Spectroscopic and computational study of a nonheme iron nitrosyl center in a biosynthetic model of nitric oxide reductase

A major barrier to understanding the mechanism of nitric oxide reductases (NORs) is the lack of a selective probe of NO binding to the nonheme Fe _B center. By replacing the heme in a biosynthetic model of NORs, which structurally and functionally mimics NORs, with isostructural ZnPP, the electronic structure and functional properties of the Fe_B nitrosyl complex was probed. This approach allowed observation of the first S=3/2 nonheme {FeNO}⁷ complex in a protein-based model system of NOR. Detailed spectroscopic and computational studies show that the electronic state of the {FeNO}⁷ complex is best described as a high spin ferrous iron (S=2) antiferromagnetically coupled to an NO radical (S= 1/2) [Fe²⁺-NO ^.]. The radical nature of the Fe_B-bound NO would facilitate N-N bond formation by radical coupling with the heme-bound NO. This finding, therefore, supports the proposed trans mechanism of NO reduction by NORs. Ironed out: A nonheme iron nitrosyl complex has been prepared at a rationally designed Fe_B site within a myoglobin-based biosynthetic model of nitric oxide reductases (NORs) that contains a zinc protoporphyrin IX. The designed Fe^II-ZnPPFe_BMb1 forms a nitrosyl complex [Fe_B²⁺-NO^.] at the nonheme site. The radical nature of NO is implied to promote N-N bond formation by radical coupling, thus supporting the trans mechanism of NORs.