  For the cofactor-free 1-H-3-hydroxy-4-oxoquinaldine-2,4-dioxygenase (HOD),
the dioxygen (O2) dependent steps are rate-limiting along with a spin state
crossover to the singlet spin state. Here, the primary triplet O2 molecule
activation on the 2-methyl-3-hydroxy-4(1H)-quinolone (MHQ) is investigated, and
the catalytic role of the intersystem crossing effects is highlighted by
directly comparing results from the Born-Oppenheimer dynamics and non-adiabatic
surface hopping dynamics. This work confirms non-adiabatic dynamical effects
are essential to modulate the O2 activation on the substrate MHQ. The time
scale of the equilibration and conversion from triplet to singlet state should
be in the range of a few hundreds of femtoseconds. We hope this work provides
us a fresh look at the underlying physics of dioxygen activation reactions
involving more than one spin state.
