Spin Transition during H₂O₂ Formation in the Oxidative Half-Reaction of Copper Amine Oxidases

Abstract

Dioxygen reduction in the oxidative half-reaction of copper amine oxidases (CAOs) has been studied quantum chemically using the hybrid density functional theory (B3LYP). The reductive activation of dioxygen is a spin-forbidden process for which substantial kinetic O-18 (but no deuterium) isotope effects have been found experimentally. The proposed mechanism was divided into three steps, and the last step was studied for two different potential energy surfaces: the quartet and the doublet surfaces. It is suggested that dioxygen reduction occurs through a spin transition that is induced by the exchange interaction between the unpaired spins of the Cu(II) ion and the O 2 -anion. The step involving this spin transition is suggested to be rate-limiting, which gives a rationalization for the puzzling experimental results when copper is substituted for other metals. The spin transition is triggered by the calculated vibronic perturbation of 5.4 (kcal/mol) Å -1 , which leads to a very fast rate of 8 × 10 10 s -1 for the spin transition. However, since the spin transition occurs at a calculated energy that is 18-20 kcal/mol higher than that of the reactant, this step could still be rate-limiting. The difference in the O-O bond distance between the resting state (free dioxygen) and the point of the spin transition provides an explanation for the oxygen isotope effect.

Understanding the Unexpected Product Distribution in the Aerial Oxidation of Carbene‐Stabilized Diphosphorus Complex

“…However, the basis of the high magnitude of the z component stems from the transfer of an electron from the lone pair residing on the p y ‐like orbital of P 2 to the π* x ‐like orbital of O 2 , coupled with a rotation of the spin (see Scheme for SOC terms and detailed electronic picture at the MECP). This origin of the high magnitude of the z component in similar molecular systems has been previously discussed in detail by Prabhakar et al . and Banerjee et al …”

“…SOC, which is dependent on the atomic number, is generally high for heavier elements (transition metals), and thus, spin transition reactions typically occur in the presence of transition metals . Thus, the significance of instances in which such allowed spin transitions occur involving only lighter elements has a direct implication for oxygen activation reactions by first‐row elements . Recently, our group unraveled the mechanistic details of the aerial oxidation of hydrazine and its methylated congeners; this proceeds through a spin transition .…”

“…Moreover, because P−P linkages are ubiquitous in various phosphorus allotropes, the mechanistic understanding of oxidative combustion would yield valuable insights into the oxidation pathways of species containing P−P linkages. The oxidation of nonmetallic singlet molecular species is intriguing because spin–orbit interactions involving lighter elements play a pivotal role in determining the reaction paradigm . Herein, we unravel the underlying mechanistic picture for the oxidation of 1 and the mechanistic underpinnings that lead to the formation of the unexpected oxidation product, with an O 2 P−PO 2 motif, over the more thermodynamically stable one containing the P−O−P motif.…”

A Serendipitous Rendezvous with a Four‐Center Two‐Electron Bonded Intermediate in the Aerial Oxidation of Hydrazine

“…The x and y components have negligibly small magnitudes. The origin of such a high magnitude of the z component in analogous molecular systems has been previously discussed in details by Prabhakar et al …”

A first principles study of the oxidation energetics and kinetics of realgar

“…Activation energy barriers due to spin transition are rarely discussed in terms of the oxidation of mineral surfaces even though the magnitude of these barriers will significantly affect kinetics and can be the major rate-controlling step. Some organisms have found a way around these barriers, using oxygenase enzymes to activate triplet dioxygen and lower the activation energy for the spin transition via multiple electron and proton transfers and multiple biradical intermediates (Prabhakar et al, 2004). Catalyst developers often utilize co-adsorbates (i.e.…”