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

2010; Elsevier BV; Volume: 23; Issue: 2 Linguagem: Inglês

10.1016/j.niox.2010.05.285

1089-8611

А. Ф. Ванин, Аlexander P. Poltorakov, V. D. Mikoyan, Lyudmila N. Kubrina, Dosymzhan Sh. Burbaev,

Electron Spin Resonance 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 (М-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 Fe2+ and thiols (рН 7.4) with gaseous nitric oxide (NO) at the thiol:Fe2+ ratio 1:1. М-DNICs are synthesized under identical conditions at the thiol:Fe2+ ratios above 20 and produce an EPR signal with an electronic configuration {Fe(NO)2}7 at gaver. = 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 рН 9–10, the solutions contain predominantly M-DNICs. The addition of thiol excess to aqueous solutions of B-DNICs synthesized at the thiol:Fe2+ ratio 1:2 results in their conversion into М-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-М-DNIC solutions results in cysteine oxidation-controlled conversion of М-DNICs first into cys-B-DNICs and then into the EPR-silent compound Х able to generate a strong absorption band at 278 nm. In the presence of glutathione or cysteine excess, compound Х is converted into B-DNIC/M-DNIC and is completely decomposed under effect of the Fe2+ chelator о-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 Х represents a polynuclear DNIC with cysteine, most probably, an appropriate Roussin's black salt thioesters and cannot be prepared by simple substitution of М-DNIC cysteine for glutathione. Treatment of М-DNIC with sodium dithionite attenuates the EPR signal at gaver. = 2.03 and stimulates the appearance of an EPR signal at gaver. = 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 gaver. = 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 М-DNIC formed in a small amount during dithionite-induced decomposition of B-DNIC.