3D Motions of Iron in Six‐Coordinate {FeNO}7 Hemes by Nuclear Resonance Vibration Spectroscopy

The vibrational spectrum of a six‐coordinate nitrosyl iron porphyrinate, monoclinic [Fe(TpFPP)(1‐MeIm)(NO)] (TpFPP=tetra‐para‐fluorophenylporphyrin; 1‐MeIm=1‐methylimidazole), has been studied by oriented single‐crystal nuclear resonance vibrational spectroscopy (NRVS). The crystal was oriented to give spectra perpendicular to the porphyrin plane and two in‐plane spectra perpendicular or parallel to the projection of the FeNO plane. These enable assignment of the FeNO bending and stretching modes. The measurements reveal that the two in‐plane spectra have substantial differences that result from the strongly bonded axial NO ligand. The direction of the in‐plane iron motion is found to be largely parallel and perpendicular to the projection of the bent FeNO on the porphyrin plane. The out‐of‐plane Fe‐N‐O stretching and bending modes are strongly mixed with each other, as well as with porphyrin ligand modes. The stretch is mixed with v₅₀ as was also observed for dioxygen complexes. The frequency of the assigned stretching mode of eight Fe‐X‐O (X=N, C, and O) complexes is correlated with the Fe?XO bond lengths. The nature of highest frequency band at ≈560 cm^?1 has also been examined in two additional new derivatives. Previously assigned as the Fe?NO stretch (by resonance Raman), it is better described as the bend, as the motion of the central nitrogen atom of the FeNO group is very large. There is significant mixing of this mode. The results emphasize the importance of mode mixing; the extent of mixing must be related to the peripheral phenyl substituents.     

Nitrosylation of Nitric‐Oxide‐Sensing Regulatory Proteins Containing [4Fe‐4S] Clusters Gives Rise to Multiple Iron–Nitrosyl Complexes

The reaction of protein‐bound iron–sulfur (Fe‐S) clusters with nitric oxide (NO) plays key roles in NO‐mediated toxicity and signaling. Elucidation of the mechanism of the reaction of NO with DNA regulatory proteins that contain Fe‐S clusters has been hampered by a lack of information about the nature of the iron‐nitrosyl products formed. Herein, we report nuclear resonance vibrational spectroscopy (NRVS) and density functional theory (DFT) calculations that identify NO reaction products in WhiD and NsrR, regulatory proteins that use a [4Fe‐4S] cluster to sense NO. This work reveals that nitrosylation yields multiple products structurally related to Roussin's Red Ester (RRE, [Fe₂(NO)₄(Cys)₂]) and Roussin's Black Salt (RBS, [Fe₄(NO)₇S₃]. In the latter case, the absence of ³²S/³⁴S shifts in the Fe?S region of the NRVS spectra suggest that a new species, Roussin's Black Ester (RBE), may be formed, in which one or more of the sulfide ligands is replaced by Cys thiolates.