Visualization on the Grid of Virus-Host Interaction

Interstitial halogen centers in X-irradiated CsBr

Measurements of reflectivity, photoluminescence and cathodoluminescence for various electron beam voltages were performed on p-type single crystals at liquid nitrogen, hydrogen and helium temperatures. By an application of the microreversibility principle and consideration of the reflectivity data, the nature and the energies of the transitions were determined. In addition, we identify the lines due to the annihilation of exciton-neutral defect complexes, in ground or excited state. When the luminescence is modulated by a CO2 laser (hνIR ~ 130 meV) effects are observed at liquid helium temperature. A quenching effect is observed for the free-to-bound and pair recombinations. Quenching of the luminescence due to the annihilation of exciton isoelectronic oxygen complexes is a direct corroboration of the binding-in-two-step theory.

A beryllium-walled cryostat is described which allows collection of nearly a full sphere of accurate three-dimensional X-ray diffractometer data at liquid-nitrogen (LN2) and liquid-helium (LHe) temperatures. Cryogen consumption is 0.3–0.4 liters of LHe per hour at 30°K and 0.1–0.2 liters of LN2 per hour at 78°K. The specimen is mounted on a copper block, the temperature of which is controlled to within 0.1°. Measurements with a powder sample of TbVO4 show that at 33°K the specimen temperature is within 2° of the temperature of the copper block. The cryostat has been used for the collection of several low-temperature data sets. Some special modifications in data-collection and data-processing software are described.

23 - THE CHANGE OF RESISTANCE OF GOLD CRYSTALS AT VERY LOW TEMPERATURES IN A MAGNETIC FIELD AND SUPRA-CONDUCTIVITY: CRITICAL REMARKS REGARDING THE RECENT WORK OF MEISSNER AND SCHEFFERS

Magnetic susceptibility measurements were carried out on octacoordinated uranium(IV) complexes, such as tetrakis(acetylacetonato)uranium (IV), tetrakis(trifluoroacetylacetonato)uranium (IV), tetrakis(benzoylacetonato)uranium(IV), tetrakis(dibenzoylmethanato)uranium(IV), tetrakis(thenoyltrifluoroacetonato)uranium(IV), tetrakis(cupferronato)uranium(IV), and uranium(IV)tetrachloride, in the temperature range from room temperature to the temperature of liquid nitrogen or liquid helium. The Curie-Weiss law holds above the temperature of liquid nitrogen, with a magnetic moment of 3.5–2.92 BM. For the latter two complexes, a temperature-independent paramagnetism was observed in the low temperature range; the other complexes also seemed to show the same trend. We tried to interpret the magnetic behavior on the basis of the crystal-field model of the square antiprism structure or the dodecahedron structure of the complexes.

Neutron diffraction studies have been made on a powdered NaUO3 sample. It has been shown that an octahedron composed of six oxygen atoms around pentavalent uranium in this compound is distorted and four of six oxygen atoms are much closer to an uranium atom (U-O distance of 2.15Å) than the other two (U-O distance of 2.24Å). A unit of the UO2 + ion does not exist in the crystal, though the pentavalent uranium has been expected to be in the form of UO2 + as in the case of UO2 2+ of a hexavalent uranium Ion. A neutron diffraction pattern at temperature of liquid helium has shown no appreciable difference from that at temperature of liquid nitrogen.

[Abstract] The objective of this work is to investigate the interlaminar fracture and damage behavior of woven glass fiber reinforced polymer (GFRP) laminates for mixed-mode bending (MMB) configuration at cryogenic temperatures. The crack length corresponding to critical load is determined using measured specimen compliance of the damage states via finite element analysis (FEA), and the critical mixed-mode energy release rates of MMB specimen at room temperature (RT), liquid nitrogen temperature (77 K) and liquid helium temperature (4 K) are calculated. The values of the critical energy release rate (mixed-mode interlaminar fracture toughness) obtained from the beam theory and FEA are then compared. A FEA coupled with damage is also employed to study the damage distributions within the MMB specimen. Moreover, the results for the special cases of pure mode I and mode II loading of woven GFRP laminates are presented.

The purpose of this research is to characterize the cryogenic delamination growth behavior in woven glass fiber reinforced polymer (GFRP) composite laminates subjected to Mode II fatigue loading. Mode II fatigue delamination tests were performed at room temperature, liquid nitrogen temperature (77 K) and liquid helium temperature (4 K) using the four-point bend end-notched flexure (4ENF) test method, and the delamination growth rate data for the woven GFRP laminates were obtained. The energy release rate range was determined by the finite element method. Microscopic examinations of the specimen sections and fracture surfaces were also carried out The present results are discussed to obtain an understanding of the fatigue delamination growth mechanisms in the woven GFRP laminates under Mode II loading at cryogenic temperatures.

Thermoreflectance spectra in the lowest singlet-singlet transition region of anthracene have been measured at liquid nitrogen temperature. The derivative optical constants are obtained by the Kramers-Kronig analysis of the spectra. The asymmetric broadening of the b -polarized 0-0 exciton absorption band with temperature is suggested. The frequency of the K =0 polariton for the 0-0 bsorpton band has been confirmed to be located at 25484 cm -1 and at 25497 cm -1 in the a - and the b -polarized spectra respectively. The K =0 polariton-exciton splittings for the a - and b -polarization are respectively 77 cm -1 and 378 cm -1 , whose magnitudes are proportional to the oscillator strength of the respective 0-0 absorption band. The b -polarized thermoreflectance spectrum at liquid helium temperature has also been measured and discussed briefly.

