Quantum scattering pathway of the cofactorless spin-forbidden O2 addition to DPA-CoA.

Certain oxidases and oxygenases catalyze the spin-forbidden incorporation of O2 into organic substrates without requiring a cofactor. A general mechanism for this reaction remains elusive; intriguingly, these enzymes accomplish this without relying on specialized catalytic machinery. Here, we report nonadiabatic quantum scattering calculations on a simplified model to investigate the nuclear quantum effects governing these processes. Our model includes six singlet and three triplet states, explicitly accounting for spin-orbit coupling. Our results indicate that the degree of spin-forbiddenness corresponds to a kinetic hindrance of only ≈2.45 kcal/mol at room temperature, slightly lower than the value predicted by nonadiabatic transition state theory (NA-TST). However, the discrepancy between the scattering results and NA-TST predictions is minor, validating the use of NA-TST for this class of reactions. Furthermore, our calculations account for the possible formation of singlet O2, a reactive oxygen species (ROS). We find that once this channel opens, singlet O2 is produced with significant probability, suggesting that ROS could be generated via the stabilization of singlet O2 within the protein cavity.