| Abstract
| - Heme proteins bind the gaseous ligands XO (X = C, N, O) via backbonding from Fe dπ electrons.Backbonding is modulated by distal interactions of the bound ligand with the surrounding protein and byvariations in the strength of the trans proximal ligand. Vibrational modes associated with FeX and XObond stretching coordinates report on these interactions, but the interpretive framework developed for COadducts, involving anticorrelations of νFeC and νCO, has seemed not to apply to NO adducts. We havenow 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 ofthe 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 areexamined, the NO adducts show large deviations from the modeled anticorrelation when there are distalH-bonds or positive charges. These deviations are proposed to result from closing of the FeNO angle dueto a shift in the valence isomer equilibrium toward the Fe(III)(NO-) form, an effect that is absent in COadducts. The differing vibrational patterns of CO and NO adducts provide complementary information withrespect to protein interactions, which may help to elucidate the mechanisms of ligand discrimination andsignaling in heme sensor proteins.
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