Biaxial deformation of dough using the bubble inflation technique. II. Numerical modelling

In Part I the bubble inflation test was used to measure the stress-strain relationship of dough. A large disagreement was found between the stress-strain curve based on experimental data and the curve derived from Bloksma's analytical model. In Part II, a numerical simulation of the bubble inflation test is performed using Finite Element Analysis, in order to obtain further information regarding the accuracy of the analytical predictions. A hyperelastic model is assumed for the dough, with a strain energy potential described by the compressible form of the Mooney-Rivlin model. Four cases were investigated, corresponding to various combinations of material parameters of the Mooney-Rivlin model. The numerical results reinforce the conclusions drawn in Part I of the study, specifically that Bloksma's analysis of the bubble inflation could lead to large errors in the stress-strain curve. It was further concluded that the accuracy of the analysis was dependent on the material properties. For a neo-Hookeian material, the analysis leads to accurate results. This is because, for this material, all the assumptions made in the analysis regarding the bubble shape, the material's incompressibility and the bubble wall thickness distribution are accurate.     

Direct measurement of mixture fraction in reacting flow using Rayleigh scattering

The mixture fraction variable, , is very useful in describing reaction zone structure in nonpremixed flames. Extinction limits and turbulent mixing are often described as a function of this variable. Experimental evaluation of is critical for improving our understanding of the influence of turbulent mixing on the chemistry process. Heretofore, the evaluation of mixture fraction in combusting flow required multiple simultaneous concentration measurements. In this paper we present a fuel designed to permit measurements of mixture fraction by Rayleigh scattering technique only. A Rayleigh intensity/mixture fraction correspondence has been obtained experimentally in a laminar coflow flame. The influence of strain rate and differential diffusion effects have been investigated using laminar counterflow diffusion flame and shifting equilibrium chemistry models. The results obtained from comparisons between experiments and these models are very encouraging and suggest that the Rayleigh/mixture fraction correspondence established is valid under both the turbulent mixing and laminar strained flamelet combustion regimes.     

Tuning Mechanism‐Based Inactivators of Neuraminidases: Mechanistic and Structural Insights

3‐Fluorosialosyl fluorides are inhibitors of sialidases that function by the formation of a long‐lived covalent active‐site adduct and have potential as therapeutics if made specific for the pathogen sialidase. Surprisingly, human Neu2 and the Trypanosoma cruzi trans‐sialidase are inactivated more rapidly by the reagent with an equatorial fluorine at C3 than by its axial epimer, with reactivation following the same pattern. To explore a possible stereoelectronic basis for this, rate constants for spontaneous hydrolysis of the full series of four 3‐fluorosialosyl fluorides were measured, and ground‐state energies for each computed. The alpha (equatorial) anomeric fluorides hydrolyze more rapidly than their beta anomers, consistent with their higher ground‐state energies. However ground‐state energies do not explain the relative spontaneous reactivities of the 3‐fluoro‐epimers. The three‐dimensional structures of the two 3‐fluoro‐sialosyl enzyme intermediates of human Neu2 were solved, revealing key stabilizing interactions between Arg21 and the equatorial, but not the axial, fluorine. Because of changes in geometry these interactions will increase at the transition state, likely explaining the difference in reaction rates.     

One‐Pot Enzymatic Synthesis of Merochlorin A and B

The polycycles merochlorin A and B are complex halogenated meroterpenoid natural products with significant antibacterial activities and are produced by the marine bacterium Streptomyces sp. strain CNH‐189. Heterologously produced enzymes and chemical synthesis are employed herein to fully reconstitute the merochlorin biosynthesis in vitro. The interplay of a dedicated type III polyketide synthase, a prenyl diphosphate synthase, and an aromatic prenyltransferase allow formation of a highly unusual aromatic polyketide‐terpene hybrid intermediate which features an unprecedented branched sesquiterpene moiety from isosesquilavandulyl diphosphate. As supported by in vivo experiments, this precursor is furthermore chlorinated and cyclized to merochlorin A and isomeric merochlorin B by a single vanadium‐dependent haloperoxidase, thus completing the remarkably efficient pathway.     

Supramolecular ssDNA Templated Porphyrin and Metalloporphyrin Nanoassemblies with Tunable Helicity

Free‐base and nickel porphyrin–diaminopurine conjugates were formed by hydrogen‐bond directed assembly on single‐stranded oligothymidine templates of different lengths into helical multiporphyrin nanoassemblies with highly modular structural and chiroptical properties. Large red‐shifts of the Soret band in the UV/Vis spectroscopy confirmed strong electronic coupling among assembled porphyrin–diaminopurine units. Slow annealing rates yielded preferentially right‐handed nanostructures, whereas fast annealing yielded left‐handed nanostructures. Time‐dependent DFT simulations of UV/Vis and CD spectra for model porphyrin clusters templated on the canonical B‐DNA and its enantiomeric form, were employed to confirm the origin of observed chiroptical properties and to assign the helicity of porphyrin nanoassemblies. Molar CD and CD anisotropy g factors of dialyzed templated porphyrin nanoassemblies showed very high chiroptical anisotropy. The DNA‐templated porphyrin nanoassemblies displayed high thermal and pH stability. The structure and handedness of all assemblies was preserved at temperatures up to +85 °C and pH between 3 and 12. High‐resolution transition electron microscopy confirmed formation of DNA‐templated nickel(II) porphyrin nanoassemblies and their self‐assembly into helical fibrils with micrometer lengths.     

