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Facilitated hydride binding in an Fe-Fe hydrogenase active-site biomimic revealed by X-ray absorption spectroscopy and DFT calculations
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Photochemistry and Molecular Science.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Photochemistry and Molecular Science.
2007 (English)In: Inorganic Chemistry, ISSN 0020-1669, Vol. 46, no 26, 11094-11105 p.Article in journal (Refereed) Published
Abstract [en]

Iron-only hydrogenases are high-efficiency biocatalysts for the synthesis and cleavage of molecular hydrogen. Their active site is a diiron center, which carries CO and CN ligands. Remarkably, the two iron atoms likely are connected by a non-protein azadithiolate (adt = S-CH2-NH-CH2-S). To dwell on the role of the adt in H-2 catalysis, a specific biomimetic diiron compound, 1 = [Fe-2(mu-adt-CH2-Ph)(CO)(4)(PMe3)(2)], with unprecedented positive reduction potential, has been synthesized and crystallized previously. It comprises two protonation sites, the N-benzyl-adt nitrogen that can hold a proton (H) and the Fe-Fe bond that will formally carry a hydride (Hy). We investigated changes in the solution structure of 1 in its four different protonation states (1', [1H](+), [1HHy](2+), and [1Hy](+)) by X-ray absorption spectroscopy at the iron K-edge. EXAFS reveals that already protonation at the adt nitrogen atom causes a change of the ligand geometry involving a significant lengthening of the Fe-Fe distance and CO and PMe3 repositioning, respectively, thereby facilitating the subsequent binding of a bridging hydride. Hydride binding clearly is discernible in the XANES spectra of [1HHy](2+) and [1Hy](+). DIFT calculations are in excellent agreement with the experimentally derived structural parameters and provide complementary insights into the electronic structure of the four protonation states. In the iron-only hydrogenases, protonation of the putative adt ligand may cause the bridging CO to move to a terminal position, thereby preparing the active site for hydride binding en route to H2 formation.

Place, publisher, year, edition, pages
2007. Vol. 46, no 26, 11094-11105 p.
National Category
Chemical Sciences
URN: urn:nbn:se:uu:diva-98033DOI: 10.1021/ic701255pISI: 000251773700024OAI: oai:DiVA.org:uu-98033DiVA: diva2:173196
Available from: 2009-02-12 Created: 2009-02-12 Last updated: 2010-03-02Bibliographically approved
In thesis
1. Synthetic [FeFe] Hydrogenase Active Site Model Complexes
Open this publication in new window or tab >>Synthetic [FeFe] Hydrogenase Active Site Model Complexes
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

[FeFe]-Hydrogenases (H2ases) are metalloenzymes that can catalyze the reversible reduction of protons to molecular hydrogen as part of the metabolism of certain cyanobacteria and green algae. Due to the low availability of the enzyme, synthetic complexes that mimic the natural active site in structure, function and activity are highly sought after. In this thesis, a number of [FeFe]-H2ases active site model complexes were synthesized to answer open questions of the active site and to develop unprecedented bio-inspired proton reduction catalysts.

The first part describes the synthesis and the protonation properties of a [Fe2(μ-adt)(CO)4(PMe3)2] (adt = azadithiolate) complex which contains two basic sites that are similar to those found in the enzyme active site. Unusual kinetic factors give rise to four discrete protonation states. The twofold protonated state is the first model complex that simultaneously carries a proton at the azadithiolate nitrogen and a bridging hydride at the Fe-Fe bond.

In the second part, a model complex with an unprecedented amine ligand was synthesized and studied. In analogy to the enzyme active site, the labile amine ligand is expelled after electrochemical reduction.

The third part describes a series of model complexes with electronically different aromatic dithiolate ligands. It is demonstrated in one case that the tuning of the ligand by electron-withdrawing substituents results in proton reduction catalysis at an overpotential that is lower than that required by the non-substituted parent compound.

The design and the synthetic work towards a new ruthenium-diiron dyad for light-driven hydrogen production are presented in the fourth part.

In the final part, differently isotope-labelled mixed valent Fe(I)-Fe(II) model complexes were synthesized, in particular the unprecedented 15N labelled analogue, with the aim to provide EPR-spectroscopic references that will allow the elucidation of the nature of the central atom in the dithiolate bridge of the [FeFe] hydrogenase active site.

Place, publisher, year, edition, pages
Uppsala: Universitetsbiblioteket, 2009. 88 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 599
hydrogenase mimic, proton reduction, bioinorganic chemistry, diiron hexacarbonyl complexes, artifical photosynthesis, hydrogen
National Category
Organic Chemistry
urn:nbn:se:uu:diva-9548 (URN)978-91-554-7404-1 (ISBN)
Public defence
2009-03-06, Å4001, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
Available from: 2009-02-12 Created: 2009-02-12 Last updated: 2010-03-04Bibliographically approved

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