This paper presents results from an analytical and experimental study of the effect of temperature and geometrical variations on the Mode II interlaminar fracture toughness in glass-cloth/epoxy laminates. The end-notched flexure (ENF) test geometry was used for Mode II experiments, which were performed at room temperature (R.T.), liquid nitrogen temperature (77 K), and liquid helium temperature (4 K). The fracture surfaces were also examined by scanning electron microscopy to verify the fracture mechanisms. A finite element model was further used to perform the delamination crack analysis. Critical load levels, and the geometric and material properties of the test specimens were input data for the analysis, which evaluated the Mode II energy release rate at onset of delamination crack propagation. The results of the finite element analysis are used to supplement the experimental data.

There is evidence that membrane proteins can serve as the functional units of ionic transport in biological membranes. Laser Raman spectroscopy has been used to probe specific molecular interactions inside two models of transport membrane proteins, valinomycin and gramicidin A. Conformational changes of these molecules, as well as specific interactions with ions, can be detected and may help elucidate how membrane transport proteins such as Na+ minus K+ ATPase and rhodopsin function. Resonance Raman spectroscopy has also been used to study conformational changes and protein-chromophore interactions in rhodopsin, the membrane protein that acts as the primary unit of visual excitation in the eye.

BackgroundProteins in their majority act rarely as single entities. Multisubunit macromolecular complexes are the actors in most of the cellular processes. These nanomachines are hold together by weak protein-protein interactions and undergo functionally important conformational changes. TFIID is such a multiprotein complex acting in eukaryotic transcription initiation. This complex is first to be recruited to the promoter of the genes and triggers the formation of the transcription preinitiation complex involving RNA polymerase II which leads to gene transcription. The exact role of TFIID in this process is not yet understood.MethodsLast generation electron microscopes, improved data collection and new image analysis tools made it possible to obtain structural information of biological molecules at atomic resolution. Cryo-electron microscopy of vitrified samples visualizes proteins in a fully hydrated, close to native state. Molecular images are recorded at liquid nitrogen temperature in low electron dose conditions to reduce radiation damage. Digital image analysis of these noisy images aims at improving the signal-to-noise ratio, at separating distinct molecular views and at reconstructing a three-dimensional model of the biological particle.ResultsUsing these methods we showed the early events of an activated transcription initiation process. We explored the interaction of the TFIID coactivator with the yeast Rap1 activator, the transcription factor TFIIA and the promoter DNA. We demonstrated that TFIID serves as an assembly platform for transient protein-protein interactions, which are essential for transcription initiation.ConclusionsRecent developments in electron microscopy have provided new insights into the structural organization and the dynamic reorganization of large macromolecular complexes. Examples of near-atomic resolutions exist but the molecular flexibility of macromolecular complexes remains the limiting factor in most case. Electron microscopy has the potential to provide both structural and dynamic information of biological assemblies in order to understand the molecular mechanisms of their functions.

The polarized absorption and emission spectra of single crystals of LaBr3 doped with nominally 0.15% and 0.75% 237Np have been recorded between 5000 and 25 000 cm−1 at temperatures of liquid helium and nitrogen. Fluorescence excitation spectra and both parallel and perpendicular Zeeman spectra were also recorded. Based on the number, polarization, and magnetic behavior of Stark components appearing in distinct groups, as well as on preliminary 5f-hydrogenic calculations, 14 absorption groups were identified as levels of the ground electronic configuration 5f4 of Np3+. The assumption of a pure 5f4 electronic configuration does not lead to a satisfactory interpretation of the spectrum; however, the inclusion of electrostatic configuration interaction permits a satisfactory fitting of the observed levels with an rms deviation in energy of 86.4 cm−1 using the parameters: E1=3394.3±29.1, E2=14.19±0.22, E3=317.5±1.9, ζ5f=1969.2±10.8, α=35.0±2.2, and β=−802.4±11.3 cm−1. Using 25 crystalline-field-split Stark components of the 5I4,5,6,7 levels of the ground multiplet, a first-order crystal-field perturbation calculation was performed. An rms deviation in energy of 8.0 cm−1 is obtained using the parameters: B20=−22.8±8, B40=−86.7±2.4, B60=−70.8±0.9, B66=+857.1±12.0 cm−1. The negative value obtained for B20 represents a crystal-field shielding in excess of 100% and is discussed in some detail. Parallel and perpendicular Zeeman interactions are also discussed in some detail.

Visible and infrared absorption of organometallic compounds of thulium at low temperatures

SKELETAL MUSCLES adjust their contractile output over an exceptionally wide dynamic range, as needed to produce movements ranging from sitting and standing, to athletics and dance. At the single fiber level, the force generated by skeletal muscles is determined, ultimately, by the formation of cycling cross bridges between actin and myosin. This interaction requires Ca 2 and is governed by the simple relationship: force is proportional to intracellular ionized Ca 2 . Cross-bridge formation is inhibited in resting muscle when Ca 2 is sequestered within the sarcoplasmic reticulum (SR); it is activated when stored Ca 2 is released into the cytosol, allowing actin-myosin interactions to proceed. Contraction stops as Ca 2 is actively resequestered by the Ca 2 -ATPase of the SR (SERCA1). Thus Ca 2 is the central messenger in muscle activation, and the amplitude and time course of the intracellular Ca 2 change is the major determinant of contractile output. The key mechanisms that activate Ca 2 release from the SR are now well defined. This sequence, termed excitation-contraction coupling, begins when an excitatory input from the motor nerve triggers a propagating action potential on the muscle membrane. The action potential rapidly depolarizes the entire sarcolemma, including transverse tubules, which invaginate from the surface membrane and form specialized junctions (triads) with the SR along much of their length. The triad junction serves as a platform for assembling sarcolemmal calcium