Showing papers by "Braunschweig University of Technology published in 1987"

Fouling of heat transfer surfaces

Abstract: Fouling and/or scaling of heat transfer surfaces causes serious problems in industry. Prevention of fouling is therefore essential for technical (considerable deterioration of heat transfer) and also economic reasons (increased costs). In order to provide successful countermeasures, it is, however, necessary to obtain more detailed information on the physical, chemical and biological processes which produce fouling. Based on the already known fouling mechanisms, it is shown that a satisfactory prediction of fouling behaviour of heat exchangers is not yet possible. This contribution presents a physical model for the description of fouling, caused by sedimentation and crystallization. It is assumed that, during fouling, deposition and removal processes overlap. In order to verify the theoretical considerations, experiments were carried out on a test rig, which could also be used for field experiments. Aqueous CaSO4 solutions served as experimental liquids. A comparison of experimental and calculated fouling factors shows a satisfactory agreement.

Optical parameters of InP-based waveguides

Abstract: The quaternary alloys InGaAsP, matched in their lattice constant to InP, are considered in view of optical-waveguide applications. The parameters, most important in this connexion, are the refractive index n and the optical absorption coefficient α. Both are calculated starting from quantities relevant to the binary constituents of the system InGaAsP. The results are compared with the relatively few experiments reported in the literature. The calculations are applied to film and rectangular waveguides, showing that they are sufficiently low in loss for integrated-optics applications.

Alkaloid patterns and biosynthetic capacity of root cultures from some pyrrolizidine alkaloid producing Senecio species.

Abstract: Root cultures of Senecio vulgaris, S. vernalis, S. erucifolius and S. squalidus were established. The patterns of pyrrolizidine alkaloids found in these root cultures were analyzed by high-resolution GC and GC-MS and compared with the alkaloids present in the respective plants. In vitro cultured roots produce alkaloid patterns and accumulate quantities which are comparable to those found in soil grown plants. With the exception of the otonecine derivative senkirkine all pyrrolizidines accumulate as N-oxides. Only senkirkine is partially released into the medium. The cultures incorporate biosynthetic precursors, e.g. 14C-labelled putrescine or spermidine with high efficiency into the alkaloids. Senecionine N-oxide was found to be the main product of biosynthesis. Evidence is presented that senecionine N-oxide is directly transformed into senkirkine, the main alkaloid of S. vernalis root cultures.

Filial cannibalism and reproductive success in the maternal mouthbrooding cichlid fish Pseudocrenilabrus multicolor

Abstract: Pseudocrenilabrus multicolor is an iteroparous maternal mouthbrooding cichlid fish living in small and shallow waters in East Africa. It is subjected to large seasonal variations of population density. Laboratory experiments show that with increasing density and time after spawning the number of offspring present in the mouth decreased. By experimentally varying density and the numbers of fertilized and unfertilized eggs in the mouth at the end of spawning the following causes of brood size reduction were identified: (1) At high densities some of the spawned eggs are stolen by conspecifics before they are taken into the mouth. Of the eggs taken up a considerable proportion is unfertilized. (2) Unfertilized eggs are selectively swallowed from day 2 to day 5 after spawning. (3) Some of the fertilized eggs are also swallowed during mouthbrooding. (4) If the number of eggs in the mouth falls below a critical value of approximately 20% of the number of eggs spawned, they are all swallowed. By contrast, predation by conspecifics is insignificant as long as the brood is in the mother's mouth. The causes and functions of the mother's filial cannibalism are discussed.

Interactive Data Interpolation by Rational Bezier Curves

Abstract: Some interpolation techniques produce curves that the designer considers unsatisfactory. Even when the data points lie on an apparently simple curve, the resulting interpolant may not be the obvious one. This article presents an algorithm that combines interactive techniques with interpolation methods to allow user intervention. Rational Bezier curves are used as local interpolants, and an approach is discussed for exploiting shape parameters.

Infinite Control Points-A Method for Representing Surfaces of Revolution Using Boundary Data

Abstract: Methode pour representer les surfaces de revolution a partir d'elements de frontiere utilisant des points de controle infinis

The RSA cryptography processor

Abstract: In commercial applications, a minimum ciphering rate of 64 K bit/sec is required which will be the transmission rate of public digital networks. In contrast, the RSA method has a very slow ciphering rate particulary when using software implementations of the algorithm. The solution of this problem is a hardware implementation of the RSA algorithm. A cryptography processor, however, consisting of standard chips like bit slice processors again does not achieve the speed necessary. Moreover, in a multi-chip processor, the security of the key management system cannot be guaranteed. Therefore, a single-chip implementation of the RSA algorithm seems to be the only solution. Such a solution is presented as an RSA Cryptography Processor (CP).

