Redox conversions of dinitrosyl iron complexes with natural thiol-containing ligands.

Using the electron paramagnetic resonance (EPR) and optical spectrophotometric methods, it has been established that biologically active, water-soluble dinitrosyl iron complexes (DNIC) with glutathione are predominantly represented by the diamagnetic binuclear form (B-DNIC) even in the presence of a 10-fold excess of glutathione non-incorporated into DNIC at neutral pH. With the increase in рН to 10-11, B-DNIC are fully converted into the paramagnetic mononuclear form (М-DNIC) with a characteristic EPR signal at g⊥=2.04, g‖=2.014 and gaver.=2.03. After treatment with a strong reducing agent sodium dithionite, both М- and B-DNIC are converted into the paramagnetic form with a characteristic EPR signal at g⊥=2.01, g‖=1.97 and gaver.=2.0. Both forms display similar absorption spectra with absorption bands at 960 and 640nm and a bend at 450nm. After oxidation by atmospheric oxygen, this situation is reversed, which manifests itself in the disappearance of the EPR signal at gaver.=2.0 and complete regeneration of initial absorption spectra of М- or B-DNIC with characteristic absorption bands at 390 or 360 and 310nm, respectively. Treatment of bovine serum albumin (BSA) solutions with gaseous NO in the presence of Fe(2+) and cysteine yields BSA-bound М-DNIC (М-DNIC-BSA). After treatment with sodium dithionite, the latter undergo transformations similar to those established for low-molecular М-DNIC with glutathione. Based on the complete coincidence of the optical and the EPR characteristics of sodium dithionite-treated М- and B-DNIC and other findings, it is suggested that sodium dithionite-reduced B-DNIC are subject to reversible decomposition into М-DNIC. The reduction and subsequent oxidation of М- and B-DNIC are interpreted in the paradigm of the current concepts of the initial electronic configurations of М- and B-DNIC (d(7) ({Fe(NO)2}(7)) and d(7)-d(7) ({Fe(NO)2}(7)-{Fe(NO)2}(7)), respectively).

Polynuclear water-soluble dinitrosyl iron complexes with cysteine or glutathione ligands: electron paramagnetic resonance and optical studies.

Electron paramagnetic resonance and optical spectrophotometric studies have demonstrated that low-molecular dinitrosyl iron complexes (DNICs) with cysteine or glutathione exist in aqueous solutions in the form of paramagnetic mononuclear (capital EM, Cyrillic-DNICs) and diamagnetic binuclear complexes (B-DNICs). The latter represent Roussin's red salt esters and can be prepared by treatment of aqueous solutions of Fe(2+) and thiols (small er, Cyrilliccapital EN, Cyrillic 7.4) with gaseous nitric oxide (NO) at the thiol:Fe(2+) ratio 1:1. capital EM, Cyrillic-DNICs are synthesized under identical conditions at the thiol:Fe(2+) ratios above 20 and produce an EPR signal with an electronic configuration {Fe(NO)(2)}(7) at g(aver.)=2.03. At neutral pH, aqueous solutions contain both M-DNICs and B-DNICs (the content of the latter makes up to 50% of the total DNIC pool). The concentration of B-DNICs decreases with a rise in pH; at small er, Cyrilliccapital EN, Cyrillic 9-10, the solutions contain predominantly M-DNICs. The addition of thiol excess to aqueous solutions of B-DNICs synthesized at the thiol:Fe(2+) ratio 1:2 results in their conversion into capital EM, Cyrillic-DNICs, the total amount of iron incorporated into M-DNICs not exceeding 50% of the total iron pool in B-DNICs. Air bubbling of cys-capital EM, Cyrillic-DNIC solutions results in cysteine oxidation-controlled conversion of capital EM, Cyrillic-DNICs first into cys-B-DNICs and then into the EPR-silent compound capital HA, Cyrillic able to generate a strong absorption band at 278 nm. In the presence of glutathione or cysteine excess, compound capital HA, Cyrillic is converted into B-DNIC/M-DNIC and is completely decomposed under effect of the Fe(2+) chelator small o, Cyrillic-phenanthroline or N-methyl-d-glucamine dithiocarbamate (MGD). Moreover, MGD initiates the synthesis of paramagnetic mononitrosyl iron complexes with MGD. It is hypothesized that compound capital HA, Cyrillic represents a polynuclear DNIC with cysteine, most probably, an appropriate Roussin's black salt thioesters and cannot be prepared by simple substitution of capital EM, Cyrillic-DNIC cysteine for glutathione. Treatment of capital EM, Cyrillic-DNIC with sodium dithionite attenuates the EPR signal at g(aver.)=2.03 and stimulates the appearance of an EPR signal at g(aver.)=2.0 with a hypothetical electronic configuration {Fe(NO)(2)}(9). These changes can be reversed by storage of DNIC solutions in atmospheric air. The EPR signal at g(aver.)=2.0 generated upon treatment of B-DNICs with dithionite also disappears after incubation of B-DNIC solutions in air. In all probability, the center responsible for this EPR signal represents capital EM, Cyrillic-DNIC formed in a small amount during dithionite-induced decomposition of B-DNIC.