Vibration Spectroscopy Of Nitrosyl Ligands In Dnic

As mentioned in Chapter 1, the frequency vno of the NO stretch vibration is strongly affected by the density of the unpaired electron in the half-occupied antibonding orbital of the nitrosyl ligand. Transfer of the unpaired electron from the antibonding orbital towards the iron stiffens the N—O bond and raises vno. The DNIC represents a four-coordinate {Fe(NO)2}n complex with n = 7, 8 or 9. The orbital diagrams were discussed in Ref. [57], which reviews the experimental data until 1974. Interestingly, the two ligands have slightly different vno vibrations near 1760 cm-1. For Cys-DNIC, the vno vibrations appear at 1770 and 1730 cm-1 [45]. The separation of ca 40 cm-1 amounts to ca 2%, and proves that the two ligands have

Fig. 10. Left panel: EPR spectra of mononitrosyl iron complex (MNIC) with maleonitriledithiol containing 14NO (curve a) or 15NO (curve b) [36]. Right panel: EPR spectra of MNIC with cysteine (triplet signal) and DNIC with cysteine (singlet signal) forming in the reaction between Fe2+, 5-nitrosocysteine and cysteine during 5 and 20 min (curves c,d, respectively) [46]. Bottom panel: EPR spectra of DNIC (a) or MNIC (b) with cysteine [47]. Recordings were made at ambient temperature (left and right panels) or 77 K (bottom panel). Reprinted with permission from McDonald CC, Phillips WD, Mower HF "An electron spin resonance study of some complexes of iron, nitric oxide and anionic ligands" (J. Am. Chem. Soc. 1965; 87: 3319-3326) © 1965, American Chemical Society.

Fig. 10. Left panel: EPR spectra of mononitrosyl iron complex (MNIC) with maleonitriledithiol containing 14NO (curve a) or 15NO (curve b) [36]. Right panel: EPR spectra of MNIC with cysteine (triplet signal) and DNIC with cysteine (singlet signal) forming in the reaction between Fe2+, 5-nitrosocysteine and cysteine during 5 and 20 min (curves c,d, respectively) [46]. Bottom panel: EPR spectra of DNIC (a) or MNIC (b) with cysteine [47]. Recordings were made at ambient temperature (left and right panels) or 77 K (bottom panel). Reprinted with permission from McDonald CC, Phillips WD, Mower HF "An electron spin resonance study of some complexes of iron, nitric oxide and anionic ligands" (J. Am. Chem. Soc. 1965; 87: 3319-3326) © 1965, American Chemical Society.

Fig. 11. EPR signals of DNIC with cysteine (curve a), DNIC with ethylxanthogenate with 56Fe (curve b) or 57Fe (curve c). Recordings were made at 77 K (left spectra) or ambient temperature (right spectra). The proposed geometrical structure of the complexes are also shown. (From Ref. [56].)

Fig. 12. Left panel: EPR spectra of monomeric DNIC with cysteine (DNIC 1:20) at g = 2.03 (curve a) and its reduced form (after addition of dithionite) at g = 2.001 with 56Fe (curve b) or 57Fe (curve c) [60]. Right panel: EPR spectra of monomeric DNIC with cysteine (DNIC 1:20) (curve a) and its reduced form (curve b) with 56Fe [47]. Recordings were made at ambient temperature (left panel) or 77 K (right panel).

Fig. 12. Left panel: EPR spectra of monomeric DNIC with cysteine (DNIC 1:20) at g = 2.03 (curve a) and its reduced form (after addition of dithionite) at g = 2.001 with 56Fe (curve b) or 57Fe (curve c) [60]. Right panel: EPR spectra of monomeric DNIC with cysteine (DNIC 1:20) (curve a) and its reduced form (curve b) with 56Fe [47]. Recordings were made at ambient temperature (left panel) or 77 K (right panel).

different degree of transfer of the unpaired electron towards the iron. Phrased otherwise, the effective charges of the nitrosyl ligands are different as well. This property is observed with various DNIC-containing ligands of low molecular weight, as well as with protein-bound DNIC [62]. Nevertheless, the nature of the non-nitrosyl ligands does influence the degree of electron transfer. The effect was studied with a range of ligands with increasing polarity of the Fe-ligand bond [63]. The vno was found to increase with bond polarity. This phenomenon is explained by charge transfer: increasing polarity of the bond increases the effective (positive) charge of the iron and promotes a compensating shift of the unpaired electron from the nitrosyl towards the iron. This stiffens the N—O bond and raises vno.

