Simulating the Shroud of Turin: A Laboratory Experiment

The Shroud of Turin, a piece of linen cloth bearing an anatomically correct image of a crucified human being, which resembles Jesus of Nazareth, has been an enigma to scientists. While recent studies including radiocarbon dating suggest the cloth is a medieval relic, few if any proposals have been put forward that satisfactorily explain how the image was generated. In combination with a lecture on the scientific method and the problem of bias, a laboratory experiment has been developed that allows science students to attempt to simulate the image on the Shroud. The experiment involves an active-learning experience in which students discover which techniques do not work to generate the image and which begin to suggest how such an image could have been generated.     

Stochastic and Deterministic Simulation of Nonisothermal Crystallization of Polymers

This paper is devoted to the numerical simulation of nonisothermal crystallization of polymers, which may be modelled as a stochastic birth-and-growth process. One of the main aims is to develop efficient algorithms for the stochastic simulation of such process. We put a special emphasis on the problem of computing the surface density of crystals, which is an important factor for the mechanical properties of the solidified material. Moreover, an averaged deterministic model, designed as an approximation in the case of many small crystals (which is very frequent in industrial applications), is presented, and the results of numerical simulations are compared with the corresponding simulations of the stochastic model.     

Comparison of different approaches to estimate the uncertainty of a liquid chromatographic assay

A measurement result cannot be properly interpreted without knowledge about its uncertainty. Several concepts to estimate the uncertainty of a measurement result have been developed. Here, four different approaches for uncertainty estimation are compared on the example of the RP-high-performance liquid chromatography (HPLC) assay for tylosin for veterinary use: the guide to the expression of uncertainty in measurement (GUM) approach, which derives the uncertainty of a measurement result by combining the uncertainties related to the uncertainty sources of the measurement process; the top-down approach, which uses the reproducibility estimate from an inter-laboratory study as uncertainty estimate; an approach recently presented by Barwick and Ellison, which combines precision, trueness and robustness data to obtain an uncertainty estimate of the measurement result and finally a further approach, which directly estimates the measurement uncertainty from a robustness test. The comparison shows that the different approaches lead to comparable uncertainty estimates.     

Surface modification of silicon nanowires via copper-free click chemistry

A two-step process based on copper-free click chemistry is described, by which the surface of silicon nanowires can be functionalized with specific organic substituents. A hydrogen-terminated nanowire surface is first primed with a monolayer of an α,ω-diyne and thereby turned into an alkyne-terminated, clickable platform, which is subsequently coupled with an overlayer of an organic azide carrying the desired terminal functionality. The reactive, electron-deficient character of the employed diyne enabled a quantitative coupling reaction at 50 °C without metal catalysis, which opens up a simple and versatile route for surface functionalization under mild conditions without any potentially harmful additives.     

Development of an organo- and enzyme-catalysed one-pot,sequential three-component reaction

A one-pot, three-component process is described which involves both organo- and enzyme-catalysed carbon–carbon bond-forming steps. In the first step, an organocatalyst catalyses the aldol reaction between acetaldehyde and a glyoxylamide. After dilution with additional aqueous buffer, and addition of pyruvate and an aldolase enzyme variant, a second aldol reaction occurs to yield a final product. Crucially, it was possible to develop a reaction in which both the organo- and enzyme-catalysed reactions could be performed in the same aqueous buffer system. The reaction described is the first example of a one-pot, three-component reaction in which the two carbon–carbon bond-forming processes are catalysed using the combination of an organocatalyst and an enzyme.     

Geometry-selective synthesis of E or Z N-vinyl ureas (N-carbamoyl enamines)

N-Vinyl ureas are emerging as a valuable class of compounds with both nucleophilic and electrophilic reactivity. They may be made by capturing the enamine tautomer of an imine with an isocyanate, a reaction which in general leads to the E isomer of the vinyl urea. Deprotonation of such a vinyl urea, or of an allyl urea, generates a dipole stabilized Z-allyl anion which may be protonated to return the Z-vinyl urea. Isomerization of an allyl urea with a Ru complex provides an alternative route to E-vinyl ureas.     

A one-pot synthesis of 1,2,4,5-tetraarylimidazoles using molecular iodine as an efficient catalyst

Molecular iodine is a cheap, nontoxic catalyst, which acts as an efficient catalyst for the synthesis of 1,2,4,5-tetraarylimidazoles using benzoin, an aromatic aldehyde and an amine in the presence of ammonium acetate. The advantage of this method is the direct use of benzoin, which is transformed into benzil in situ and condenses further to generate the imidazole in a one-pot sequence.     

