Ambidentate H-bonding by heme-bound NO: structural and spectral effects of –O versus –N H-bonding

Abstract

Resonance Raman studies have uncovered puzzling complexities in the structures of NO adducts of heme proteins. Although CO adducts of heme proteins obey well-behaved anti-correlations between Fe-C and C-O stretching frequencies, which reflect changes in backbonding induced by distal H-bonding residues, the corresponding NO data are scattered. This scatter can be traced to distal influences, since protein-free NO-hemes do show well-behaved anti-correlations. Why do distal effects produce irregularities in vFeN/ vNO plots but not in vFeC/vCO plots? We show via density functional theory (DFT) computations on model systems that the response to distal H-bonding differs markedly when the NO acceptor atom is N versus O. Backbonding is augmented by H-bonding to O, but the effect of H-bonding to N is to weaken both N-O and N-Fe bonds. The resulting downward deviation from the vFeN/vNO backbonding line increases with increasing H-bond strength. This effect explains the deviations observed for a series of myoglobin variants, in which the strength of distal H-bonding is modulated by distal pocket residue substitutions. Most of the data follow a positive vFeN/vNO correlation with the same slope as that calculated for H-bonding to N. Such deviations are not observed for CO adducts, because the CO pi* orbital is unoccupied, and serves as a delocalized acceptor of H-bonds. H-bonding to N primes NO-heme for reduction to the HNO adduct, a putative intermediate in NO-reducing enzymes.

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

“…There are interesting parallels between distal lone pair interactions with Fe III (NO) adducts and distal H-bond interactions with Fe II (NO) adducts, investigated earlier,36,55 which can best be appreciated with the aid of the valence bond diagrams in Figure 12. In both cases, interaction at the O NO atom polarizes the π bonds.…”

“…This orientation is stable for Fe II (CO)37,52–54 and Fe II (NO)55,56 adducts, but for (ImH)Fe III (NO)P no stable structure could be found with this orientation. However, a stable structure was found when the imidazole N: lone pair was oriented toward the NO (Figure 9a).…”

Covalent attachment of the heme to Synechococcus hemoglobin alters its reactivity toward nitric oxide

“…Additionally, differences in hydrogen bonding could alter the propensity for NO reduction. For example, H-bonding to the proximal nitrogen of Fe II –NO has been proposed to bias the electron density of the {FeNO} 7 adduct in such a way as to promote HNO formation [94]. The intermediate Fe II –NO GlbN complex is of interest for its ability to activate NO for reduction and is accessible for future mechanistic studies using a gentle reductant such as the flavin-containing diaphorase NR domain (Figure 8B).…”

Reductive activation of the heme iron–nitrosyl intermediate in the reaction mechanism of cytochrome c nitrite reductase: a theoretical study

“…At the same time, the charge on O(NO) becomes more positive, which is a consequence of hydrogen-bond formation. Xu and Spiro [78] have argued that hydrogen bonding to oxygen pulls electron density into the NO p * orbital, thus strengthening the Fe-N bond and weakening the N-O bond. However, hydrogen bonding to nitrogen withdraws bonding electrons from both the Fe-N and the N-O bonds into a (partial sp 2 ) nitrogen-based nonbonding orbital.…”