A Co(III) complex in a mixed sulfur/nitrogen ligand environment: modeling the substrate- and product-bound forms of the metalloenzyme thiocyanate hydrolase.

Thiocyanate hydrolase (TCH) is a pink bacterial metalloenzyme found in Thiobacillus thioparus THI 115 that catalyzes the conversion of thiocyanate to carbonyl sulfide and ammonia.1,2 Although TCH has not been well characterized spectroscopically, an analysis of the genes encoding for the enzyme has shown a high active-site sequence homology to the metalloenzyme nitrile hydratase (NHase), including the residues that bind the metal center.2 NHases catalyze the hydration of nitriles to amides and fall into a rare class of cysteine-ligated non-corrinoid Co(III) or non-heme Fe(III) enzymes.3 NHase has been characterized by a number of physical techniques including EPR,4 EXAFS,5,6 Mössbauer,7 Resonance Raman,8 and X-ray crystallography.9,10 The metal in NHase is low-spin and is ligated by three cysteinates, two amide nitrogens (from the peptide backbone), and a hydroxide or water. Given the large amount of sequence homology, it is likely that the metal center of TCH is ligated in a similar fashion. Previously, our group showed that the spin-state and spectroscopic properties of iron-containing NHases can be modeled by six-coordinate iron complexes incorporating cis-thiolates and imine nitrogens.11,12 Furthermore, we were able to model the reactivity of NHase with five-coordinate iron complexes that possess an open site allowing for substrate binding.13-15 In our efforts to model cobalt-containing NHase, we have synthesized a model which binds both SCNand ammonia, the substrate and product of TCH. Six-coordinate [CoS2,N3(Pr,Pr)(SCN)] (1) was synthesized by adding tetrakis-(n-butyl)ammonium thiocyanate to fivecoordinate [CoS2N3(Pr,Pr)](PF6) (2)16 in acetonitrile. This