First principles total energy studies of the adsorption of germane on Ge(001)-c(2 × 4)

We perform first principles total energy calculations to study the energetics, and the atomic structure of the adsorption of germane (GeH₄) molecules on the Ge(001)-c(2 × 4) surface. The adsorption of a GeH₄ unit occurs after its dissociation into a germanium trihydride (GeH₃) and a hydrogen atom and a subsequent decomposition into a germanium dihydride (GeH₂) subunit and H atoms. Consequently, we first consider the adsorption of GeH₂ in two different configurations; the on-dimer and the intra-row geometries. Similar to the adsorption of SiH₂ and GeH₂ on Si(001), it is found that the on-dimer site is more stable than the intra-row geometry by 0.13 eV. However, in the adsorption of a GeH₂ fragment together with two H atoms we find that the intra-row geometry is energetically more favorable, again, similar to the adsorption of SiH₂ and GeH₂ (plus two H atoms) on the Si(001) surface.     

High Power CW Wattmeter Calibration at NIST

The National Institute of Standards and Technology has established a measurement capability to support high power systems and devices. The automated wattmeter calibration system operates at power levels of 1 to 1000 W for frequencies from 1 to 30 MHz and 1 to 500 W from 30 to 400 MHz. A cascaded coupler technique is used to extend power measurements to high levels which are traccable to a 10 mW standard thermistor mount. This technique uses an arrangement of nominal 10, 20, 30, 40, and 50 dB couplers with sidearm power meters. The initial step transfers the calibration of the 10 mW standard to the 10 dB coupler/power meter. The standard is then replaced with a wattmeter to be calibrated. RF power is increased 10 dB and the calibration is transferred to the adjacent 20 dB eoupler/power meter. This sequence is repeated with the remaining coupler/power meters until the wattmeter is calibrated at the desired power levels and frequencies. Power ratios calculated from simultaneous power measurements made at each transfer are used to calculate the incident power at the wattmeter. Due to nonideal components, corrections are made for nonlinearities, mismatch, and other errors. Two types of wattmeters have been evaluated at selected frequencies and power levels. Total uncertainties are based on the random and systematic components.     

The NH2 terminus of titin spans the Z-disc: its interaction with a novel 19-kD ligand (T-cap) is required for sarcomeric integrity

Titin is a giant elastic protein in vertebrate striated muscles with an unprecedented molecular mass of 3-4 megadaltons. Single molecules of titin extend from the Z-line to the M-line. Here, we define the molecular layout of titin within the Z-line; the most NH2-terminal 30 kD of titin is located at the periphery of the Z-line at the border of the adjacent sarcomere, whereas the subsequent 60 kD of titin spans the entire width of the Z-line. In vitro binding studies reveal that mammalian titins have at least four potential binding sites for alpha-actinin within their Z-line spanning region. Titin filaments may specify Z-line width and internal structure by varying the length of their NH2-terminal overlap and number of alpha-actinin binding sites that serve to cross-link the titin and thin filaments. Furthermore, we demonstrate that the NH2-terminal titin Ig repeats Z1 and Z2 in the periphery of the Z-line bind to a novel 19-kD protein, referred to as titin-cap. Using dominant-negative approaches in cardiac myocytes, both the titin Z1-Z2 domains and titin-cap are shown to be required for the structural integrity of sarcomeres, suggesting that their interaction is critical in titin filament-regulated sarcomeric assembly.     

An unusual foreign body in the rectum

This is the report of an unusual foreign body in the rectum which was a complication of the migration of an esophageal Celestin's prosthesis.     

The Caenorhabditis elegans RAD51 homolog is transcribed into two alternative mRNAs potentially encoding proteins of different sizes

In prokaryotes, the RecA protein plays a pivotal role in homologous recombination, catalyzing the transfer of a single DNA strand into an homologous molecule. Structural homologs of the bacterial RecA protein, called Rad51, have been found in different eukaryotes (from yeast to man), suggesting a certain level of conservation in recombination pathways among living organisms. We have cloned the homolog of RAD51 in Caenorhabditis elegans. The CeRAD51 gene is transcribed into two alternative mRNAs and potentially codes for two proteins of 395 and 357 amino acids in length, respectively. We discuss the evolutionary implications of these findings.     

An electrochemical bioassay for dichlorvos analysis in durum wheat samples

The use of an acetylcholinesterase inhibition assay for the detection of dichlorvos in durum wheat samples by a simplified extraction procedure is reported. After an incubation step, the residual activity was determined with an amperometric biosensor using a portable potentiostat. The use of electric eel and recombinant acetylcholinesterase was compared. The effect of the matrix extract was evaluated by using various sample:solvent ratios, 1:2.5, 1:5, 1:10, and 1:20. The optimal extraction ratio, considering the electrochemical interferences and the effect on enzyme activity and bioavailability of the pesticide, was 1:10. Calibrations were performed in buffer and durum wheat extract. The calculated detection limits in buffer solution were 10 ng/ ml and 0.045 ng/ml for electric eel and recombinant acetylcholinesterase, respectively, whereas operating in the matrix extract they increased up to 45 ng/ml and 0.07 ng/ml, corresponding to 0.45 mg/kg (extraction ratio 1:10) and 0.07 mg/kg in samples. These characteristics allowed the detection of contaminated samples at the maximum residue limit, which is 2 mg/kg and well below. Fortified samples of durum wheat were obtained with both dichlorvos and the commercial product Didivane, which contains dichlorvos as an active molecule. At all the tested levels, the occurrence of contaminant was detected with an average recovery of 75%. The total assay time, including the extraction step, was 30 min. Because several extractions as well as most of the assay steps can be run simultaneously, the throughput for one operator is 12 determinations per hour.