Facilitated hydride binding in an Fe-Fe hydrogenase active-site biomimic revealed by X-ray absorption spectroscopy and DFT calculations
2007 (English)In: Inorganic Chemistry, ISSN 0020-1669, Vol. 46, no 26, 11094-11105 p.Article in journal (Refereed) Published
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.
IdentifiersURN: urn:nbn:se:uu:diva-98033DOI: 10.1021/ic701255pISI: 000251773700024OAI: oai:DiVA.org:uu-98033DiVA: diva2:173196