Gesetzmäßigkeiten von Funkenentladungen im Nanosekundenbereich

In the experimental section of this paper it is shown how the shape of the electrical spark discharge pulse (in air) depends on the discharged capacitor C, the initial field strength E0, and the inductance of the electrodes and of the spark L0. In order to get nanosecond pulses, the time constant C must be some nanoseconds and the circuit has almost to be damped critically. The experiments confirm that this can be achieved for each capacitor C by choosing a suitable spark length l (or field strength E0). In the theoretical section the current is calculated for a discharge circuit with a time-dependent spark resistance R(t)=g/t. Under certain assumptions a solution for a “hard” discharge can be calculated according to the theory of WEIZEL and ROMPE. Thus formulae for imax, (di/dt)max, the spark resistance constant g0 and the risetime τan are obtained, and these prove to be in fair agreement with experimental results. The half-width of the light pulse exceeds that of the envelope of the electrical pulse approximately 1 ... 2 nanoseconds. Thus the design of nanosecond light sources can be based on this theory.

Messungen der Elektronenbeweglichkeit bei hohen Gasdrucken in Ar, He, N2 und H2

By means of a time of flight method the electron drift velocity ν _ in argon, helium, nitrogen, and hydrogen was determined as function of E/p [V/Torr cm] at room temperature and for high pressure up to 42 at, at which no measurements were as yet available. In He and H2 E/p was decreased to that range where the electrons are in thermal equilibrium with thet gas. In N2 measurements were extended nearly to this range, and in Ar measurements were carried out for E/p well above this range of thermal equilibrium.

The accuracy from 1% to 1.5% in the available pressure range enabled an examination of the similarity rule. For the diatomic gases N2 and H2 the measurements show that for constant E/p, in the range E/p < 1, the drift velocities decrease with increasing pressure. This effect was not found in Ar. In He no effect was found in the pressure range up to 8400 Torr (or it lies within the limits of error).

Peptide bond formation mediated by substrate mimetics. Structure-guidedoptimization of trypsin for synthesis

Substrate mimetics are excellent tools for protease-mediated peptide synthesis that enable the coupling of peptides independently of the primary specificity of the enzyme without undesired cleavages of the newly formed peptide bonds. However, the synthetic utility of this beneficial approach is limited to reactions with nonspecific amino-acid-containing peptides while the coupling of specific ones leads to unwanted cleavages due to the native proteolytic activity of the biocatalyst. This paper reports on the use of site-directed mutagenesis to design trypsin variants with decreased cleavage activity. Starting from the variant D189S, which is known for its low proteolytic potential, Ser189 and Ser190 were exchanged for Ala to further repress the inherent amidase activity of trypsin D189S. The effect of mutations was analysed by model synthesis reactions using specific amino-acid-containing peptides and substrate mimetics as the reactants. Finally, computer-assisted protein-ligand docking studies were performed to get closer insight into the molecular basis of the experimental results.

Spo0A directly controls the switch from acid to solvent production in solvent-forming clostridia

The spo0A genes of Clostridium beijerinckii NCIMB 8052 and Clostridium cellulolyticum ATCC 35319 were isolated and characterized. The C-terminal DNA-binding domains of the predicted products of spo0A from these two organisms, as well as 16 other taxonomically diverse species of Bacillus and Clostridium, show extensive amino acid sequence conservation (56% identity, 65% similarity over 104 residues). A 12-amino-acid motif (SRVERAIRHAIE) that forms the putative DNA recognition helix is particularly highly conserved, suggesting a common DNA target. Insertional inactivation of spo0A in C. beijerinckii blocked the formation of solvents (as well as spores and granulose). Sequences resembling Spo0A-binding motifs (TGNCGAA) are found in the promoter regions of several of the genes whose expression is modulated at the onset of solventogenesis in Clostridium acetobutylicum and C. beijerinckii. These include the upregulated adc gene, encoding acetoacetate decarboxylase (EC 4.1.1. 4), and the downregulated ptb gene, encoding phosphotransbutyrylase (EC 2.3.1.c). In vitro gel retardation experiments using C. acetobutylicum adc and C. beijerinckii ptb promoter fragments and recombinant Bacillus subtilis and C. beijerinckii Spo0A suggested that adc and ptb are directly controlled by Spo0A. The binding affinity was reduced when the 0A boxes were destroyed, and enhanced when they were modified to conform precisely to the consensus sequence. In vivo analysis of wild-type and mutagenized promoters transcriptionally fused to the gusA reporter gene in C. beijerinckii validated this hypothesis. Post-exponential phase expression from the mutagenized adc promoter was substantially reduced, whereas expression from the mutagenized ptb promoter was not shut down at the end of exponential growth.

Messungen des Anlagerungskoeffizienten von Elektronen in Sauerstoff

The attachment coefficient η for the formation of negative ions in low energy electron swarms was measured over an E/p-range of 0.1 ... 30 V/Torr cm by a new and accurate method. Thus earlier measurements up to 54 Torr of other authors could be extended to 880 Torr.

The shape of the minimum in the η/p-curves between the three-body process (e+2O2 → O2- + O2) and the dissociative process (e + O2 → O- + O) and its shift to higher E/p with increasing pressure was measured. Behind the minimum a maximum at E/p = 14 was found. Between the minimum and this maximum the dissociative process is predominant but the three-body process is still of influence. For E/p > 14 the η/p-values are slowly decreasing with increasing E/p.

For the higher pressures above 44 Torr deviations from the relation η proportional to p2 were found for the three-body process. These deviations are discussed.