Optical absorption and photoluminescence studies on heavily doped (Ga,Mn)Sb crystals

Ga1-xMnxSb crystals are grown with different Mn doping concentrations by the horizontal Bridgman method (x = 0 - 0.04). Optical absorption and photoluminescence studies are carried out in the temperature range 3-300 K. Optical absorption studies reveal that the inter-valence band transition from the spin-orbit split-off band to the light/heavy hole bands is dominant over the fundamental valence band to conduction band absorption. In higher doped crystals, the fundamental absorption peak is merged with the inter-valence band transition and could not be resolved. Photoluminescence measurements in heavily doped crystals reveal the band gap narrowing and band filling effects due to the Fermi level shifting into the valence band.

Corona Aging Studies on Silicone Rubber Nanocomposites

In EHV and UHV power transmission lines, corona could occur even on well designed transmission line hardware and insulators especially under wet conditions. Corona if allowed to occur continuously can significantly damage the polymeric insulators used in such lines in the long run. This paper presents the experimental results of corona aging studies conducted on unfilled silicone rubber as well as filled silicone rubber nanocomposites. Corona aging studies were conducted on silicone rubber samples with filler concentrations of 0, 1, 2 and 3 % by wt of nanosilica for 25 h and 50 h. Needle-plane electrode geometry has been used to create the corona on the samples. Different characterization techniques such as Scanning Electron Microscopy, Energy Dispersive X-ray analysis, Hydrophobicity, Fourier Transform Infrared Spectroscopy, and Optical Profilometry have been used to assess the relative performance of the samples with respect to corona aging. Results indicate that at 3 wt %, the performance of the nanocomposite is much better than the unfilled silicon rubber which can be attributed to the modifications in the material caused by the size factor of the filler.

Interaction of metal ions with nucleic acids and related compounds. II. Studies on Au(III)-nucleic acid system

The nature of interaction of Au(III) with nucleic acids was studied by using methods such as uv and ir spectrophotometry, viscometry, pH titrations, and melting-temperature measurements. Au(III) is found to interact slowly with nucleic acids over a period of several hours. The uv spectra of native calf-thymus DNA 9pH 5.6 acetate buffer containing (0.01M NaCIO4) showed a shift in λ max to high wavelengths and an increase in optical density at 260 nm. There was a fourfold decrease in viscosity (expressed as ηsp/c). The reaction was faster at pH 4.0 and also with denatured DNA (pH 5.6) and whole yeast RNA (pH 5.6). The order of preference of Au(III) (as deduced from the time of completion of reaction) for the nucleic acids in RNA > denatured DNA > DNA. The reaction was found to be completely reversible with respect KCN. Infrared spectra of DNA-Au(III) complexes showed binding to both the phosphate and bases of DNA. The same conclusions were also arrived at by melting-temperature studies of Au(III)-DNA system. pH titrations showed liberation of two hydroxylions at r = 0.12 [r = moles of HAuCl4 added per mole of DNA-(P)] and one hydrogen ion at r = 0.5. The probable binding sites could be N(1)/N(7) of adenine, N(7) and/or C(6)O of guanine, N(3) of cytosine and N(3) of thymine. DNAs differing in their (G = C)-contents [Clostridium perfingens DNA(G = C, 29%), salmon sperm DNA (G + C, 42%) and Micrococcus lysodeikticus DNA(G + C, 29%), salmon sperm DNA (G = C, 72%)] behaved differently toward Au(III). The hyperchromicity observed for DNAs differing in (G + C)-content and cyanide reversal titrations indicate selectivity toward ( A + T)-rich DNA at lw values of r. Chemical analysis and job's continuous variation studies indicated the existence of possible complexes above and below r = 1. The results indicate that Au(III) ions probably bind to hte phosphate group in the initial stages of the reaction, particularly at low values of r, and participation of the base interaction also increases. Cross-linking of the two strands by Au(III) may take place, but a complete collapse of the doulbe helix is not envisaged. It is probable that tilting of the bases or rotaiton of the bases around the glucosidic bond, resulting in a significant distrotion of the double helix, might take place due to binding of Au(III) to DNA.

