The kinetics of radical polymerization—XXVII. Investigation of the radical reactivity of 3,3′- and 4,4′-disubstituted azobenzenes in the polymerization of vinylacetate

A study was made on the inhibition by some 3,3′- and 4,4′-disubstituted azobenzenes of the polymerization of vinylacetate initiated by azoisobutyronitrile at 50°. The inhibitory effects of these substances can be attributed to their ability to engage in radical addition giving a less reactive hydrazyl type radical. The mechanism of the inhibition has been established by ESR and kinetic (stoichiometric) measurements. The value of k₅/k₂ (characteristic of the reactivity of an inhibitor) was determined for 9 substituents. The radical reactivity of the aromatic azo group was decreased by electron donor substituents and increased by electron acceptors. The substituent effect can be well interpreted by the Hammett equation; the value of the reaction constant was ? = +0.53.     

Qualitative and quantitative sensitivity in flame photometry

Concepts of sensitivity in flame photometry are discussed. A distinction is made between sensitivity itself, concentration limits and dilution limits applied to qualitative and quantitative analysis by flame photometry. Sensitivity values, as well as the concentration limits, are considered from two different aspects: as a function of the slope of calibration curves-percentual values-and as a function of fluctutions-fluctuational values. The concepts are applied to the two main branches of flame photometry, emission and absorption.     

Effect of the specific activity of the tracer on rat luteinizing hormone radioimmunoassay

Rat luteinizing hormone /LH/ was labelled with¹²⁵I by the Chloramine T method.¹²⁵I-LH, used as tracer in radioimmunoassay, was separated from the labelling reaction mixture by gel filtration. By using the proper protein/radioiodine ratio in the labelling reaction mixture the specific activity of¹²⁵I-LH was adjusted to 2.5–20.5 MBq g^–1. The influence of the specific activity on the assay parameters as well as on the tracer stability was investigated.     

Rapid determination of PAHs in soil samples by HPLC with fluorimetric detection following sonication extraction

A rapid method for the determination of PAHs in soil samples based on their extraction with methylene chloride by sonication and subsequent separation by HPLC with fluorimetric detection is proposed. A Hypersil Green PAH column was used with a gradient of acetonitrile/water as the mobile phase, together with a program of nine excitation and emission wavelength pairs. Recoveries were in the range 70-98%, except for acenaphthene and naphthalene, at concentration levels 1.08-442 microg/kg with relative standard deviations in the range 2-15% (n = 4). Total PAHs found in soil samples were in the range 15-282 microg/kg. The results were compared with those obtained by applying the 3540 EPA method for two samples.     

Characterisation of Tokaj wines based on free amino acids and biogenic amines using ion-exchange chromatography

Summary Tokaj wines (Szamorodni and Aszu wines) of Hungarian origin were investigated on the basis of free amino acids and biogenic amines. The separation and determination of these compounds was performed by an amino acid analyser equipped with an ion-exchange resin column. The total amount of free amino acids and biogenic amines was higher in Aszu wines than in Szamorodni wines. The main amino acids were proline and arginine, while the major biogenic amines were tyramine and putrescine. The free amino acid and biogenic amine content of Aszu wines depended on the vineyards the wines originated from. Presented at Balaton Symposium '01 on High-Performance Separation Methods, Siófok, Hungary, September 2–4, 2001     

Natural radioactivity of building materials

Experiments were designed to measure trace uranium concentration and the rate of radon exhalation from masonry structural materials, both bare and surface finished and coated. LR115 cellulose nitrate track detectors were used to record the alpha emission from structural material surface. Fission track, neutron activation and fluorometric analysis methods were used to determine the uranium content. Most types of paints studied will reduce alpha contribution and radon emanation from building materials.     

Thermal rearrangements of 2-vinylcyclopropylidene to cyclopentadiene and vinylallene: a theoretical investigation

In an attempt to clarify the favored rearrangement reaction of vinylcyclopropylidenes, the prototype thermal rearrangements of singlet 2-vinylcyclopropylidene (1) leading to 1,3cyclopentadiene (2) and 1,2,4-pentatriene (vinylallene) (3) were investigated by means of ab initio quantum-mechanical electronic-structure calculations. The B3LYP functional with the 6-31G(d) basis set was employed for geometry optimization of the equilibrium and transition-state structures relevant to the two reaction pathways and for computing their harmonic vibrational frequencies. Final energies were evaluated by single-point calculations at the CCSD(T) level of theory with the 6-311 + G(3df,2p) basis set. The rearrangement of s-cis 1 to 2 is found to occur by a three-step pathway. The first step involves the formation of a nonclassical carbene (5), which is an internal pi complex between the pi molecular orbital of the double bond and the empty p atomic orbital of the carbene carbon. In the second step, the nonplanar five-membered ring geometry of 5 flattens to reach the planar structure of 3-cyclopentenylidene (4). The last step is the 1,2-migration of a alpha-hydrogen atom to the carbene center in 4. The rate-determining step for the rearrangement of s-cis 1 to 2 is the formation of 5, with a predicted global deltaG++(220 K) of only 0.6 kcalmol(-1). The rearrangement of s-trans 1 to 2 requires an initial conversion of s-trans 1 to the s-cis conformer, with a predicted deltaG++(220 K) of 1.8 kcalmol(-1). The transition structure for the ring-opening of s-trans 1 into s-trans 3 (deltaG++(220 K)=4.7 kcalmol(-1)) is more energetic than that for the ring-opening of s-cis 1 into s-cis 3 (deltaG++(220 K)=2.5 kcalmol(-2)) due to larger repulsive nonbonded H...H interactions in the former transition structure. On the basis of these results, it is suggested that if the reaction of 1,1-dibromo-2-vinylcyclopropane with methyllithium at -78 degrees C leads to the initial formation of carbene 1, then the reaction should yield 2 as the main product together with small amounts of 3. This theoretical prediction nicely agrees with experimental findings.     

