Nickel-mediated hydrogenolysis of C-O bonds of aryl ethers: what is the source of the hydrogen?

Mechanistic studies of the hydrogenolysis of aryl ethers by nickel were undertaken with (diphosphine)aryl methyl ethers. A Ni(0) complex containing Ni-arene interactions adjacent to the aryl-O bond was isolated. Heating led to aryl-O bond activation and generation of a nickel aryl methoxide complex. Formal β-H elimination from this species produced a nickel aryl hydride which can undergo reductive elimination in the presence of formaldehyde to generate a carbon monoxide adduct of Ni(0). The reported complexes map out a plausible mechanism of aryl ether hydrogenolysis catalyzed by nickel. Investigations of a previously reported catalytic system using isotopically labeled substrates are consistent with the mechanism proposed in the stoichiometric system, involving β-H elimination from a nickel alkoxide rather than cleavage of the Ni-O bond by H(2).     

Planar or nonplanar: what is the structure of urea in aqueous solution?

A combined quantum chemical statistical mechanical method has been used to study the solvation of urea in water, with emphasis on the structure of urea. The model system consists of three parts: a Hartree-Fock quantum chemical core, 99 water molecules described with a polarizable force-field, and a dielectric continuum. A free-energy profile along the transition of urea from planar to a nonplanar structure is calculated. This mode in aqueous solution is found to be floppy. That is, the structure of urea in water is not well-defined because the planar to nonplanar transition requires an energy of the order of the thermal energy at room temperature. We discuss the implications of this finding for simulation studies of urea in polar environments like water and proteins.     

The third edition of ISO Guide 34: what were the drivers for the revision and what is new?

After the International Laboratory Accreditation Cooperation (ILAC) had taken in 2004, the resolution to conduct accreditation of producers of reference materials according to ISO Guide 34 ‘General requirements for the competence of reference material producers’ in combination with ISO/IEC 17025 ‘General requirements for the competence of testing and calibration laboratories’, ISO/REMCO, the ISO Committee on Reference Materials, decided in 2005 to revise ISO Guide 34 to align it closer with ISO/IEC 17025 and to clarify certain issues for accreditors and producers seeking accreditation without adding new requirements. Moreover, the publication in 2007 of ISO/IEC Guide 99 ‘International vocabulary of metrology—Basic and general concepts and associated terms (VIM)’ triggered additional adaptations of the guide.     

Surface-enhanced vibrational spectroscopy of B vitamins: what is the effect of SERS-active metals used?

Surface-enhanced Raman scattering (SERS) spectroscopy and surface-enhanced infrared absorption (SEIRA) spectroscopy are analytical tools suitable for the detection of small amounts of various analytes adsorbed on metal surfaces. During recent years, these two spectroscopic methods have become increasingly important in the investigation of adsorption of biomolecules and pharmaceuticals on nanostructured metal surfaces. In this work, the adsorption of B-group vitamins pyridoxine, nicotinic acid, folic acid and riboflavin at electrochemically prepared gold and silver substrates was investigated using Fourier transform SERS spectroscopy at an excitation wavelength of 1,064 nm. Gold and silver substrates were prepared by cathodic reduction on massive platinum targets. In the case of gold substrates, oxidation–reduction cycles were applied to increase the enhancement factor of the gold surface. The SERS spectra of riboflavin, nicotinic acid, folic acid and pyridoxine adsorbed on silver substrates differ significantly from SERS spectra of these B-group vitamins adsorbed on gold substrates. The analysis of near-infrared-excited SERS spectra reveals that each of B-group vitamin investigated interacts with the gold surface via a different mechanism of adsorption to that with the silver surface. In the case of riboflavin adsorbed on silver substrate, the interpretation of surface-enhanced infrared absorption (SEIRA) spectra was also helpful in investigation of the adsorption mechanism.     

Natural and biotech-derived therapeutic proteins: what is the future?

A myriad of novel proteins and ligands of unknown function will be generated by the Human Genomic Project. Due to differences in post-translational processing, proteins produced by recombinant DNA technology may not possess proper biological activity. One way to find their function is to search for their natural counterparts. Proteins are produced in the tissues, and many of them are secreted into plasma and excreted into urine. There is a virtually "unlimited" array of human proteins in our plasma and urine, many of them in a fully active form. They include small molecules like steroids, peptides, and large glycoproteins like human menopausal gonadotropin. A library of plasma and urinary proteins could be developed to serve as a reference for the novel proteins generated by the functional genomic projects.     

Prediction of the phase structure in polymer blends: what is needed for success?

Rules for prediction of the phase structure in immiscible polymer blends from the knowledge of their composition, component properties and the flow field in a mixing or processing device are discussed. The reliability of qualitative prediction of the dependence of phase structure on system parameters is used as a criterion of plausibility of the rules. No general reliable rule for prediction of the phase structure type (continuity of phases) is available in the literature. Dependence of the droplet break‐up frequency on its size, contribution of simultaneous collisions of three or more droplets to coalescence and the effect of complex flow field on coalescence must be better described for a reliable qualitative prediction of the dependence of the droplet size on the system parameters.     

