Point charge models for LiH,CH4, and H2O

Point charge models for LiH. CH₄, and H₂O are presented. The models preserve the correct total charge and dipole moment of the molecules. Relations between spherical Gaussian wave function values and point charge model values of a variety of one-electron molecular properties are derived. The errors inherent in some of the point charge model values are of two types: those which may be large but are easily evaluated and those which are small and diminish rapidly as the distance from the molecule increases. The models are shown to be a reliable means of calculating one-electron properties and possible uses of the models are suggested.     

Trigger mechanism for the catalytic hydrogen activation by monoiron (iron-sulfur cluster-free) hydrogenase

A fully optimized model for the resting state of the active site of the third type of phylogenetically unrelated (monoiron) hydrogenase, iron-sulfur cluster-free hydrogenase (Hmd), was constructed based on density functional calculations. This resting state structure shows good agreement with the experimental IR spectra. The calculations predict that the barrier for H2 cleavage in the presence of MPT+ is 18 kcal/mol lower than that in the absence of MPT+, a result that explains why the isotopic H2/D2O exchange catalyzed by Hmd is strictly dependent on the presence of MPT+. This difference is a result of the MPT+ triggering the pyridone to provide electron density to allow the Fe to take a proton while transferring a hydride to the MPT+. These active site models and catalytic mechanism are useful in understanding this hydrogen activation for the design of novel hydrogenation catalysts and for low cost, high efficiency hydrogen generation.     

Regioselective (12)CO/(13)CO exchange activity of a mixed-valent Fe(ii)Fe(i) model of the H(ox) state of [FeFe]-hydrogenase

A mixed-valent Fe(ii)Fe(i) model of the H(ox) state of [FeFe]-hydrogenase is shown, under certain conditions, to exhibit regioselective (12)CO/(13)CO exchange activity similar to that observed for H(ox).     

Biotemplated synthesis of an ordered macroporous superconductor with high critical current density using a cuttlebone template

Periodic superconducting, porous self-supporting monoliths were synthesized using cuttlebone as a morphological template; this produced a lightweight, structurally stable superconductor with a greatly improved critical current density.     

Use of thin collodion films to prevent recoil-ion contamination of alpha-spectrometry detectors

Recoil ions from alpha-particle emission can contaminate surface-barrier detection systems. This contamination results in increased measurement uncertainty, and may require the replacement of expensive detectors. Disposable thin Collodion films are easily prepared and effectively retard the recoil ions when either directly applied to the surface of alpha-sources or as catcher foils between the source and the detector. The thin films are particularly effective for relatively low-level sources, but can sustain structural damage when exposed to high levels of recoil ions (tens of thousands per second) over extended periods of time.     

Comment on "Free energy simulations of single and double ion occupancy in gramicidin A" [J. Chem. Phys. 126, 105103 (2007)

In a recent article published by Bastug and Kuyucak [J. Chem. Phys.126, 105103 (2007)] investigated the microscopic factors affecting double ion occupancy in the gramicidin channel. The analysis relied largely on the one-dimensional potential of mean force of ions along the axis of the channel (the so-called free energy profile of the ion along the channel axis), as well as on the calculation of the equilibrium association constant of the ions in the channel binding sites. It is the purpose of this communication to clarify this issue.     

Benzoboroxoles as efficient glycopyranoside-binding agents in physiological conditions: structure and selectivity of complex formation

In contrast to normal boronic acids, o-hydroxymethyl phenylboronic acid (benzoboroxole) has the capability of complexing glycopyranosides efficiently in neutral water. The measurement of association constants with a panel of model hexopyranosides indicates that the preferred mode of binding is through a cis-3,4-diol, such as that found in galactopyranosides, and mass spectrometric studies support a 1:1 binding stoichiometry. The complexation of glucopyranosides is weaker, and they are bound through their 4,6-diol unit. Although several factors may explain the exceptional carbohydrate-binding behavior of this class of hemiboronic acids, the relatively high Lewis acidity of benzoboroxoles is a likely contributing factor along with subtle factors such as intramolecular hydrogen bonds with other hydroxyl groups in the resulting anionic complex. These results with hexopyranosides suggest that biologically relevant cell-surface oligosaccharides could be targeted in water using oligomeric benzoboroxole receptors.     

Presentations of some 3-transposition groups

We classify all 3-transposition groups which are generated by at most five of their 3-transpositions. Modulo a center these are 27 specific groups plus various quotients of a particular group of order 2(3⁴⁹). For each of the 27 groups at least one presentation is given. We also give presentations for many groups of importance in the recent classification of 3-transposition groups with trivial center. Our presentations include ones for the three sporadic 3-transposition groups of Fischer, each on a 3-transposition generating set of minimal size.