Reactivity of Five-Coordinate Models for the Thiolate-Ligated Fe Site of Nitrile Hydratase

Abstract

To examine inhibitor binding to an iron site resembling that of the metalloenzyme nitrile hydratase (NHase), a coordinatively unsaturated, five-coordinate FeIII thiolate complex was synthesized, and its reactivity examined. Ferricinium hexafluorophosphate induced oxidation of gem-dimethyl-protected [FeIIS2 Me2N3(Pr,Pr)] affords the chiral, five-coordinate complex [FeIIIS2 Me2N3(Pr,Pr)]+ (2) in reasonable yields. The magnetic properties and EPR of 2 are consistent with an S = 1/2 ground state. This unusual spin state, in conjunction with the low coordination number, of 2 result in unusually short Fe−S bonds (2.15(2) Å). Ligand constraints distort the S−Fe−N angles in 2 and create an open (132.3(1)°) reactive site. Azide binds to this site to afford a model for the azide-inhibited form of NHase [FeIIIS2 Me2N3(Pr,Pr)(N3)] (3). In MeOH azide binds reversibly, whereas in MeCN it binds irreversibly. This demonstrates that the secondary coordination sphere (i.e., the solvent, or possibly a protein binding pocket) can have a dramatic influence on the substrate binding properties of a metal complex. A variable-temperature equilibrium study in MeOH afforded thermodynamic parameters (ΔH = −5.2 ± 0.9 kcal/mol and ΔS = −12.4 ± 0.4 eu) for the binding of this inhibitor. The electronic spectrum of 3 displays an intense band at 708 (1600) nm similar to that (710 (∼1200) nm) of the pH = 7.3 form of NHase, and other six-coordinate cis-dithiolate ligated FeIII complexes synthesized by our group. EPR parameters for 3 (g = 2.23, 2.16, 1.99) are nearly identical to those of the azide-inhibited form of NHase (g = 2.23, 2.14, 1.99), suggesting that (1) the iron site of our model closely resembles that of the enzyme, and (2) azide binds directly to the metal ion in NHase. Reactivity is oxidation-state dependent, and the reduced analogue of 2, [FeIIS2N3(Pr,Pr)] (4), reversibly binds CO, but not azide, whereas oxidized 2 binds azide, but not CO.

How does cyanide inhibit superoxide reductase? Insight from synthetic Fe III N 4 S model complexes

“…2, the acetate is coordinated in a monodentate, as opposed to bidentate, fashion trans to an imine nitrogen [N(1), Table 1]. As is the case with structures 3 and 5, the FeOS bond in 6 (Table 1) falls on the short end of reported low-spin Fe(III) thiolate structures (21,22,30). Again, this most likely reflects the cationic molecular charge and the weaker trans influence of the amine nitrogen [N(2)] relative to anionic ligands such as N 3 Ϫ or SCN Ϫ .…”

The structure and spin-states of some Fe(III) mimics of nitrile hydratase, studied by DFT and ONIOM(DFT:PM3) calculations

“…Also, an ambient temperature solution magnetic moment of 3.5 m B indicates the presence of a spin mixture at higher temperatures. Below 125 K the EPR spectrum of complex 3 confirms that the low spin-state is significantly populated [53]. Hence an S ¼ 1/2 ground state was expected from the corresponding in vacuo geometry optimization.…”

“…This complex exhibits an absorption spectrum that shows two bands with maxima at 718 and 438 nm, which closely resemble the absorption spectrum of Fe-NHase [52]. Complex 3 [53] is coordinatively unsaturated and shows reactivity towards azide (N À 3 ) and NO to generate the corresponding azide and nitrosyl adducts, which present spectral features that make them good models for the azide-inhibited form and NO-inactivated form of Fe-NHase, respectively [53,54]. The average Fe-S bond length in this complex is 2.14 Å , much shorter than the one observed in the enzyme and in the previous models.…”

“…Azide binds reversibly to complex 3 to afford an azide-bound complex that displays an intense, low-energy sulphur--to-iron charge-transfer band, which is not seen with trigonal bipyramidal complex 3 [49,53]. With the exception of the NO-inactivated Fe-NHase, all of the other forms of the enzyme that have been characterized to date display an intense low-energy charge-transfer band.…”

CoIII Complexes with Square‐Planar N2S2‐ and N2(SO2)2‐Type Ligands as An Active Site Structural Model for Nitrile Hydratase – Biological Implications of an Amidate Coordination

“…[13] Most studies that use model complexes have focused on the effect of the oxidation of sulfur. For example, several Co III and Fe III complexes with pentadentate ligands that contain an imine, [14] an amide, [15,16] or an amine [17] group and sulfur atoms have been synthesized and studied. Among them, Mascharak and coworkers [15a] have prepared the complex [Co(PyPS)(CN)] with the use of the amide-type ligand, which was easily replaced by a H 2 O molecule.…”