Molecular dynamics simulation of the equilibrium liquid–vapor interphase with solidification

The equilibrium structure of the finite, interphase interfacial region that exists between a liquid film and a bulk vapor is resolved by molecular dynamics simulation. Argon systems are considered for a temperature range that extends below the melting point. Physically consistent procedures are developed to define the boundaries between the interphase and the liquid and vapor phases. The procedures involve counting of neighboring molecules and comparing the results with boundary criteria that permit the boundaries to be precisely established. Two-dimensional radial distribution functions at the liquid and vapor boundaries and within the interphase region demonstrate the physical consistency of the boundary criteria and the state of transition within the region. The method developed for interphase boundary definitions can be extended to nonequilibrium systems. Spatial profiles of macroscopic properties across the interphase region are presented. A number of interfacial thermodynamic properties and profile curve-fit parameters are tabulated, including evaporation/condensation coefficients determined from molecular flux statistics. The evaporation/condensation coefficients away from the melting point compare more favorably with transition state theory than those of previous simulations. Near the melting point, transition theory approximations are less valid and the present results differ from the theory. The effects of film substrate wetting on evaporation/condensation coefficients are also presented.     

Toward the assembly of heparin and heparan sulfate oligosaccharide libraries: efficient synthesis of uronic acid and disaccharide building blocks

The monosaccharide moieties found in heparin (HP) and heparan sulfate (HS), glucosamine and two kinds of uronic acids, glucuronic and iduronic acids, were efficiently synthesized by use of glucosamine hydrochloride and glucurono-6,3-lactone as starting compounds. In the synthesis of the disaccharide building block, the key issues of preparation of uronic acids (glucuronic acid and iduronic acid moieties) were achieved in 12 steps and 15 steps, respectively, without cumbersome C-6 oxidation. The resulting monosaccharide moieties were utilized to the syntheses of HP/HS disaccharide building blocks possessing glucosamine-glucuronic acid (GlcN-GlcA) or iduronic acid (GlcN-IdoA) sequences. The disaccharide building blocks were also suitable for further modification such as glycosylation, selective deprotection, and sulfation.     

5‐(3,4‐Dimethoxybenzyl)‐7‐isopropyl‐1,3,5‐triazepane‐2,6‐dione acetonitrile solvate refined using a multipolar atom model

The crystal structure of the title compound, C₁₆H₂₃N₃O₄·CH₃CN, was refined using a multipolar atom model transferred from an experimental electron‐density database. The refinement showed some improvement in crystallographic statistical indices compared with the independent atom model. The triazepane ring adopts a twist‐boat conformation. In the crystal structure, the molecule forms intermolecular contacts with 14 different neighbours. There are two N—H...O and one C—H...O intermolecular hydrogen bond.     

The facile synthesis of an aldehyde-containing graft copolymer membrane for covalent protein capture with retention of protein functionality

The immobilization of biomolecules onto an insoluble carrier surface has always been a subject of great interest to enhance their resistance to pH and temperature, which aids in an increased activity lifespan as well as easy reuse of the said biomolecules. However, traditional methods are only able to provide single-layer biomolecular binding and require multiple chemical reactions to prepare the final substrate before the immobilization can be carried out properly. Here we report a facile one-step chemical synthesis of a new aldehyde-bearing graft copolymer via atom transfer radical polymerization (ATRP) for covalent protein capture in a multilayered approach to covalently capture bovine serum albumin (BSA) onto a polymeric membrane. The resultant protein-bound membrane illustrated the retention of BSA's stereoselective discrimination ability by binding to an excess of 2 mol of tryptophan/mol of BSA and demonstrated an enantioresolution of a 0.184 mM racemic tryptophan mixture with a time-averaged-separation factor of 2.9.     

Origin of Stability in Branched Alkanes

The potential origins of stability in branched alkanes are investigated, paying close attention to two recent hypotheses: geminal steric repulsion and protobranching. All alkane isomers through C₆H₁₄ along with heptane and octane were investigated at the MPW1B95/6‐311++G(d,p) level. Their geminal steric repulsion, total steric repulsion, and orbital interactions were evaluated by using natural bond orbital analysis. All measures of steric repulsion fail to explain the stability of branched alkanes. The extra stability of branched alkanes and protobranching, in general, is tied to stabilizing geminal σ→σ* delocalization, particularly of the type that involves adjacent C? C bonds and, thus, preferentially stabilizes branched alkanes. This picture is corroborated by valence bond calculations that attribute the effect to additional ionic structures (e.g., CH₃⁺ :CH₂ :CH₃^? and CH₃:^? CH₂: CH₃⁺ for propane) that are not possible without protobranching.     

