Is there potential for use of the Hall effect in analytical science?

This article describes the basic principles of the Hall effect and how it has been applied in solid state applications to measure both the magnetic field and material properties. This is followed by an outline of research undertaken in aqueous ionic solutions as a possible precursor to application in chemical studies. However, previous research has shown that factors such as the Nernst effect have reduced the success of reliably measuring the Hall voltage under these conditions. The possibility of a multi-sensing approach using both electrical and acoustic Hall voltages to overcome these challenges is discussed. The article concludes with a brief look at some early efforts to detect biomolecular interactions.     

Is there a carboxylic group in the sodium channel of excitable biomembranes?

A model describing the effects of H⁺, Ca²⁺ and Cd²⁺ ions on the sodium channels of excitable membranes was developed. On the basis of this model, it was shown that the decrease in the amplitude of the sodium current in the membrane of the neurones of the mollusc Helix pomatia caused by extracellular Cd²⁺, Ca²⁺ and H⁺ ions is related to the change in the near-membrane concentration of sodium ions due to a decrease of the outer surface membrane potential. The data obtained allow us to suggest that the sodium channel, contrary to the calcium channel, does not contain the carboxylic group in its selectivity filter.     

Is there a future for computational chemistry in drug research?

Improvements in computational chemistry methods have had a growing impact on drug research. But will incremental improvements be sufficient to ensure this continues? Almost all existing efforts to discover new drugs depend on the classic one target, one drug paradigm, although the situation is changing slowly. A new paradigm that focuses on a more systems biology approach and takes account of the reality that most drugs exhibit some level of polypharmacology is beginning to emerge. This will bring about dramatic changes that can significantly influence the role that computational methods play in future drug research. But these changes require that current methods be augmented with those from bioinformatics and engineering if the field is to have a significant impact on future drug research.     

Liquid dewetting of ultrathin polystyrene films: Is there a molecular architecture effect?

We have used a liquid dewetting method to investigate the glass transition temperature T_g of high molecular weight linear, long branched 3-arm star, and short branched 8-arm star polystyrene (PS) in the form of ultrathin films. The results of these dewetting experiments are consistent with prior studies of dewetting of linear PS films and show that, independent of molecular architecture, the glass transition temperature T_g reductions with decreasing film thickness, while important below about 20 nm, are weaker than those observed for linear PS supported on a rigid substrate and as well as those observed in freely standing films. The lack of a strong molecular architecture effect on the T_g-reductions is consistent with the T_g reductions for the dewetting from a liquid substrate reflects changes in segmental dynamics upon confinement rather than chain effects. This contrasts with changes, including increases seen in dewetting from a rigid substrate, for different molecular architectures reported in the literature.     

Is there a future for sequential chemical extraction?

Since their introduction in the late 1970s, sequential extraction procedures have experienced a rapid increase in use. They are now applied for a large number of potentially toxic elements in a wide range of sample types. This review uses evidence from the literature to consider the usefulness and limitations of sequential extraction and thereby to assess its future role in environmental chemical analysis. It is not the intention to provide a comprehensive survey of all applications of sequential extractions or to consider the merits and disadvantages of individual schemes. These aspects have been covered adequately in other, recent reviews. This review focuses in particular on various key issues surrounding sequential extractions such as nomenclature, methodologies, presentation of data and interpretation of data, and discusses typical applications from the recent literature for which sequential extraction can provide useful and meaningful information. Also covered are emerging developments such as accelerated procedures using ultrasound- or microwave energy-assisted extractions, dynamic extractions, the use of chemometrics, the combination of sequential extraction with isotope analysis, and the extension of the approach to non-traditional analytes such as arsenic, mercury, selenium and radionuclides.     

Is there a "chain-foldability" effect without neighboring moiety-assistance?

