Understanding the Role of Hyponitrite in Nitric Oxide Reduction

Herein, we review the preparation and coordination chemistry of cis and trans isomers of hyponitrite, [N2O2](2-). Hyponitrite is known to bind to metals via a variety of bonding modes. In fact, at least eight different bonding modes have been observed, which is remarkable for such a simple ligand. More importantly, it is apparent that the cis isomer of hyponitrite is more reactive than the trans isomer because the barrier of N2O elimination from cis-hyponitrite is lower than that of trans-hyponitrite. This observation may have important mechanistic implications for both heterogeneous NOx reduction catalysts and NO reductase. However, our understanding of the hyponitrite ligand has been limited by the lack of a general route to this fragment, and most instances of its formation have been serendipitous

Studies of Iron(III) Porphyrinates Containing Silanethiolate Ligands

The chemistry of several iron­(III) porphyrinates containing silanethiolate ligands is described. The complexes are prepared by protonolysis reactions of silanethiols with the iron­(III) precursors, [Fe­(OMe)­(TPP)] and [Fe­(OH)­(H₂O)­(TMP)] (TPP = dianion of <i>meso-</i>tetraphenylporphine; TMP = dianion of <i>meso-</i>tetramesitylporphine). Each of the compounds has been fully characterized in solution and the solid state. The stability of the silanethiolate complexes versus other iron­(III) porphyrinate complexes containing sulfur-based ligands allows for an examination of their reactivity with several biologically relevant small molecules including H₂S, NO, and 1-methylimidazole. Electrochemically, the silanethiolate complexes display a quasi-reversible one-electron oxidation event at potentials higher than that observed for an analogous arenethiolate complex. The behavior of these complexes versus other sulfur-ligated iron­(III) porphyrinates is discussed

Studies of Iron(III) Porphyrinates Containing Silanethiolate Ligands

The chemistry of several iron­(III) porphyrinates containing silanethiolate ligands is described. The complexes are prepared by protonolysis reactions of silanethiols with the iron­(III) precursors, [Fe­(OMe)­(TPP)] and [Fe­(OH)­(H₂O)­(TMP)] (TPP = dianion of <i>meso-</i>tetraphenylporphine; TMP = dianion of <i>meso-</i>tetramesitylporphine). Each of the compounds has been fully characterized in solution and the solid state. The stability of the silanethiolate complexes versus other iron­(III) porphyrinate complexes containing sulfur-based ligands allows for an examination of their reactivity with several biologically relevant small molecules including H₂S, NO, and 1-methylimidazole. Electrochemically, the silanethiolate complexes display a quasi-reversible one-electron oxidation event at potentials higher than that observed for an analogous arenethiolate complex. The behavior of these complexes versus other sulfur-ligated iron­(III) porphyrinates is discussed

<i>Treponema denticola</i> Superoxide Reductase: In Vivo Role, in Vitro Reactivities, and a Novel [Fe(Cys)₄] Site

In vitro and in vivo results are presented demonstrating that superoxide reductase (SOR) from the air-sensitive oral spirochete, <i>Treponema denticola</i> (Td), is a principal enzymatic scavenger of superoxide in this organism. This SOR contains the characteristic non-heme [Fe­(His)₄Cys] active sites. No other metal-binding domain has been annotated for Td SOR. However, we found that Td SOR also accommodates a [Fe­(Cys)₄] site whose spectroscopic and redox properties resemble those in so-called 2Fe-SORs. Spectroscopic comparisons of the wild type and engineered Cys → Ser variants indicate that three of the Cys ligands correspond to those in [Fe­(Cys)₄] sites of “canonical” 2Fe-SORs, whereas the fourth Cys ligand residue has no counterpart in canonical 2Fe-SORs or in any other known [Fe­(Cys)₄] protein. Structural modeling is consistent with iron ligation of the “noncanonical” Cys residue across subunit interfaces of the Td SOR homodimer. The Td SOR was isolated with only a small percentage of [Fe­(Cys)₄] sites. However, quantitative formation of stable [Fe­(Cys)₄] sites was readily achieved by exposing the as-isolated protein to an iron salt, a disulfide reducing agent and air. The disulfide/dithiol status and iron occupancy of the Td SOR [Fe­(Cys)₄] sites could, thus, reflect intracellular redox status, particularly during periods of oxidative stress