Substrate- and stereospecificity, induction and metallo-dependence of a microbial hydantoinase

Abstract: D, L-5-monosubstituted hydantoins can be used as substrates for a two-step-enzymatic production of optically active aminoacids. The substrate- and stereospecificity of the first enzyme — a hydantoinase -, investigations on its induction and on its dependence upon metallo-ions are described. It is shown, that the activity of this hydantoinase, which is not identical with the well-known enzyme D-hydantoinase, depends on manganese-ions. Of synthetic and natural compounds tested as inductors, D, L-5-indolylmethylhydantoin showed the best effect. The hydantoinase has a wide substrate-specificity. Its stereoselectivity seems to depend on the structure of the side chain in 5-position of the hydantoin.

Sila‐Pharmaka, 371) Darstellung und Eigenschaften der Enantiomere der Antimuskarinika Sila‐Procyclidin und Sila‐Tricyclamol‐iodid: Optisch aktive Silanole mit Silicium als Chiralitätszentrum2)

Abstract: Durch Racematspaltung mit L-(+)- bzw. D-(-)-Weinsaure wurden die Enantiomere des Sila-Procyclidins (R)-1b und (S)-1b erhalten [> 97% ee (NMR), 99.7% ee (DSC)]. Daraus wurden die Hydrochloride (R)-2b und (S)-2b und durch Umsetzung mit CH3I die Enantiomere des Sila-Tricyclamol-iodids (R)-3b und (S)-3b [ > 96% ee (NMR)] hergestellt. Die optisch aktiven Silanole sind in kristalliner Form und in inerten Losungsmitteln konfigurationsstabil, wahrend sie in wasseriger Losung racemisieren (3b schneller als 1b). In Analogie zur Stereoselektivitat der antimuskarinischen Wirkung der Enantiomere der Kohlenstoff-Analoga Procyclidin (1a) und Tricyclamol-iodid (3a) besitzen die (R)-Enantiomere von 1b und 3b eine grosere Affinitat zu den ilealen M2β- und atrialen M2α- Muskarinrezeptoren des Meerschweinchens als die (S)-Antipoden. Alle Silicium-Verbindungen sind starker antimuskarinisch wirksam als ihre Kohlenstoff-Analoga, deren Stereoselektivitat jedoch starker ausgepragt ist. Die Unterschiede in der Affinitat von (R)-1b und (S)-1b zu den ilealen und atrialen Muskarinrezeptoren bestatigen das Konzept der Heterogenitat muskarinischer M2-Rezeptoren (M2α: atrialer Typ; M2β: ilealer Typ). Sila-Pharmaca, 371). – Preparation and Properties of tbe Enantiomers of the Antimuscarinic Agents Sila-Procyclidine and Sila-Tricyclamol Iodide: optically Active Silanols with Silicon as the Centre of Chirality2) The enantiomers of sila-procyclidine (R)-1b and (S)-1b [ > 97% ee (NMR), 99.7% ee (DSC)] were obtained by resolution with L-(+)- and D-(-)-tartaric acid, respectively. Starting from (R)-1b and (S)-1b, the hydrochlorides (R)-2b and (S)-2b were prepared and the enantiomers of sila-tricyclamol iodide (R)-3b and (S)-3b [ > 96% ee (NMR)] were synthesized by reaction with CH3I. The optically active silanols show configurational stability in the crystalline state and in inert solvents, whereas they racemize in aqueous solution (3b faster than 1b). By analogy with the stereoselectivity of antimuscarinic action of the enantiomers of the carbon analogues procyclidine (1a) and tricyclamol iodide (3a), the (R) enantiomers of 1b and 3b show a greater affinity for the ileal M2β and atrial M2α muscarinic receptors of the guinea pig than the corresponding (S) antipodes. All silicon compounds exhibit a greater antimuscarinic potency than their carbon analogues, whereas the stereoselectivity of action is more pronounced for the carbon compounds. The differences in affinity for (R)-1b and (S)-1b for ileal and atrial muscarinic receptors confirm the present concept of heterogeneity in muscarinic M2 receptors (M2α: atrial type; M2β: ileal type).

Switching genes in Schizosaccharomyces pombe: their influence on cell viability and recombination

Abstract: In Schizosaccharomyces pombe, mutations in 10 swi genes are known which reduce but not abolish mating-type (MT) switching in homothallic (h 90) strains. Three classes (Ia, Ib, and II) of swi genes are distinguished. Three swi1 alleles are nonsense mutations. In strains with mutations in two or three swi genes, a cumulative reduction of MT switching only occurs if the genes belong to different classes. The class 1a combinations swi1 swi7 and swi3 swi7 are lethal. Besides its influence on MT switching, swi5 also reduces the frequencies of meiotic intragenic recombination and gene conversion in the ade6 locus.

