Iron (II/III) Halide Complexes Promote the Interconversion of Nitric Oxide and S-nitrosothiols through Reversible Fe-S Interaction
Heme and non-heme iron in biology mediate the storage/release of NO<sup>•</sup> from <i>S</i>-nitrosothiols as a means to control the biological concentration of NO<sup>•</sup>. Despite their importance in many physiological processes, the mechanisms of N-S bond formation/cleavage at Fe centers have been controversial. Herein, we report the interconversion of NO<sup>•</sup> and <i>S</i>-nitrosothiols mediated by Fe<sup>II</sup>/Fe<sup>III</sup> chloride complexes. The reaction of two equivalents of <i>S</i>-nitrosothiol (Ph<sub>3</sub>CSNO) with [Cl<sub>6</sub>Fe<sup>II</sup><sub>2</sub>]<sup>2</sup><sup>-</sup> results in facile release of NO<sup>•</sup> and formation of iron(III) halothiolate. Detailed spectroscopic studies, including in situ UV-vis, IR, and Mössbauer spectroscopy, support the interaction of the S−atom with the Fe<sup>II</sup> center. This is in contrast to the proposed mechanism of NO<sup>•</sup> release from the well-studied “red product” <i>k</i><sup>1</sup>-N bound <i>S</i>-nitrosothiol Fe<sup>II</sup> complex, [(CN)<sub>5</sub>Fe(<i>k</i><sup>1</sup>-N-RSNO)]<sup>3</sup><sup>-</sup>. Additionally, Fe<sup>III</sup> chloride can mediate NO<sup>•</sup> storage through the formation of <i>S</i>-nitrosothiols. Treatment of iron(III) halothiolate with two equivalents of NO<sup>•</sup> regenerates Ph<sub>3</sub>CSNO with the Fe<sup>II</sup> source trapped as the <i>S</i> = 3/2 {FeNO}<sup>7</sup> species [Cl<sub>3</sub>FeNO]<sup>-</sup>, which is inert towards further coordination and activation of <i>S</i>-nitrosothiols. Our work demonstrates how labile iron can mediate the interconversion of NO<sup>•</sup>/thiolate and <i>S</i>-nitrosothiol, which has important implications for how Nature manages the biological concentration of free NO<sup>•</sup>.