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 (M-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 (pH 7.4) with gaseous nitric oxide (NO) at the thiol:Fe2+ ratio 1:1. M-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 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 pH 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 M-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-M-DNIC solutions results in cysteine oxidation-controlled conversion of M-DNICs first into cys-B-DNICs and then into the EPR-silent compound X able to generate a strong absorption band at 278 nm. In the presence of glutathione or cysteine excess, compound X is converted into B-DNIC/M-DNIC and is completely decomposed under effect of the Fe2+ chelator o-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 X represents a polynuclear DNIC with cysteine, most probably, an appropriate Roussin's black salt thioesters and cannot be prepared by simple substitution of M-DNIC cysteine for glutathione. Treatment of M-DNIC with sodium dithionite attenuates the EPR signal at g = 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 gayer. = 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 M-DNIC formed in a small amount during dithionite-induced decomposition of B-DNIC. (C) 2010 Elsevier Inc. All rights reserved.