Showing papers by "Technical University of Berlin published in 1981"

Nucleoside Syntheses, XXIII1) On the Mechanism of Nucleoside Synthesis

Abstract: During the silyl-Hilbert-Johnson nucleoside synthesis with Friedel-Crafts catalysts three processes occur simultanously: A) The formation of the electrophilic sugar cation. B) The σ-complex formation between th silylated base and the Friedel-Crafts catalyst and finally. C) The reaction of the electrophilic sugar cation with the silylated base to the nucleoside. Mode and rate of σ-complex formation between the silylated base and the Friedel-Crafts catalysts were monitored by 13C-NMR. The probable mechanism of nucleoside synthesis and the factors which influence course and yield of nucleoside synthesis are discussed. Nucleosid-Synthesen, XXIII1) Uber den Mechanismus der Nucleosid-Synthese In der Silyl-Hilbert-Johnson-Reaktion geschehen drei Prozesse gleichzeitig: A) Die Bildung des elektrophilen Zucker-Kations. B) Die σ-Komplex-Bildung zwischen der silylierten Base und dem Friedel-Crafts-Katalysator. C) Die Reaktion des elektrophilen Zucker-Kations mit der silylierten Base zum Nucleosid. Art und Geschwindigkeit der σ-Komplex-Bildung zwischen der Silylierten Base und den Friedel-Crafts-Katalysatoren wurden mit Hilfe der 13C-NMR verfolgt. Der wahrscheinliche Mechanismus der Nucleosid-Synthese und die Faktoren, die Verlauf und Ausbeute der Nucleosid-Synthese beeinflussen, werden diskutiert.

Organic acid synthesis and release of hydrogen ions in response to Fe deficiency stress of mono‐ and dicotyledonous plant species

Abstract: Comparing three mono‐ and dicotyledonous plant species (barley, oat, millet; pea, sugar beet, bean) it will be shown that under Fe stress conditions monocots do not lower the pH of the nutrient solution if nitrate is the only form of nitrogen supplied, whereas dicots do. During the iron stress‐induced H+ excretion there is a striking increase in organic acids (particularly citrate and malate) in the roots of dicots. Therefore, it is proposed that these acids could be the source of hydrogen ions released by the roots. When severe chlorosis has developed the organic acid content of the monocots is raised too, but without any drop of the nutrient solution pH. The results are discussed in relation to a possible involvement of growth regulators and differences between the NO3 assimilation rate in roots of mono‐ and dicotyledonous plant species.

On Automorphic Functions

Abstract: The paper begins with a brief account of Christoffel’s work on abelian integrals and its historical context and then describes the close connection between Riemann surfaces and Fuchsian groups. This is followed by a survey of some modern developments in the theory of infinitely generated Fuchsian groups, namely the Bers spaces (the generalization of the space of abelian differentials) and the classification of Fuchsian groups.

A model for the electronic transport in hydrogenated amorphous silicon

Abstract: We present a model for the steady-state transport in glow-discharge (g.d.)-deposited silicon that can account in detail for the ‘activated mobility’ observed from a comparison of thermopower and co...

Rates of change in vegetation during secondary succession

Abstract: For a number of experiments on secondary succession on different soils (lasting 9–23 yr) calculations were made of: a) rates of floristic change as measured with the community coefficient of Sorensen (CC), b) rates of cover-based change by the percentage similarity coefficient of Dahl & Hadac (PS), c) rates of change of stages (dominant growth forms). In the calculation of CC and PS the first year of observation and the preceding year have been used as reference years for a given year. Rates of >65% have been ranked as very rapid, rates of 35–65% as rapid, rates of 5–35% as slow, whereas rates of 95%) indicate temporal stability. Rapid changes of CC are to a large extent confined to the first year only, rapid changes of PS may occur in the following years too. After 5 yr, in many cases CC and PS are as high as ca. 90%, but the values do not exceed 95%. The examples show that rates of change are a useful tool in the description of the succession process.

In vitro cell growth inhibition by metallocene dichlorides.

Abstract: The in vitro growth-inhibiting potencies of titanocene dichloride (TDC), zirconocene dichloride (ZDC), hafnocene dichloride (HDC), vanadocene dichloride (VDC), and molybdocene dichloride (MDC) against Ehrlich ascites tumor (EAT) cells cultured in viro as permanently growing suspension cultures were determined. The most striking growth-suppression activity was detected for VDC. A VDC concentration as low as 5. 10(-6) mol/l effects a highly significant diminution of cell proliferation. TDC and MDC inhibit cellular growth only concentration of 5 . 10(-4) or 10(-3) mol/l, respectively, whereas ZDC and HDC, which are ineffective against EAT cells in vivo, require higher concentration levels. The growth inhibition is caused by a cytotoxic action of the metallocene dichlorides, as is demonstrated in the case of VDC and TDC by differentiation of live and dead EAT cells by means of the dye lissamine green.

