Heme proteins bind the gaseous ligands XO (X = C, N, O) via backbonding from Fe d π electrons. Backbonding is modulated by distal interactions of the bound ligand with the surrounding protein and by variations in the strength of the trans proximal ligand. Vibrational modes associated with FeX and XO bond stretching coordinates report on these interactions, but the interpretive framework developed for CO adducts, involving anticorrelations of νFeC and νCO, has seemed not to apply to NO adducts. We have now obtained an excellent anticorrelation of νFeN and νNO, via resonance Raman spectroscopy on (N-methylimidazole)Fe(II)TPP-Y(NO), where TPP-Y is tetraphenylporphine with electron donating or withdrawing substituents, Y, that modulate the backbonding; the problem of laser-induced dissociation of the axial base was circumvented by using frozen solutions. New data are also reported for CO adducts. The anticorrelations are supported by DFT calculations of structures and spectra. When protein data are examined, the NO adducts show large deviations from the modeled anticorrelation when there are distal H-bonds or positive charges. These deviations are proposed to result from closing of the FeNO angle due to a shift in the valence isomer equilibrium toward the Fe(III)(NO -) form, an effect that is absent in CO adducts. The differing vibrational patterns of CO and NO adducts provide complementary information with respect to protein interactions, which may help to elucidate the mechanisms of ligand discrimination and signaling in heme sensor proteins.

Transition metal oxo-species have been the focus of extensive studies because of their relevance to the redox biochemistry of dioxygen as well as myriads of oxidative catalytic processes. High valent oxo-manganese complexes have been described for porphyrin, 1 salen, 2 corrole, 3 corrolazine, 4 and non-heme systems. 5 The O=Mn V moiety has been suggested in the photosynthetic water oxidation process, 6 and a bridged Mn V porphyrin dimer has recently been demonstrated to oxidize water into dioxygen. 7 We have previously described low-spin d 2 oxomanganese(V) porphyrin complexes that display an extraordinary range of reactivity toward oxo-transfer as a function of prototropic equilibria involving the axial ligand. 8 A prediction of that work was that oxo-aqua and oxo-hydroxomanganese(V) intermediates are reactive oxidants while the stable species observed at high pH are trans-dioxo complexes. Here we provide the first definitive spectroscopic evidence for trans-dioxomanganese(V) porphyrins [O=Mn V =O]. Further, we show that protonation of these species affords the reactive intermediates usually associated with these catalytic systems (Scheme 1).

Visible and ultraviolet resonance Raman (RR) spectra are reported for Fe III (NO) adducts of myoglobin variants with altered polarity in the distal heme pockets. The stretching frequencies of the Fe III -NO and N-O bonds, ν FeN and ν NO , are negatively correlated, consistent with backbonding. However, the correlation shifts to lower ν NO for variants lacking a distal histidine. DFT modeling reproduces the shifted correlations, and shows the shift to be associated with the loss of a lone-pair donor interaction from the distal histidine that selectively strengthens the N-O bond. However, when the model contains strongly electron-withdrawing substituents at the heme β-positions, ν FeN and ν NO become positively correlated. This effect results from Fe III -N-O bending, which is induced by lone pair donation to the N NO atom. Other mechanisms for bending are discussed, which likewise lead to a positive ν FeN /ν NO correlation, including thiolate ligation in heme proteins and electrondonating meso-substituents in heme models. The ν FeN /ν NO data for the Fe(III) complexes are reporters of heme pocket polarity and the accessibility of lone pair, Lewis base donors. Implications for biologically important processes, including NO binding, reductive nitrosylation and NO reduction, are discussed.