Structural model studies for the high-valent intermediate Q of methane monooxygenase from broken-symmetry density functional calculations

Mössbauer isomer shift parameters have been obtained for both density functional theory (DFT) OPBE and OLYP functionals by linear regressions between the measured isomer shifts and calculated electron densities at Fe nuclei for a number of Fe2+,2.5+ and Fe2.5+,3+,3.5+,4+ complexes grouped separately. The calculated isomer shifts and quadrupole splittings on the sample Fe complexes from OPBE and OLYP functionals are similar to those of PW91 calculations [J. Comput. Chem. 27 (2006) 1292], however the fit parameters from the linear regressions differ between PW91 and OPBE, OLYP. Four models for the active site structure of intermediate Q of the hydroxylase component of soluble methane monooxygenase (MMOH) have been studied, using three DFT functionals OPBE, OLYP, and PW91, incorporated with broken-symmetry methodology and the conductor-like screening (COSMO) solvation model. The calculated properties, including optimized geometries, electronic energies, pKa’s, Fe net spin populations, and Mössbauer isomer shifts and quadrupole splittings, have been reported and compared with available experimental values. The high-spin antiferromagnetically (AF) coupled Fe4+ sites are correctly predicted by OPBE and OLYP methods for all active site models. PW91 potential overestimates the Fe–ligand covalencies for some of the models because of spin crossover. Our calculations and data analysis support the structure (our current model II shown in Fig. 8) proposed by Friesner and Lippard’s group [J. Am. Chem. Soc. 123 (2001) 3836–3837], which contains an Fe4+(μ-O)2Fe4+ center, one axial water which also H-bonds to both side chains of Glu243 and Glu114, and one bidentate carboxylate group from the side chain of Glu144, which is likely to represent the active site of MMOH-Q. A new model structure (model IV shown in Fig. 9), which has a terminal hydroxo and a protonated His147 which is dissociated from a nearby Fe, is more asymmetric in its Fe(μ-O)2Fe diamond core, and is another very good candidate for intermediate Q.

Mössbauer isomer shift parameters have been obtained for both DFT OPBE and OLYP functionals. Four models for the active site structure of intermediate Q of the hydroxylase component of soluble methane monooxygenase (MMOH) have been studied, using three DFT functionals OPBE, OLYP, and PW91, incorporated with broken-symmetry methodology and COSMO solvation model. Possible Fe4+(μ-O)2Fe4+ structures of the MMOH-Q active site are proposed.Figure optionsDownload as PowerPoint slide