Modeling the Correlation between and in an X-ray Crystal Structure Refinement.

We have examined how the refined -factor changes as a function of (the atomic number of a scatterer) at the sulfur site of the [4Fe:4S] cluster of the nitrogenase iron protein by refinement. A simple model is developed that quantitatively captures the observed relationship between and , based on a Gaussian electron density distribution with a constant electron density at the position of the scatterer. From this analysis, the fractional changes in and are found to be similar.

Selenocyanate derived Se-incorporation into the nitrogenase Fe protein cluster.

The nitrogenase Fe protein mediates ATP-dependent electron transfer to the nitrogenase MoFe protein during nitrogen fixation, in addition to catalyzing MoFe protein-independent substrate (CO) reduction and facilitating MoFe protein metallocluster biosynthesis. The precise role(s) of the Fe protein FeS cluster in some of these processes remains ill-defined. Herein, we report crystallographic data demonstrating ATP-dependent chalcogenide exchange at the FeS cluster of the nitrogenase Fe protein when potassium selenocyanate is used as the selenium source, an unexpected result as the Fe protein cluster is not traditionally perceived as a site of substrate binding within nitrogenase.

Structural Characterization of Two CO Molecules Bound to the Nitrogenase Active Site.

As an approach towards unraveling the nitrogenase mechanism, we have studied the binding of CO to the active-site FeMo-cofactor. CO is not only an inhibitor of nitrogenase, but it is also a substrate, undergoing reduction to hydrocarbons (Fischer-Tropsch-type chemistry). The C-C bond forming capabilities of nitrogenase suggest that multiple CO or CO-derived ligands bind to the active site.

Rethinking the Nitrogenase Mechanism: Activating the Active Site.

Metalloenzymes called nitrogenases (Nases) harness the reactivity of transition metals to reduce N to NH. Specifically, Nases feature a multimetallic active site, called a cofactor, which binds and reduces N. The seven Fe centers and one additional metal center (Mo, V, or Fe) that make up the cofactor are all potential substrate binding sites.

N -to-NH Conversion by a triphos-Iron Catalyst and Enhanced Turnover under Photolysis.

Bridging iron hydrides are proposed to form at the active site of MoFe-nitrogenase during catalytic dinitrogen reduction to ammonia and may be key in the binding and activation of N via reductive elimination of H . This possibility inspires the investigation of well-defined molecular iron hydrides as precursors for catalytic N -to-NH conversion. Herein, we describe the synthesis and characterization of new P Fe(N )(H) systems that are active for catalytic N -to-NH conversion.

Stereoelectronic Model To Explain Highly Stereoselective Reactions of Seven-Membered-Ring Oxocarbenium-Ion Intermediates.

Nucleophilic attack on seven-membered-ring oxocarbenium ions is generally highly stereoselective. The preferred mode of nucleophilic attack forms the product in a conformation that minimizes transannular interactions, thus leading to different stereoselectivity as compared to that of reactions involving six-membered-ring oxocarbenium ions.