Structure and reactivity of a mononuclear non-haem iron(III)-peroxo complex

Oxygen-containing mononuclear iron species--iron(III)-peroxo, iron(III)-hydroperoxo and iron(IV)-oxo--are key intermediates in the catalytic activation of dioxygen by iron-containing metalloenzymes. It has been difficult to generate synthetic analogues of these three active iron-oxygen species in identical host complexes, which is necessary to elucidate changes to the structure of the iron centre during catalysis and the factors that control their chemical reactivities with substrates. Here we report the high-resolution crystal structure of a mononuclear non-haem side-on iron(III)-peroxo complex, [Fe(III)(TMC)(OO)](+). We also report a series of chemical reactions in which this iron(III)-peroxo complex is cleanly converted to the iron(III)-hydroperoxo complex, [Fe(III)(TMC)(OOH)](2+), via a short-lived intermediate on protonation. This iron(III)-hydroperoxo complex then cleanly converts to the ferryl complex, [Fe(IV)(TMC)(O)](2+), via homolytic O-O bond cleavage of the iron(III)-hydroperoxo species. All three of these iron species--the three most biologically relevant iron-oxygen intermediates--have been spectroscopically characterized; we note that they have been obtained using a simple macrocyclic ligand. We have performed relative reactivity studies on these three iron species which reveal that the iron(III)-hydroperoxo complex is the most reactive of the three in the deformylation of aldehydes and that it has a similar reactivity to the iron(IV)-oxo complex in C-H bond activation of alkylaromatics. These reactivity results demonstrate that iron(III)-hydroperoxo species are viable oxidants in both nucleophilic and electrophilic reactions by iron-containing enzymes.     

Extreme deuterium enrichment in stratospheric hydrogen and the global atmospheric budget of H2

Molecular hydrogen (H2) is the second most abundant trace gas in the atmosphere after methane (CH4). In the troposphere, the D/H ratio of H2 is enriched by 120 per thousand relative to the world's oceans. This cannot be explained by the sources of H2 for which the D/H ratio has been measured to date (for example, fossil fuels and biomass burning). But the isotopic composition of H2 from its single largest source--the photochemical oxidation of methane--has yet to be determined. Here we show that the D/H ratio of stratospheric H2 develops enrichments greater than 440 per thousand, the most extreme D/H enrichment observed in a terrestrial material. We estimate the D/H ratio of H2 produced from CH4 in the stratosphere, where production is isolated from the influences of non-photochemical sources and sinks, showing that the chain of reactions producing H2 from CH4 concentrates D in the product H2. This enrichment, which we estimate is similar on a global average in the troposphere, contributes substantially to the D/H ratio of tropospheric H2.     

The breakdown of continuum models for mechanical contacts

Forces acting within the area of atomic contact between surfaces play a central role in friction and adhesion. Such forces are traditionally calculated using continuum contact mechanics, which is known to break down as the contact radius approaches atomic dimensions. Yet contact mechanics is being applied at ever smaller lengths, driven by interest in shrinking devices to nanometre scales, creating nanostructured materials with optimized mechanical properties, and understanding the molecular origins of macroscopic friction and adhesion. Here we use molecular simulations to test the limits of contact mechanics under ideal conditions. Our findings indicate that atomic discreteness within the bulk of the solids does not have a significant effect, but that the atomic-scale surface roughness that is always produced by discrete atoms leads to dramatic deviations from continuum theory. Contact areas and stresses may be changed by a factor of two, whereas friction and lateral contact stiffness change by an order of magnitude. These variations are likely to affect continuum predictions for many macroscopic rough surfaces, where studies show that the total contact area is broken up into many separate regions with very small mean radius.     

Anodic dissolution of metals in ionic liquids

The anodic dissolution of metals is an important topic for battery design, material finishing and metal digestion. Ionic liquids are being used in all of these areas but the research on the anodic dissolution is relatively few in these media. This study investigates the behaviour of 9 metals in an ionic liquid [C4mim][Cl] and a deep eutectic solvent, Ethaline, which is a 1:2 mol ratio mixture of choline chloride and ethylene glycol. It is shown that for the majority of metals studied a quasi-passivation of the metal surface occurs, primarily due to the formation of insoluble films on the electrode surface. The behaviour of most metals is different in [C4mim][Cl] to that in Ethaline due in part to the differences in viscosity. The formation of passivating salt films can be decreased with stirring or by increasing the electrolyte temperature, thereby increasing ligand transport to the electrode surface.     

