A photoemission study of 4H---SiC(0001)

A photoemission study using synchrotron radiation of the (0001) surface of 4H-SiC is reported. The investigations were concentrated on the (√3 × √3)-R30° and (6√3 × 6√3)-R30° reconstructed surfaces, prepared by resistive heating at a temperature of about 1000°C and 1250°C, respectively. Results from surfaces heated at intermediate temperatures, exhibiting a mixture of these reconstructions, and after heating at a higher temperature, when graphitisation is clearly observed, are also presented. The √3 and 6√3 reconstructed surfaces exhibit characteristic core level and valence band spectra. High resolution core level spectra show unambiguously the presence of surface shifted components in both the Si 2p and C 1s core levels. For the √3 reconstruction, two surface shifted components are observed both in the Si 2p and C 1s level. For the 6√3 reconstruction, the surface region is found to contain a considerably larger amount of carbon. This carbon is found not to be graphitic since surface C 1s components with binding energies different from a graphitic C 1s peak are observed. Graphitisation, as revealed by the appearance of a graphitic C 1s peak, is observed only after heating to a higher temperature than that required for obtaining a well developed 6√3 diffraction pattern.     

Classification of fungi by means of pyrolysis-gas chromatography-pattern recognition

Repetitive samples of three strains of the mould Penicillium were subjected to pyrolysis-gas chromatography (Py-GC). From the chromatograms, 26 peak heights were used in a subsequent SIMCA pattern recognition analysis. This data analysis gives a marked improvement in the classification of the samples (100% correct, 85% unique) in comparison with the traditional analysis based on the average chromatogram of each class (92% correct, 45% unique). The data analytical method is described in detail using the Py-GC data as an illustration.     

Rapid coulometric method for the Kjeldahl determination of nitrogen

A rapid coulometric method for the Kjeldahl determination of nitrogen is described. The samples are digested by means of the Tecator AB digestion system which permits forty samples to be digested at the same time. The digestion products are diluted to 75 ml and 1 ml is coulometrically titrated in 1-2 min: 20-30 determinations can be performed per hour. For substances containing nitrogen in the per cent range the relative standard deviations for eight different substances were 0.1-1%.     

Partial donor-donor energy migration (PDDEM) as a fluorescence spectroscopic tool for measuring distances in biomacromolecules

A theoretical model is presented, tested and applied for determining the rates of energy migration and distances within pairs of chemically identical fluorophores, so-called donors (D), which are exposed to different physical properties. The model is a general extension of the recently developed donor-donor energy migration (DDEM) model [J. Chem. Soc., Faraday Trans. 92 (1996)1563; J. Chem. Phys. 105 (1996) 10896] that applies to examining structure-function of biomacromolecules, such as proteins. Most fluorescent groups of the same kind incorporated at different positions (alpha and beta) in a macromolecule exhibit shifts of the absorption and/or emission spectra, as well as different relaxation rates of the photophysics. As a consequence, the energy migration between the D(alpha) and D(beta) groups will be partially reversible. We refer to this case, as the partial donor-donor energy migration (PDDEM). The models of PPDEM presented can be used for analysing time-resolved fluorescence relaxation, as well as fluorescence depolarisation experiments. To explore the limitations of the PDDEM model, we have generated and re-analysed synthetic data that mimic time-correlated single photon counting (TCSPC) experiments. It was found that slow and fast rates of energy migration are most accurately recovered from the fluorescence relaxation and the depolarisation experiments, respectively. At comparable transfer and fluorescence rates, both kinds of experiments are equally useful. Real experiments on PDDEM were performed on an asymmetrically quenched bichromophoric molecule (1,32-dihydroxy-dotriacontane-bis-(Rhodamine 101) ester), that spans across the lipid bilayer of a vesicle. The depolarisation data were analysed by the PDDEM model and provide a distance between Rhodamine 101 groups, which agrees with independent studies.     