An efficient approach for the calculation of Franck-Condon integrals of large molecules

A general and efficient approach for the calculation of Franck-Condon integrals (FCIs) of large molecules is presented. In a first step, by exploiting the diagonally dominant and sparse structure of the Duschinsky matrix, a model system is constructed for which the Duschinsky matrix takes a block-diagonal form. For each of these blocks separately, the FCIs are calculated discarding all below a certain threshold. From those integrals retained the FCIs of the model system are obtained by simple multiplication. These serve as an estimate for the FCIs of the exact system which are calculated for those integrals which lie above a certain threshold. By systematically decreasing the threshold, the simulation can be reliably converged to the exact result with an arbitrary accuracy. Using this scheme, a considerable reduction of the number of FCIs which have to be calculated is achieved which leads to an improved scaling behavior of the computational effort with system size. The approach has been tested thoroughly for a set of molecules including difficult cases. For the larger systems a speedup of up to three orders of magnitude compared to an exact calculation is observed while the errors can be kept negligible. With this approach accurate calculations of FCIs are feasible also for large molecules encountered in "real-life" chemistry, especially biochemistry and material science.     

Porous Electrodes with Immobilized Enzymes: The Problem of Development of Nanocomposites with High Concentrations of Molecules of Active Enzymes

The currents that are generated in a porous electrode with an immobilized enzyme increase with increasing concentration of molecules of an electrochemically active enzyme. However, a finely divided composite, which is manufactured from colloidal particles of a support that have nanodimensions and molecules of the enzyme with the aid of methods of colloid chemistry, has a peculiar structure: it consists of a set of fractal clusters, which are capable of adsorbing only a limited number of enzyme molecules. The paper is devoted to computer simulation of all the stages of immobilization of the enzyme, specifically, producing random fractal clusters of required dimensions and deploying molecules of the enzyme on them. An analysis of the link of the concentration of molecules of an active enzyme with the structure and characteristics of a porous composite makes it possible to give an interpretation to experimental facts obtained by other authors for an oxygen electrode consisting of finely divided colloidal graphite and laccase.     

Effects of ionic dissociation of membrane polymer on pervaporation performance

Pervaporation of a water/alcohol mixture through a membrane which has ion-exchange capacity has been investigated. Theoretical equations are introduced which relate the degree of ionic dissociation of the polymer to quantities of water and alcohol dissolved in the polymer. From these equations, an equation for selective dissolution R is derived which does not contain an explicit term for ionic dissociation. Dissociation affects selective dissolution only by changing the degree of swelling of the polymer. Reformulating R asymptotically obtains a reciprocal relationship between permselectivity and permeability for a water-selective membrane. Experiments to check the validity of the relationship have been carried out using chitosan membranes neutralized by several acids. The effect of degree of neutralization also has been investigated. Results can be well understood on the supposition that ionic dissociation depends upon the water/alcohol composition, the kind of acid, and the degree of neutralization. Experimental results indicate that the reciprocal relationship is maintained over an appropriate range of feed compositions which confirm the validity of the theoretical equations for the swelling equilibrium of an ionic membrane. © 1994 John Wiley & Sons, Inc.     

Greener Synthesis of Pristane by Flow Dehydrative Hydrogenation of Allylic Alcohol Using a Packed-Bed Reactor Charged by Pd/C as a Single Catalyst

Our previous work established a continuous-flow synthesis of pristane, which is a saturated branched alkane obtained from a Basking Shark. The dehydration of an allylic alcohol that is the key to a tetraene was carried out using a packed-bed reactor charged by an acid–silica catalyst (HO-SAS) and flow hydrogenation using molecular hydrogen via a Pd/C catalyst followed. The present work relies on the additional propensity of Pd/C to serve as an acid catalyst, which allows us to perform a flow synthesis of pristane from the aforementioned key allylic alcohol in the presence of molecular hydrogen using Pd/C as a single catalyst, which is applied to both dehydration and hydrogenation. The present one-column-two-reaction-flow system could eliminate the use of an acid catalyst such as HO-SAS and lead to a significant simplification of the production process.     

激光诱导铝纳米膜熔化机理的分子动力学模拟

采用耦合一维双温模型的分子动力学方法研究了纳米级的铝膜在飞秒激光辐照下的熔化机制.这种方法不仅能够在原子水平上展现金属膜的各种微观行为,还能有效地描述金属膜的激光能量吸收、传递和金属电子热传导等过程.模拟结果表明,与其它金属相比,铝膜在飞秒激光辐照下的电子温度、晶格温度以及内部压力等呈现出不同的变化.铝膜在较高强度激光辐照下会很快发生全局一致的熔化,这与镍膜上下非均匀的熔化不同.并且由于铝的电子-声子耦合强度较高导致铝膜较镍膜和金膜熔化得更快.模拟结果显示,铝膜的熔化时间与实验测量的超快激光诱导的铝膜熔化时间一致.进而从理论上支持激光诱导的铝膜熔化是一个热力学熔化过程.     