Molecular characterization of h − mutants of Schizosaccharomyces pombe

Abstract: Meade and Gutz (1976) have described mat2:2 mutants of Schizosaccharomyces pombe having various defects in the Plus (P) function; four different classes were distinguished. Of special interest are the class II mutants in which none of the P mating-type functions is expressed. We made Southern analyses of 23 “class II” strains. In most of these, the P cassette and the K region are deleted as in h −s strains, however, some distinct differences were found as to the intensity of the bands in the blots (classes Ila, Ilb 1, and IIb2). The class Ilb mutants have strong bands characteristic of lethal deletions (h −L) and mat2:1 0 plasmids. Two “class II” mutants turned out to have a typical h90 mating-type region with an intact P cassette, but they seem to have a completely defective switching signal at matl:1 (new class V). Mutants of classes I, III, and IV yielded band patterns identical to those of an h 90 strain; they obviously have point mutations in the P cassette.

Enantioselective reduction of acetyldimethylphenylsilane by Trigonopsis variabilis (DSM 70714)

Abstract: Growing and resting cells of the yeast Trigonopsis variabilis (DSM 70714) can be used for the enantioselective reduction of the organosilicon compound acetyldimethylphenylsilane (1) to give optically active (R)-(1-hydroxyethyl)dimethylphenylsilane [(R)-2] in good yields. The enantiomeric purity of the isolated product was determined to be 62–86% ee depending on the substrate concentration used. Both substrate and product caused an inhibition of the reaction at concentrations higher than 0.35 and 0.5 g/l, respectively. Besides, higher substrate and product concentrations led to increased formation of the by-product 1,1,3,3-tetramethyl-1,3-diphenyldisiloxane. Considering the limiting substrate and product concentrations, it was possible to use the same biomass at least 5 times without significant loss of enzyme activity. 3-Methyl-3-phenyl-2-butanone (5) and acetyldimethylphenylgermane (7), which represent carbon and germanium analogues of 1, were also found to be accepted as substrates by Trigonopsis variabilis (DSM 70714). The reduction rates of the silicon (1) and germanium compound (7) were much higher than the transformation rate of the corresponding carbon analogue 5.

Screening method for microorganisms producing L-aminoacids from D,L-5-monosubstituted hydantoins

Abstract: The ability of microorganisms to produce hydantoinase and L-N-carbamoylase could be established by an overlay assay. Enzyme producing strains form clear areas around their colonies caused by the cleavage of D,L-indolylmethylhydantoin. A second overlayer with a tryptophan-auxotroph yeast enables us to detect microorganisms which are able to produce L-tryptophan from D,L-indolylmethylhydantoin.

Chemie des 4-Chlor-1,3,5,7-tetramethyl-1,3,5,7-tetraaza-4λ5-phosphaspiro[3.3]heptan-2,6-dions; Kristall- und Molekülstruktur eines Chlortetraaza- und zweier Pentaazaphosphorane

Abstract: Durch Umsetzung des Chlorphosphorans 4-Chlor-1,3,5,7-tetramethyl-1,3,5,7-tetraaza-4λ5-phosphaspiro[3.3]heptan-2,6-dion (1) mit Methanol bzw. Phenol/Triethylamin, Trimethylsilylthioethern, (Dimethylamino)trimethylsilan und Natriumazid wurde eine Reihe von N,N′-Dimethylharnstoff-substituierten Phosphoranen, 2–7, mit dem oben genannten Ringsystem dargestellt. Die Azidgruppe in 7 reagierte mit Trimethylphosphan im Sinne einer Staudinger-Reaktion unter Bildung der neuartigen gemischtvalenten Diphosphorverbindung 8 mit dem λ5PNP(λ4)-Strukturelement. Die neuen Verbindungen wurden analytisch, NMR-und IR-spektroskopisch sowie durch ihre Massenspektren charakterisiert. – Fur die Verbindungen 1, 6 und 7 wurden Rontgenstrukturanalysen durchgefuhrt. In allen Fallen liegt ein stark verzerrtes, trigonal-bipyramidales Gerust vor. Signifikant unterschiedliche aquatoriale PN-Bindungslangen wurden fur 6 (164.4 pm) und 7 (169.8 pm) beobachtet. Fur die aquatorialen P–N(Harnstoff)-Bindungen wurden Werte von 166.8 bis 169.1 pm, fur alle drei Verbindungen, 1, 6 und 7, axiale PN-Bindungslangen um 176 pm gefunden. Chemistry of 4-Chloro-1,3,5,7-tetramethyl-1,3,5,7-tetraaza-4λ5-phosphaspiro[3.3]heptane-2,6-dione; Crystal and Molecular Structure of a Chlorotetraaza-and of Two Pentaazaphosphoranes The reaction of the chlorophosphorane, 4-Chlor-1,3,5,7-tetramethyl-1,3,5,7-tetraaza-4λ5-phosphaspiro[3.3]heptan-2,6-dione (1) with methanol or phenol/triethylamine, trimethylsilyl thioethers, (dimethylamino)trimethylsilane, and sodium azide has furnished a series of N,N′-dimethylurea-substituted phosphoranes, 2–7, involving the above ring system. The azide group of 7 was found to react with trimethylphosphane in a Staudinger-type reaction to give the novel, mixed-valence diphosphorus compound 8 with the λ5PNP(λ4) grouping. The new compounds were characterized by analysis, NMR and IR spectra as well as by their mass spectra. – X-ray crystal structural analyses have been performed for the compounds 1, 6, and 7. In all cases a strongly distorted, trigonal-bipyramidal framework was found. Significantly different equatorial PN bond lengths have been observed for 6 (164.4 pm) and 7 (169.8 pm), and for PN(urea) (166.8–169.1 pm). For all three compounds, 1, 6, and 7, the axial PN bond lengths are near 176 pm.

