Oscillator Strength of Metallic Carbon Nanotubes

Based on the tight binding method with hopping integral between the nearest-neighbor atoms, an oscillator strength \int_0^{\infty} \d \omega {\rm Re} \sigma (\omega) is discussed for armchair and metallic zigzag carbon nanotubes. The formulae of the oscillator strength are derived for both types of nanotubes and are compared with the result obtained by a linear chain model. In addition, the doping dependence is investigated in the absence of Coulomb interaction. It is shown that the oscillator strength of each carbon nanotube shows qualitatively the same doping dependence, but the fine structure is different due to it's own peculiar band structure. Some relations independent of the radius of the tube are derived, and a useful formula for determining the amount of doping is proposed.Comment: 4 pages, 4 figures, submitted to J. Phys. Soc. Jpn. at June 30, 200

Excitonic Effects and Optical Spectra of Single-Walled Carbon Nanotubes

Many-electron effects often dramatically modify the properties of reduced dimensional systems. We report calculations, based on an many-electron Green's function approach, of electron-hole interaction effects on the optical spectra of small-diameter single-walled carbon nanotubes. Excitonic effects qualitatively alter the optical spectra of both semiconducting and metallic tubes. Excitons are bound by ~ 1 eV in the semiconducting (8,0) tube and by ~ 100 meV in the metallic (3,3) tube. These large many-electron effects explain the discrepancies between previous theories and experiments.Comment: 6 pages, 3 figures, 2 table

Ultrafast Optical Spectroscopy of Micelle-Suspended Single-Walled Carbon Nanotubes

We present results of wavelength-dependent ultrafast pump-probe experiments on micelle-suspended single-walled carbon nanotubes. The linear absorption and photoluminescence spectra of the samples show a number of chirality-dependent peaks, and consequently, the pump-probe results sensitively depend on the wavelength. In the wavelength range corresponding to the second van Hove singularities (VHSs), we observe sub-picosecond decays, as has been seen in previous pump-probe studies. We ascribe these ultrafast decays to intraband carrier relaxation. On the other hand, in the wavelength range corresponding to the first VHSs, we observe two distinct regimes in ultrafast carrier relaxation: fast (0.3-1.2 ps) and slow (5-20 ps). The slow component, which has not been observed previously, is resonantly enhanced whenever the pump photon energy resonates with an interband absorption peak, and we attribute it to radiative carrier recombination. Finally, the slow component is dependent on the pH of the solution, which suggests an important role played by H$^+$ ions surrounding the nanotubes.Comment: 6 pages, 8 figures, changed title, revised, to be published in Applied Physics

Aharonov-Bohm spectral features and coherence lengths in carbon nanotubes

The electronic properties of carbon nanotubes are investigated in the presence of disorder and a magnetic field parallel or perpendicular to the nanotube axis. In the parallel field geometry, the $\phi_{0}(=hc/e)$-periodic metal-insulator transition (MIT) induced in metallic or semiconducting nanotubes is shown to be related to a chirality-dependent shifting of the energy of the van Hove singularities (VHSs). The effect of disorder on this magnetic field-related mechanism is considered with a discussion of mean free paths, localization lengths and magnetic dephasing rate in the context of recent experiments.Comment: 22 pages, 6 Postscript figures. submitted to Phys. Rev.

BMP2 commitment to the osteogenic lineage involves activation of Runx2 by DLX3 and a homeodomain transcriptional network

Several homeodomain (HD) proteins are critical for skeletal patterning and respond directly to BMP2 as an early step in bone formation. RUNX2, the earliest transcription factor proven essential for commitment to osteoblastogenesis, is also expressed in response to BMP2. However, there is a gap in our knowledge of the regulatory cascade from BMP2 signaling to the onset of osteogenesis. Here we show that BMP2 induces DLX3, a homeodomain protein that activates Runx2 gene transcription. Small interfering RNA knockdown studies in osteoblasts validate that DLX3 is a potent regulator of Runx2. Furthermore in Runx2 null cells, DLX3 forced expression suffices to induce transcription of Runx2, osteocalcin, and alkaline phosphatase genes, thus defining DLX3 as an osteogenic regulator independent of RUNX2. Our studies further show regulation of the Runx2 gene by several homeodomain proteins: MSX2 and CDP/cut repress whereas DLX3 and DLX5 activate endogenous Runx2 expression and promoter activity in non-osseous cells and osteoblasts. These HD proteins exhibit distinct temporal expression profiles during osteoblast differentiation as well as selective association with Runx2 chromatin that is related to Runx2 transcriptional activity and recruitment of RNA polymerase II. Runx2 promoter mutagenesis shows that multiple HD elements control expression of Runx2 in relation to the stages of osteoblast maturation. Our studies establish mechanisms for commitment to the osteogenic lineage directly through BMP2 induction of HD proteins DLX3 and DLX5 that activate Runx2, thus delineating a transcriptional regulatory pathway mediating osteoblast differentiation. We propose that the three homeodomain proteins MSX2, DLX3, and DLX5 provide a key series of molecular switches that regulate expression of Runx2 throughout bone formation. <br/

CRISPR-Cas9 screens in human cells and primary neurons identify modifiers of C9ORF72 dipeptide-repeat-protein toxicity.

