Showing papers by "Academy of Sciences of the Czech Republic published in 1989"

Photochemistry of colloidal semiconductors 30. Reactions and fluorescence of AgI and AgI — Ag2S colloids

Abstract: When Ag+ ions are precipitated with I− ions in the presence of poly (N-vinyl pyrrolidone) or poly (vinyl alcohol) the AgI colloid forms slowly (during minutes or hours, depending on conditions), the changes in the absorption spectrum indicate the presence of unusual AgI structures during the first stages of particle growth. Size quantization effects are partly responsible for these changes. — AgI colloids with 10% excess of I− do not photolyse under continuous illumination. However, intermediates can be detected in flash photolysis experiments. In the presence of sulfite, silver is formed, the quantum yield lying in the 10−4 range. Greater yields of silver (10−3 to 10−2) are observed with solutions containing excess Ag+ ions. Less silver is produced in acidic solutions and the silver signal in the flash experiments decays more rapidly. These effects are attributed to the re-oxidation of small silver clusters formed on the colloidal particles by H+ ions. — AgI colloids have a weak fluorescence close to the threshold of absorption. It is blue-shifted with decreasing particle size (size quantization effect). The fluorescence is quenched by small amounts of silver deposited on the particles. It is also quenched by an Ag2S deposit. Under these circumstances a new fluorescence having three maxima in the red and infrared appears. This new fluorescence is red-shifted with increasing size of the silver deposit (size quantization of Ag2S). The fluorescence is explained in terms of electron transfer from AgI (conduction band) to the hole captured at the AgI—Ag2S interface. Size quantization effects are also believed to play a role in the sulfursensitization of photographic plates.

Compatibility stresses in deformed bicrystals

Abstract: Compatibility stresses have been calculated in the model of a bicrystal composed of two semi-infinite crystals. The elastic anisotropy of the component crystals is fully taken into account. Various particular cases chosen for the cubic bicrystals with the 〈100〉 and 〈110〉 rotation axes commonly used in experiments are discussed in detail. The effects of elastic and plastic deformation have also been compared. The model provides compatibility stresses as functions of grain boundary geometrical parameters and of the loading axis orientation.

Simulation of continuous solid phase synthesis: synthesis of methionine enkephalin and its analogs.

Abstract: The method of continuous solid phase synthesis, i.e., synthesis performed on a continual carrier, transferable from one synthetic compartment to another by means of a mechanical device, allows one to perform multiple synthetic steps simultaneously on different regions of the carrier. This procedure was tested on the synthesis of methionine enkephalin and its analogs. Products, obtained in reasonable yields, were fully characterized. Possible arrangements and the use of continuous solid phase synthesizers are discussed.

Carbocyclic nucleosides and nucleotides

Abstract: The invention relates to 1-B-4-phosphonylmethoxycyclopentane compounds having Formula I or Formula II wherein R¹ and R² are independently hydrogen, hydroxy, chlorine, bromine, or an organic substituent having 1 to 5 carbon atoms and selected from carbacyloxy, alkoxy, alkylthio, amino, alkylamino and dialkylamino, R³ and R⁴ are independently hydrogen, or organic phosphonic ester substituents having 1 to 12 carbon atoms and selected from alkyl, alkenyl, aryl, and aralkyl, B is a heterocyclic group having at least one nitrogen heteroatom and up to three additional heteroatoms selected from nitrogen, oxygen and sulfur, said heterocyclic group being attached through a nitrogen heteroatom thereof, and the pharmaceutically acceptable acid addition, metal, and amine salts thereof. These compounds are useful for treating viral diseases and diseases of microbial origin. They are also effective against infections conditions and tumors.

Sulfhydryl groups of the uncoupling protein of brown adipose tissue mitochondria. Distinction between sulfhydryl groups of the H+ channel and the nucleotide binding site.

