Dnic With Sulfide And Neocuproine Anionic Ligands

As well known, sulfide dianions S2- often appear in biological systems as inorganic sulfur bridges between iron ions in polynuclear iron-sulfur clusters [65]. Disruption of such multinuclear clusters is one mechanism by which nitric oxide may interact with iron-sulfur proteins (ISPs) [3,66]. More details of such reactions are given in Chapters 5 and 6. The disruption can lead to the formation of iron-nitrosyl complexes, but the inorganic sulfide ions may remain bound to the iron. Incorporation of sulfide anions into non-heme nitrosyl-iron complexes was demonstrated for the first time by Goodman and Raynor by exposing an aqueous alcoholic solution of ferrous sulfate and excess sodium sulfide to gaseous NO [67]. At 77 K, EPR of frozen aliquots showed the formation of a paramagnetic MNIC containing sulfide ions (complex I). The spectrum had axial symmetry with gx = 2.01, g|| = 2.049 and a resolved HF coupling to the nitrogen nucleus of A||(14N) = 0.75 mT (Fig. 15, curve a). At room temperature, the spectra appear as a motionally averaged triplet at g = 2.021 with HFS of 0.48 mT.

The parameters of the EPR absorption changed sharply when ferrous ions were added to the solution in excess over sulfide ions (Fe2+:sulfide > 10:1) [68]. The NO treatment of the solution for 10 min led to the formation of a new paramagnetic ferrous iron-nitrosyl species (complex II). In frozen solution, it had a narrow axial EPR spectrum with gx = 2.035, g|| = 2.02, gav = 2.03 (Fig. 15, curve b) [68]. Room temperature spectra showed a single k b i i

Cysteine Ferrous Complex

Fig. 15. EPR signals of nitrosyl-iron complexes with sulphide [66]. (Curve a): Mononitrosyl iron complex with sulphide synthesized at the ratio of [sulphide]:[Fe2+] > 10 (complex I, g^ = 2.01, g|| = 2.049); (curve b) iron-nitrosyl complex with sulphide synthesized at the ratio of [Fe2+]:[sulphide] > 10 (complex II, g^ = 2.035, g|| = 2.020); (curve c) EPR signal of complex II with 57Fe; (curves d-f) respectively, EPR signals from DNIC (g± = 2.041, g|| = 2.014) and complexes II bound with nitrogenase enzyme. Recordings at 77 K. (From Ref. [68].)

Fig. 15. EPR signals of nitrosyl-iron complexes with sulphide [66]. (Curve a): Mononitrosyl iron complex with sulphide synthesized at the ratio of [sulphide]:[Fe2+] > 10 (complex I, g^ = 2.01, g|| = 2.049); (curve b) iron-nitrosyl complex with sulphide synthesized at the ratio of [Fe2+]:[sulphide] > 10 (complex II, g^ = 2.035, g|| = 2.020); (curve c) EPR signal of complex II with 57Fe; (curves d-f) respectively, EPR signals from DNIC (g± = 2.041, g|| = 2.014) and complexes II bound with nitrogenase enzyme. Recordings at 77 K. (From Ref. [68].)

line at g = 2.03 and half-width of 0.5 mT, without resolved HFS structure. Labeling of this species with the 57Fe isotope produces spectra with resolved 1.2 mT doublet HFS (Fig. 15, curve c). This HFS is resolved at 77 K as well as ambient temperature, and its magnitude shows that the unpaired electron is mostly localized on the iron atom.

The balance between the complexes I and II could be reversibly shifted by the addition of competing ligands for the iron. The shifts as induced with cysteine or cyanide are given in Scheme 1:

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