For paramagnetic {Fe(NO)2}7 (S = 1/2), the experimental vno is found to be positively correlated with the ^-factor of EPR spectroscopy [63]. High vibration frequencies (large electron transfer) bring high ^-factors, and this relationship confirms that the unpaired electron of the {Fe(NO)2}7 resides in the ^ orbital of the iron atom. A more detailed discussion of the electronic structure of DNIC is given later in this chapter.

MOSSBAUER (y-RESONANCE) PROPERTIES OF LOW-MOLECULAR DNICs WITH THIOL-CONTAINING LIGANDS

Mossbauer spectroscopic studies recoilless excitation of nuclear excited states by y-rays. The Mossbauer effect requires a nucleus with low-lying excited states and has been detected in 43 different elements. The technique provides a particularly valuable analytical tool for iron complexes since the shape of the Mossbauer spectrum is affected by the redox state and electronic configuration of the complex as well as by the ligand field surrounding the iron. 57Fe is a stable isotope with only 2.2% natural abundance. In 57Fe Mossbauer spectroscopy, one observes the dipole transitions between the nuclear ground state (I = 1/2, nuclear ^-factor gN = 0.181) and the excited state at 14.4 keV (I = 3/2, gN = -0.106). The shape of the spectrum is determined by the interaction of the nuclear quadrupole moment with the electrical field gradient (EFG) tensor of the ligand field in the complex. After enriching the iron with the 57Fe isotope, the ligand field of DNIC complexes may be investigated with y-resonance (GR) spectroscopy.

Depending on the redox state and dimerization, Cys-DNICs reveal three types of GR spectra [43,44]. Dimeric DNIC-cysteine (Fe:Cys = 1:2) is characterized by an isomeric shift (IS) of 0.14 mm/s and quadrupole splitting (QS) of 1.1 mm/s (Fig. 13a, 14A). (The IS value being relative to the metallic iron (a-Fe) at room temperature). After the addition of cysteine in a 30-40-fold excess to the complex solution, the solution changed its color from yellowish to green, characteristic of monomeric paramagnetic state of Cys-DNIC (DNIC 1:20). The corresponding GR spectrum had a complicated shape (Fig. 13e). A slightly different GR-spectrum is obtained if a solution with 57Fe:Cys = 1:20 is treated with gaseous NO (Fig. 13d, 14B). Despite the non-identity of the shape, the spectra share the presence of GR absorption in a wide range of rates ("blurred" absorption). This indicates the magnetic HF interaction in these complexes, i.e. the presence of a magnetic moment of the electron shell interacting with the nucleus spin of 57Fe in ground and excited states. Such absorption is typical for paramagnetic complexes with the time of electronic relaxation of 10-7-10-8 s. This timescale is close to the characteristic time of the GR method, as defined by the lifetime of the excited state of the 57Fe nucleus (t ~ 10-7 s). From the GR spectra, it is estimated that the electrons produce a magnetic HF field strength of ca 180 kG at the 57Fe nucleus. This field strength is compatible with electronic spin S = 1/2.

At higher concentrations of the monomeric DNIC-cysteine, the GR spectrum simplifies into a doublet spectrum [60] (Fig. 14C). This phenomenon is interpreted as motional narrowing caused by the increase of rapid stochastic fluctuations in the magnetic field at the iron due

Fig. 13. Shape of Mossbauer spectra of dimeric DNIC with cysteine (DNIC 1:2) (curve a); of the same complex treated with dithionite (curve b); DNIC with cysteine synthesized for an Fe2+ :Cys ratio of 1:1 (curve c); monomeric DNIC with cysteine (DNIC 1:20) (curve d); dimeric DNIC with cysteine after its contact with an excess of cysteine (Fe2+:Cys = 1:30) (curve e) and monomeric DNIC treated with dithionite (curve f). Recordings at 77 K. In (curves d-f), the arrows indicate the maximum values for Heff. I, II and Fe2+ denote the positions of the doublet spectra, respectively for dimeric DNIC, reduced dimeric DNIC and the high spin Fe2+ complex. (From Ref. [44].)