Sterility (ste) genes of Schizosaccharomyces pombe

Abstract: In previous experiments of Girgsdies (1982), eight sterile (ste) mutants of Schizosaccharomyces pombe did not sporulate when fused with h+ or h− protoplasts. We succeeded in achieving sporulation with these mutants. Two hitherto unknown ste genes, ste7 and ste8, were found.

Basic theory and experimental techniques of the strain-gradient method

Abstract: The theories of presently used experimental methods of stress and deformation analysis which employ radiant energy as a detector are based on the assumption that light propagates rectilinearly within both undeformed and deformed bodies which are initially homogeneous and isotropic when diffraction phenomena are negligible. This assumption is not correct: light propagation within deformed bodies is nonrectilinear in a general case. Although this has already been observed and applied practically by some researchers in photoelasticity, it has not so far been generally acknowledged and accepted in experimental mechanics. On the basis of empirical data produced by the authors in the period 1948–1983, we present theories and foundations of the techniques of a new experimental method which is based on the relations between stress/strain gradients and curvatures of light beams. This method is called the strain-gradient method or, less rigorously, gradient photoelasticity.

Review of optical fiber couplers

Abstract: The present trends are toward implementation of fiber optics in data-distribution networks such as data buses, community antenna television, and local area networks. The thrust is, therefore, on high-volume and economical production of good quality optical fiber couplers. This paper reviews the various techniques for making fiber couplers. Important characteristics of various constructions and configurations together with their merits and demerits are discussed.

Continuous production of L-phenylalanine by transamination.

Abstract: L-Phenylalanine was produced continuously from L-as-partate and phenylpyruvate by transaminase from a newly screened Pseudomonas putida strain. The process was carried out with an isolated enzyme in homogeneous phase in an enzyme membrane reactor and with immobilized whole cells in a stirred tank reactor, respectively. Due to the difference in transport resistance, the productivity of the free enzyme in homogeneous phase (72 mmol/L h) was about 3 times higher than the productivity achieved using immobilized cells. However, a better stability of the biocatalyst was observed with immobilized cells.

Determination of the glass transition temperature of metallic glasses by low-frequency internal friction measurements*

Abstract: Based on the “isoviscous” definition of the glass transition (η(Tg) = 1012 Ns/m2), it is shown that the glass transition temperature Tg can be determined by low-frequency internal friction measurements. By comparing the anelastic and viscoelastic contributions to internal friction, an upper frequency limit for this kind of Tg determination is derived; for metallic glasses, a rough estimate of 0.2 Hz is given. In corresponding experiments at 0.1 Hz on amorphous Pd40Ni40P20, a cross-over of the internal friction Q−1 vs temperature is observed for different heating rates Ṫ: whereas below the glass transition range Q−1 is lower for lower Ṫ due to structural relaxation, the opposite is true for higher temperatures due to the shift in Tg with Ṫ. This shift is found to be linear in log Ṫ both for Tg values obtained by internal friction and by DSC. However, a single straight line of Tg (log Ṫ) for both methods is obtained from 0.03 to 100 K/min only if in DSC Tg is defined at the end of the corresponding increase in specific heat. These findings are discussed with respect to the compatibility of the different definitions of Tg. There seems to be some independent evidence that the results obtained on Pd40Ni40P20 might be of more general importance.

Electronically tunable all-pole low-pass leapfrog ladder filter with operational transconductance amplifier

Abstract: A general theory of the synthesis of all-pole low-pass filters using only operational transconductance amplifiers and capacitors with the leapfrog design method is presented. Especially the possibility of a simple method for tuning the cut-off frequency will be shown. An estimation of the worst-case sensitivity and a comparison of theory and experiment will complete the paper.