)(iPrNH2)(iPrNO)] was identified by X-ray crystallography.

Recent mechanistic developments for cytochrome c nitrite reductase, the key enzyme in the dissimilatory nitrate reduction to ammonium pathway

Cytochrome c nitrite reductase, NrfA, is a soluble, periplasmic pentaheme cytochrome responsible for the reduction of nitrite to ammonium in the Dissimilatory Nitrate Reduction to Ammonium (DNRA) pathway, a vital reaction in the global nitrogen cycle. NrfA catalyzes this six-electron and eight-proton reduction of nitrite at a single active site with the help of its quinol oxidase partners. In this review, we summarize the latest progress in elucidating the reaction mechanism of ammonia production, including new findings about the active site architecture of NrfA, as well as recent results that elucidate electron transfer and storage in the pentaheme scaffold of this enzyme.