Fig. 13. Shape of Mossbauer spectra of dimeric DNIC with cysteine (DNIC 1:2) (curve a); of the same complex treated with dithionite (curve b); DNIC with cysteine synthesized for an Fe2+ :Cys ratio of 1:1 (curve c); monomeric DNIC with cysteine (DNIC 1:20) (curve d); dimeric DNIC with cysteine after its contact with an excess of cysteine (Fe2+:Cys = 1:30) (curve e) and monomeric DNIC treated with dithionite (curve f). Recordings at 77 K. In (curves d-f), the arrows indicate the maximum values for Heff. I, II and Fe2+ denote the positions of the doublet spectra, respectively for dimeric DNIC, reduced dimeric DNIC and the high spin Fe2+ complex. (From Ref. [44].)

Fig. 14. Mössbauer spectra of dimeric DNIC with cysteine (A), monomeric DNIC with cysteine (B,C). The amount of 57Fe was 0.5 (A,B) or 2 mg/ml (C). Recordings were made at 77 K [60].

Fig. 14. Mössbauer spectra of dimeric DNIC with cysteine (A), monomeric DNIC with cysteine (B,C). The amount of 57Fe was 0.5 (A,B) or 2 mg/ml (C). Recordings were made at 77 K [60].

to the spin-spin interaction between the DNIC monomers in solution. The fluctuations are sufficiently rapid to cause averaging of the HFS during the characteristic timescale of the GR method. The IS and QS of this simplified monomer spectrum coincide with those for the dimeric form (DNIC 1:2) (Fig. 14A,C). This suggests that the electronic configuration of the iron be identical in both forms. It implies that the dimeric form can be considered as a structure where two monomeric forms located in parallel planes and very weak electronic coupling between the iron centers (this structure will be considered below). The optical properties of dimeric DNIC are compatible with this interpretation.

Interestingly, the solutions of DNIC-cysteine with Fe:Cys = 1:1 show the characteristic dimeric optical absorption bands at 310 and 360 nm, except that the optical density is only half as large as expected from the iron concentration. The GR spectrum of DNIC-cysteine with 57Fe:Cys = 1:1 appears as a superposition of two doublet spectra of approximately equal intensity: the first spectrum has the shape of dimeric DNIC with 57Fe:Cys and the second spectrum was due to a high-spin 57Fe2+ complex with IS = 1.4 and QS = 3 mm/s (Fig. 13c). Therefore, because of the cysteine shortage, only half of the total iron is sequestered in the form of Cys-DNIC. The other half remained in high-spin complex with non-thiol ligands. These data indicate unequivocally that each of the two iron atoms in dimeric DNIC-cysteine is coordinated with two cysteine ligands. These dimer comprises two monomeric DNICs and its formula in solution can be presented as {(RS-)2Fe+(NO+)2)2+. It is noteworthy that the proposed structure differs from the characteristic Roussin's Red Salt in crystalline state.

This dimeric iron-dinitrosyl complex has the formula Na2[Fe2S2(NO)4] [64a]. In this structure, the two sulfur atoms act as bridging ligands between two iron atoms.

The proposition that the dimeric DNIC-cysteine is composed of two monomeric DNIC is corroborated by the changes in the optical and EPR spectra if the complexes are reduced with dithionite [43]. The addition of excess dithionite to dimeric Cys-DNIC rapidly changes the color from yellowish to bright green and, after 2-3 s, to stable raspberry-violet. A frozen aliquot of the intermediate bright green solution showed that the EPR spectrum is a mixture of monomeric Cys-DNIC with {Fe(NO)2}7 and {Fe(NO)2}9 configurations. The final raspberry-violet solution did not have an EPR signal, and showed that the reduced complexes may form diamagnetic dimers as well. The GR spectrum of the raspberry violet solution showed a species with doublet structure having QS = 0.8 mm/s and IS = 0.1 mm/s respectively (Fig. 13b).

The behavior was different when dithionite was added to monomeric Cys-DNIC: the color changed from green to bright green and persisted for several minutes [43]. The EPR spectrum of this bright green solution appears as a mixture of monomeric DNIC {Fe(NO)2}7 and {Fe(NO)2 }9. Its GR spectrum retained the complex blurred shape characteristic of the original monomeric DNIC (Fig. 13f).

The spectroscopic changes in DNIC 1:2 and DNIC 1:20 by reduction were reversible by admission of oxygen: Upon exposure to ambient air, the raspberry-violet color of reduced DNIC 1:2 solution transformed again to yellowish characteristic of the initial DNIC 1:2 solution without EPR absorption. Similarly, the final bright green color of DNIC 1:20 solution changed to green, and the EPR signal from the solution became identical to that from initial DNIC 1:20 solution. The cycle of reduction with dithionite and reoxidation could be repeated without apparent loss of signal intensity in optical and EPR spectra.

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