Hexanucleotide-repeat expansions in the C9ORF72 gene are the most common cause of amyotrophic lateral sclerosis and frontotemporal dementia (c9ALS/FTD). The nucleotide-repeat expansions are translated into dipeptide-repeat (DPR) proteins, which are aggregation prone and may contribute to neurodegeneration. We used the CRISPR-Cas9 system to perform genome-wide gene-knockout screens for suppressors and enhancers of C9ORF72 DPR toxicity in human cells. We validated hits by performing secondary CRISPR-Cas9 screens in primary mouse neurons. We uncovered potent modifiers of DPR toxicity whose gene products function in nucleocytoplasmic transport, the endoplasmic reticulum (ER), proteasome, RNA-processing pathways, and chromatin modification. One modifier, TMX2, modulated the ER-stress signature elicited by C9ORF72 DPRs in neurons and improved survival of human induced motor neurons from patients with C9ORF72 ALS. Together, our results demonstrate the promise of CRISPR-Cas9 screens in defining mechanisms of neurodegenerative diseases

Left ventricular non-compaction: clinical features and cardiovascular magnetic resonance imaging

Background: It is apparent that despite lack of family history, patients with the morphological characteristics of left ventricular non-compaction develop arrhythmias, thrombo-embolism and left ventricular dysfunction. METHODS: Forty two patients, aged 48.7 +/- 2.3 yrs (mean +/- SEM) underwent cardiovascular magnetic resonance (CMR) for the quantification of left ventricular volumes and extent of non-compacted (NC) myocardium. The latter was quantified using planimetry on the two-chamber long axis LV view (NC area). The patients included those referred specifically for CMR to investigate suspected cardiomyopathy, and as such is represents a selected group of patients. RESULTS: At presentation, 50% had dyspnoea, 19% chest pain, 14% palpitations and 5% stroke. Pulmonary embolism had occurred in 7% and brachial artery embolism in 2%. The ECG was abnormal in 81% and atrial fibrillation occurred in 29%. Transthoracic echocardiograms showed features of NC in only 10%. On CMR, patients who presented with dyspnoea had greater left ventricular volumes (both p < 0.0001) and a lower left ventricular ejection fraction (LVEF) (p < 0.0001) than age-matched, healthy controls. In patients without dyspnoea (n = 21), NC area correlated positively with end-diastolic volume (r = 0.52, p = 0.0184) and end-systolic volume (r = 0.56, p = 0.0095), and negatively with EF (r = -0.72, p = 0.0001). CONCLUSION: Left ventricular non-compaction is associated with dysrrhythmias, thromboembolic events, chest pain and LV dysfunction. The inverse correlation between NC area and EF suggests that NC contributes to left ventricular dysfunction

Chlorotetracycline As An Indicator Of The Interaction Of Calcium With Brain Membrane Fractions

The fluorescence of chlorotetracycline (CTC) in the presence of synaptosomes isolated from sheep brain is selectively increased by Ca2+ under conditions in which Mg2+, Na+, K+, Li+ or choline have only a small effect. The monovalent cations release bound Ca2+ from synaptosomes, and this effect is reflected by a decrease in the CTC fluorescence. Under optimal conditions there is a near parallelism between Ca2+ and CTC binding to the synaptosomes membranes, and Li+ is the monovalent cation tested which interferes the most with the binding of both substances. These results obtained in a predominantly sucrose medium become less distinct when media simulating physiological composition are utilized, which limits the usefulness of the method. Brain mitochondria and myelin also bind Ca2+ and CTC. The ratio of the fluorescence signal (or CTC bound) to Ca2+ bound is highest of all for mitochondrial membranes, and the apparent fluorescence quantum yield of CTC is also the highest in these membranes, which suggests that the Ca2+ in these membranes is localized in a more apolar region than is the case for synaptosomes and myelin

Osmium Vapor Pretreatment of Gnomonia Infected Leaves

A study was conducted to determine if pretreatment with osmium tetroxide (OsO4) vapor prior to the conventional preparation procedure would increase the retention of fungal structures on leaf surfaces as observed with scanning electron microscopy (SEM). Leaves of black walnut (Juglans nigra L.) were inoculated with conidia of Gnomoma lepfostyla (Fr.) Ces. and de Not., the etiological agent that causes anthracnose of walnut. Following lesion development, leaves were either conventionally prepared with immersion in fixative, ethanol and critical-point dried or vapor-fixed with OsO4 before conventional specimen preparation. Data indicate that significantly more fungal structures were present on OsO4 vapor-fixed leaf samples than on conventionally prepared samples

Progression in the bacterial load during the breeding season in nest boxes occupied by the Blue Tit and its potential impact on hatching or fledging success

The Blue Tit Cyanistes caeruleus prefer to use nest boxes to raise their young rather than nests in natural tree cavities. However, nest boxes provide a warm, humid microclimate that is favourable to the growth of rich bacterial communities. In this study we investigated how the bacterial community developed throughout the breeding season and whether it had any efect on egg or nestling mortality. Samples were collected across six sites and three breeding seasons at the completion of nest building, clutch completion and immediately post fedging. Bacterial counts were obtained for each sample, including a total bacterial count using non-selective media and bacterial counts on selective media for Staphylococcus spp. and Enterobacter spp, which may indicate pathogenicity to the birds. There were signifcantly more bacteria (total counts and counts for Staphylococcus spp. and Enterobacter spp. specifcally) present within the nest box at the post fedging stage compared to the two earlier stages, likely due to increased activity by the adult birds (feeding of the nestlings and defecating) and less time by the adult birds for self-preening and nest sanitation. No positive relationship was found between bacterial counts and either egg mortality, nestling mortality or brood size; however, a negative relationship between egg mortality and total bacterial count was identifed. Although somewhat unexpected, this negative relationship may indicate the presence of a greater number of symbiotic bacteria, which could ofer a greater level of protection