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We have performed hybrid density functional theory (DFT) calculations on the geometric and electronic structures of low-lying doublet and quartet ferryl-oxo [Fe(IV)═O] oxidants and a doublet perferryl-oxo [Fe(V)═O] oxidant in Cytochrome P450. Fully optimized structures of compound I models have been determined, and the proper symmetry of wave functions has been restored by the spin-projection technique. The results show that the perferryl-oxo species is relatively low lying, as compared with the excited state of the ferryl-oxo species, if the iron-oxo bond is properly described as the mixing of several appropriate excited electronic configurations to minimize electron repulsion. This means that the perferryl-oxo species is virtually in a mixed-valent resonance state, ↑Fe(V)═O ↔ ↑Fe(IV)•↑-↓•O, containing a highly reactive pπ atomic oxygen radical. The anionic thiolate ligand acts as a Lewis σ base and functions to achieve the stability of the perferryl-oxo complex and to activate the oxo ligand trans to it by asymmetric bond distortion along the O-Fe-S axis by lengthening the Fe-O bond and shortening the Fe-S bond, prior to the hydrogen-atom abstraction from the substrate., (© 2011 American Chemical Society)

Thermal decomposition of peroxidized coelenterazines with and without external perturbations has been studied theoretically using the hybrid density functional theory (B3LYP) and the Coulomb-attenuating method (CAM). Possible roles of a hydrogen-bonding interface constituted by amino acid residues in the coelenterazine-biding site of aequorin are addressed by using simple model clusters with a polarizable continuum model to grasp some important aspects that may affect the electronic mechanism operating within the photoprotein. Calculations have revealed that the electronic property and stability of the peroxide are greatly affected by its protonation state and/or environmental effects, such as a polarizing medium and specific (localized) short-range electrostatic interactions, which may be critical for the bioluminescence activity. Theory highlights two mechanisms by which the neutral species can be activated, which otherwise decomposes by a homolytic O-O dissociation with a high barrier. In the first mechanism, the Tyr82-His16-Trp86 triad motif facilitates the deprotonation process of the phenolic OH group at the C(6) position of the coelenterazine and thereby makes it a sufficiently good electron donor to activate the O-O bond. In the second mechanism, intramolecular charge transfer is accomplished within the neutral peroxide by a proton delivery mediated via another triad motif, Tyr184-His169-Trp173, without the activation of the substrate itself. The combination of the first and second mechanisms leads to complete electron transfer for the formation of a radical pair as a local intermediate stabilized by the nearby triad motifs.

The spin transition in the reactions of the derivatives of imidazo[1,2-a]pyrazin-3(7H)-one (1H) with a triplet molecular oxygen (3O2) has been investigated by the geometry optimization at the B3LYP/6-31+G(d) level and the evaluation of the electronic matrix elements for spin-orbit coupling (SOC) using the full Pauli-Breit SOC operator. The reductive activation for the 3O2 reaction is affected by the proton activity and solvent polarity of a surrounding reaction field. In a polar aprotic solvent, a base-prompted anionic substrate may react with 3O2 in a stepwise manner through complete electron transfer from the substrate anion to 3O2, while the irreversible concerted 3O2 addition via intersystem crossing may become complete in a less polar solvent. SOC in the thermal decomposition of a resulting peroxide adduct can be controlled by the protonation state of the substrate. There exists an optimal protonation state for the suppression of SOC in the charge-transfer-induced luminescence (CTIL) of the peroxide, which is closely related with the ability of a substituent to donate an electron. This will constitute a necessary condition for the high efficiency of chemi- and bioluminescence.