Comparative Assessment of the Composition and Charge State of Nitrogenase FeMo-Cofactor
详细信息 查看全文
- 作者：Travis V. Harris ; Robert K. Szilagyi
- 刊名：Inorganic Chemistry
- 出版年：2011
- 出版时间：June 6, 2011
- 年：2011
- 卷：50
- 期：11
- 页码：4811-4824
- 全文大小：1102K
- 年卷期：v.50,no.11(June 6, 2011)
- ISSN：1520-510X
文摘
A significant limitation in our understanding of the molecular mechanism of biological nitrogen fixation is the uncertain composition of the FeMo-cofactor (FeMo-co) of nitrogenase. In this study we present a systematic, density functional theory-based evaluation of spin-coupling schemes, iron oxidation states, ligand protonation states, and interstitial ligand composition using a wide range of experimental criteria. The employed functionals and basis sets were validated with molecular orbital information from X-ray absorption spectroscopic data of relevant iron-sulfur clusters. Independently from the employed level of theory, the electronic structure with the greatest number of antiferromagnetic interactions corresponds to the lowest energy state for a given charge and oxidation state distribution of the iron ions. The relative spin state energies of resting and oxidized FeMo-co already allowed exclusion of certain iron oxidation state distributions and interstitial ligand compositions. Geometry-optimized FeMo-co structures of several models further eliminated additional states and compositions, while reduction potentials indicated a strong preference for the most likely charge state of FeMo-co. M枚ssbauer and ENDOR parameter calculations were found to be remarkably dependent on the employed training set, density functional, and basis set. Overall, we found that a more oxidized [MoIV-2FeII-5FeIII-9S2鈥?/sup>-C4鈥?/sup>] composition with a hydroxyl-protonated homocitrate ligand satisfies all of the available experimental criteria and is thus favored over the currently preferred composition of [MoIV-4FeII-3FeIII-9S2鈥?/sup>-N3鈥?/sup>] from the literature.