The coaggregating tendencies of five methyl esters of saturated or uusaturated single-chain carboxylic acids,namely,octanoic (8-Me),dodecanoic (12-Me),stearic (18-Me),oleic (c-18:l-Me) and elaidic (t-18:l-Me) acids,were investigated in five aquiorgano binary solvent systems,V/V DX-H2O (φ=0.50,0.45,0.40,0.30,0.20),using naphthylethyl octyl(Np-8),dodecyl (Np-12),hexadecyl (Np-16) ether as fluorescent probes.The orders of their coaggregating tendencies changed in solvents of different φvalues.Our results could be rationalized by a chain-foldability effect without neighboring moiety participation.     

Z/E isomerism in Nα-Nα-disubstituted hydrazides and the amidoxy bond: application to the conformational analysis of pseudopeptides built of hydrazinoacids and α-aminoxyacids

We have investigated the Z/E isomerism of the hydrazide link (CO-NH-N) and amidoxy link (CO-NH-O). The study was first focused on small molecular models using NMR and X-ray diffraction. It allowed determination of simple NMR criterions to differentiate easily the Z and E forms, which were then applied to investigate the behavior of these links inside the corresponding oligomers. Our results concerning the hydrazide link corroborate pioneering work that had been done in the 1970s except in the case were it is located inside aza-β(3)-cyclopeptides, where the old empirical rules failed to predict the right geometry of the link. The geometrical preference of the amidoxy bond is also unambiguously established and differs clearly from recent theoretical calculations. Our findings help rationalize the close self-organization ability of aza-β(3)-peptides and α-aminoxypeptides, two recently described foldamers.     

Is there computational support for an unprotonated carbon in the E4 state of nitrogenase?

In the key enzyme for nitrogen fixation in nature, nitrogenase, the active site has a metal cluster with seven irons and one molybdenum bound by bridging sulfurs. Surprisingly, there is also a carbon in the center of the cluster, with a role that is not known. A mechanism has been suggested experimentally, where two hydrides leave as a hydrogen molecule in the critical E₄ state. A structure with two hydrides, two protonated sulfurs and an unprotonated carbon has been suggested for this state. Rather recently, DFT calculations supported the experimental mechanism but found an active state where the central carbon is protonated all the way to CH₃. Even more recently, another DFT study was made that instead supported the experimentally suggested structure. To sort out the origin of these quite different computational results, additional calculations have here been performed using different DFT functionals. The conclusion from these calculations is very clear and shows no computational support for an unprotonated carbon in E₄. © 2017 Wiley Periodicals, Inc.     

Is there a unique melting temperature for two-state proteins?

Thermal unfolding (or folding) in many proteins occurs in an apparent two-state manner, suggesting that only two states, unfolded and folded, are populated. At the melting temperature, Tm, the two states coexist. Using lattice models with side chains we show that individual residues become structured at temperatures that deviate from Tm, which implies that partially folded conformations make substantial contribution to thermodynamic properties of two-state proteins. We also find that the folding cooperativity for a given residue is linked to its accessible surface area. These results are consistent with the experiments on GCN4-like zipper peptide, which showed that local melting temperatures differ from Tm. Analysis of thermal unfolding of six proteins shows that deltaT/Tm approximately N(-1), where deltaT is the transition width and N is the number of residues. This scaling allows us to conclude that, when corrected for finite size effects, folding cooperativity can be captured using coarse grained models.     

Is there a dipolar interaction of like charges in thin aqueous films?

The dipole moment and the screened interaction of interfacial charges are discussed. We compare this interaction with that of parallel dipoles and show that the dipolar picture fails for charges in thin aqueous films.     

Can acyclic conformational control be achieved via a sulfur–fluorine gauche effect?

The gauche conformation of the 1,2-difluoroethane motif is known to involve stabilising hyperconjugative interactions between donor (bonding, σ_C–H) and acceptor (antibonding, σ*C–F) orbitals. This model rationalises the generic conformational preference of F–C_β–C_α–X systems (φ_FCCX ≈ 60°), where X is an electron deficient substituent containing a Period 2 atom. Little is known about the corresponding Period 3 systems, such as sulfur and phosphorus, where multiple oxidation states are possible. Conformational analyses of β-fluorosulfides, -sulfoxides and -sulfones are disclosed here, thus extending the scope of the fluorine gauche effect to the 3rd Period (F–C–C–S(O)_n; φ_FCCS ≈ 60°). Synergy between experiment and computation has revealed that the gauche effect is only pronounced in structures bearing an electropositive vicinal sulfur atom (S⁺–O^–, SO₂).     

