135 research outputs found
Joining-splitting interaction of non-critical string
The joining--splitting interaction of non-critical bosonic string is analyzed
in the light-cone formulation. The Mandelstam method of constructing tree
string amplitudes is extended to the bosonic massive string models of the
discrete series. The general properties of the Liouville longitudinal
excitations which are necessary and sufficient for the Lorentz covariance of
the light-cone amplitudes are derived. The results suggest that the covariant
and the light-cone approach are equivalent also in the non-critical dimensions.
Some aspects of unitarity of interacting non-critical massive string theory are
discussed.Comment: 38 pages, 4 embedded figures, discussion in the Introduction
clarified, Appendix D and some material from Section 5 remove
Topologically confined states at corrugations of gated bilayer graphene
We investigate the electronic and transport properties of gated bilayer
graphene with one corrugated layer, which results in a stacking AB/BA boundary.
When a gate voltage is applied to one layer, topologically protected gap states
appear at the corrugation, which reveal as robust transport channels along the
stacking boundary. With increasing size of the corrugation, more localized,
quantum-well-like states emerge. These finite-size states are also conductive
along the fold, but in contrast to the stacking boundary states, which are
gapless, they present a gap. We have also studied periodic corrugations in
bilayer graphene; our findings show that such corrugations between AB- and
BA-stacked regions behave as conducting channels that can be easily identified
by their shape
Interface States in Carbon Nanotube Junctions: Rolling up graphene
We study the origin of interface states in carbon nanotube intramolecular
junctions between achiral tubes. By applying the Born-von Karman boundary
condition to an interface between armchair- and zigzag-terminated graphene
layers, we are able to explain their number and energies. We show that these
interface states, costumarily attributed to the presence of topological
defects, are actually related to zigzag edge states, as those of graphene
zigzag nanoribbons. Spatial localization of interface states is seen to vary
greatly, and may extend appreciably into either side of the junction. Our
results give an alternative explanation to the unusual decay length measured
for interface states of semiconductor nanotube junctions, and could be further
tested by local probe spectroscopies
Controlling the layer localization of gapless states in bilayer graphene with a gate voltage
Experiments in gated bilayer graphene with stacking domain walls present
topological gapless states protected by no-valley mixing. Here we research
these states under gate voltages using atomistic models, which allow us to
elucidate their origin. We find that the gate potential controls the layer
localization of the two states, which switches non-trivially between layers
depending on the applied gate voltage magnitude. We also show how these bilayer
gapless states arise from bands of single-layer graphene by analyzing the
formation of carbon bonds between layers. Based on this analysis we provide a
model Hamiltonian with analytical solutions, which explains the layer
localization as a function of the ratio between the applied potential and
interlayer hopping. Our results open a route for the manipulation of gapless
states in electronic devices, analogous to the proposed writing and reading
memories in topological insulators
Massive annotation of bacterial L-asparaginases reveals their puzzling distribution and frequent gene transfer events
l-Asparaginases, which convert l-asparagine to l-aspartate and ammonia, come in five types, AI-AV. Some bacterial type AII enzymes are a key element in the treatment of acute lymphoblastic leukemia in children, but new l-asparaginases with better therapeutic properties are urgently needed. Here, we search publicly available bacterial genomes to annotate l-asparaginase proteins belonging to the five known types. We characterize taxonomic, phylogenetic, and genomic patterns of l-asparaginase occurrences pointing to frequent horizontal gene transfer (HGT) events, also occurring multiple times in the same recipient species. We show that the reference AV gene, encoding a protein originally found and structurally studied in Rhizobium etli, was acquired via HGT from Burkholderia. We also describe the sequence variability of the five l-asparaginase types and map the conservation levels on the experimental or predicted structures of the reference enzymes, finding the most conserved residues in the protein core near the active site, and the most variable ones on the protein surface. Additionally, we highlight the most common sequence features of bacterial AII proteins that may aid in selecting therapeutic l-asparaginases. Finally, we point to taxonomic units of bacteria that do not contain recognizable sequences of any of the known l-asparaginase types, implying that those microorganisms most likely contain new, as yet unknown types of l-asparaginases. Such novel enzymes, when properly identified and characterized, could hold promise as antileukemic drugs
The effects of nature-inspired amino acid substitutions on structural and biochemical properties of the E. coli L-asparaginase EcAIII
Octagonal defects at carbon nanotube junctions,”
We investigate knee-shaped junctions of semiconductor zigzag carbon nanotubes. Two dissimilar octagons appear at such junctions; one of them can reconstruct into a pair of pentagons. The junction with two octagons presents two degenerate localized states at Fermi energy ( ). The reconstructed junction has only one state near , indicating that these localized states are related to the octagonal defects. The inclusion of Coulomb interaction splits the localized states in the junction with two octagons, yielding an antiferromagnetic system
Glioblastoma-derived spheroid cultures as an experimental model for analysis of EGFR anomalies
Glioblastoma cell cultures in vitro are frequently used for investigations on the biology of tumors or new therapeutic approaches. Recent reports have emphasized the importance of cell culture type for maintenance of tumor original features. Nevertheless, the ability of GBM cells to preserve EGFR overdosage in vitro remains controversial. Our experimental approach was based on quantitative analysis of EGFR gene dosage in vitro both at DNA and mRNA level. Real-time PCR data were verified with a FISH method allowing for a distinction between EGFR amplification and polysomy 7. We demonstrated that EGFR amplification accompanied by EGFRwt overexpression was maintained in spheroids, but these phenomena were gradually lost in adherent culture. We noticed a rapid decrease of EGFR overdosage already at the initial stage of cell culture establishment. In contrast to EGFR amplification, the maintenance of polysomy 7 resulted in EGFR locus gain and stabilization even in long-term adherent culture in serum presence. Surprisingly, the EGFRwt expression pattern did not reflect the latter phenomenon and we observed no overexpression of the tested gene. Moreover, quantitative analysis demonstrated that expression of the truncated variant of receptor—EGFRvIII was preserved in GBM-derived spheroids at a level comparable to the initial tumor tissue. Our findings are especially important in the light of research using glioblastoma culture as the experimental model for testing novel EGFR-targeted therapeutics in vitro, with special emphasis on the most common mutated form of receptor—EGFRvIII
Formation of Amyloid-Like Fibrils by Y-Box Binding Protein 1 (YB-1) Is Mediated by Its Cold Shock Domain and Modulated by Disordered Terminal Domains
YB-1, a multifunctional DNA- and RNA-binding nucleocytoplasmic protein, is involved in the majority of DNA- and mRNA-dependent events in the cell. It consists of three structurally different domains: its central cold shock domain has the structure of a β-barrel, while the flanking domains are predicted to be intrinsically disordered. Recently, we showed that YB-1 is capable of forming elongated fibrils under high ionic strength conditions. Here we report that it is the cold shock domain that is responsible for formation of YB-1 fibrils, while the terminal domains differentially modulate this process depending on salt conditions. We demonstrate that YB-1 fibrils have amyloid-like features, including affinity for specific dyes and a typical X-ray diffraction pattern, and that in contrast to most of amyloids, they disassemble under nearly physiological conditions
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