Erica J. Lyon

Carbon Monoxide and Cyanide Ligands in a Classical Organometallic Complex Model for Fe‐Only Hydrogenase

The Fe(I) organometallic complex [(µ-SCH(2)CH(2)CH(2)S)Fe(2)(CO)(6)] provides a structural model for the cyano-carbonyl diiron site of Fe-only hydrogenase as characterized by X-ray crystallography (the picture shows the structure (black) of the model overlaid with that of the Fe-Fe dimetallic site in the hydrogenase isolated from Desulfovibrio desulfuricans). Cyanide substitution of CO occurs readily and provides spectroscopic references for the active site.

The bio-organometallic chemistry of active site iron in hydrogenases ☆

Abstract The recent X-ray crystal structure determinations of several hydrogenase enzymes have engaged the organometallic community owing to the presence of CN − and CO ligands bound to iron in the active sites. This review focuses primarily on the structural features of these metalloproteins and discusses synthetic efforts to develop small molecule models of the active sites. Specific attention is given to the use of infrared spectroscopy as an additional tool to probe different enzyme states. In addition, structurally dissimilar complexes which show some ability to facilitate either dihydrogen uptake or production, are reviewed as possible functional models. Insights from earlier work with metal hydride chemistry and recent theoretical studies are discussed in terms of functional mechanistic proposals.

The organometallic active site of [Fe]hydrogenase: Models and entatic states

The simple organometallic, (μ-S2)Fe2(CO)6, serves as a precursor to synthetic analogues of the chemically rudimentary iron-only hydrogenase enzyme active site. The fundamental properties of the (μ-SCH2CH2CH2S)[Fe(CO)3]2 compound, including structural mobility and regioselectivity in cyanide/carbon monoxide substitution reactions, relate to the enzyme active site in the form of transition-state structures along reaction paths rather than ground-state structures. Even in the absence of protein-based active-site organization, the ground-state structural model complexes are shown to serve as hydrogenase enzyme reaction models, H2 uptake and H2 production, with the input of photo- or electrochemical energy, respectively.

Fundamental properties of small molecule models of Fe-only hydrogenase: computations relative to the definition of an entatic state in the active site

Abstract Well-studied organometallic complexes (μ-SRS)Fe 2 (CO) 6 that serve as structural models of the active site of Fe-only hydrogenases have been employed in DFT computational studies with the goal of understanding the fundamental nature of the active site of this biological catalyst. Intramolecular CO site exchange processes, experimentally observable in variable temperature (VT) NMR studies were modeled. The transition state structure of the Fe(CO) 3 unit rotation looks very similar to the structure that the active site has adopted in the protein environment. That is, a semi-bridging CO is formed upon Fe(CO) 3 rotation partially disrupting the FeFe bonding interaction and leaving an open site trans to this semi-bridging CO. The CN − /CO substitution reaction of these complexes which yields the disubstituted derivatives, (μ-SRS)[Fe(CO) 2 (CN)] 2 2− , was also examined as experimental results found a complicated, R-dependent, reactivity pattern for the second CN − addition. The connection of the above rotation process to the CN − /CO substitution was supported by the fact that an intermediate with a μ-CO group, like that resulting from the Fe(CO) 3 unit rotation, is formed upon CN − attack. The assumption that the Fe(CO) 3 rotational barrier is an important contributor to the overall activation energy of CN − attack, explains the experimental observation that generally the second CN − addition finds a lower Fe(CO) 3 rotational barrier due to the presence of the already coordinated CN − ligand.

The iron-sulfur-cluster-free hydrogenase (Hmd) is a metalloenzyme with a novel iron binding motif

The iron-sulfur cluster-free hydrogenase (Hmd) from methanogenic archaea harbors an iron-containing cofactor of yet unknown structure. X-ray absorption spectroscopy of the active, as isolated enzyme from Methanothermobacter marburgensis (mHmd) and of the active, reconstituted enzyme from Methanocaldococcus jannaschii (jHmd) revealed the presence of mononuclear iron with two CO, one sulfur and one or two N/O in coordination distance. In jHmd, the single sulfur ligand is most probably provided by Cys176, as deduced from a comparison of the activity and of the x-ray absorption and Mössbauer spectra of the enzyme mutated in any of the three conserved cysteines. In the isolated Hmd cofactor, two CO, one sulfur, and two nitrogen/oxygen atoms coordinate the iron, the sulfur ligand being most probably provided by mercaptoethanol, which is absolutely required for the extraction of the iron-containing cofactor from the holoenzyme and for the stabilization of the extracted cofactor. In active mHmd holoenzyme, the number of iron ligands increased by one when one of the Hmd inhibitors (CO or KCN) were present, indicating that in active Hmd, the iron contains an open coordination site, which is proposed to be the site of H2 interaction.