Calidad de la electricidad en el suministro eléctrico de los consumidores de transporte ferroviario sin tracción.

El artículo presenta los resultados del estudio de la influencia de las lámparas LED con fuentes de alimentación conmutadas que contienen correctores del factor de potencia sobre la calidad de la electricidad en el sistema de suministro de energía de los consumidores sin tracción del transporte ferroviario. Se observa que la emisión de componentes armónicos de corriente por cargas electrónicas puede ocasionar desviaciones en la calidad de la electricidad hasta la ocurrencia de situaciones de emergencia por sobrecargas de redes de baja (0.4 kV) y media (6/10 kV) de tensión. Una carga electrónica no lineal (lámpara LED), aunque equipada con un corrector de factor de potencia, provoca la generación de un nivel significativo de armónicos de corriente, que afectan significativamente el funcionamiento del transformador. Como resultado, el transformador, calculado teniendo en cuenta el factor de carga de 0,7 - 0,8, no se puede operar normalmente en las condiciones de uso de dispositivos de carga electrónicos. Se realiza el modelado matemático de la corrección de los modos del sistema de alimentación con cargas monofásicas no lineales, siempre que los dispositivos de corrección realizados en base a varios condensadores y reactores con características no lineales se utilicen para equilibrar los modos

Vacuum Expectation Value of a Wegner-Wilson Loop near the Light Cone

Vacuum expectation values for one Wegner-Wilson loop representing a moving quark-antiquark pair are calculated in four-dimensional Euclidean and Minkowski space-time. The calculation uses gluon field strength correlators with perturbative gluon exchange and non-perturbative correlations from the stochastic vacuum model. The expectation value of a Wegner-Wilson loop forming a hyperbolic angle in Minkowski space-time is connected by an analytical continuation to the expectation value of the Wegner-Wilson loop in Euclidean four-space. The obtained result shows how confinement enters into the light-cone Hamiltonian for valence quarks independently of the chosen model.Comment: 12 pages, 2 figures, sections 2 and 3 shortened, more details added in section

Evolution of glyoxylate cycle enzymes in Metazoa: evidence of multiple horizontal transfer events and pseudogene formation

BACKGROUND: The glyoxylate cycle is thought to be present in bacteria, protists, plants, fungi, and nematodes, but not in other Metazoa. However, activity of the glyoxylate cycle enzymes, malate synthase (MS) and isocitrate lyase (ICL), in animal tissues has been reported. In order to clarify the status of the MS and ICL genes in animals and get an insight into their evolution, we undertook a comparative-genomic study. RESULTS: Using sequence similarity searches, we identified MS genes in arthropods, echinoderms, and vertebrates, including platypus and opossum, but not in the numerous sequenced genomes of placental mammals. The regions of the placental mammals' genomes expected to code for malate synthase, as determined by comparison of the gene orders in vertebrate genomes, show clear similarity to the opossum MS sequence but contain stop codons, indicating that the MS gene became a pseudogene in placental mammals. By contrast, the ICL gene is undetectable in animals other than the nematodes that possess a bifunctional, fused ICL-MS gene. Examination of phylogenetic trees of MS and ICL suggests multiple horizontal gene transfer events that probably went in both directions between several bacterial and eukaryotic lineages. The strongest evidence was obtained for the acquisition of the bifunctional ICL-MS gene from an as yet unknown bacterial source with the corresponding operonic organization by the common ancestor of the nematodes. CONCLUSION: The distribution of the MS and ICL genes in animals suggests that either they encode alternative enzymes of the glyoxylate cycle that are not orthologous to the known MS and ICL or the animal MS acquired a new function that remains to be characterized. Regardless of the ultimate solution to this conundrum, the genes for the glyoxylate cycle enzymes present a remarkable variety of evolutionary events including unusual horizontal gene transfer from bacteria to animals. REVIEWERS: Arcady Mushegian (Stowers Institute for Medical Research), Andrey Osterman (Burnham Institute for Medical Research), Chris Ponting (Oxford University)

