New Light on NO Bonding in Fe(III) Heme Proteins from Resonance Raman Spectroscopy and DFT Modeling

摘要

Visible and ultraviolet resonance Raman (RR) spectra are reported for FeIII(NO) adducts ofnmyoglobin variants with altered polarity in the distal heme pockets. The stretching frequencies of the FeIII-NOnand N-O bonds, νFeN and νNO, are negatively correlated, consistent with backbonding. However, thencorrelation shifts to lower νNO for variants lacking a distal histidine. DFT modeling reproduces the shiftedncorrelations and shows the shift to be associated with the loss of a lone-pair donor interaction from thendistal histidine that selectively strengthens the N-O bond. However, when the model contains stronglynelectron-withdrawing substituents at the heme u0002-positions, νFeN and νNO become positively correlated. Thisneffect results from FeIII-N-O bending, which is induced by lone-pair donation to the NNO atom. Othernmechanisms for bending are discussed, which likewise lead to a positive νFeN/νNO correlation, includingnthiolate ligation in heme proteins and electron-donating meso-substituents in heme models. The νFeN/νNOndata for the Fe(III) complexes are reporters of heme pocket polarity and the accessibility of lone pair,nLewis base donors. Implications for biologically important processes, including NO binding, reductivennitrosylation, and NO reduction, are discussed.