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J Am Chem Soc. 2005 Oct 5;127(39):13611-21.

Principal active species of horseradish peroxidase, compound I: a hybrid quantum mechanical/molecular mechanical study.

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  • 1Institute of Chemistry and the Lise-Meitner-Minerva Center for Computational Quantum Chemistry, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel.


The active species, Compound I, of horseradish peroxidase (HRP) has been investigated by quantum mechanical/molecular mechanical (QM/MM) calculations using 10 different QM regions. In accord with experimental data, the lowest doublet and quartet states are found to be virtually degenerate, with two unpaired electrons on the FeO moiety and one localized on the porphyrin in an a(2u)-dominant orbital with a minor, but nonnegligible, a(1u) component. The proximal ligand appears to be imidazole rather than imidazolate. The hydrogen-bonding network around the FeO moiety (i.e., Arg38 and His42) has significant influence on the axial bonds and the spin density distribution in the FeO moiety. Including this network in the QM region was found to be essential for reproducing the experimental Mössbauer parameters. The protein environment shapes most of the subtle features of Compound I of HRP.

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