Unsteady axisymmetric stagnation flow on a circular cylinder

Unsteady, axisymmetric stagnation flow about a circular cylinderis examined when the far-field flow is a periodic function oftime with a fixed time average and an oscillatory part of prescribedamplitude and frequency. Solutions are computed for arbitraryvalues of the Reynolds number, quantifying the effects of surfacecurvature, and a frequency parameter based on the period ofthe far-field flow. It is found that solutions remain regularand periodic provided that the far-field amplitude lies belowa critical value. Above this value, solutions terminate in afinite-time singularity. The blow-up time is delayed by increasingthe curvature of the surface. These results are corroboratedby asymptotic predictions valid in the limits of small and largeamplitude and frequency. For large Reynolds number, the problemreduces to the two-dimensional stagnation-point flow againsta plane wall studied by previous authors.     

Polyethylene glycol matrix reduces the rates of photochemical and thermal release of nitric oxide from S-nitroso-N-acetylcysteine

S -nitrosothiols have many biological activities and may act as nitric oxide (NO) carriers and donors, prolonging NO half-life in vivo. In spite of their great potential as therapeutic agents, most S -nitrosothiols are too unstable to isolate. We have shown that the S -nitroso adduct of N -acetylcysteine (SNAC) can be synthesized directly in aqueous and polyethylene glycol (PEG) 400 matrix by using a reactive gaseous (NO/O₂) mixture. Spectral monitoring of the S–N bond cleavage showed that SNAC, synthesized by this method, is relatively stable in nonbuf-fered aqueous solution at 25°C in the dark and that its stability is greatly increased in PEG matrix, resulting in a 28-fold decrease in its initial rate of thermal decomposition. Irradiation with UV light (λ= 333 nm) accelerated the rate of decomposition of SNAC to NO in both matrices, indicating that SNAC may find use for the photogeneration of NO. The quantum yield for SNAC decomposition decreased from 0.65 ± 0.15 in aqueous solution to 0.047 ± 0.005 in PEG 400 matrix. This increased stability in PEG matrix was assigned to a cage effect promoted by the PEG microenvironment that increases the rate of geminated radical pair recombination in the homolytic S–N bond cleavage process. This effect allowed for the storage of SNAC in PEG at −20°C in the dark for more than 10 weeks with negligible decomposition. Such stabilization may represent a viable option for the synthesis, storage and handling of S -nitrosothiol solutions for biomedical applications.     

Photochemie substituierter cycloheptatriene—VI : Diarylsubstituierte cycloheptatriene

The substituent effect of the dimethylamino group in diarylcycloheptatrienes, on the one hand, leads to a strict selection of the reaction way of the sigmatropic hydrogen shift. On the other hand, these substituents in 1,4-positions give rise to the reversible photo valence isomerization cycloheptatriene-bicycloheptadiene. The quantum yield of the sigmatropic H-shift depends strongly on the wavelength of the exciting light. It has been shown by sensitizing and quenching experiments that the intramolecular cyclisation does not proceed through triplet manifold or fluorescent minimum, but through an independent “funnel” in the singlet state. The ring opening is possible not only photoinduced, but also thermically with small activation energy.     

Synthetic analogues of glycosylphosphatidylinositol-anchored proteins and their behavior in supported lipid bilayers

Positioned at the C-terminus of many eukaryotic proteins, the glycosylphosphatidylinositol (GPI) anchor is a posttranslational modification that anchors the modified proteins in the outer leaflet of the plasma membrane. GPI-anchored proteins play vital roles in signal transduction, the vertebrate immune response, and the pathobiology of trypanosomal parasites. While many GPI-anchored proteins have been characterized, the biological functions of the GPI anchor have yet to be elucidated at a molecular level. We synthesized a series of GPI-protein analogues bearing modified anchor structures that were designed to dissect the contribution of various glycan components to the GPI-protein's membrane behavior. These anchor analogues were similar in length to native GPI anchors and included mimics of the native structure's three domains. A combination of expressed protein ligation and native chemical ligation was used to attach these analogues to the green fluorescent protein (GFP). These modified GFPs were incorporated in supported lipid bilayers, and their mobilities were analyzed using fluorescence correlation spectroscopy. The data from these experiments suggest that the GPI anchor is more than a simple membrane-anchoring device; it also may prevent transient interactions between the attached protein and the underlying lipid bilayer, thereby permitting rapid diffusion in the bilayer. The ability to generate chemically defined analogues of GPI-anchored proteins is an important step toward elucidating the molecular functions of this interesting post-translational modification.     

广义Bethe树上奇偶马尔可夫链场上的若干极限性质

通过构造两个非负鞅证明了一个强极限定理,然后把它应用到本文所定义的广义Bethe树上的奇偶马尔可夫链场上,从而获得了此马氏链场上的一类强极限定理.     

Nuclear spin relaxation in a liquid alloy with segregation tendency

In liquid In_0.83Te_0.17 which exhibits demixing at T_c=835 K, we have measured the nuclear spin relaxation rate R of the isomeric probe 117msb by a TDPAD technique. Within an interval of 50 K, an increase of R by about 30% is observed as T T_c. This phenomenon is interpreted as indicating a marked increase of concentration fluctuations over small regions in r-space.Guest of Sfb 161We are obliged to Gesellschaft für Kernforschung Karlsruhe for machine time and technical assistance. This work is supported by Deutsche Forschungsgemeinschaft through Sonderforschungsbereich 161.     

Suppression of plasma turbulence during optimized shear configurations in JET

Correlation of density turbulence suppression and reduced plasma transport is observed in the internal transport barrier (ITB) region of JET tokamak discharges with optimized magnetic shear. The suppression occurs in two stages. First, low frequency turbulence and ion transport are reduced across the plasma core by a toroidal velocity shear generated by intense auxiliary heating. Then with the ITB formation, high frequency turbulence and electron transport are reduced locally within the steep pressure gradient region of the ITB.     

Test of the Wu-Feenberg and F2 approximations in neutron matter and liquid 3He

It is shown that the Wu-Feenberg (WF) approximation for the radial distribution of a Slater-Jastrow function can be reformulated in a way which should improve the approximation in low orders, as judged by the test case where the Jastrow factor is replaced by the boson ground state of the Hamiltonian of interest. It is also shown that the lowest order of another approximation scheme due to Paulick and Campbell appears to be a partial resummation of the WF approximation. The energy expectation value is evaluated for “homework” neutron matter and liquid ³He using these approximations and compared to the Monte Carlo evaluation of the same quantity by Ceperley, Chester and Kalos (CCK) and the FHNC/4 approximation. There is excellent agreement for neutron matter in the density range 0.17 to 4.0 fm^?3; the agreement for liquid ³He is fair but has room for improvement.