Hydrogenases are prime candidates for bioengineering or biomimetic efforts in bio-fuel cells or the industrial production of “bio-hydrogen”. Knowledge of their structures and catalytic mechanisms are actively being pursued toward the development of efficient synthetic catalysts for hydrogen-fueled processes. Although the density functional theory (DFT) in particular can be utilized to characterize the enzymatic mechanism of hydrogenases by exploring structural, energetic and spectroscopic properties, challengea are in determining redox energies of diiron analogues of the active site of [FeFe]-hydrogenases. Our recent study revealed that the calculated energetic properties of the diiron models were extremely sensitive density functionals and the calculated relative energy between two isomers of diiron models ranged from +10 to -60 kcal/mol. To find out a reliable DFT method for exploring structural and redox properties of the active site of [FeFe]-hydrogenases the active site of [FeFe]-hydrogenases, based on recent excellent experimental findings in X-ray crystallographic, photoelectron spectroscopic (PES), and FTIR studies, a series of optimized density functionals for simulating redox energies of [1Fe]-protein analogs, including GGA, meta-GGA, hybrid-GGA, hybrid-GGA, hybrid-meta-GGA, and long range corrected hybrid-GGA functionals, have been further examined for structural, redox and spectroscopic properties of the five model complexes: [(μ-PDT)Fe2(CO)4(CN)2]2- (I), [Fe2(CO)4{PhCH2SCH2C(Me)(CH2)2}(CN)]- (II), [Fe2(CO)6(μ-SCH2CH2S)]0, (III), [Fe2(CO)6S2]0 (IV), and [Fe2(CO)3(μ-CO){MeSCH2C(Me)(CH2S)2}(CN)2]2- (V) (PDT = SCH2CH2CH2S, Me =CH3). We found that the different kinds of functionals exhibited different performances. Overall, The TPSS functional gave the best descriptions of structures, IR vibrational frequencies, and electron binding energies. Remarkably, calculations with the LRC-ωPBEh functional and B3LYP and M06 functionals with approximate projection model found a spin-polarized, symmetry-broken solution with a slightly longer Fe-Fe distance. This solution may be related to a near-degeneracy problem and influence the accuracy of DFT calculations. It would give rise to an interesting question about if the existence of this solution be also related to the broad X bands observed by PES.

Assessing density functionals for simulations of structural, redox, and spectroscopic properties of [FeFe]-hydrogenases

251st National Meeting (2016)

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