Home About us Contact
|
Home About us Contact | ||
|
|
||
Recombination Mechanisms (recombination + mechanism)
Selected AbstractsRecombination mechanism of luminescence excitation of self-trapped excitons in BeOPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 1 2005S.V. Gorbunov Abstract Time-resolved spectra, polarization of the luminescence and transient optical absorption (TOA) induced by irradiation of beryllium oxide crystals with electron pulses have been studied. Exponential stages with decay time , = 6.5 ms in luminescence bands at 4.0, 5.0 and 6.7 eV coinciding in spectral and polarized properties with the luminescence of two different types of self-trapped excitons (STE) have been found. Efficiency of formation of centers with decay time 6.5 ms appeared to be comparable with that of formation of triplet STE. The electron fluence increase does not change the exponential character and relaxation of TOA decaying with time 6.5 ms and results in a linear growth of optical density. These data are typical of monomolecular recombination processes. Similarity of TOA spectra of centers with decay time 6.5 ms with those of V-type hole centers and hole component of STE shows that the stage 6.5 ms is due to the recombination of cation Frenkel defects. The recombination of close spatially correlated Frenkel pairs Be+ V, manifests itself in a form of exponential components with decay time 6.5 ms in the luminescence of two types of STE in BeO. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] The phylogenetic origins of the antigen-binding receptors and somatic diversification mechanismsIMMUNOLOGICAL REVIEWS, Issue 1 2004John P. Cannon Summary:, The adaptive immune system arose in ancestors of the jawed vertebrates approximately 500 million years ago. Homologs of immunoglobulins (Igs), T-cell antigen receptors (TCRs), major histocompatibility complex I (MHC I) and MHC II, and the recombination-activating genes (RAGs) have been identified in all extant classes of jawed vertebrates; however, no definitive homolog of any of these genes has been identified in jawless vertebrates or invertebrates. RAG-mediated recombination and associated junctional diversification of both Ig and TCR genes occurs in all jawed vertebrates. In the case of Igs, somatic variation is expanded further through class switching, gene conversion, and somatic hypermutation. Although the identity of the ,primordial' receptor that was interrupted by the recombination mechanism in jawed vertebrates may never be established, many different families of genes that exhibit predicted characteristics of such a receptor have been described both within and outside the jawed vertebrates. Recent data from various model systems point toward a continuum of immune receptor diversity, encompassing many different families of recognition molecules whose functions are integrated in an organism's response to pathogenic invasion. Various approaches, including both genomic and protein-functional analyses, currently are being applied in jawless vertebrates, protochordates, and other invertebrate deuterostome systems and may yield definitive evidence regarding the presence or absence of adaptive immune homologs in species lacking adaptive immune systems. Such studies have the potential for uncovering previously unknown mechanisms of generating receptor diversity. [source] Mechanistic studies of intramolecular CH insertion reaction of arylnitrenes: isotope effect, configurational purity and radical clock studiesJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 1 2005Shigeru Murata Abstract In order to reveal the mechanism of the intramolecular CH insertion of arylnitrenes, three experiments were carried out: measurement of isotope effects, determination of the extent of configurational retention and radical clock studies. Irradiation of the deuterium-substituted azide 4 - d in an inert solvent exclusively afforded the indolines 5 - h and 5 - d, in which the kinetic isotope effect kH/kD on the intramolecular CH insertion of the nitrene was evaluated as 12.6,14.7 at room temperature. A chiral chromatographic analysis of the indoline 11 obtained from the optically active azide (S)- 6 revealed that the enantiomeric purity of the starting azide was almost completely lost during the intramolecular CH insertion of the photolytically generated nitrene (enantiomeric excess <10%). The thermolysis of the azide 7 at 180°C mainly gave a mixture of the cyclopropyl ring-opened products 20,22, together with the intramolecular CH insertion product with an intact cyclopropyl ring 19. On the basis of these observations, we concluded that the intramolecular CH insertion of the nitrene proceeds primarily by the hydrogen abstraction,recombination mechanism. We propose, however, a small