The vibrational structures for two possible peroxo intermediates of soluble methane monooxygenase (sMMO), in which O2 binds into the diiron(II) active site with end-on and side-on bridging modes, are analyzed from quantum chemical calculations at the B3LYP DFT level of theory. Dioxygen bridging modes should be controlled by carboxylate coordinations around the diiron active site. Two bridging carboxylato ligands are contained in the end-on peroxo complex, whereas one bridging carboxylato ligand is contained in the side-on peroxo complex. DFT calculations demonstrate that the end-on bridging mode into the diiron active site is energetically more stable than the side-on bridging mode. The end-on peroxo complex has a vibrational mode of O-O stretching at 909 cm-1, the isomer shift of which upon 18O2 substitution is 57 cm-1. The side-on peroxo complex exhibits a lower O-O stretching mode at 841 cm-1, and the isotope shift is 45 cm -1. The frequency of the O-O stretching mode and the 18O2 isotope shift are dependent on the dioxygen bridging modes in the active site of sMMO. The O-O stretching frequency in the end-on complex falls within the range measured in peroxo intermediates of other non-heme diiron enzymes and cis-μ-1,2 peroxo-bridged diiron(III) synthetic complexes. From the viewpoint of energetics and the vibrational frequencies, it is predicted that O2 should coordinate into the diiron active site of sMMO in an end-on cis-μ-1,2 fashion rather than a side-on fashion.
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