Simultaneous synthesis of both rings of chromenes via a benzannulation/o-quinone methide formation/electrocyclization cascade

A new route to the chromene ring system has been developed which involves the reaction of an α,β-unsaturated Fischer carbene complex of chromium with a propargyl ether bearing an alkenyl group on the propargylic carbon. This transformation involves a cascade of reactions that begins with a benzannulation reaction and is followed by the formation of an o-quinone methide, and finally results in the emergence of a chromene upon an electrocyclization. This reaction was extended to provide access by employing an aryl carbene complex. This constitutes the first synthesis of chromenes in which both rings of the chromene system are generated in a single step and is highlighted in the synthesis of lapachenole and vitamin E.     

Structure and Properties of Polymer Biocomposite Materials

Results of studying the structure and properties of biocomposite materials are summarized. The materials in question include an enzyme (laccase, peroxidase), an ion- or electron-conducting polymer (Nafion, polymethylpyrrole), and a carbon substrate (compact, disperse). It is shown that the orientation of a large number of enzyme molecules in an enzyme/Nafion composite material on the substrate surface favors direct bioelectrocatalysis. During co-immobilization of an enzyme and polymethylpyrrole, conditions are realized under which the polymer takes part in the electron transfer between the active center of the enzyme and the surface of the electroconducting substrate. A fresh approach to constructing a biocomposite material is developed. The material is based on an extremely finely divided carbon material (colloidal graphite), which ensures a high specific activity of laccase immobilized on it. The size of colloidal-graphite particles is commensurate with that of the laccase molecule, owing to which the enzyme macromolecule is surrounded by carbon particles. As a result, practically all adsorbed enzyme molecules are electrochemically active and participate in direct bioelectrocatalysis.     

A general process for the haloamidation of olefins. Scope and mechanism

A methodology is described for the addition of a bromine atom and an amide nitrogen in a trans sense to an olefinic double bond. The process, which is illustrated by numerous examples, involves the use of an N-bromoamide and a Lewis acid as a source of Br+ which reacts with the olefin. The amide group is derived from a nitrile and a water molecule which serve as nucleophiles for the overall three-component reaction. The bromoamidation is general for a broad range of olefins and nitriles. This reaction pathway provides access not only to vicinal bromoamides but also to N-acyl aziridines and oxazolines. From these, many types of amines and amino alcohols can be prepared. Examples are provided which delineate diastereo- and regioselectivity preferences. An analogous chloroamidation reaction is also described.     

Enantioselective Total Synthesis of Cymoside through a Bioinspired Oxidative Cyclization of a Strictosidine Derivative

The first total synthesis of the caged monoterpene indole alkaloid cymoside is reported. This natural product displays a unique hexacyclic‐fused skeleton whose biosynthesis implies an early oxidative cyclization of strictosidine. Our approach to the furo[3,2‐b]indoline framework relied on an unprecedented biomimetic sequence which started by the diastereoselective oxidation of the indole ring into a hydroxyindolenine which triggered the addition of an enol ether and was followed by the trapping of an oxocarbenium intermediate.     

A framework for the evaluation of chemical structure databases

Access to desk-top structure and reaction databases through applications such as Chemical Abstracts' SciFinder, MDL's Beilstein CrossFire, and ISIS Reaction Browser has led to changes in information seeking habits of research chemists, the impact of which has implications when database purchasing decisions are made. A semiquantitative assessment is proposed which takes into account key aspects of structure and reaction databases. Assessment criteria are identified which can be weighted according to an organization's information needs. Values are then assigned to criteria for each data source, after which a formula is applied which leads to an indication of the relative value of systems under consideration. The formula takes into account the cost of database products and also the incremental benefit of adding a new system to an existing collection. This work is presented as a generic approach to the evaluation of databases and is not limited in scope to only structure and reactions databases.     

Synthesis of purine-acridine hybrid molecules related to artificial endonucleases

In the course of a program devoted to the synthesis of artificial endonucleases, we have previously reported a series of hybrid molecules in which a purine is linked to an intercalating drug by a polyamino chain. These molecules recognize and cleave selectively abasic sites in DNA with very high efficiency. In order to get insight into the mechanism of recognition and cleavage, we have prepared a new series of molecules in which the purine is linked to an amino-acridine by an aliphatic chain containing amido or/and amino groups. The key intermediates are -halo-ω-amino polyaza which may be of general use as linkers in bioconjugate chemistry.     

An assembly concept for the consecutive introduction of unsymmetrical disulfide bonds: synthesis of a releasable multidrug conjugate of folic acid

We describe the development of methodology which allows for the introduction of a second disulfide bond into a molecular framework with a pre-existing disulfide linker system. Compounds which contain an S-9-fluorenylmethyl-protected thiol and an additional disulfide linkage are deprotected in situ and trapped with an activated thiophile. This methodology allowed for the synthesis of the first molecule possessing two different biologically active agents covalently attached to a folate receptor targeting ligand unit via two disulfide-based release systems.