A computational investigation into the catalytic activity of a diselenolene sulfite oxidase biomimetic complex

Molybdenum is the only 4d metal found in almost all life. One such molybdenum-containing enzyme is sulfite oxidase, which also contains the dithiolene-molybdopterin ligand. Sulfite oxidase is essential in the degradation of sulfur-containing compounds such as cysteine and methionine. Past work has shown parallels in the chemistry of dithiolene-metal and diselenolenemetal complexes. Thus, in this present work, the oxygen atom transfer mechanism for a diselenolene sulfite oxidase biomimetic complex was investigated using computational tools, the results of which were compared to the analogous dithiolene biomimetic complex. From the results obtained, the molybdenum-diselenolene sulfite oxidase biomimetic complex is able to catalyse the oxygen atom transfer and does so with a marginally lower value of Delta(r)G(double dagger) than that for the analogous dithiolene complex. In particular, it was found that on average, the diselenolene complex had an activation energy 1.2 kJ mol(-1) lower in energy than the analogous dithiolene complex. However, the calculated value of Delta(r)G suggests that the oxidation of sulfite is more favourable for the dithiolene complex where the average difference in reaction aqueous Gibbs reaction energy was -9.4 kJ mol(-1) relative to the diselenolene complex. It is noted that with the use of D3 and D3BJ corrections in combination with the B3LYP functional, the barrier for oxygen atom transfer is lowered by more than 30.0 kJ mol(-1) for both the diselenolene and dithiolene complexes. Such results suggest that to study such oxo-transfer reactions, the proper treatment of dispersion interaction is necessary.