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Internet Electronic Journal of Molecular Design - IEJMD, ISSN 1538-6414, CODEN IEJMAT
ABSTRACT - Internet Electron. J. Mol. Des. August 2006, Volume 5, Number 8, 447-459

Electronic Structures of Heme(Fe)-Dioxygen Complex as an Intermediate Model of Dioxygen Reduction in Cytochrome c Oxidase
Yasunori Yoshioka, Masaki Mitani, and Hiroyuki Satoh
Internet Electron. J. Mol. Des. 2006, 5, 447-459

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Abstract:
We have previously proposed the O2 reduction mechanism that the protons transfer from the K-channel to the active site of CcO. The second proton transfer to the hydroperoxy intermediate FeOOH did not lead the OO bond cleavage. It has been recently reported that the addition of the proton induces the OO bond cleavage to yield the productive H2O molecule and the oxo intermediate Fe=O, being inconsistent with our results. We have started this study to confirm whether the OO bond cleavage occurs or not upon the proton addition. We concentrated our study to the changes of the electronic structures of the heme a3 part throughout the sequential additions of the protons and an electron during the process of the O2 reduction. We employed a model that is composed of porphyrin without any substituents, Fe, and an imidazole as His376. We added sequentially protons and an electron to the heme(Fe)-dioxygen complex. The geometrical parameters were fully optimized without any constraints. The unrestricted hybrid exchange-correlation functional B3LYP method was used. The Wachters double zeta basis set was employed for Fe atom. The 6-31G* basis set was used for C, N, and H atoms, and the 6-311+G* for O atom. All calculations were carried out using the program package Gaussian 98. The proton addition to FeOO (H2) to yield FeOOH (H3) causes the electron transfer from the porphyrin ring to FeOOH moiety, giving the porphyrin-radical cation. The additional proton to FeOOH (H4) leads to the OO bond cleavage, yielding the hydrogen bonded complex of an oxo heme (compound I) and a H2O molecule. The bonding characters of FeOOH in H3 and H4 are identical, even though the electronic structures of the porphyrin rings in H3 and H4 are different. However, the proton addition to FeOOH (H3) does not lead to the OO bond cleavage. The protonated heme(Fe)-dioxygen complexes are effective models to investigate the reduction mechanism of a O2 molecule in CcO.

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