Effect of cholesterol on the properties of phospholipid membranes. 4. Interatomic voids

The properties of the interatomic voids present in fully hydrated dimyristoylphosphatidylcholine (DMPC)-cholesterol mixed membranes of different compositions are analyzed in detail using a generalized variant of the Voronoi-Delaunay method on the basis of computer simulation results. The systems investigated are chosen from both sides of the DMPC-cholesterol miscibility gap; the pure DMPC bilayer has also been included in the analysis as a reference system. The results obtained show that the empty space is organized in a more compact way, forming larger voids in the presence than in the absence of cholesterol. The voids located in the region of the rigid cholesterol rings become, on average, less spherical, oriented more parallel with the membrane normal axis with increasing cholesterol concentration, whereas an opposite effect of cholesterol is observed in the middle of the membrane among the chain terminal methyl groups. In general, the preferential orientation of the voids is found to strongly correlate with that of the molecules in the hydrocarbon phase of the membranes. The membranes are found to contain rather large voids, the volume of which can be an order of magnitude larger than the largest spherical cavities present in the systems. These voids are elongated or branching channels rather than big empty holes. The voids located among the DMPC and cholesterol molecules are lying preferably parallel with the membrane normal axis. The existence of such empty channels can be of great importance in the cross-membrane permeation of small, uncharged penetrants, in particular, of polar molecules.     

Isomerization of substituted tricyclic 4,5-dihydropyrazoles

Summary The acid and base catalyzed isomerization of some tricyclic 2-pyrazolines with N-Carbamoyl-,N-thiocarbamoyl-and N-phenyl substituents was investigated. Starting fromcis ortrans 3-H, 3a-H diastereomers, equilibrium mixtures ofcis andtrans diastereomers were prepared which were separated and subsequently studied by¹H NMR and¹³C NMR spectroscopy. A mechanism for the isomerization of the pyrazolines is suggested, supported by a deuterium exchange at C-3a.

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Isomerisierung von einigen substituierten 4,5-Dihydropyrazolen

Zusammenfassung Die Isomerisierung einiger tricyclischer 2-Pyrazoline mit N-Carbamoyl-, N-Thiocarbamoyl-und N-Phenyl-substituenten unter saurer und basischer Katalyse wurde untersucht. Ausgehend von dencis odertrans 3-H,3a-H-Diastereomeren wurdencis- undtrans Gleichgewichtsgemische gewonnen, die getrennt und durch¹H- und¹³C-NMR-Spektroskopie untersucht wurden. Ein Mechanismus für die Isomerisierung von Pyrazolinen wird vorgeschlagen, der durch den Deuteriumaustausch in Position 3a-C unterstützt wird.

     

Assessment of mobility of heavy metals in two soil types by use of column leaching experiments and chemometric evaluation of elution curves

The objectives of this study were to evaluate the mobility of heavy metals (HMs) in two types of soils (acidic forest soil and neutral agricultural soil) by leaching with calcium chloride solution in column experiments. The screening properties of neutral agricultural soil towards pollution by heavy metals (Ni, Cu, Zn and Cd) are approximately 10 times higher than those of acid forest soil. The neutral agricultural soil, polluted artificially by one pore volume (PV) of an HMs solution of concentration 200 mg L^?1, can screen the leaching of these metals over several hundreds of years. The higher apparent desorption rate and per cent desorption of HMs (especially Cd) in acid forest soil indicated a higher potential of intensive migration of the metals across the profile and indicated potential risk of Cd pollution for this type of soil. The latest approach of artificial neural networks to describe transport of HMs in soil has been also evaluated. Using a simple three-layer perceptron topology with three hidden neurons, the experimental data could be simulated. The results suggested that the pH of soil is a major factor controlling the retention of the heavy metals in the soils.     

