Stability of a fluid-fluid interface in a biconical pore segment

Pore networks that include biconical pore segments are frequently used to model two-phase flow. In this work, we describe in detail the displacement of a fluid-fluid interface in such a pore segment. We assume sharp edges in the throat, inlet, and outlet of the pore segment to be the limiting cases of round edges, the radii of which vanish. We account for interfacial and lineal tensions that cause nonconstant contact angles. For zero lineal tension, we provide analytical solutions for flow induced by changing infinitesimally slowly either capillary pressure or the volume of one fluid. In diverging and converging cones, the common line among the two fluids and the solid phase slides while it is pinned in the throat, inlet, and outlet. We observe hysteresis within the pore segment, and drainage entry pressures deviate from prior work.     

Protein adsorption on nanoporous TiO2 films: a novel approach to studying photoinduced protein/electrode transfer reactions

We have investigated the use of nanoporous TiO2 films as substrates for protein immobilisation. Such films are of interest due to their high surface area, optical transparency, electrochemical activity and ease of fabrication. These films moreover allow detailed spectroscopic study of protein/electrode electron transfer processes. We find that protein immobilisation on such films may be readily achieved from aqueous solutions at 4 degrees C with a high binding stability and no detectable protein denaturation. The nanoporous structure of the film greatly enhances the active surface area available for protein binding (by a factor of up to 850 for an 8 microns thick film). We demonstrate that the redox state of proteins such as immobilised cytochrome-c (Cyt-c) and haemoglobin (Hb) may be modulated by the application of an electrical bias potential to the TiO2 film, without the addition of electron transfer mediators. The binding of Cyt-c on the TiO2 films is investigated as a function of film thickness, protein concentration, protein surface charge and ionic strength. We demonstrate the potential use of immobilised Hb on such TiO2 films for the detection of dissolved CO in aqueous solutions. We further show that protein/electrode electron transfer may be initiated by UV bandgap excitation of the TiO2 electrode. Both photooxidation and photoreduction of the immobilised proteins can be achieved. By employing pulsed UV laser excitation, the interfacial electron transfer kinetics can be monitored by transient optical spectroscopy, providing a novel probe of protein/electrode electron transfer kinetics. We conclude that nanoporous TiO2 films may be useful both for basic studies of protein/electrode interactions and for the development of novel bioanalytical devices such as biosensors.     

Defective transport of thymidine by cultured cells resistant to 5-bromodeoxyuridine.

A line of HeLa cells resistant to 5-bromo-2'-deoxyuridine (BUdR) was established by continuous culture in growth medium containing BUdR; during the selection period, BUdR concentrations, initially 15 micrometer, were gradually increased to 100 micrometer. Cells of a clone (HeLa/B5) established from this line were also resistant to 5-fluoro-2'-deoxyuridine (FUdR), but not to the free base, 5-fluorouracil. Although extracts of HeLa/B5 cells exhibited levels of thymidine kinase activity comparable to those of parental cells, rates of uptake of BUdR, FUdR, and thymidine into intact cells were much reduced. The kinetics of uptake of uridine and adenosine, nucleosides which appear to be transported independently of thymidine in HeLa cells, were similar for HeLa/B5 and the parental line (HeLa/O). Relative to thymidine uptake by HeLa/O cells, that by HeLa/B5 cells was distinctly less sensitive to nitrobenzylthioinosine (NBMPR), a specific inhibitor of nucleoside transport in various types of animal cells. Despite this difference in NBMPR sensitivity, both cell lines possessed the same number of high affinity NBMPR binding sites per mg cell protein. The altered kinetics of thymidine uptake and the NBMPR insensitivity of that function in HeLA/B5 cells suggest that resistance to BUdR is due to an altered thymidine transport mechanism.     