Cationic polymerization of dienes VIII: Is the elimination of cross-linking by a bulky electron donor a general behavior in the presence of aluminium trichloride?

Previous works on the polymerization of 1,3-pentadiene initiated by aluminium trichloride in non polar solvent at room temperature in the presence of bulky electron donor (ED) as tri-p-tolylamine have highlighted a stabilization of the polymerizing actives centres by ED, which allowed a reduction of some side reactions and the formation of more precisely defined polypentadienes than ever by cationic polymerization in non polar medium. The aim of this research was to investigate the role of bulky EDs such as tri-p-tolylamine and similar compounds in polar medium in order to obtain if possible a complete control of the polymerization of isoprene and 1,3-pentadiene. The beneficial effect of tri-p-tolylamine was shown in the case of isoprene polymerization at room temperature, with an important reduction of the cross-linked fraction for long reaction times and strong reduction of termination reactions. At −30 °C in the presence of tri-p-tolylamine, polypentadienes more controlled than in non polar solvent could be obtained, with a nearly complete elimination of the cross-linked fraction, while keeping the microstructure approximately constant.     

Is anthracene cofactor or spectator for the thermolysis of anthracenyl acylnitroso cycloadducts in the presence of a diene?

Acylnitroso species are very reactive dienophiles broadly used in organic synthesis. They are traditionally prepared from the oxidation of hydroxamic acids at low temperature. When this strategy fails, acylnitroso compounds may be generated from other stable sources, for example, their (9,10-dimethyl)anthracenyl cycloadducts. The thermolysis of such compounds performed in the presence of a diene leads generally to the corresponding hetero Diels-Alder (HD-A) cycloadduct in high yield with concomitant release of anthracene. The most widely accepted mechanism is the postulated slow release of acylnitroso via a retro Diels-Alder process, although the anthracenyl acylnitroso cycloadducts bear intrinsic structural features suggesting another pathway, that is, an exchange process. By using computational chemistry, we have hereby clarified the exact role of anthracene in this reaction.     

Is there a higher-order mode coupling transition in polymer blends?

We present simulations on a binary blend of bead-spring polymer chains. The introduction of monomer size disparity yields very different relaxation times for each component of the blend. Competition between two different arrest mechanisms, namely, bulklike dynamics and confinement, leads to an anomalous relaxation scenario for the fast component, characterized by sublinear time dependence for mean squared displacements, or logarithmic decay and convex-to-concave crossover for density-density correlators. These anomalous dynamic features, which are observed over time intervals extending up to 4 decades, strongly resemble predictions of mode coupling theory for nearby higher-order transitions. Chain connectivity extends anomalous relaxation over a wide range of blend compositions.     

An unexpected directing effect in the asymmetric transfer hydrogenation of α,α-disubstituted ketones

α,α-Disubstituted ketones containing an aromatic ring or alkene are reduced in high enantiomeric excess using an asymmetric transfer hydrogenation catalyst. The sense of reduction indicates that the unsaturated region of the ketone adopts a position adjacent to the Ru-bound η(6)-arene ring in the reduction transition state.     

Is there a minimum size for aqueous doubly charged metal cations?

A major feature of the chemistry of multiply charged solvated metal ions is dissociative charge transfer. It happens because the second ionization potential (IP) of a metal atom usually exceeds the first IP of a solvent molecule. This raises the issue of whether there is a minimum number of ligands below which the species would charge-separate spontaneously. To elucidate this, doubly charged aqueous cations of most common divalent metals (group 2 elements Mn, Fe, Co, Ni, Zn, Cd, and Cu) have been generated using electrospray and examined by collision-induced dissociation in a triple-quadrupole mass spectrometer. We have clearly observed the monoaqua complexes for all aforementioned doubly charged metal ions, except Be for which the smallest complex found is the dihydrate. We have also systematically revisited the matter of critical size--the maximum number of ligands at which dissociative charge transfer is competitive with simple ligand loss.