Flavo-diiron proteins (FDPs) contain non-heme diiron and proximal flavin mononucleotide (FMN) active sites and function as terminal components of a nitric oxide reductase (NOR) and/or a four-electron dioxygen reductase (O2R). Despite conservation of most structural, spectroscopic and redox properties, O2R and NOR activities vary significantly among FDPs. A potential source of this variability is the iron ligation status of a conserved His residue, which provides an iron ligand in all but one known FDP structure, where this His residue is rotated away from iron and replaced by a solvent ligand. In order to test the effect of this His ligation status, we changed this ligating His residue (H90) in Thermotoga maritima (Tm) FDP to either Asn or Ala. The wild type Tm FDP shows significantly higher O2R than NOR activity. Single crystal X-ray crystallography revealed a remarkably conserved diiron site structure in the H90N and -A variants, differing mainly by either Asn or solvent coordination, respectively, in place of H90. The steady state activities were minimally affected by the H90 substitutions, remaining significantly higher for O2R vs NOR. The pre-steady state kinetics of the fully reduced FDP with O2 were also minimally affected by the H90 substitutions. The results indicate that the coordination status of this His ligand does not significantly modulate the O2R or NOR activities, and that FDPs can retain these activities when the individual iron centers are differentiated by His ligand substitution. This differentiation may have implications for the O2R and NOR mechanisms of FDPs.

Caught in transition: an efficient catalytic system is the key to the successful development of the first highly diastereo- and enantioselective annulation reaction between indoles and transient nitrosoalkenes. This robust reaction affords structurally unique architectures with up to three new chiral centers. The products can be readily elaborated into other indoline-based chiral heterocyclic motifs, including those of pyrrolidinoindoline alkaloids.

No abstract

For the first time a highly enantioselective catalytic [4+2] annulation of unstable and extremely reactive in situ prepared nitrosoalkenes is developed by using the synergistic combination of copper catalyst and a silver base. The obtained heterocycles can be used as precursor to natural products and their derivatives. -(ZHANG, Y.; STEPHENS, D.; HERNANDEZ, G.; MENDOZA, R.; LARIONOV*, O. V.; Chem.--Eur.