The influence of water on the memory effect of the amylose tris(3,5-dimethylphenyl carbamate) stationary phase

Acid/base modifiers are sometimes used as additives in normal phase elution on columns packed with CHIRALPAK^® AD^®. These modifiers affect enantioseparations in ways that are not yet fully understood for the lack of systematic studies. Shifts of the selectivity of certain pairs of enantiomers upon exposure of the column to these modifiers is amply documented. Furthermore, once the modifier has been removed from the mobile phase, the modified selectivity remains, which has been named the Memory Effect. After a column has been exposed to an eluent stream containing acidic/basic modifiers, this particular column no longer separates certain enantiomeric pairs with the same selectivity as a modifier naive column. This makes the transfer of developed methods from one to other CHIRALPAK AD columns difficult to predict, if the selectivity needs to be similar between the two columns. We selected four enantiomeric pairs for a systematic study of this Memory Effect. The selectivity of 4-chlorophenylalanine ethyl ester improves after a solution of ethanesulfonic acid (ESA) is percolated through the column. The selectivity of Propranolol and Tröger's base increases after a solution of Diiospropylamine is percolated through the column. The selectivity of Propranolol and Tröger's base enantiomers is inversely affected by percolation of the acid solution. The 4-chlorophenylalanine ethyl ester enantiomers is inversely affected by percolation of the base solution. In contrast, the selectivity of trans-stilbene oxide (TSO) is not affected by either modifier. Analytical studies of the stationary phase suggest that slow protonation/deprotonation of water molecules attached to the carbamate moiety may be responsible for the acid/base Memory Effect. To further the understanding of the effect of water on the Memory Effect, mobile phases – spiked with water (0.01–0.43%) – were used to measure changes in the Memory Effect. Finally, we showed that the influence of water on the Memory Effect can be minimized by percolating through the column a sufficiently concentrated solution of the appropriate base while using dried mobile phases.     

Real-time increase in depth of field of an uncooled thermal camera using several phase-mask technologies

Imaging systems that combine a phase mask in the pupil and digital postprocessing may have better performance than conventional ones. We have built such a system to enhance the depth of field of an uncooled thermal camera. The phase masks are binary, their structures are optimized thanks to an image quality criterion, and they have been realized with three different technologies that give equivalent results. The deconvolution postprocessing is performed in real time with a graphics processing unit. A significant increase of the depth of field of a factor 3 has been obtained.     

Adaptive optics wide-field microscopy using direct wavefront sensing

We report a technique for measuring and correcting the wavefront aberrations introduced by a biological sample using a Shack-Hartmann wavefront sensor, a fluorescent reference source, and a deformable mirror. The reference source and sample fluorescence are at different wavelengths to separate wavefront measurement and sample imaging. The measurement and correction at one wavelength improves the resolving power at a different wavelength, enabling the structure of the sample to be resolved.     

Convergence results for a class of spectrally hyperviscous models of 3-D turbulent flow

We consider the spectrally hyperviscous Navier–Stokes equations (SHNSE) which add hyperviscosity to the NSE but only to the higher frequencies past a cutoff wavenumber _m0$m_0$. In Guermond and Prudhomme (2003) [18], subsequence convergence of SHNSE Galerkin solutions to dissipative solutions of the NSE was achieved in a specific spectral-vanishing-viscosity setting. Our goal is to obtain similar results in a more general setting and to obtain convergence to the stronger class of Leray solutions. In particular we obtain subsequence convergence of SHNSE strong solutions to Leray solutions of the NSE by fixing the hyperviscosity coefficient μ$\mu$ while the spectral hyperviscosity cutoff _m0$m_0$ goes to infinity. This formulation presents new technical challenges, and we discuss how its motivation can be derived from computational experiments, e.g. those in Borue and Orszag (1996, 1998)  and . We also obtain weak subsequence convergence to Leray weak solutions under the general assumption that the hyperviscous coefficient μ$\mu$ goes to zero with no constraints imposed on the spectral cutoff. In both of our main results the Aubin Compactness Theorem provides the underlying framework for the convergence to Leray solutions.