Application of complex reaction kinetic models in thermal analysis

The complexity of the phenomena which arise during the heating of the various substances seldom can be described by a single reaction kinetic equation. As a consequence, sophisticated models with several unknown parameters have to be developed. The determination of the unknown parameters and the validation of the models requires the simultaneous evaluation of whole series of experiments. We can accept a model and its parameters if, and only if we get a reasonable fit to several experiments carried out at different experimental conditions. In the field of the thermal analysis the method of least squares alone seldom can select abest model or abest set of parameter values. Nevertheless, the careful evaluation of the experiments may help in the discerning between various chemical or physical assumptions by the quality of the corresponding fit between the experimental and the simulated date. The problem is illustrated by the thermal de-composition of cellulose under various experimental conditions.This research program was funded by the National Science Foundation (grant INT 8914934), the US Hungarian Science and Technology Joint Fund (grants 90b-22 and 93b-375), the Hungarian National Research Fund (OTKA, grant 3077/91) and the Coral Industries Endowment.     

Substituted 1,2-Thiazetidine 1,1-Dioxides. Synthesis of (<Emphasis Type="Italic">RS</Emphasis>)- and (<Emphasis Type="Italic">S</Emphasis>)-1,2-Thiazetidine-3-acetic Acid 1,1-Dioxide and its Reactions with Amino Acids and Dipeptides

Summary. (RS)-2-tert-Butyldimethylsilyl-1,2-thiazetidine-3-acetic acid 1,1-dioxide prepared from (RS)-S-benzyl--homocysteine was condensed via DCC/NHS with various L-amino acid esters or dipeptide esters yielding N-silylated -sultam peptides. A -sultam active ester was isolated as an intermediate. Desilylation with TBAF in THF yielded stable N-unsubstituted products, and deprotection of the benzyl esters was achieved by catalytic hydrogenation. (S)-S-Benzyl--homocysteine was obtained by fractional crystallization of the brucine salt of the racemate and transformed into benzyl (S)-1,2-thiazetidine-3-acetate, which was on the other hand synthesized by an enantiospecific route from -benzyl Boc-L-aspartate. Some -sultam peptides were prepared from the (S)-enantiomer, and finally some -sultam peptides containing D-Ala units were obtained.     

Thermochemical data of barium peroxide from thermogravimetric measurements

Decomposition temperatures of barium peroxide in equilibrium with the oxygen pressure in the gas phase have been determined by thermogravimetric measurements at temperatures from 670 to 843. Enthalpies and entropies of the reaction BaO₂= =BaO+0.5O₂ and of the formation of BaO₂ have been calculated.

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Zusammenfassung Zersetzungstemperaturen von mit dem Sauerstoffdruck in der Gasphase in Gleichgewicht stehendem Bariumperoxyd wurden thermogravimetrisch im Temperaturbereich von 670 bis zu 843 ermittelt. Enthalpie- und Entropiewerte der Reaktion BaO₂= =BaO+0.5 O₂ und der Bildung von BaO₂ wurden errechnet.

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Résumé On a déterminé les températures de décomposition du peroxyde de baryum en équilibre avec la pression d'oxygène de la phase gazeuse, par mesures thermogravimétriques entre 670 et 843. On a calculé les enthalpies et les entropies correspondant à la réaction BaO₂=BaO+0.5 O₂ et à la formation de BaO₂.

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, , , 670 843. BaO₂= + +0.5 O₂ BaO₂.

     

Rotational brownian motion and fluorescence intensify fluctuations

A theory, which connects rotational brownian motion with intensity fluctuations of the light emitted from fluorescent molecules excited by linearly polarized light, is given. Analysis of rotational diffusion in this way does not depend on the close relationship between fluorescence lifetime and rotational relaxation times, which is necessary in present methods and thus makes an enlarged time range available for fluorescence spectroscopy.When short fluorescence lifetimes are used the rotational diffusion of the molecule in its ground state will be observed.     

Many-to-many location-routing with inter-hub transport and multi-commodity pickup-and-delivery

In this paper, we consider a variant of the many-to-many location-routing problem, where hub facilities have to be located and customers with either pickup or delivery demands have to be combined in vehicle routes. In addition, several commodities and inter-hub transport processes are taken into account. A practical application of the problem can be found in the timber-trade industry, where companies provide their services using hub-and-spoke networks. We present a mixed-integer linear model for the problem and use CPLEX 12.4 to solve small-scale instances. Furthermore, a multi-start procedure based on a fix-and-optimize scheme and a genetic algorithm are introduced that efficiently construct promising solutions for medium- and large-scale instances. A computational performance analysis shows that the presented methods are suitable for practical application.     

The Effect of Lipid Composition on the Permeability of Fluorescent Markers from Photosensitized Membranes

There is evidence indicating that the cellular locus of PDT action by amphiphilic sensitizers are the cellular membranes. The photosensitization process causes oxidative damage to membrane components that can result in the cell's death. However, it was not yet established whether lipid oxidation can cause free passage of molecules through the membrane and, as a result, be the primary cause of the cell's death. In this work, we studied the effect of liposomes' lipid composition on the kinetics of the leakage of three fluorescent dyes, calcein, carboxyfluorescein and DTAF, which were trapped in the intraliposomal aqueous phase, after photosensitization with the photosensitizer deuteroporphyrin. We found that as the degree of fatty acid unsaturation increased, the photosensitized passage of these molecules through the lipid bilayer increased. We also found that the rate of leakage of these molecules was affected by their size and bulkiness as well as by their net electric charge. In liposomes that are composed of a lipid mixture similar to that of natural membranes, the observed passage of molecules through the membrane is slow. Thus, the photodynamic damage to lipids does not appear to be severe enough to be an immediate, primary cause of cell death in biological photosensitization.     

Total Synthesis of a Partial Structure from Arabinogalactan and Its Application for Allergy Prevention

Arabinogalactan, a microheterogeneous polysaccharide occurring in plants, is known for its allergy-protective activity, which could potentially be used for preventive allergy treatment. New treatment options are highly desirable, especially in a preventive manner, due to the constant rise of atopic diseases worldwide. The structural origin of the allergy-protective activity of arabinogalactan is, however, still unclear and isolation of the polysaccharide is not feasible for pharmaceutical applications due to a variation of the activity of the natural product and contaminations with endotoxins. Therefore, a pentasaccharide partial structure was selected for total synthesis and subsequently coupled to a carrier protein to form a neoglycoconjugate. The allergy-protective activity of arabinogalactan could be reproduced with the partial structure in subsequent in vivo experiments. This is the first example of a successful simplification of arabinogalactan with a single partial structure while retaining its allergy-preventive potential.     

Heterocycles from α‐Aminonitriles

Owing to their various modes of reactivity, α‐aminonitriles represent versatile building blocks for the construction of a wide range of nitrogen heterocycles. The present Concept article focuses on synthetic methodologies using their bifunctional nature which is the basis of their reactivity as α‐amino carbanions and as iminium ions. Reactions exclusively taking place on either the amine or on the nitrile moiety will not be considered.