Dinitrosyl Iron Complexes with Persulfide Ligands: EPR and Optical Studies

Using the electron paramagnetic resonance (EPR) method, it was established that under aerobic conditions thiol (RS⁻) groups of thiol-containing ligands within the composition of paramagnetic mononuclear dinitrosyl iron complexes (M-DNIC) are converted into persulfide (RSS⁻) ligands in the presence of Na₂S in water-ethanolic solutions thereby initiating the formation of paramagnetic M-DNIC with persulfide ligands. Correspondingly, the EPR signal of M-DNIC with thiol-containing ligands at g_⊥ = 2.04, g_∥ = 2.014, and g_aver. = 2.031 is substituted for the EPR signal characteristic of M-DNIC with persulfide ligands at g_⊥ = 2.04, g_∥ = 2.02, and g_aver. = 2.033. With the increase in the registration temperature to ambient temperature, the anisotropic shape of the EPR signal of M-DNIC with persulfide ligands recorded at 77 K and determined by the anisotropy of the g-factor turns into the symmetric shape with a halfwidth of 0.3–0.4 mT. The lack of the five-component hyperfine structure (HFS) of this EPR signal caused by the interaction of the unpaired electron with the nitrogen nuclei of two nitrosyl ligands is attributed to the compensation of the differently charged contributions of isotropic HFS formed as a result of the spin polarization of appropriate molecular orbitals. It is suggested that similar to M-DNIC with thiol-containing ligands, M-DNIC with persulfide ligands are characterized by the d⁷ electronic configuration of the iron atom and the localization of the unpaired electron on MO(d_z2). The incorporation of ⁵⁷Fe into M-DNIC with persulfide ligands is accompanied by doublet HFS splitting of the EPR signal of M-DNIC by virtue of the mononuclear nature of the latter. It is concluded that M-DNIC with persulfide ligands may exist in the form of both low-molecular and protein-bound complexes and have relevance to the recently established biological activity of hydrogen sulfide and its derivatives.