The State of the Kenyan Cotton Growing Industry

From the early 1960s to late 1980s, the Kenyan cotton growing industry played a vital role in the Kenyan economy in terms of provision of employment and creation of wealth in the rural areas. It also played a central role in the textile industry which was thriving during the above mentioned period. Over the years, cotton production in Kenya has fallen steadily, such that by the year 2000, the country experienced a severe cotton fiber deficit. This study was undertaken to investigate the trend of the cotton growing industry in Kenya. Selected aspects of the industry like cost of production, cotton seed distribution, the operation of cotton gins and the quality of cotton lint were considered. Kohonen Self Organizing Maps （SOM） and K-means clustering techniques were used in data analysis. The results of this study show that Kenyan cotton farmers produced seed cotton at a break-even price of US $ 0.31 per kilogram, while the price offered was US $ 0.29 per kilogram.     

Single artificial-atom lasing

Solid-state superconducting circuits are versatile systems in which quantum states can be engineered and controlled. Recent progress in this area has opened up exciting possibilities for exploring fundamental physics as well as applications in quantum information technology; in a series of experiments it was shown that such circuits can be exploited to generate quantum optical phenomena, by designing superconducting elements as artificial atoms that are coupled coherently to the photon field of a resonator. Here we demonstrate a lasing effect with a single artificial atom--a Josephson-junction charge qubit--embedded in a superconducting resonator. We make use of one of the properties of solid-state artificial atoms, namely that they are strongly and controllably coupled to the resonator modes. The device is essentially different from existing lasers and masers; one and the same artificial atom excited by current injection produces many photons.     

Synthesis and characterization of copper foams through a powder metallurgy route using a compressible and lubricant space-holder material

In the present work, a compressible and lubricating space-holder material commonly known as “acrawax” was used to process Cu foams with various pore sizes and various porosities. The foams were processed without using binders to avoid contamination of their metal matrices. The lubricant space-holder material was found to facilitate more uniform flow and distribution of metal powder around the surface of the space holder. In addition, the use of acrawax as a space-holder material yielded considerably dense cell walls, which are an essential prerequisite for better material properties. The foams processed with a smaller-sized space holder were found to exhibit better electrical and mechanical properties than those processed with a coarser-sized space holder. The isotropic pore shape, uniform pore distribution throughout the metal matrix, and uniform cell wall thickness were found to enhance the properties pertaining to fine-pore foam samples. The processed foams exhibit properties similar to those of the foams processed through the lost-carbonate sintering process.     

Characterization of carrot pectin methylesterase

The most alkaline form of pectin methylesterase was purified from ripe carrot roots and used for structural analysis. Determination of an N-terminal blocking group and of the primary structure allowed comparisons with other forms, and facilitated crystallographic determination of the three-dimensional structure. The mature enzyme has 319 residues and the N-terminal blocking group was shown to be a pyroglutamyl residue derived from a glutaminyl cyclization. Few other methylesterases have been isolated and assigned to exact mature forms, and together with the present enzyme, only two have been analyzed in three-dimensional structure. However, comparison of 39 forms, mainly from GenBank data, reveals clear relationships and identifies sub-groups of this enzyme type, deviating in structure but centering around two functionally important and conserved Asp residues at positions 136 and 157 in the carrot enzyme. Received 2 January 2002; accepted 4 January 2002     

Targetted correction of a mutant HPRT gene in mouse embryonic stem cells

Two recent developments suggest a route to predetermined alterations in mammalian germlines. These are, first, the characterization of mouse embryonic stem (ES) cells that can still enter the germline after genetic manipulation in culture and second, the demonstration that homologous recombination between a native target chromosomal gene and exogenous DAN can be used in culture to modify specifically the target locus. We here use gene targetting functionally to correct the mutant hypoxanthine-guanine phosphoribosyl transferase (HPRT) gene in the ES cell line which has previously been isolated and used to produce an HPRT-deficient mouse. This modification of a chosen gene in pluripotent ES cells demonstrates the feasibility of this route to manipulating mammalian genomes in predetermined ways.