contribution of the concerted mechanism to the intramolecular CH insertion, based on the solvent dependence of the isotope effect and the extent of the configurational retention. Copyright © 2004 John Wiley & Sons, Ltd. [source] MicroCommentary: Smarter than the average phageMOLECULAR MICROBIOLOGY, Issue 4 2004Garry W. Blakely Summary The seventh cholera pandemic emerged in the poorer nations of the world towards the end of the 20th century and continues to kill thousands of people per year. The causative agent of cholera, the Gram-negative bacterium Vibrio cholera, is only pathogenic when it contains a lysogenic bacteriophage, CTX,, that encodes the toxin responsible for inducing massive fluid loss from the human host. Site-specific integration of CTX, into chromosome I of V. cholera occurs at a site, dif, that is normally required for resolution of chromosome dimers generated by homologous recombination. An article in this issue of Molecular Microbiology reports the analysis of interactions between two host encoded recombinases, XerC and XerD, and the recombination sites involved in lysogeny. Surprisingly, recombination between the CTX,attP site and the chromosomal dif site requires additional recombinase binding sites, downstream from the positions of strand exchange, which might play an architectural role. The positions of strand cleavage also differ significantly between the two sites, suggesting a novel recombination mechanism that implicates additional host factors in resolution of the Holliday junction intermediate. [source] Growth and characterization of isotopic natGa15N by molecular-beam epitaxyPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue S2 2009Yong-zhao Yao Abstract Isotopically enriched gallium nitride films, Ga14N and Ga15N, have been fabricated by molecular-beam epitaxy to study the effects of nitrogen atomic mass variation on structures and properties of GaN. The phonon frequency shift due to the isotopic substitution was clearly observed using Raman spectroscopy. The lattice constants of Ga15N differed from those of Ga14N; the unit cell volume of Ga15N was approximately 0.06% less than that of Ga14N. Temperature-dependent photoluminescence measurements revealed that the recombination mechanism in Ga14N and Ga15N was the same in the temperature range of 4-50 K, and the band gap energy difference was Eg15 -Eg14 = 6.0 ± 0.1 meV. This Eg difference is discussed in terms of volume shrinkage and change in phonon-electron interaction due to isotopic substitution. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Admittance spectroscopy defect density of electrodeposited CuIn(S,Se)2 and its correlation with solar cells performancesPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 11 2008A. Darga Abstract Electrodeposited CuIn(S,Se)2 based solar cells with varying CdS buffer layer thicknesses were studied by admittance spectroscopy. An electrically active defect was identified. Its density of states which varies with CdS layer deposition process was found to be correlated with solar cell performance. This defect seems to be CdS/CuIn(S,Se)2 interface defect or to be located within the grain boundaries of the absorber layer. Direct dark I,V measurements reveal that the dominant recombination mechanism is a tunnelling assisted process. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Antigenic Variation in Pneumocystis,THE JOURNAL OF EUKARYOTIC MICROBIOLOGY, Issue 1 2007JAMES R. STRINGER ABSTRACT. Pneumocystis is a genus containing many species of non-culturable fungi, each of which infects a different mammalian host. Pneumonia caused by Pneumocystis is a problem in immunodeficient humans, but not in normal humans. Nevertheless, it appears that Pneumocystis organisms cannot survive and proliferate outside of their mammalian hosts, suggesting that Pneumocystis parasitizes immunocompetent mammals. Residence in immunocompetent hosts may rely on camouflage perpetrated by antigenic variation. In P. carinii, which is found in rats, there exist three families of genes that appear to be designed to create antigenic variation. One gene family, which encodes the major surface glycoprotein (MSG), contains nearly 100 members. Expression of the MSG family is controlled by restricting transcription to the one gene that is linked to a unique expression site. Changes in the sequence of the MSG gene linked to the expression site occur and appear to be caused by recombination with MSG genes not at the expression site. Preliminary evidence suggests that gene conversion is the predominant recombination mechanism. [source] Aspects of stability and phenomenology in type IIA orientifolds with intersecting D6-branesFORTSCHRITTE DER PHYSIK/PROGRESS OF PHYSICS, Issue 1 2004T. OttArticle first published online: 14 JAN 200 