Recent studies on the behaviour of interference fit pins in composite plates

Composites are finding increasing application in many advanced engineering fields like aerospace, marine engineering, hightech sports equipment, etc., due to their high specific strength and/or specific stiffness values. The use of composite components in complex situations like airplane wing root or locations of concentrated load transfer is limited due to the lack of complete understanding of their behaviour in the region of joints. Joints are unavoidable in the design and manufacture of complex structures. Pin joints are one of the most commonly used methods of connection. In regions of high stresses like airplane wing root joints interference fit pins are used to increase its fatigue life and thereby increase the reliability of the whole structure. The present contribution is a study on the behaviour of the interference fit pin in a composite plate subjected to both pull and push type of loads. The interference fit pin exhibits partial contact/separation under the loads and the contact region is a non-linear function of the load magnitude. This non-linear behaviour is studied by adopting the inverse technique and some new results are presented in this paper.

Theoretical and Experimental Studies on an Ammonia-Based Loop Heat Pipe With a Flat Evaporator

An ammonia loop heat pipe (LHP) with a flat plate evaporator is developed and tested. The device uses a nickel wick encased in an aluminum-stainless steel casing. The loop is tested for various heat loads and different sink temperatures, and it demonstrated reliable startup characteristics. Results with the analysis of the experimental observation indicate that the conductance between the compensation chamber and the heater plate can significantly influence the operating temperatures of the LHP. A mathematical model is also presented which is validated against the experimental observations.

Directing role of functional groups in selective generation of C-H center dot center dot center dot pi interactions: In situ cryo-crystallographic studies on benzyl derivatives

The in situ cryo-crystallization study of benzyl derivatives reveals that the molecular packing in these compounds is either through methylene (sp(3)) C-H center dot center dot center dot pi or aromatic (sp(2)) C-H center dot center dot center dot pi interactions depending on the level of acidity of the benzyl proton. These studies of low melting compounds bring out the subtle features of such weak interactions and point to the directional preferences depending on the nature (electron withdrawing, polarizability) of the neighbouring functional group.

Synthetic and conformational studies on dehydrophenylalanine containing model peptides

Four model dipeptides containing a Z-dehydrophenylalanine residue (ΔZPhe) at the C-terminal, Boc-X-ΔZ Phe-NHMe (X = Ala (1), Gly (2), Pro (3), and Val (4)), have been synthesised and their solution conformations investigated by 270 MHz 1H n.m.r. and i.r. spectroscopy. N.m.r. studies on these peptides clearly show the presence of intramolecularly hydrogen bonded structures in CHCl3 solutions while such structures appear to be absent in the corresponding saturated peptides. This conclusion is also supported by i.r. studies. Studies of the nuclear Overhauser effect provided evidence for the occurrence of a significant population of β-turn structures in solvents like CDCl3 and (CD3)2SO. The observed NOES are consistent with a major contribution from Type II β-turn structure in CDCl3, while in (CD3)2SO solutions there is evidence of a partially extended structure also.

Thermal and electrical switching studies on Ge20Se80-xBix (1 <= x <= 13) ternary chalcogenide glassy system