On an Extension Problem for Polynomials

Consider the following problem: given complex numbers a₁, ...,a_n, find an L function f of minimum norm whose Fourier coefficientsc_k(f) are equal to a_k for k between 0 and n. We show the uniquenessof this function, and we estimate its norm. The operator-valuedcase is also discussed. 2000 Mathematics Subject Classification30E05, 47A20, 47A56, 47A57.     

Pressure induced quantum critical point and non-fermi-liquid behavior in BaVS3

We report on experiments giving evidence for quantum effects of electromagnetic flux in barium alumosilicate glass. In contrast to expectation, below 100 mK the dielectric response becomes sensitive to magnetic fields. The experimental findings include both lifting of the dielectric saturation by weak magnetic fields and oscillations of the dielectric response in the low temperature resonant regime. As the origin of these effects we suggest that the magnetic induction field violates the time reversal invariance leading to a flux periodicity in the energy levels of tunneling systems. At low temperatures, this effect is strongly enhanced by the interaction between tunneling systems and thus becomes measurable.     

A study of the exposure of various population groups to platinum in Vienna (Austria) through urine analysis by inductively coupled plasma-mass spectrometry

Platinum is emitted to environment by automobile catalytic converters (ACCs), hospitals, noble metal refineries, and other industries. Due to abrasion of ACCs platinum leaves the cars with the exhaust fumes and can thus be found in urban and rural atmospheres. To study the exposure to this element its concentration in urine is determined. Urine represents an often used specimen for monitoring studies, as it plays an important role in the elimination of various substances from the body and in addition it can be collected easily. It is supposed that the Pt-intake depends on the physical activity of a person, i.e. the amount of air inhaled and food consumed. Therefore subjects without and with one or two hip-endoprostheses were selected for this monitoring study. The medians obtained for the Pt-concentration of the urine-samples of these three groups were 3.7 µg/g, 3.3 µg/g and 1.9 µg/g for persons with no, one and two prostheses, respectively. Applying a Mann-Whitney-U-test on these data leads to a p-value of 0.051 comparing the groups with n = 0 and n = 2, and a p-value of 0.052 for n = 1 and n = 2 (n = number of prostheses).     

Measurement of time histories of stable intermediates during first stage ignition of n-heptane and its two isomers in a shock tube

We report the first shock tube measurements of formaldehyde (CH₂O) during the first stage ignition of n-heptane, 2-methylhexane and 3,3-dimethylpentane, in highly diluted fuel/oxygen mixtures in the pressure range of 7–10 atm and temperature range of 700–880 K. Combined time histories of all carbonyl (–C = O) species, CO and fuel were also measured simultaneously in an effort to study the impact of fuel structure on the concentration and the rate of evolution of first stage ignition products. Of the three isomers studied in this work, n-heptane was found to be the fastest, while 3,3-dimethylpentane was found to be the slowest. The differences in the time scale of formation, and plateau concentration of the intermediates between the isomers across the entire range of test conditions suggests a strong dependency of the measured time histories to fuel structure. These species therefore act as markers of the Negative Temperature Coefficient (NTC) behavior of fuels and can be used as targets for developing semi-empirical, hybrid chemistry models of complex, multi-component petroleum derived gasoline and jet fuels. The time histories reported in this work should prove very useful in the refinement of detailed kinetic models of n-heptane, and development of rate rules for branched alkane isomers.     

Dérivés C-glycosyliques. VII. Synthèse et réactions de nitrones dérivées de sucres. Communication préliminaire

The syntheses of three types of sugar nitrones (aldonitrone, ketonitrone and α-β unsaturated aldonitrone) are described. On 1,3-dipolar cycloaddition with phenylacetylene, the aldonitrone gave two Δ₄-isoxazolines epimeric at the new asymetric carbon, while the same reaction on the ketonitrone led to a spiro-Δ₄-isoxazoline. The reaction of these nitrones with carbon nucleophiles like phenylethynylmagnesium bromide constitutes a novel chain-extension reaction in carbohydrate chemistry.