Spin-orbit coupling effects in O ₂ activation by cofactor-independent 2,4-dioxygenase

The O ₂ (dioxygen) is paramagnetic molecule with two non-paired electron spins and triplet ground state (S = 1) while majority of organic molecules are diamagnetic species; they have all electron spins paired and the singlet ground state with the total spin S = 0. Oxygenases catalyze a concerted insertion of the triplet dioxygen into organic (diamagnetic) molecules in a strictly spin-forbidden process and this puzzle is not solved so far in modern enzymology. Many oxidases and oxygenases utilize the π-conjugated organic cofactor (like flavins, pterins) in a singlet ground state and reaction of cofactor with O ₂ is still spin-forbidden. It is clear that the protein environment in the enzyme active-site “helps” in some way to overcome spin prohibition, but this environment is definitely diamagnetic and the spin-puzzle still exists. Some oxidases and oxygenases use paramagnetic metal ions as a cofactor; in this case the spin prohibition is formally reduced. In recent years, a numbers of oxidative enzymes are discovered which do not contain any cofactor. In the present work, we considered a rather popular cofactor-free bacterial 2,4-dioxygenase and its oxygenolytic reactions with 2-n-alkyl-3-hydroxy-4(1H)-quinolones (AHQ’s). We presented results of quantum-chemical calculations of intermediate diradical proposed recently for direct reaction of dioxygen with AHQ substrate and made conclusion about the mechanism of spin-catalysis.