Abstract: Mersalyl, 5,5'-dithio-bis(2-nitrobenzoate) (Nbs2) and fluorescent Thiolyte DB react with SH groups in the H+ channel (SHc) of the uncoupling protein of brown adipose tissue mitochondria, as inferred from their inhibition of H+ transport. Cl- transport by the uncoupling protein was unaffected. Using these modifiers and N-ethylmaleimide (MalNEt), distinct SH groups (SHB) in the purine nucleotide binding site were identified. Nbs2 reacts more readily with the SHB than with the SHc groups, but mersalyl and Thiolyte DB are more reactive with the SHc groups. MalNEt reacts exclusively with the SHB. GDP inhibition is fully prevented after sufficient modification of the SHB. Pretreatment with p-diazobenzenesulfonate (N2PhSO2) suppresses only 20-25% of fluorescence of Thiolyte-DB-labeled uncoupling protein on SDS/PAGE gels, while MalNEt suppresses 66% and Nbs2 80-90%. Since N2PhSO2 also affects the GDP binding site, these results demonstrate that the N2PhSO2-reactive residue is not identical with the SHB.

Spectrophotometric identification of 8-hydroxy-5-deazaflavin: NADPH oxidoreductase activity in streptomycetes producing tetracyclines

Abstract: Cell-free extracts of vegetative mycelia of Streptomyces aureofaciens and Streptomyces rimosus were found to reduce streptomycete-origin 8-hydroxy-5-deazaisoalloxazine derivatives (SF420) using NADPH as a dnor of hydrogen and electrons. 7,8-didemethyl-8-hydroxy-5-deazariboflavin (F0) also was a substrate, although with a lower reaction rate than that for SF420. NADH could not substitute for NADPH. The F420-reductase activity was also observed in homogenates of S. aureofaciens spores.

Light scattering effects in CPM and PDS measurement on a-Si:H films

Abstract: We show that scattering may influence the measurement of the subband-gap absorption by the CPM technique. We present a simple method to get the true absorption value, non-affected by scattering, and to measure the scattering itself. Relationship between microstructure of material and scattering coefficient is suggested.

Phase-lock loops of higher orders

Abstract: Phase-lock loops in frequency synthesizer applications often put contradictory requirements on the designers such as: optimization of the noise behaviour and switching speed, leakage of spurious signals, to mention only the most important. The authors discuss cases where phase-lock loops of 3rd, 4th, and 5th order should be used. Since phase margin is small at the 3rd and 5th order loops they are not suitable in instances where a transportation delay is present. Their advantage is a rather large attenuation at high normalized frequencies, i.e. x>>1. On the other hand the 4th order loops with their large phase margin are rather insensitive to additional transportation delay. Their advantage is either notch properties or additional attenuation, however, only in instances where an increased output noise can be tolerated. >

Study of A-Si:H drift mobility in subnanosecond time scale

Abstract: The new integral TOF method in SCLC region is suggested for study of transport in subnanosecond time scale and discussed in detail. First results on undoped a-Si:H show the increase of electron drift mobility with electric field and strong decrease of its activation energy. These results are important for understanding of transport and density of states near the mobility edge.

OnΛ-hyperon(s) in the nuclear medium

Abstract: Relativistic mean field theory is tested in reproducing the novel experimentalΛ single particle (π+, K+) spectra of Λ=12/90 hypernuclei (and extended to Λ 209 Pb). The adjusted model is then applied to multistrange systems\({}_{n_\Lambda \Lambda }^{16 + n_\Lambda } O,_{n_\Lambda \Lambda }^{40 + n_\Lambda } Ca\); no anomalous behaviour of radii and densities in those multi-Λ hypernuclei is encountered.

NMR study of three heteroyohimbine derivatives from Rauwolfia serpentina: Stereochemical aspects of the two isomers of reserpiline hydrochloride

Abstract: 1H NMR data for isoreserpiline and tetraphylline and 1H and 13C NMR data for reserpiline hydrochloride are presented. New insights into the 1H NMR coupling constants of the heteroyohimibines provide an easy means of assigning their configuration and conformation, allowing the elucidation of the stereochemical features of the two isomers of reserpiline hydrochloride.

