The Thiolate Trans Effect in Heme {FeNO}⁶ Complexes and Beyond: Insight into the Nature of the Push Effect

Cyt P450 nitric oxide (NO) reductase (P450nor) is an important enzyme in fungal denitrification, responsible for the large-scale production of the greenhouse gas N₂O. In the first step of catalysis, the ferric heme-thiolate active site of P450nor binds NO to produce a ferric heme-nitrosyl or {FeNO}⁶ intermediate (in the Enemark-Feltham notation). In this paper, we present the low-temperature preparation of six new heme-thiolate {FeNO}⁶ model complexes, [Fe(TPP)(SPh*)(NO)], using a unique series of electron-poor thiophenolates (SPh*^-), and their detailed spectroscopic characterization. Our data show experimentally, for the first time, that a direct correlation exists between the thiolate donor strength and the Fe-NO and N-O bond strengths, evident from the corresponding stretching frequencies. This is due to a σ-trans effect of the thiolate ligand, which manifests itself in the population of an Fe-N-O σ-antibonding (σ*) orbital. Via control of the thiolate donor strength (using hydrogen bonds), nature is therefore able to exactly control the degree of activation of the FeNO unit in P450nor. Vice versa, NO can be used as a sensitive probe to quantify the donor strength of a thiolate ligand in a model system or protein, by simply measuring the Fe-NO and N-O frequencies of the ferric NO adduct and then projecting those data onto the correlation plot established here. Finally, we are able to show that the σ-trans effect of the thiolate is the electronic origin of the "push" effect, which is proposed to mediate O-O bond cleavage and Compound I formation in Cyt P450 monooxygenase catalysis.