Immobilized purine nucleoside phosphorylase from Schistosoma mansoni for specific inhibition studies

The parasite Schistosoma mansoni (Sm) depends exclusively on the salvage pathway for its purine requirements. The enzyme purine nucleoside phosphorylase (PNP) is, therefore, a promising target for development of antischistosomal agents and an assay for screening of inhibitors. To enable this, immobilized SmPNP reactors were produced. By quantification of hypoxanthine by liquid chromatography, kinetic constants (K _M) for the substrate inosine were determined for the free and immobilized enzyme as 110 ± 6.90 μmol?L ^?1 and 164 ± 13.4 μmol?L ^?1 , respectively, indicating that immobilization did not affect enzyme activity. Furthermore, the enzyme retained 25 % of its activity after four months. Non-Michaelis kinetics for the phosphate substrate, and capacity for P_i-independent hydrolysis were also demonstrated, despite the low rate of enzymatic catalysis. Use of an SmPNP immobilized enzyme reactor (IMER) for inhibitor-screening assays was demonstrated with a small library of 9-deazaguanine analogues. The method had high selectivity and specificity compared with screening by use of the free enzyme by the Kalckar method, and furnished results without the need for verification of the absence of false positives.

Streptavidin binding bifunctional aptamers and their interaction with low molecular weight ligands

This paper describes the measurement of the binding affinities of two bifunctional RNA aptamers to their respective ligands. The aptamers comprise either a theophylline or malachite green binding sequence fused to a streptavidin binding sequence. These bifunctional aptamers are shown to bind simultaneously to both the small ligand and to streptavidin whether in free solution or on gold surfaces. Binding isotherms for both interactions were measured by different physiochemical techniques: surface plasmon resonance, fluorescence spectroscopy and dynamic light scattering. Both qualitatively and quantitatively there is little difference in binding affinities between the bifunctional aptamers and their monofunctional components. The respective K_d values for streptavidin binding in the monofunctional aptamer and in the theophylline bifunctional aptamer were 12 nM and 65 nM, respectively whilst the K_d values for theophylline binding in the monofunctional aptamer and the streptavidin bifunctional aptamer were 300 nM and 120 nM. These results are consistent with treating each aptamer sequence as a module that can be combined with others without significant loss of function. This allows for the use of streptavidin based immobilization strategies without either the cost of biotinylated dNTPs or the variable yields associated with the chemical biotinylation of RNA.     

Double prompt photon production at high transverse momentum byπ − on protons at 280 GeV/c

A search for pairs of highp _T prompt photons produced in hydrogen by a 280 GeV/c incidentπ ^- beam has been carried out using a fine-grained electromagnetic calorimeter and the Omega spectrometer at the CERN SPS. Clear evidence for the existence of such events is found with a six standard deviation signal forp _T >3.0 GeV/c. The cross-sections are consistent with beyond leading order QCD calculations. A discussion on the determination of α _s is also presented.     

The structure of events triggered by direct photons in π− p, π+ p andpp collisions at 280 GeV/c

The structure of events associated with the production of direct photons in π^? p, π⁺ p andpp reactions of 280 GeV/c has been studied using data from the WA 70 experiment at the CERN SPS. Results are presented on the distributions of the fractional momenta of the colliding partons and on the fragmentation of the recoil jet and a comparison is made with predictions using the structure functions of Duke and Owens in the Lund Monte Carlo with string fragmentation.     

EPR Studies of Tl0.5Pb0.5Sr1.6Ba0.4CaCu2?xRuxO7?δ Superconductor

Bulk superconducting samples of type Tl_0.5Pb_0.5Sr_1.6Ba_0.4CaCu_2−x Ru _x O_7−δ, (Tl, Pb)/Sr-1212, with 0.0 ≤ x ≤ 0.525 were prepared by the conventional one-step solid-state reaction technique. The prepared samples were investigated using X-ray powder diffraction, electrical resistivity and electron paramagnetic resonance (EPR) measurements. Enhancement of the phase formation, superconducting transition temperature T _c and hole carriers concentration P was observed up to x = 0.075. For x > 0.075, a reverse trend was observed. EPR spectra were measured at different temperatures (120–290 K) for all prepared samples. The number of spins N participating in the resonance and the paramagnetic susceptibility χ were calculated as a function of both Ru-content and temperature. N and χ increased as the Ru-content increased. A linear relationship between logN and 1/T was established, from which the activation energy E _a was calculated as a function of the Ru-content. The temperature dependence of χ was fitted according to Curie–Weiss type of magnetic behavior. Curie constant C, Curie temperature θ, the effective magnetic moment μ and the electronic specific heat γ were estimated as a function of the Ru-content.