Structure of the Michaelis Complex and Function of the Catalytic Center in the Reductive Half‐Reaction of Computational and Synthetic Models of Sulfite Oxidase

By using frontier‐molecular‐orbital and electrostatic (nucleophilic) interactions as well as relaxed potential‐energy surface scans, it is shown that the initial step in the oxygen‐atom transfer (OAT) reaction of Mo^VIO₂‐(S₂C₂Me₂)SMe]^?1 ( 1 ) and Mo^VIO₂‐{(S₂C₂(CN)₂}₂]^2? ( 2 ) with HSO₃^? takes place by oxoanionic binding of the substrate to the Mo^VI center with the formation of a stable Michaelis complex. The gas‐phase and solvent‐corrected enthalpy profile with fully optimized minima and transition states for the OAT reaction of 1 and 2 with HSO₃^? showed the release of reaction energy for both complexes. The optimized geometries of 1 and 2 in the respective enzyme–substrate complexes showed a common feature with the participation of hydrogen bonding of the substrate with the axial (spectator) oxo group in the subsequent formation of the six‐membered MoO₂HOS transition state. The enzyme–substrate complex of 2 shows heptacoordination as proposed earlier, although the trans (to axial oxo)‐Mo? S(dithiolene) bond is elongated to 2.948 Å.