Ambidentate H-bonding of NO and O2 in heme proteins

The affinity and reactivity of the gaseous molecules CO, NO and O2 (XO) in heme protein adducts are controlled by secondary interactions, especially by H-bonds donated from distal protein residues. Vibrational spectroscopy, supported by DFT (Density Functional Theory) modeling, has revealed that for NO and O2, but not for CO, a critical issue is whether the H-bond is donated to the outer or inner atom of the bound diatomic ligand. DFT modeling shows that bound NO and O2 are ambidentate, both atoms separately acting as H-bond acceptors. This is not the case for CO, whose π* orbital acts as a delocalized H-bond acceptor. Vibrational spectra of heme-XO adducts reveal a general pattern of backbonding variations, marked by families of negative correlations between frequencies associated with FeX and XO bond stretches. For heme-CO adducts, H-bonding increases backbonding, the νFeX/νXO points moving up the backbonding correlation established with model compounds. For NO and O2 adducts, however, increased backbonding is only observed when the outer atom is the H-bond acceptor. H-bonding to the inner (X) atom instead produces a positive νFeX/νXO correlation. This effect can be reproduced by DFT modeling. Its mechanism is polarization of the sp2 orbital on the X atom, on the back side of the bent FeXO unit, drawing electrons from both the FeX and XO bonds and weakening them together. Thus, the positioning of H-bond donors in the protein differentially affects bonding and reactivity in heme adducts of NO and O2.

Graphical abstractThe positioning of H-bond donors by the protein differentially controls bonding and reactivity in heme‐NO and -O2 adducts. This article reviews the results of vibrational spectroscopy and DFT modeling, revealing a critical issue of whether the H-bond is donated to the outer or inner atom of the bound ligand.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► H-bonding in heme protein adducts of XO (X=C, N, O) are analyzed with DFT and RR. ► Different response of FeNO and FeO2 to the position of H-bond donors is revealed. ► H-bonding from heme pocket residues to the outer oxygen increases backbonding. ► H-bonding to the inner atom of FeXO weakens the Fe-X and X-O bonds in concert. ► Positioning of H-bond donors by the protein controls bonding in heme FeNO and FeO2.