To what question is the clamped-nuclei electronic potential the answer?

An examination is made of how the nuclear motion Hamiltonian arises from a consideration of solutions to the eigenvalue problem for the full Coulomb Hamiltonian and the role played by the usual clamped-nuclei electronic Hamiltonian in the construction of such solutions.     

Elongated dihydrogen complexes: what remains of the H-H bond?

Of the several hundred examples of transition metal dihydrogen complexes that have been reported to date, the vast majority have H-H distances of less than 1.0 Angstrom. A small number of complexes have been reported with distances in the range of 1.1 to 1.5 Angstrom. These complexes have been termed elongated dihydrogen complexes. In this review, experimental methods for structure determination of such complexes are summarized, along with computational approaches which have proven useful in understanding the structures of these molecules.     

What is an element? What is the periodic table? And what does quantum mechanics contribute to the question?

This article considers two important traditions concerning the chemical elements. The first is the meaning of the term “element” including the distinctions between element as basic substance, as simple substance and as combined simple substance. In addition to briefly tracing the historical development of these distinctions, I make comments on the recent attempts to clarify the fundamental notion of element as basic substance for which I believe the term “element” is best reserved. This discussion has focused on the writings of Fritz Paneth which are here analyzed from a new perspective. The other tradition concerns the reduction of chemistry to quantum mechanics and an understanding of chemical elements through their microscopic components such as protons, neutrons and electrons. I claim that the use of electronic configurations has still not yet settled the question of the placement of several elements and discuss an alternative criterion based on maximizing triads of elements. I also point out another possible limitation to the reductive approach, namely the failure, up to now, to obtain a derivation of the Madelung rule. Mention is made of some recent similarity studies which could be used to clarify the nature of ‘elements’. Although it has been suggested that the notion of element as basic substance should be considered in terms of fundamental particles like protons and electrons, I resist this move and conclude that the quantum mechanical tradition has not had much impact on the question of what is an element which remains an essentially philosophical issue.     

Spectral characterization of self-assemblies of aldopyranoside amphiphilic gelators: what is the essential structural difference between simple amphiphiles and bolaamphiphiles?

An aldopyranoside-based gelators (dodecanoyl-p-aminophenyl-beta-D-aldopyranoside)s and [1,12-dodecanedicarboxylic-bis(p-aminophenyl-beta-D-aldopyranoside)]s 1-4 were synthesized, and their gelation ability was evaluated in organic solvents and water. Simple aldopyranoside amphiphiles 1 and 2 were found to gelate organic solvents as well as water in the presence of a small amount of alcoholic solvents. More interestingly, not only extremely dilute aqueous solutions (0.05 wt%) of the bolaamphiphiles 3 and 4, but solutions of 3 and 4 in several organic solvents could be gelatinized. These results indicate that 1-4 can act as versatile amphiphilic gelators. We characterized the superstructures of the aqueous gels and organogels prepared from 1-4 using SEM, TEM, NMR and IR spectroscopy, and XRD. The aqueous gels 1 and 2 formed a three-dimensional network of puckered fibrils diameters in the range 20-200 nm, whereas the aqueous gels 3 and 4 produced filmlike lamellar structures with 50-100 nm thickness at extremely low concentrations (0.05 wt%). Powder XRD experiments indicate that the aqueous gels 1 and 2 maintain an interdigitated bilayer structure with a 2.90 nm period with the alkyl chain tilted, while the organogels 1 and 2 take a loosely interdigitated bilayer structure with a 3.48 nm period. On the other hand, the aqueous- and the organogels 3 and 4 have 3.58 nm spacing, which corresponds to a monolayered structure. The XRD, 1H NMR and FT-IR results suggest that 1-4 are stabilized by a combination of the hydrogen-bonding, pi-pi interactions and hydrophobic forces.     

Fracture of polymer interfaces: what are the relevant length scales?

While it is tempting to relate directly the molecular structure of an interface (between glassy or between semi‐cristalline polymers) with its fracture toughness, these two parameters are simply the two end‐points of a complex network which needs to be understood in order to control the mechanical strength of the interface. The important mechanisms occur at three different length scales: the molecular scale (stress‐transfer across the interface), the microscopic scale (plastic deformation at the crack tip) and the macroscopic scale (loading geometry and elastic constants of the polymers). The couplings existing between these length scales in glassy polymer interfaces are reviewed in this paper in light of the latest experimental studies.     

To what extent is the magnitude of the Cole-Cole α of the β-dielectric dispersion of cell suspensions explicable in terms of the cell size distribution?

