Indian Journal of Chemistry Vol. 42A, September 2003, pp. 2223-2227 Redox reaction between [BU4Nh[Mo VI 02 (mnt)2] and thiophenol in relevance to the autoreduction in the crystallization process of oxidized form of sulfite oxidase Pradeep K Chaudhury", Kowliki Nagarajan b , Anil Kumar b , Rabindrana th Maiti C , Samar K Das d & Sabyasachi Sarkar b * "Department of Chemistry , University of Pune, Pune. India "Department of Chemistry , Indian Institute of T ec hnology, Kanpur 208 016, India Email:abya@iirk.ac.in c Inuian Institute of Technology Kharagpur, Kharagpur, India dDepartment of Che mi stry, Central University of Hyderabad, Hyderabad, India Received 3 February 2003 [BU4Nb[Mo v'0 2( mnt)2J readily reacts with thiophenol lead in g to its reduction to (BU4Nh [MolvO(mnt hJ. Detailed kinetics of this reaction is presented. The kinetics consists of thr ee consecutive irreversible first ord er reactions (in the presence of excess of thiophenol) with k,. kz, kJ = 2.82(6) x 10" s" , 6.09(5) X 10. 2 s", 1.66(8) X 10') s·, and 2.91(4) x 10" s ", 6.15(3) x s" , 3.41(6) X 10- 3 s" at 298 K in acetonitrile and aqueous-acetonitrile media respectively. Sulfite oxidase catalyzes physiologically vital oxidation of sulfite to sulfate' . The enzyme residing in the mitochondrial inter-membrane space, is dimeric with a subunit mass of abo ut 52000. Each monomer contains molybdenum associated with a single molybdopterin and a cytochrome b type heme. The two electron oxidation of sulfite to sulfate is known to occur at the molybdenum site, which is reduced from Mo(VI) to Mo(JV) in the process, and the cata lytic cycle is completed with the reoxidation of the molybdenum in sequential two one electron transfer reactions {Mo(JV)-Mo(V)-Mo(VI)} associated with intramolecular electron transfer to the cytochrome b site 2 : Enzyme- Mo(VI ) + sot Enzyme-Mo(l V) + SO}' +2H+ ,. i- E",ym,.Mo'·' ... _J __ ,. With the exception of nitrogenase, all molybdenum enzymes that have been described to date 3 contain a novel pterin cofactor in which the molybdenum is bound by the pyranodithiolene of the molybdopterin cofactor 3 ,4 (Fig. 1) . Until recently, structural information on molybdenum enzymes was derived almost entirely from spectroscopy of the enzyme and of model compounds 3 . As one of the most intensively studied molybdenum enzymes, sulfite oxidase can be regarded as prototypical member of one class of molybdenum enzymes-those possessing dioxo molybdenum sites when the enzyme is in the fully oxidized Mo(VJ) form s , s . Recently, the crystal structure of chicken liver sulfite oxidase has been reported at 1.9A resolution 9 . Tn contrast to the information available from previous X-ray absorption spectroscopic studies, the active site as revealed by cry tallography was found to contain monoxo species where molybdenum is in the reduced {Mo (IV)} state. Although the X-ray diffraction quality crystals were grown from the fully oxidized state of the protein (Fig. 2a), its reduction to Mo(JV) state (Fig. 2b) has been attributed presumably to long standing in the presence of by trace amounts of sulfite present in the precipitant, lithium sulfate, used in such crystal- lization processes. Our model complex containing bis-dithiolene coordination in [Bu4Nh[MoOz(mnt)21 (1) uniquely mimics enzymatic reaction of sulfite oxidase'o. In this M= Mo,W Fig. I-The minimal coo rdinati on unit of a molybdenum cofactor, showing the structure of molybdopterin (MPT).