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

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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 νFeN/νNO plots but not in νFeC/νCO 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 νFeN/νNO 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 νFeN/νNO correlation with the same slope as that calculated for H-bonding to N. Such deviations are not observed for CO adducts, because the CO π* 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.

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