How cofactor-free oxygenases can overcome spin prohibition in substrates oxygenation by dioxygen

Oxidases and oxygenases can activate the triplet ground state O-2 molecule for the controlled chemical synthesis initiated by virtue of electron-transfer process which leads to dioxygen reduction. Typically they use organic cofactor or paramagnetic metal ion to initiate dioxygen activation and overcome spin prohibition for triplet O-2 reactivity. A number of mono- and di-oxygenases are found recently which can operate even in the absence of any cofactor. Analysis of spin-orbit coupling in such enzymes helps to unravel some mechanistic puzzles and explains why aromatic substrate works instead cofactor. A general attribute of such enzymes, O-2(-) radical, provides a good opportunity to induce internal magnetic perturbation and fulfill the spin flip as a common way to overcome spin prohibition for dioxygen activation. The driving force for spin flip - spin-orbit coupling inside superoxide ion - does not depend on the radical-pair partner; thus pi-delocalized substrate can operate instead of flavin cofactor.