Energetics of Oxidized and Reduced Methane Monooxygenase Active Site Clusters in the Protein Environment

Using the density functional optimized active site geometries obtained in the accompanying paper (Lovell, T.; Li, J.; Noodleman, L. Inorg. Chem. 2001, 40, 5251), a combined density functional and electrostatics approach has been applied to further address attendant uncertainties in the protonation states of the bridging ligands for MMOHox. The acidities (pKas) associated with the bridging H2O ligand in Methylococcus capsulatus and corresponding energetics of each active site cluster interacting with the protein environment have been evaluated. The pKa calculations in combination with the results of the gas phase DFT studies allow the active site cluster in Methylosinus trichosporium to be best described as a diiron unit bridged by 2OH- ligands having an overall neutral net cluster charge. The presence of the exogenous acetate in M. capsulatus reveals a diiron unit bridged by 1OH- and 1H2O which asymmetrically shares its proton with a second-shell acetate in a very short strong AcO··H···OH hydrogen bond. F...