Ambidentate H-bonding of NO and O-2 in heme proteins

作者：Spiro Thomas G^*; Soldatova Alexandra V

来源：Journal of Inorganic Biochemistry, 2012, 115: 204-210.

DOI：10.1016/j.jinorgbio.2012.05.013

• Heme
• Nitric oxide
• Dioxygen
• H-bonding
• Backbonding
• DFT

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 pi* 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 Fe-X and X-O bond stretches. For heme-CO adducts. H-bonding increases backbonding, the nu FeX/nu XO 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 nu FeX/nu XO 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 Fe-X and X-O 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.

• 出版日期2012-10