Alternating Differential Scanning Calorimetric (ADSC) and electrical switching studies have been undertaken on Ge20Se80-xBix glasses (1 <= x <= 13), to understand the effect of topological thresholds on thermal properties and electrical switching behavior. It is found that the compositional dependence of glass transition temperature (Tg), crystallization temperature (T-c1) and thermal stability (AT) of Ge20Se80-xBix glasses show anomalies at a composition x= 5, the rigidity percolation/stiffness threshold of the system. Further, unusual variations are also observed in different thermal properties, such as T-g, T-c1, Delta T, Delta C-p and Delta H-NR, at the composition x= 10, which indicates the occurrence of chemical threshold in these glasses at this composition. Electrical switching studies indicate that Ge20Se8o_RBig glasses with 5 11 exhibit threshold switching behavior and those with x = 12 and 13 show memory switching. A sharp decrease has been noticed in the switching voltages with bismuth concentration, which is due to the more metallic nature of bismuth and the presence of Bi+ ions. Further, a saturation is seen in the decrease in V-T around x = 6, which is related to bismuth phase percolation at higher concentrations of Bi. (C) 2010 Elsevier B.V. All rights reserved.

Stereochemically constrained peptides. Theoretical and experimental studies on the conformations of peptides containing 1-aminocyclohexane carboxylic acid

Conformational energy calculations on the model system N-acetyl- 1 -aminocyclohexanecarboxylic acid N'methylamide (Ac-Acc6-NHMe), using an average geometry derived from 13 crystallographic observations, establish that the Acc6 residue is constrained to adopt conformations in the helical regions of In contrast, the a,a-dialkylated residue with linear hydrocarbon side chains, a,a-di-n-propylglycine favors fully extended backbone structures (6 1= $ = 180'). The crystal structures of two model peptides, Boc-(Acc6),-OMe (type 111 @-turn at -Acc6(1)-Acc6(2)-) and Boc-Pro-Acc6-Ala-OMe (type I1 P-turn at -Pro-Acc6-), establish that Acc6 residues can occupy either position of type 111 P-turns and the i + 2 position of type I1 @-turns. The stereochemical rigidity of these peptides is demonstrated in solution by NMR studies, which establish the presence of one intramolecular hydrogen bond in each peptide in CDCI, and (CDJ2S0. Nuclear Overhauser effects permit characterization of the @-turn conformations in solution and establish their similarity to the solid-state structures. The implications for the use of Acc6 residues in conformational design are considered.

Conformations of dehydrophenylalanine containing peptides. NMR studies on three tripeptides with a central dehydrophenylalanyl residue.

Three tripeptides containing a central Z-dehydrophenylalanine residue (Δz-Phe), Boc-L-Phe-Δz-Phe-X-OMe (X = L-Val 1, L-Leu 2 and X = L-Ala 3) have been synthesized and their solution conformations investigated by 270 MHz 1H NMR spectroscopy. In all three peptides, conformations involving the X residue NH in an intramolecular hydrogen bond were favoured in CDCl3 solutions. Studies of the nuclear Overhauser effect (NOE) provided support for a Type II β turn conformation in these peptides with Phe and Δz-Phe occupying the i + 1 and i + 2 positions, respectively. Significantly different conformations lacking any intramolecular hydrogen bonds were observed for peptide 1 in (CD3)2SO. NOE results were consistent with a significant population of molecules having semi-extended conformations (ø > 100°) at the Δz-Phe residue.

X-ray and molecular-dynamics studies on Mycobacterium leprae single-stranded DNA-binding protein and comparison with other eubacterial SSB structures

The crystal structures of two forms of Mycobacterium leprae single-stranded DNA-binding protein (SSB) have been determined at 2.05 and 2.8 A resolution. Comparison of these structures with the structures of other eubacterial SSBs indicates considerable variation in their quaternary association, although the DNA-binding domains in all of them exhibit the same OB-fold. This variation has no linear correlation with sequence variation, but could be related to variation in protein stability. Molecular-dynamics simulations have been carried out on tetrameric molecules derived from the two forms and the prototype Escherichia coli SSB and the individual subunits of both proteins. Together, the X-ray studies and molecular-dynamics simulations yield information on the relatively rigid and flexible regions of the molecule and on the effect of oligomerization on flexibility. The simulations provide insight into the changes in subunit structure on oligomerization. They also provide insight into the stability and time evolution of the hydrogen bonds/water bridges that connect the two pairs of monomers in the tetramer.