A glucose sensor made by chemically crosslinking glucose oxidase directly on the surface of a carbon electrode modified with Pd/Au for hydrogen peroxide electrocatalysis

Glucose oxidase, mutarotase and bovine serum albumin were chemically crosslinked with glutaraldehyde directly on the surface of a carbon rod, covered with a layer of sputtered palladium/gold to produce a catalytic electrode for hydrogen peroxide oxidation. The polymerization resulted in glucose sensors with response factors that were stable for months under different storage conditions. The thin membranes had a fast response so that flow injection peaks with a width at half peak height of 3 s could be recorded accurately. The detection limit was 0.5M D-glucose and the response was linear up to 3 mM D-glucose.     

Convenient stereoselective synthesis of some 3-aminosteroid scaffolds

Abstract

An efficient stereoselective synthesis 3α- and 3β-aminoandrostan-17-one and 3α-amino dehydroepiandrosten-17-one based on a Mitsunobu reaction has been developed, using azide as the ammonia equivalent. All the products were isolated in high yield.     

Quantitative determination of the ligand content in Benzamidine Sepharose 4 Fast Flow media with ion-pair chromatography

A quantitative hydrochloric acid hydrolysis-HPLC method was developed for the analysis of the ligand content of Benzamidine Sepharose 4 Fast Flow media. The method requires about 100 mg of dried sample and simple reaction vials can be utilised. Release of the ligand (p-aminobenzamidine) from the base matrix (Sepharose 4 Fast Flow) was obtained after hydrolysis for 180min at 70 degrees C in concentrated hydrochloric acid. When Benzamidine Sepharose 4 Fast Flow media were treated this way p-aminobenzoic acid and p-aminobenzamidine were the only products released from the ligand. A chromatographic system based on ion-pair reversed phase separation was used to quantify these ligand products. The mobile phase was made acidic enough to make p-aminobenzoic acid and p-aminobenzamidine positively charged in order to make ion-pair formation with hexanesulfonic acid possible. The relative standard deviation of th e method was below 2% and no systematic errors could be detected when the results were compared to an independent method based on elemental analysis (nitrogen). The new HPLC method was used to analyse ligand densities in the range of 2-20 micromol/ml medium.     

Effect of dextran- and poly(ethylene glycol)-bound procion yellow HE-3G on the partition of membranes from calf brain synaptosomes within an aqueous two-phase system

Membranes obtained by lysis and Yeda-press treatment of synaptosomes (nerve endings) from calf brain cortex have been partitioned within the aqueous phases (and the interface between them) of a Ficoll-dextran-poly(ethylene glycol)-water two-phase system. By introducing the dye Procion yellow HE-3G in the upper phase, bound to poly(ethylene glycol), or in the lower phase, bound to dextran, the partition of the membranes could be strongly affected. The influence on the partition was more pronounced when the dye was bound to dextran. By using a number of two-phase systems in a counter-current distribution process, it was shown that the membrane preparation was inhomogeneous and that the fractions obtained differed in their contents of acetylcholinesterase, succinate dehydrogenase and ATPase. The affinity partitioning effect depended strongly on the concentration of polymer-bound dye. An optimum dye concentration was found when Procion yellow HE-3G was bound to poly(ethylene glycol). When the same dye was bound to dextran, the number of dye molecules per dextran molecule influenced the effectiveness of the extraction.     

Solid-phase synthesis of serine-based glycosphingolipid analogues for preparation of glycoconjugate arrays

Synthetic glycolipids with defined structures are important tools in the study of glycolipid biology. In this paper we describe a solid-phase synthesis of three galactosylated serine-based glycosphingolipid analogues using the novel linker 2-fluoro-4-(hydroxymethyl)-phenoxyacetic acid. Gel-phase (19)F-NMR spectroscopy was used to measure the yield and stereochemical outcome of the solid-phase glycosylations. Under NIS-TfOH promotion, alpha- and beta-selective glycosylations were performed at room temperature with thioglycoside donors carrying fluorine labelled protective groups. Finally, the glycolipids were covalently linked to microtiter plates and labelled lectins with different selectivity for alpha- and beta-galactosides could bind to the glycolipid arrays.