An inorganic helix in the supramolecular structure of trimethyltin diphenylarsinate, [me3sn(o2asph2)]n: a structural comparison of triorganotin arsinates and phosphinates

The crystallographic structure analysis of [Me3Sn(O2AsPh2)]n reveals supramolecular association owing to symmetric bridges between Me3Sn centres afforded by O2AsPh2 groups. The resulting polymer has a helical topology. A systematic analysis of triorganotin arsinato and phosphinato analogues indicates a role for the remote substituents on aggregations patterns

Engineering tyrosine-based electron flow pathways in proteins: The case of aplysia myoglobin

Tyrosine residues can act as redox cofactors that provide an electron transfer ("hole-hopping") route that enhances the rate of ferryl heme iron reduction by externally added reductants, for example, ascorbate. Aplysia fasciata myoglobin, having no naturally occurring tyrosines but 15 phenylalanines that can be selectively mutated to tyrosine residues, provides an ideal protein with which to study such through-protein electron transfer pathways and ways to manipulate them. Two surface exposed phenylalanines that are close to the heme have been mutated to tyrosines (F42Y, F98Y). In both of these, the rate of ferryl heme reduction increased by up to 3 orders of magnitude. This result cannot be explained in terms of distance or redox potential change between donor and acceptor but indicates that tyrosines, by virtue of their ability to form radicals, act as redox cofactors in a new pathway. The mechanism is discussed in terms of the Marcus theory and the specific protonation/deprotonation states of the oxoferryl iron and tyrosine. Tyrosine radicals have been observed and quantified by EPR spectroscopy in both mutants, consistent with the proposed mechanism. The location of each radical is unambiguous and allows us to validate theoretical methods that assign radical location on the basis of EPR hyperfine structure. Mutation to tyrosine decreases the lipid peroxidase activity of this myoglobin in the presence of low concentrations of reductant, and the possibility of decreasing the intrinsic toxicity of hemoglobin by introduction of these pathways is discussed. © 2012 American Chemical Society

Six-Coordinate Nitrito and Nitrato Complexes of Manganese Porphyrin

Reaction of small increments of NO2 gas with sublimed amorphous layers of Mn(II)(TPP) (TPP = meso-tetra-phenylporphyrinato dianion) in a vacuum cryostat leads to formation of the 5-coordinate monodentate nitrato complex Mn(III)(TPP)(η(1)-ONO2) (II). This transformation proceeds through the two distinct steps with initial formation of the five coordinate O-nitrito complex Mn(III)(TPP)(η(1)-ONO) (I) as demonstrated by the electronic absorption spectra and by FTIR spectra using differently labeled nitrogen dioxide. A plausible mechanism for the second stage of reaction is offered based on the spectral changes observed upon subsequent interaction of (15)NO2 and NO2 with the layered Mn(TPP). Low-temperature interaction of I and II with the vapors of various ligands L (L = O-, S-, and N-donors) leads to formation of the 6-coordinate O-nitrito Mn(III)(TPP)(L)(η(1)-ONO) and monodentate nitrato Mn(III)(TPP)(L)(η(1)-ONO2) complexes, respectively. Formation of the 6-coordinate O-nitrito complex is accompanied by the shifts of the ν(N═O) band to lower frequency and of the ν(N-O) band to higher frequency. The frequency difference between these bands Δν = ν(N═O) - ν(N-O) is a function of L and is smaller for the stronger bases. Reaction of excess NH3 with I leads to formation of Mn(TPP)(NH3)(η(1)-ONO) and of the cation [Mn(TPP)(NH3)2](+) plus ionic nitrite. The nitrito complexes are relatively unstable, but several of the nitrato species can be observed in the solid state at room temperature. For example, the tetrahydrofuran complex Mn(TPP)(THF)(η(1)-ONO2) is stable in the presence of THF vapors (∼5 mm), but it loses this ligand upon high vacuum pumping at RT. When L = dimethylsulfide (DMS), the nitrato complex is stable only to ∼-30 °C. Reactions of II with the N-donor ligands NH3, pyridine, or 1-methylimidazole are more complex. With these ligands, the nitrato complexes Mn(III)(TPP)(L)(η(1)-ONO2) and the cationic complexes [Mn(TPP)(L)2](+) coexist in the layer at room temperature, the latter formed as a result of NO3(-) displacement when L is in excess

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

Fatalism and Inaction Associations with the Romanian Ballad of the Little Ewe

The ballad of the Little Ewe is generally noted as one of the distinctive features of the Romanian culture. Its facts are simple and few, if any: a little ewe warns its shepherd that his two associates intend to kill him for his possessions. The discussion that follows between the shepherd and the ewe is what draws attention – in that there is no mention whatsoever on any precise plans to resist the foretold murder; instead, the shepherd cares to provide instructions for things to happen after his death, in a particularly long lyrical monologue. This latter attitude has prompted many critics to label it as fatalism – and wide cultural implications have been claimed on that account – going to the point where public requests have been made to dismiss the ballad from public conscience as toxic. Discussing the text, we argue here that any such fatalism has to do with the eye of the respective critic – and that if indeed toxicity and eradication need to be mentioned, they would at best be associated with the respective critics rather than with the ballad itself