The mole is an Avogadro number of entities,the macroscopic unit for chemical amount

Just as the gram is an Avogadro number of daltons, g = (g/Da) Da, the current definition of the mole can be interpreted as an Avogadro number of entities, as it is often thought of by chemists—and it should be explicitly redefined this way. By introducing a hitherto missing atomic-scale unit for the amount of any substance, the entity (symbol ent), analogous to the dalton for mass, we can then define the mole as: mol = (g/Da) ent—i.e., an Avogadro number of entities. It is argued that the name for the base quantity should be “chemical amount,” by analogy with “electric current.” Recent proposals and counterproposals for redefining the mole (and renaming “amount of substance”) are critically discussed in light of the above intuitively obvious definition.     

Dynamic behavior of interfaces: Modeling with nonequilibrium thermodynamics

In multiphase systems the transfer of mass, heat, and momentum, both along and across phase interfaces, has an important impact on the overall dynamics of the system. Familiar examples are the effects of surface diffusion on foam drainage (Marangoni effect), or the effect of surface elasticities on the deformation of vesicles or red blood cells in an arterial flow. In this paper we will review recent work on modeling transfer processes associated with interfaces in the context of nonequilibrium thermodynamics (NET). The focus will be on NET frameworks employing the Gibbs dividing surface model, in which the interface is modeled as a two-dimensional plane. This plane has excess variables associated with it, such as a surface mass density, a surface momentum density, a surface energy density, and a surface entropy density. We will review a number of NET frameworks which can be used to derive balance equations and constitutive models for the time rate of change of these excess variables, as a result of in-plane (tangential) transfer processes, and exchange with the adjoining bulk phases. These balance equations must be solved together with mass, momentum, and energy balances for the bulk phases, and a set of boundary conditions coupling the set of bulk and interface equations. This entire set of equations constitutes a comprehensive continuum model for a multiphase system, and allows us to examine the role of the interfacial dynamics on the overall dynamics of the system. With respect to the constitutive equations we will focus primarily on equations for the surface extra stress tensor.     

Synthesis of 4‐Aryl‐8‐fluoro‐3a,4,5,9b‐tetrahydro‐3H‐cyclopenta[c]quinolines and Their Ozonides

4‐Aryl‐8‐fluoro‐3a,4,5,9b‐tetrahydro‐3H‐cyclopenta[c]quinolines are synthesized by acid‐catalyzed (CF₃CO₂H) three‐component cyclocondensation of 4‐fluoroaniline with aromatic aldehydes and cyclopentadiene. Stable ozonides with (1R*,4S*,5aR*,6S*,11bS*)‐configurations are obtained by ozonolysis of corresponding trifluoroacetyl derivatives.     

Achieving dynamic behaviour and thermal expansion in the organic solid state via co-crystallization

Thermal expansion involves a response of a material to an external stimulus that typically involves an increase in a crystallographic axis (positive thermal expansion (PTE)), although shrinking with applied heat (negative thermal expansion (NTE)) is known in rarer cases. Here, we demonstrate a means to achieve dynamic molecular motion and thermal expansions in organic solids via co-crystallizations. One co-crystal component is known to exhibit dynamic behaviour in the solid state while the second, when varied systematically, affords co-crystals with linear thermal expansion coefficients that range from colossal to nearly zero. Two co-crystals exhibit rare NTE. We expect the approach to guide the design of molecular solids that enable predesigned motion related to thermal expansion processes.     

Kinetic model of olefin hydroalumination by HAlBui2 and AlBui3 in the presence of Cp2ZrCl2 catalyst

The mechanism of α‐olefin hydroalumination by HAlBuⁱ₂, ClAlBuⁱ₂, and AlBuⁱ₃ in the presence of Cp₂ZrCl₂ catalyst has been studied. It was established that the key intermediate of the reaction is a bimetallic complex [Cp₂ZrH₂ · ClAlBuⁱ₂]₂, which reacts with olefins and yields higher diisobutylalkylalanes. In parallel with this stage, the key complex can readily react with XAlBuⁱ₂ (X = H, Cl, Buⁱ) and form a stable trihydride complex Cp₂ZrH₂ · ClAlBuⁱ₂ · HAlBuⁱ₂, which does not exhibit reactivity in olefin hydroalumination. A kinetic model of α‐olefin hydroalumination with HAlBuⁱ₂ and AlBuⁱ₃ has been developed. The model explains the causes of the low stability of the key intermediate [Cp₂ZrH₂ · ClAlBuⁱ₂]₂ and of the different activities of organoaluminum compounds (OACs) in the olefin hydroalumination reaction. Moreover, the model gives information about the limit stages of the reaction and explains the influence of the length of the initial olefins on the rate of the whole catalytic process. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 333–339, 2007