Iron(II/III) Halide Complexes Promote the Interconversion of Nitric Oxide and -Nitrosothiols through Reversible Fe-S Interaction.

Heme and non-heme iron in biology mediate the storage/release of NO from -nitrosothiols as a means to control the biological concentration of NO. 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 and -nitrosothiols mediated by Fe/Fe chloride complexes. The reaction of 2 equiv of -nitrosothiol (PhCSNO) with [ClFe] results in facile release of NO 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 center. This is in contrast to the proposed mechanism of NO release from the well-studied "red product" κ-N bound -nitrosothiol Fe complex, [(CN)Fe(κ-N-RSNO)]. Additionally, Fe chloride can mediate NO storage through the formation of -nitrosothiols. Treatment of iron(III) halothiolate with 2 equiv of NO regenerates PhCSNO with the Fe source trapped as the = 3/2 {FeNO} species [ClFeNO], which is inert toward further coordination and activation of -nitrosothiols. Our work demonstrates how labile iron can mediate the interconversion of NO/thiolate and -nitrosothiol, which has important implications toward how Nature manages the biological concentration of free NO.