Abstract Intersecting branes have been the subject of an elaborate string model building for several years. After a general introduction into string theory, this work introduces in detail the toroidal and -orientifolds. The picture involving D9-branes with B-fluxes is shortly reviewed, but the main discussion employs the T-dual picture of intersecting D6-branes. The derivation of the R-R and NS-NS tadpole cancellation conditions in the conformal field theory is shown in great detail. Various aspects of the open and closed chiral and non-chiral massless spectrum are discussed, involving spacetime anomalies and the generalized Green-Schwarz mechanism. An introduction into possible gauge breaking mechanisms is given, too. Afterwards, both ,, = 1 supersymmetric and non-supersymmetric approaches to low energy model building are treated. Firstly, the problem of complex structure instabilities in toroidal ,R -orientifolds is approached by a -orbifolded model. In particular, a stable non-supersymmetric standard-like model with three fermion generations is discussed. This model features the standard model gauge groups at the same time as having a massless hypercharge, but possessing an additional global B - L symmetry. The electroweak Higgs mechanism and the Yukawa couplings are not realized in the usual way. It is shown that this model descends naturally from a flipped SU(5) GUT model, where the string scale has to be at least of the order of the GUT scale. Secondly, supersymmetric models on the -orbifold are discussed, involving exceptional 3-cycles and the explicit construction of fractional D-branes. A three generation Pati-Salam model is constructed as a particular example, where several brane recombination mechanisms are used, yielding non-flat and non-factorizable branes. This model even can be broken down to a MSSM-like model with a massless hypercharge. Finally, the possibility that unstable closed and open string moduli could have played the role of the inflaton in the evolution of the universe is being explored. In the closed string sector, the important slow-rolling requirement can only be fulfilled for very specific cases, where some moduli are frozen and a special choice of coordinates is taken. In the open string sector, inflation does not seem to be possible at all. [source] Molecular characterization of the spectrum of genomic deletions in the mismatch repair genes MSH2, MLH1, MSH6, and PMS2 responsible for hereditary nonpolyposis colorectal cancer (HNPCC)GENES, CHROMOSOMES AND CANCER, Issue 2 2005Heleen van der Klift A systematic search by Southern blot analysis in a cohort of 439 hereditary nonpolyposis colorectal cancer (HNPCC) families for genomic rearrangements in the main mismatch repair (MMR) genes, namely, MSH2, MLH1, MSH6, and PMS2, identified 48 genomic rearrangements causative of this inherited predisposition to colorectal cancer in 68 unrelated kindreds. Twenty-nine of the 48 rearrangements were found in MSH2, 13 in MLH1, 2 in MSH6, and 4 in PMS2. The vast majority were deletions, although one previously described large inversion, an intronic insertion, and a more complex rearrangement also were found. Twenty-four deletion breakpoints have been identified and sequenced in order to determine the underlying recombination mechanisms. Most fall within repetitive sequences, mainly Alu repeats, in agreement with the differential distribution of deletions between the MSH2 and MLH1 genes: the higher number and density of Alu repeats in MSH2 corresponded with a higher incidence of genomic rearrangement at this disease locus when compared with other MMR genes. Long interspersed nuclear element (LINE) repeats, relatively abundant in, for example, MLH1, did not seem to contribute to the genesis of the deletions, presumably because of their older evolutionary age and divergence among individual repeat units when compared with short interspersed nuclear element (SINE) repeats, including Alu repeats. Moreover, Southern blot analysis of the introns and the genomic regions flanking the MMR genes allowed us to detect 6 novel genomic rearrangements that left the coding region of the disease-causing gene intact. These rearrangements comprised 4 deletions upstream of the coding region of MSH2 (3 cases) and MSH6 (1 case), a 2-kb insertion in intron 7 of PMS2, and a small (459-bp) deletion in intron 13 of MLH1. The characterization of these genomic rearrangements underlines the importance of genomic deletions in the etiology of HNPCC and will facilitate the development of PCR-based tests for their detection in diagnostic laboratories. © 2005 Wiley-Liss, Inc. [source] RecA-mediated excision repair: a novel mechanism for repairing DNA lesions at sites of arrested DNA synthesisMOLECULAR MICROBIOLOGY, Issue 1 2007Marc Bichara