Density Functional Study on the Electronic Structures of Model Peroxidase Compounds I and II

Abstract

The electronic structures of [Fe(Por)(Im)O] 1+ and [Fe(Por)(Im)O] (model compounds I and II, respectively) have been studied on the basis of density functional theory or DFT (Por ) porphine, Im ) imidazole). The a 2u π-cation radical state ( 4 A 2u ) was determined to be the ground state of compound I with total spin equal to 3 / 2 , while the a 1u π-cation state ( 4 A 1u ) was found to be 0.15 eV higher in energy than the 4 A 2u state. Since, in both states, the spins were localized to the porphyrin ring (S ) 1 / 2 ) and the Fe-O center (S ) 1), the magnetic coupling interaction between the two spin sites was examined by using a broken symmetry method. The calculated J value revealed very weak magnetic coupling for the A 2u state, which corresponded to the experimental data. The calculated J value revealed strong antiferromagnetic coupling for the A 1u state. The calculated Mössbauer spectrum parameters (quadrupole splitting and asymmetry) were similar for both the A 1u and A 2u states, and both agreed well with experimental values. On the other hand, the calculated hyperfine coupling constants for the nitrogen and the proton of the porphyrin ring were different in the two states. Although the experimental coupling constant values of the pyrrole nitrogen atoms were intermediate between the calculated values for the A 2u and A 1u states, the experimental values for the meso protons were closer to the values calculated for the A 2u state. These results suggest that the electronic structure of compound I is closer to the A 2u state than to the A 1u state. However, these results also suggest that there is the possibility that the electronic structure of compound I is an admixture of the A 1u state and the A 2u state. The electronic structure of compound II was calculated and compared with the electronic structure of compound I. The energetics of the redox reaction between the two compounds is discussed.

Effect of the Axial Ligand on Substrate Sulfoxidation Mediated by Iron(IV)–Oxo Porphyrin Cation Radical Oxidants

“…[16] Further computational studies showed that the degeneracy of the oxidant states also has consequences for substrate-activation mechanisms, which tend to lead to two-state reactivity (TSR) patterns with different rate constants and reaction mechanisms on both spin-state surfaces. [17] This TSR profile can result in the masquerade of kinetic isotope effects and the seemingly observance of multiple oxidants in the reaction mixture.…”

Unravelling novel synergies between organometallic and biological partners: a quantum mechanics/molecular mechanics study of an artificial metalloenzyme

“…This result clearly shows how the distortion of the cofactor, the modification of the first coordination sphere of the metal and the rearrangement of the structure of the receptor are intrinsically correlated and not individual phenomena. The energy of the transition state is 20.7, 9.8 and 18.6 kcal mol 21 , above the absolute minimum of the system, for low-, intermediate-and high-spin systems, respectively (figure 4). They are sufficiently large to discard the possibility that the atomic motions at room temperature naturally allow the transition from glutamate bound (6) to glutamate unbound configurations (5 and 5 0 ).…”

“…However, it is still interesting to note that, in most cases, this part of the system remains almost unaffected (Fe(II) cases) or even stabilized (Fe(III) cases) for Fe -O glu going from reactant to a few steps after the transition state. From then on, a destabilization of the system is observed, reaching up to ca 15 kcal mol 21 (electronic supplementary material, figure S4). Structural analysis showed us that the first part of the transition implies a slight relaxation of helix A while the glutamate is removed.…”

“…catalase [20]) or an octahedral low-spin configuration (i.e. peroxidases [21], cytochromes P450 [22] or haem oxygenase [23]) with only one residue of their apo-protein bound to the metal in the axial position and, eventually, one labile water molecule coordinated on the distal site of the haem to complete the octahedral configuration of the metal (figure 1). The strength of the coordination of this water molecule can be influenced by its environment and, in turn, conditions the transition from hexacoordinated resting states to active pentacoordinated species [24].…”

Symmetry states of metalloporphyrin π-cation radicals, models for peroxidase compound I

“…Electron abstraction from the porphyrin a 2u orbital results in high unpaired p-orbital spin density at the meso-carbons, while abstraction from the a 1u orbital results in appreciable unpaired p-orbital spin density at the pyrrole b-carbons. In general, alkyl or aryl substituents at the meso positions destabilize the a 2u orbital, favoring oxidation of porphyrins to p-cation radicals with 2 [24] (a downfield-shift of the meso-H signal may be attributed to induced spin polarization at the meso-carbon [70,71]). …”