Biotehnološka primjena kvasca Yarrowia lipolytica uzgojenog u uvjetima nedostatka tiamina

During the cultivation of a thiamine-auxotrophic yeast strain Yarrowia lipolytica VKM Y-2412 on ethanol, the growth limitation by thiamine leads to the production of α-ketoglutaric acid. The α-ketoglutaric acid synthesis has been studied in dependence on pH, oxygen supply and ethanol, zinc and iron concentrations. Under optimal conditions, Y. lipolytica produced 88.7 g/L of α-ketoglutaric acid. The culture broth containing α-ketoglutaric acid was subjected to chemical treatment with hydrogen peroxide, which led to the formation of succinic acid in significant quantities (71.7 g/L). Further direct esterification of succinic acid with excess absolute ethanol yielded diethyl succinate. Biomass of Y. lipolytica, a superproducer of α-ketoglutaric acid, was characterized by a high content of protein and essential amino acids, free amino acids, and unusually large amount of γ-aminobutyric acid. The unique amino acid composition of the producer makes it possible to use this biomass as a component of parenteral nutrition mixtures and as a basis for neuroleptics.Auksotrofni kvasac Yarrowia lipolytica VKM Y-2412 uzgojen je na podlozi s etanolom u uvjetima nedostatka tiamina, što je dovelo do nastajanja α-ketoglutarne kiseline. Ispitana je ovisnost njezine sinteze o pH-vrijednosti, opskrbljenosti kisikom i etanolom, te koncentracijama cinka i željeza. U optimalnim je uvjetima kvasac Y. lipolytica proizveo 88,7 g/L α-ketoglutarne kiseline. Podloga za uzgoj koja je sadržavala α-ketoglutarnu kiselinu kemijski je obrađena vodikovim peroksidom, što je dovelo do nastajanja značajne koncentracije sukcinske kiseline (71,7 g/L). Daljnjom izravnom esterifikacijom sukcinske kiseline uz suvišak apsolutnog etanola dobiven je dietil sukcinat. Biomasu kvasca Y. lipolytica, koji proizvodi velike količine α-ketoglutarne kiseline, karakterizira veliki udjel proteina i esencijalnih aminokiselina, slobodnih aminokiselina i neuobičajeno velika količina γ-aminomaslačne kiseline. Jedinstveni aminokiselinski sastav kvasca omogućuje njegovu primjenu u intravenoznoj prehrani i pri liječenju pacijenata antipsihoticima

Vibrating the QCD string

The large distance behaviour of the adiabatic hybrid potentials is studied in the framework of the QCD string model. The calculated spectra are shown to be the result of interplay between potential-type longitudinal and string-type transverse vibrations.Comment: LaTeX2e, 9 pages, 2 Postscript figures, final version to appear in Yad.Fi

Spectroscopy of Baryons Containing Two Heavy Quarks in Nonperturbative Quark Dynamics

We have studied the three quark systems in an Effective Hamiltonian approach in QCD. With only two parameters: the string tension sigma and the strong coupling constant alpha_s we obtain a good description of the ground state light and heavy baryons. The prediction of masses of the doubly heavy baryons not discovered yet are also given. In particular, a mass of 3620 MeV for the lightest (ccu) baryon is found by employing the hyperspherical formalism to the three quark confining potential with the string junction.Comment: 8 pages, LaTe

Comment on the proper QCD string dynamics in a heavy-light system

The string correction to the inter-quark interaction at large distances is derived using the field theory approach to a heavy-light quark-antiquark system in the modified Fock-Schwinger gauge.Comment: LaTeX2e, 6 pages, no figures, to appear in JETP Let

From scalar to string confinement

We outline a connection between scalar quark confinement, a phenomenologically successful concept heretofore lacking fundamental justification, and QCD. Although scalar confinement does not follow from QCD, there is an interesting and close relationship between them. We develop a simple model intermediate between scalar confinement and the QCD string for illustrative purposes. Finally, we find the bound state masses of scalar, time-component vector, and string confinement analytically through semi-classical quantization.Comment: ReVTeX, 9 pages, 5 figure