Experimental Evidence on Time-Dependent Specific Heat in Vitreous Silica

Abstract: Measurements have been made of the heat capacity of thin plates of vitreous silica between 0.35 and 1.6 K. In an adiabatic sample arrangement the time-resolved specific heat was measured over a time scale from 2 \ensuremath{\mu}sec to 0.5 msec. The observed time dependence of the heat capacity is in quite good agreement with the tunneling model, which predicts a logarithmic increase in time. In addition, a much stronger time dependence was observed above 1 K, which was interpreted as a decoupling of excess modes from the Debye phonons.

Specificity of a Serum Peptidase Hydrolyzing Thyroliberin at the Pyroglutamyl‐Histidine Bond

Abstract: The substrate specificity of a serum enzyme which degrades thyroliberin (< Glu-His-Pro-NH2) into pyroglutamic acid and His-Pro-NH2 has been investigated and compared with that of the pyroglutamyl aminopeptidase from calf liver. The latter enzyme has a broad specificity, causing rapid degradation of thyroliberin, pyroglutamyl β-naphthylamide and luliberin. In contrast, the serum enzyme causes rapid stereospecific cleavage only of the pyroglutamyl-histidine bond of thyroliberin and closely related peptides. Compounds such as < Glu-Ala, < Glu-His and pyroglutamyl β-naphthylamide, which are known substrates of the pyroglutamyl atninopeptidases (such as the liver enzyme), are not substrates of the serum enzyme, and inhibit it only poorly. Pyroglutamyl-containing peptides such as luliberin and neurotensin and thyroliberin analogues such as lld-thyroliberin, < Glu-His-Pro-NHCH3, < Glu-His-Pro-Gly-NH2 and < Glu-Phe-Pro-NH2 inhibit effectively the degradation of thyroliberin by the serum enzyme, but are not hydrolyzed by this enzyme. The high specificity of the serum enzyme implies a physiological function.

Air Pollution Control Equipment

Abstract: Man is the architect of his environment and has to accept full responsibility for his environment. With respect to this chapter on air pollution control equipment, man is responsible for air pollution and all its effects on man, fauna, flora, and all kinds of buildings. To meet this responsibility, man has to observe the following general rules: 1. Stop the production of pollutants, or 2. reduce the production of pollutants to the lowest possible level. 3. Produce the unavoidable amount of pollutants in such a way, that the properties of the pollutants will guarantee easy separation from the carrier fluid, or effective conversion into harmless products. 4. Develop processes and equipment for effective separation of the pollutants from the carrier fluid or conversion into harmless products.

Turbulence measurements in an adverse-pressure-gradient three-dimensional turbulent boundary layer along a circular cylinder

Abstract: Measurements, including the six components of the Reynolds-stress tensor, have been made along three generators of a centre-mounted circular cylinder with an elliptical nose cone. The pressure distribution was axisymmetric upstream and asymmetric downstream. The streamwise adverse pressure gradient led to almost zero skin friction in the direction of the limiting streamline, and the circumferential pressure gradient led to skew angles up to 30° in the vicinity of the wall. Special emphasis was laid on measurements in the wall region (y+ > 4), on the repeatability of these measurements and on an estimate of error bounds. The turbulence level encountered (up to 60% close to the wall) was much higher than in other threedimensional boundary-layer measurements. It has been shown that available measuring techniques have to be improved considerably or even abandoned if used under these conditions. Previous measurements of collateral velocity profiles in three-dimensional boundary layers will probably now have to be corrected for severe aerodynamic interference effects.It has been shown that the normal stresses , both characteristic of three-dimensional flow and caused by the circumferential pressure gradient, are influenced in different ways by the streamwise and circumferential pressure gradients. Spectra of u′-fluctuations are again similar to those obtained from two-dimensional boundary layers.Mean velocity profiles obey the linear and logarithmic law of the wall known from two-dimensional boundary layers both along a line of symmetry and in the threedimensional boundary layer. This may be because the streamwise pressure gradient dominates over the circumferential pressure gradient in this experiment.Finally it has been found that the skew angle γ of the Reynolds shear stress vector leads the skew angle ζ of the resultant velocity gradient or ‘mean shear’, both having the opposite sign of the skew angle β of the mean velocity vector except close to the wall. The ratio of Reynolds shear stress and turbulent kinetic energy is no longer ‘approximately’ constant as is assumed for two-dimensional boundary layers.

Transformations of structures: An algebraic approach

Abstract: This paper introduces a new mathematical approach to transformations of structures, where the concept of “structure” is extremely general. Many structures and transformations that arise in biology as well as computer science are special cases of our concepts. A structure may be changed by finding an occurrence of a pattern and replacing it by another pattern as specified by a rule. To prove theorems about long sequences of applications of complicated rules, we need precise and tractable mathematical definitions of rules and how to apply them. This paper presents such definitions and three fundamental theorems, together with brief remarks on applications to control flow analysis, record handling, and evaluation of recursively defined functions. Unlike some previous efforts toward a rigorous theory of transformations of structures, this paper uses ideas and results from abstract algebra to minimize the need for elaborate constructions.

Effect of aluminium toxicity on root morphology of barley

Abstract: Light- and electronmicroscopic investigations were carried out in order to find the differences, in order to explain the differential Al tolerance of two barley varieties [2], Kearney (Al-sensitive) and Dayton (Al-tolerant). The plants were exposed to 9 ppm Al in an acidified nutrient solution (pH 4,8).