Reversible Conversion of Disulfide/Dithiolate Occurring at a Vanadium(IV) Center: A Biomimetic System for Redox Exchange in Vanabin.

Ascidians use a class of cysteine-rich proteins generally referred to as vanabins to reduce vanadium ions, one of the many biological processes that involve the redox conversion between disulfide and dithiolate mediated by transition-metal ions. To further understand the nature of disulfide/dithiolate exchange facilitated by a vanadium center, we report herein a six-coordinate non-oxido V complex containing an unbound disulfide moiety, [V(PS3″)(PS1″)] () (PS3″ = [P(CH-3-MeSi-2-S)], where PS1″ is a disulfide form of PS3″). Complex is obtained from a reaction of previously reported [V(PS3″)(PS2″S)] () (PS2″S = [P(CH-3-MeSi-2-SH)(CH-3-MeSi-2-S)] with TEMPO (TEMPO = 2,2,6,6-tetramethylpiperidin-1-yl)oxyl) via hydrogen atom transfer. Importantly, complex can be reduced by two electrons to form an eight-coordinate V complex, [V(PS3″)] (). The reaction can be reversed through a two-electron oxidation process to regenerate complex . The redox pathways both proceed through a common intermediate, [V(PS3″)] (), that has been previously reported as a resonance form of V-dithiolate and a V-(thiolate)(thiyl-radical) species. This work demonstrates an unprecedented example of reversible disulfide/dithiolate interconversion mediated by a V center, as well as provides insights into understanding the function of V reductases in vanabins.