The Cole-Cole α is a number that is often used to describe the divergence of a measured dielectric dispersion from the ideal dispersion exhibited by a Debye type of dielectric relaxation, and is widely assumed to be related to a distribution of the relaxation times in the system involved. The magnitude and relaxation time of the β-dielectric dispersion due to the charging of the plasma membrane capacitance of cell suspensions depend, inter alia, on the cell radius. An investigation was carried out to determine whether there might therefore be a relationship between the Cole-Cole α of the β-dispersion of yeast cell suspensions and the distribution of cell sizes. Changes in the Cole-Cole α during the batch culture of baker's yeast were recorded, showing an increase in the Cole-Cole α during the exponential phase (more than 0.3) relative to those of the lag phase (about 0.28) and the stationary phase (about 0.2). Although the cell size distribution, measured by flow cytometry, also showed an increase in breadth during the exponential phase, this was not strictly related to the changes in the Cole-Cole α observed. Further, the Cole-Cole α calculated from the measured cell size distribution was significantly smaller than that obtained experimentally. Simulations in which the internal conductivity or membrane capacitance per unit area of individual cells were allowed to vary substantially did not account for the “excessive” Cole-Cole α. Thus the magnitude of the Cole-Cole α of the β-dispersion of yeast cells cannot be ascribed simply to the charging of a static membrane capacitance in cells of differing sizes and/or internal conductivities.     

What is the structure of the RecA-DNA filament?

The bacterial RecA protein has been a model system for understanding how a protein can catalyze homologous genetic recombination. RecA-like proteins have now been characterized from many organisms, from bacteriophage to humans. Some of the RecA-like proteins, including human RAD51, appear to function as helical filaments formed on DNA. However, we currently have high resolution structures of inactive forms of the protein, and low resolution structures of the active complexes formed by RecA-like proteins on DNA in the presence of ATP or ATP analogs. Within a crystal of the E. coli RecA protein, a helical polymer exists, and it has been widely assumed that this polymer is quite similar to the active helical filament formed on DNA. Recent developments have suggested that this may not be the case.     

What is the enthalpy of formation of acrylonitrile?

Acrylonitrile is a key industrial compound with numerous uses. Despite its importance, its enthalpy of formation is still contentious. There is a 12 kJ mol⁻¹ range of values reported for the gas phase quantity: 173–185 kJ mol⁻¹. Quantum chemical calculations, using current methodologies and defining reactions, suggest values between 185 and 191 kJ mol⁻¹: the recommended value, an average, is 188 ± 7 kJ mol⁻¹.     

Oxygen economy of cytochrome P450: what is the origin of the mixed functionality as a dehydrogenase-oxidase enzyme compared with its normal function?

The economy of dioxygen consumption by enzymes constitutes a fundamental problem in enzymatic chemistry (ref 1). Sometimes, the enzyme converts ALL the oxygen into water, without affecting the organic substrate, thereby acting as an "oxidase" (ref 1). Other times, the enzyme converts all the oxygen into water and causes desaturation in the substrate, thus exhibiting a mixed function as both "oxidase" and "dehydrogenase" (refs 2-5). The present paper describes density functional calculations demonstrating that the oxidase-dehydrogenase mixed activity occurs from the cationic intermediate species and requires electro-steric inhibition of the rebound process. Furthermore, the calculations reveal that the carbocation is formally nascent from an excited state of the active species of the enzyme (2Cpd I), in which the Fe=O moiety is singlet coupled as in the 1Deltag state of dioxygen! Thus, our results resolve an important mechanism and reveal the factors that underlie its observability.     

Hydrogen exchange mass spectrometry: what is it and what can it tell us?

Proteins are undoubtedly some of the most essential molecules of life. While much is known about many proteins, some aspects still remain mysterious. One particularly important aspect of understanding proteins is determining how structure helps dictate function. Continued development and implementation of biophysical techniques that provide information about protein conformation and dynamics is essential. In this review, we discuss hydrogen exchange mass spectrometry and how this method can be used to learn about protein conformation and dynamics. The basic concepts of the method are described, the workflow illustrated, and a few examples of its application are provided.     

Stabilities and Biological Activities of Vanadium Drugs: What is the Nature of the Active Species?

Diverse biological activities of vanadium(V) drugs mainly arise from their abilities to inhibit phosphatase enzymes and to alter cell signaling. Initial interest focused on anti‐diabetic activities but has shifted to anti‐cancer and anti‐parasitic drugs. V‐based anti‐diabetics are pro‐drugs that release active components (e.g., H₂VO₄⁻) in biological media. By contrast, V anti‐cancer drugs are generally assumed to enter cells intact; however, speciation studies indicate that nearly all drugs are likely to react in cell culture media during in vitro assays and the same would apply in vivo. The biological activities are due to V^V and/or V^IV reaction products with cell culture media, or the release of ligands (e.g., aromatic diimines, 8‐hydroxyquinolines or thiosemicarbazones) that bind to essential metal ions in the media. Careful consideration of the stability and speciation of V complexes in cell culture media and in biological fluids is essential to design targeted V‐based anti‐cancer therapies.