Faujasite Embedded Platinum: Zeolite Framework Texture and Effect of Metal Dispersion on Syngas Conversion

Abstract: Platinum phases of fine (1–2 nm mean particle size) and coarse (4–5 nm mean particle size) dispersion are grown within a NaX matrix. The hysteresis loops of the cyclopentane isotherms show the generation of mesopore textures exhibiting average pore sizes which correspond to the mean metal particle sizes. This demonstrates that the formation of metal crystals being larger than the supercage dimension is accompanied by lattice fragmentations around the growing metal particles. The fine dispersion shows higher selectivities for the conversion of non-dissociatively chemisorbed carbon monoxide in the water gas shift reaction accompanying the syngas conversion, i.e. lower values for turnover frequency ratios TOF(CH 4 )/TOF(CO 2 ), as compared to the coarse dispersion.

RF generation of the plasma jet channel for the jet PCVD

Abstract: Breakdown conditions for creation of the hollow cathode discharge in the nozzle passed through the rf powered electrode and creation of the plasma jet channel in PCVD reactor are studied. Pure nitrogen is used for measurements. The creation of jet channel is easier for smaller rf electrodes. The breakdown depends on the pressure and on the gas inflow rate. The plasma potential and the self-bias potential is influenced by the covering of reactor walls and the rf electrode by a dielectric layer.

Hard processes in general factorisation scheme

Abstract: A framework for the formulation, in a general factorization scheme of QCD predictions for physical quantities involving parton distribution and/or fragmentation functions, is suggested. The main advantage of this method, based on the use of Jacobi polynomials and implying a novel parametrization of nonperturbative properties of hadrons, is that it allows an easy and straightforward transformation of the results of phenomenological analyses performed in arbitrary factorization scheme, into any other one. The study of nonsinglet nucleon structure functions furthermore demonstrates that the specification of the factorization convention, i.e. the way parton densities beyond the leading order are defined, may be as important as the choice of the factorization and hard scattering scalesM and μ.

Micellar enzymology: potentialities in fundamental and applied areas.

Abstract: Micellar enzymology, a new trend in molecular biology, studies catalysis by enzymes entrapped in hydrated reversed micelles of surfactants (phospholipids, detergents) in organic solvents. In this review, the key research problems of micellar enzymology are formulated and examples of biocatalysis in microheterogeneous media are discussed. In particular, new applications are presented of micellar enzymology in fine organic syntheses, in clinical and chemical analyses (bioluminescence and enzyme immunoassays), in bioconversion of energy and mass, in therapy (engineering of new drugs capable of targeted penetration into cells), as well as in biotechnology (processes using nanogranulated or nanocapsulated enzymes).

Si K-emission bands of nitrogen rich a-SiNx:H

Abstract: X-ray emission spectroscopic (XES) studies of the Si K-emission band of a-SiNx:H alloys, prepared by plasma deposition from NH3/SiH4 mixtures indicate that SiSi bonds exist at least up to x = 10 With increasing x an increasing Si 1s core level shift is observed in the emission bands, directly testifying charge transfer from Si to N atoms Comparison of Si emission bands with available ultraviolet photoemission spectra (UPS) makes possible to identify all features observable in XES and UPS with respect to N 2s, N 2p, Si 3s and Si 3p derived states Contrary to UPS interpretation we have found that at the top of valence band also Si states are present

Parameter distortion by unappropriate nonisothermal treatment

Abstract: The aim of this contribution is to draw attention to the following problems: (i) boundary conditions of kinetic models, (ii) distortion of kinetic data by cutting-off the peak ends, (iii) regression analysis involved in the evaluation of kinetic data.

A-Si:H drift mobility - study of isotropy

Abstract: The systematic difference has been observed between (μτ)cc and (μτ)ss - the mobility-lifetime product found in a-Si:H from TOF charge collection or steady state photoconductivity. Its recently suggested explanation (μτ-anisotropy) is briefly discussed. The new TOF measurements made under clear experimental conditions on unique a-Si:H samples in both “perpendicular” and “planar” directions convincingly show that a-Si:H is isotropic material with μ⊥ = μ∥. Even (μτ)⊥t (μτ)∥ when the same method (TOF change collection) is used.

Infrared absorption spectrum of a small-polaron system

Abstract: Characteristic features and the temperature dependence of the small-polaron absorption spectrum are discussed using a recently published model. The physical interpretation of formal results is introduced, in particular a connection of small-polaron states with oscillator coherent states is emphasized. The continuous change of absorption curves with temperature is illustrated by numerical calculations.