Summary In Escherichia coli, bulky DNA lesions are repaired primarily by nucleotide excision repair (NER). Unrepaired lesions encountered by DNA polymerase at the replication fork create a blockage which may be relieved through RecF-dependent recombination. We have designed an assay to monitor the different mechanisms through which a DNA polymerase blocked by a single AAF lesion may be rescued by homologous double-stranded DNA sequences. Monomodified single-stranded plasmids exhibit low survival in non-SOS induced E. coli cells; we show here that the presence of a homologous sequence enhances the survival of the damaged plasmid more than 10-fold in a RecA-dependent way. Remarkably, in an NER proficient strain, 80% of the surviving colonies result from the UvrA-dependent repair of the AAF lesion in a mechanism absolutely requiring RecA and RecF activity, while the remaining 20% of the surviving colonies result from homologous recombination mechanisms. These results uncover a novel mechanism , RecA-mediated excision repair , in which RecA-dependent pairing of the mono-modified single-stranded template with a complementary sequence allows its repair by the UvrABC excinuclease. [source] Carrier recombination mechanisms in mid-infrared GaInAsSb quantum well lasersPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 1 2007K. O'Brien Abstract Hydrostatic pressure and spontaneous emission techniques have been used to examine the important recombination mechanisms in type-I GaInAsSb/GaSb quantum well lasers. High pressure results indicate that Auger recombination dominates the threshold current of 2.11 ,m and 2.37 ,m devices and is the origin of their temperature sensitivity around room temperature. While the characteristics of the 2.37 ,m devices are much improved by the suppression of the CHSH Auger process, since its spin,orbit splitting energy is greater than its band gap, other important Auger processes such as CHHL and CHCC persist. In the larger band gap 2.11 ,m devices, an increase in threshold current with pressure is observed suggesting that CHSH Auger is present in these devices at atmospheric pressure and contributes to performance degradation at these shorter wavelengths. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Impurity levels in the layered semiconductor p-GaSe doped with group V elements As, Bi and SbPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 15 2005S. Shigetomi Abstract The radiative and non radiative recombination mechanisms in the As, Sb and Bi-doped GaSe have been investigated on the basis of photoluminescence (PL) and Hall effect measurements. The PL features (at 77 K) related to the impurity levels coming from the As, Sb and Bi atoms are dominated by a broad emission band at about 1.7 eV. From the temperature dependences of the peak energy and PL intensity and the dependence of excitation intensity of peak energy, it was found that the 1.7 eV emission band is due to the transition from the shallow donor level at about 0.08 eV below the conduction band to the deep acceptor. In addition it was found, from the temperature dependence of hole concentration, that a deep acceptor level at about 0.6 eV above the valence band is formed by the doping atoms. It is associated with defects or defect complexes. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Performance of thin-film transistors fabricated by sequential lateral solidification crystallization techniquesPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 12 2008M. A. Exarchos Abstract The performance of Excimer Laser Annealed (ELA) Thin-Film Transistors (TFTs), in terms of drain current behaviour in unstressed as well as in DC stressed devices, is presented. The transistors studied were fabricated under different irradiation schemes of a novel Sequential Lateral Solidification (SLS) process. As far as unstressed transistors concerned, drain current transients relaxed through stretched exponential law. Fitting results disclosed that both gate dielectric polarization and carrier recombination mechanisms occurred through transient relaxation. Deep Level Transient Spectroscopy (DLTS) technique was called forth in order to investi- gate the origin of carrier recombination mechanisms. DC hot carrier stress measurements, under "worst ageing condition" regime, were conducted in order to probe degradation mechanisms and device reliability standards. Crystal domain size significantly affects threshold voltage degradation. The latter increases with decreasing crystal domain size, due to increased concentration of protrusions in the polysilicon film. Transconductance degradation also depends on crystal domain size, attributed mainly to bulk polysilicon trap generation. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | ||||||||||||||||