Abstract
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 VIV complex containing an unbound disulfide moiety, [VIV(PS3″)(PS1″S-S)] (1) (PS3″ = [P(C6H3-3-Me3Si-2-S)3]3-, where PS1″S-S is a disulfide form of PS3″). Complex 1 is obtained from a reaction of previously reported [VV(PS3″)(PS2″SH)] (2) (PS2″SH = [P(C6H3-3-Me3Si-2-SH)(C6H3-3-Me3Si-2-S)2] with TEMPO (TEMPO = 2,2,6,6-tetramethylpiperidin-1-yl)oxyl) via hydrogen atom transfer. Importantly, complex 1 can be reduced by two electrons to form an eight-coordinate VIV complex, [VIV(PS3″)2]2- (4). The reaction can be reversed through a two-electron oxidation process to regenerate complex 1. The redox pathways both proceed through a common intermediate, [V(PS3″)2]- (3), that has been previously reported as a resonance form of VV-dithiolate and a VIV-(thiolate)(thiyl-radical) species. This work demonstrates an unprecedented example of reversible disulfide/dithiolate interconversion mediated by a VIV center, as well as provides insights into understanding the function of VV reductases in vanabins.
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