title: Ambidentate H-bonding of NO and O2 in heme proteins.
abstract: The affinity and reactivity of the gaseous molecules CO, NO and O(2) (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 O(2), 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 O(2) 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 O(2) 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 sp(2) 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 O(2).
authors: Department of Chemistry, University of Washington, Box 351700, Seattle, WA 98195, USA. spiro@chem.washington.edu
authors: Thomas G
authors: TG
authors: Spiro
authors: Alexandra V
authors: AV
authors: Soldatova
mesh_terms: D006418:Heme / Q000737:chemistry*; D006420:Hemeproteins / Q000737:chemistry*; D006860:Hydrogen Bonding; D007501:Iron / Q000737:chemistry*; D008958:Models, Molecular; D009569:Nitric Oxide / Q000737:chemistry*; D010100:Oxygen / Q000737:chemistry*
chemical_list: D006420:Hemeproteins; D009569:Nitric Oxide; D006418:Heme; D007501:Iron; D010100:Oxygen