Toward modeling the high chloride, low pH form of sulfite oxidase: Ka-band ESEEM of equatorial chloro ligands in oxomolybdenum(V) complexes

Two oxomolybdenum(V) complexes, (dttd)MoOCl and [(bdt)MoOCl2]− (where dttd = 2,3:8,9-dibenzo-1,4,7,10-tetrathiadecane and bdt = 1,2-benzenedithiolate), which contain one or two equatorial chloro ligands, respectively, were studied by electron spin echo envelope modulation (ESEEM) spectroscopy in the microwave Ka-band (∼29 GHz). The ESEEM amplitude from the chloro ligands in both compounds is significantly greater than that tentatively attributed to chloride in the vicinity of the oxomolybdenum active site in the high chloride, low-pH (lpH) form of sulfite oxidase (SO). Thus, these ESEEM results rule out equatorial coordination of chloride in the enzyme, although the possibility for a weakly bound chloride in the trans axial position or nearby non-coordinated chloride(s) remains for lpH SO in solution.