Selective liquid adsorption and structural properties of montmorillonite and its hexadecylpyridinium derivatives

Summary An X-ray diffraction method was used to study the structures of montmorillonites, the surfaces of which were covered to a systematically increasing extent with hexadecylpyridinium (HDP) cations. It was found that, depending on the degree of cover of the surface, the HDP cations form a mono- or bimolecular layer in the interlayer space. A detailed investigation was made of the selective sorption of binary mixtures on montmorillonite and on HDP-montmorillonites with different surface coverages. The adsorption excess isotherms were determined, as were the adsorption capacities from analysis of the isotherms, and also the equivalent specific surface areas.It was established that from benzene-n-heptane mixtures the benzene is adsorbed only on the external surface of montmorillonite, while in the case of organocomplexes it causes moderate disaggregation.From benzene, alcohols with different numbers of carbon atoms are also preferentially adsorbed on the internal surfaces of montmorillonite; with the rise in the number of carbon atoms, the extent of interlamellar adsorption decreases. In the case of HDP-montmorillonites in methanol-benzene mixtures, the adsorption capacity exhibits a monotonous increase with the degree of organopbility in the range under consideration. In the event of optimum organophility the disaggregation of the organocomplex is complete, and the equivalent specific surface area is 764-778 m² · g^–1. The swelling of the organocomplex, which can be characterized by X-ray diffraction measurement, is in close correlation with the interlamellar liquid adsorption. The results show modification of the surface of montmorillonite with HDP cations fundamentally determines the nature and extent of the selective liquid adsorption.

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Zusammenfassung Die Struktur von mit Hexadecyl-pyridinium(HDP)-Kationen im systematisch gesteigerten Maße bedeckten Montmorilloniten wurde mit der Röntgendiffraktions-Methode untersucht. Es wurde festgestellt, daß die HDP-Kationen, von der Bedeckung der Oberfläche abhängend, im Schichtzwischenraum eine mono- bzw. bimolekulare Schicht bilden. Die selektive Sorption binärer Gemische wurde an Montmorilloniten und an HDP-Montmorilloniten verschiedener Bedeckung eingehend untersucht. Es wurden die Adsorptionsisothermen von Flüssigkeitsgemischen und aus deren Analyse die Adsorptionskapazitäten und die äquivalenten spezifischen Oberflächen bestimmt.Es wurde festgestellt, daß aus Benzol-n-Heptan-Gemischen das Benzol nur an der äußeren Montmorillonite-Oberfläche adsorbiert wird, während es bei Organokomplexen eine mäßige Desaggregation verursacht. Die Adsorption von Alkoholen verschiedener Kohlenstoffzahl aus Benzol erscheint auch an inneren Montmorillonit-Oberflächen bevorzugt; mit zunehmender Kohlenstoffzahl nimmt das Ausmaß der interlamellaren Adsorption ab. Bei HDP-Montmorilloniten zeigt die Adsorptionskapazität aus Methanol-Benzol-Gemischen mit zunehmender Organophilität im untersuchten Gebiet eine monotone Steigerung. Im Falle optimaler Organophilität ist die Desaggregation vollständig, die äquivalente spezifische Oberfläche beträgt 764-778 m²/g. Die durch Röntgenn-diffraktionsmessung charakterisierbare Quellung des Organokomplexes steht mit der interlamellaren Flüssig-keitsadsorption in engern Zusammenhang. Unsere Ergebnisse zeigen, daß Charakter und Ausmaß der selektiven Flüssigkeitsadsorption durch die Modifizierung der Montmorillonit-Oberfläche mit HDP-Kationen grundlegend verändert wird.

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With 14 figures and 4 tables     

Molecular dynamics simulations of the mononuclear zinc-beta-lactamase from Bacillus cereus complexed with benzylpenicillin and a quantum chemical study of the reaction mechanism

Herein, we present results from MD simulations of the Michaelis complex formed between the B. cereus zinc-beta-lactamase enzyme and benzylpenicillin. The structural and dynamical effects induced by substrate-binding, the specific role of the conserved residues, and the near attack conformers of the Michaelis complex are discussed. Quantum chemical methods (HF/6-31G* and B3LYP/6-31G*) are also applied to study the hydrolysis reaction of N-methylazetidinone catalyzed by a monozinc system consisting of the side chains of the histidine residues (His86, His88, and His149) complexed with Zn-OH and the side chains of Asp90 and His210. From this model system, we built molecular-mechanics representations of the prereactive complex and transition state configurations docked into the active site. Linear-scaling semiempirical calculations coupled with a continuum solvent model were then performed on these static models. We propose that the experimental rate data for the B. cereus enzyme is compatible with a one-step mechanism for the hydrolysis of beta-lactam substrates in which His210 acts as a proton donor.     

Enzyme catalysis in organic solvents: a promising field for optical biosensing

The ability of enzymes to work in non-aqueous media offers new and almost unexploited possibilities for the development of new optical biosensors. The advantages of performing biocatalytic reactions in non-aqueous media are discussed in relation to their possible application in optical biosensor design. Attention is focused on the factors that influence enzymatic catalysis in organic solvents, including the role of enzyme-associated water, criteria for solvent selection and the alteration of enzyme specificity. Recent examples of relevant applications and future prospects of organic-phase optical biosensing are discussed.