title: Influence of thiolate ligands on reductive N-O bond activation. Probing the O2(-) binding site of a biomimetic superoxide reductase analogue and examining the proton-dependent reduction of nitrite.
abstract: Nitric oxide (NO) is frequently used to probe the substrate-binding site of "spectroscopically silent" non-heme Fe(2+) sites of metalloenzymes, such as superoxide reductase (SOR). Herein we use NO to probe the superoxide binding site of our thiolate-ligated biomimetic SOR model [Fe(II)(S(Me(2))N(4)(tren))](+) (1). Like NO-bound trans-cysteinate-ligated SOR (SOR-NO), the rhombic S = 3/2 EPR signal of NO-bound cis-thiolate-ligated [Fe(S(Me(2))N(4)(tren)(NO)](+) (2; g = 4.44, 3.54, 1.97), the isotopically sensitive ν(NO)(ν((15)NO)) stretching frequency (1685(1640) cm(-1)), and the 0.05 Å decrease in Fe-S bond length are shown to be consistent with the oxidative addition of NO to Fe(II) to afford an Fe(III)-NO(-) {FeNO}(7) species containing high-spin (S = 5/2) Fe(III) antiferromagnetically coupled to NO(-) (S = 1). The cis versus trans positioning of the thiolate does not appear to influence these properties. Although it has yet to be crystallographically characterized, SOR-NO is presumed to possess a bent Fe-NO similar to that of 2 (Fe-N-O = 151.7(4)°). The N-O bond is shown to be more activated in 2 relative to N- and O-ligated {FeNO}(7) complexes, and this is attributed to the electron-donating properties of the thiolate ligand. Hydrogen-bonding to the cysteinate sulfur attenuates N-O bond activation in SOR, as shown by its higher ν(NO) frequency (1721 cm(-1)). In contrast, the ν(O-O) frequency of the SOR peroxo intermediate and its analogues is not affected by H-bonds to the cysteinate sulfur or other factors influencing the Fe-SR bond strength; these only influence the ν(Fe-O) frequency. Reactions between 1 and NO(2)(-) are shown to result in the proton-dependent heterolytic cleavage of an N-O bond. The mechanism of this reaction is proposed to involve both Fe(II)-NO(2)(-) and {FeNO}(6) intermediates similar to those implicated in the mechanism of NiR-promoted NO(2)(-) reduction.
authors.forename: Gloria
authors.initials: G
authors.lastname: Villar-Acevedo
authors.forename: Elaine
authors.initials: E
authors.lastname: Nam
authors.forename: Sarah
authors.initials: S
authors.lastname: Fitch
authors.forename: Jason
authors.initials: J
authors.lastname: Benedict
authors.forename: John
authors.initials: J
authors.lastname: Freudenthal
authors.forename: Werner
authors.initials: W
authors.lastname: Kaminsky
authors.forename: Julie A
authors.initials: JA
authors.lastname: Kovacs
mesh_terms: D001665:Binding Sites; D040761:Biomimetic Materials / Q000737:chemistry*; D004583:Electrons; D008024:Ligands; D008280:Magnetics; D008958:Models, Molecular; D008968:Molecular Conformation; D009569:Nitric Oxide / Q000737:chemistry; D009573:Nitrites / Q000737:chemistry*; D009584:Nitrogen / Q000737:chemistry*; D010084:Oxidation-Reduction; D010088:Oxidoreductases / Q000737:chemistry*; D010100:Oxygen / Q000737:chemistry*; D011522:Protons; D013481:Superoxides / Q000737:chemistry*; D014732:Vibration
chemical_list: D008024:Ligands; D009573:Nitrites; D011522:Protons; D013481:Superoxides; D009569:Nitric Oxide; D010088:Oxidoreductases; C121849:superoxide reductase; D009584:Nitrogen; D010100:Oxygen