Nitric oxide coupling to generate NO promoted by a single-heme system as examined by density functional theory.

Bacteria utilize a heme/non-heme enzyme system to detoxify nitric oxide (NO) to NO. In order to probe the capacity of a single-heme system to mediate this NO-to-NO transformation, various scenarios for addition of electrons, protons, and a second NO molecule to a heme nitrosyl to generate NO were explored by density functional theory calculations. We describe, utilizing this single-heme system, several stepwise intermediates along pathways that enable the critical N-N bond formation step yielding the desired Fe-NO product. We also report a hitherto unreported directional second protonation that results in either productive NO formation with loss of water, or formation of a non-productive hyponitrous acid adduct Fe{HONNOH}.