Engineered dimer interface mutants of triosephosphate isomerase: the role of inter-subunit interactions in enzyme function and stability

The role of inter-subunit interactions in maintaining optimal catalytic activity in triosephosphate isomerase (TIM) has been probed, using the Plasmodium falciparum enzyme as a model. Examination of subunit interface contacts in the crystal structures suggests that residue 75 (Thr, conserved) and residue 13 (Cys, variable) make the largest number of inter-subunit contacts. The mutants Cys13Asp (C13D) and Cys13Glu (C13E) have been constructed and display significant reduction in catalytic activity when compared with wild-type (WT) enzyme (similar to 7.4-fold decrease in k(cat) for the C13D and similar to 3.3-fold for the C13E mutants). Analytical gel filtration demonstrates that the C13D mutant dissociates at concentrations < 1.25 mu M, whereas the WT and the C13E enzymes retain the dimeric structure. The order of stability of the mutants in the presence of chemical denaturants, like urea and guanidium chloride, is WT > Cys13Glu > Cys13Asp. Irreversible thermal precipitation temperatures follow the same order as well. Modeling studies establish that the Cys13Asp mutation is likely to cause a significantly greater structural perturbation than Cys13Glu. Analysis of sequence and structural data for TIMs from diverse sources suggests that residues 13 and 82 form a pair of proximal sites, in which a limited number of residue pairs may be accommodated.

Observations on the role of the steel disc counterface during the sliding of various structural ceramics

Sliding wear characteristics and mechanisms of structural ceramics, namely Al2O3, zirconia-toughened alumina, tetragonal zirconia polycrystals (TZP) and Si3N4 against a steel counterface are influenced by mechanical and tribochemical interactions, specific to the combinations studied. The present paper studies the role of the disc in the sliding wear process of the above ceramics. Experiments were conducted at a pressure of 15.5 MPa between 0.1 and 12.0 m s(-1) with ceramic pins sliding against an EN-24 steel disc. Except in the case of TZP, the disc morphology is sensitive to variations in speed rather than to the pin material. The disc track is (i) mildly abraded at low speeds (about 0.1-0.75 m s(-1)), (ii) severely abraded at intermediate speeds (about 1.0-3.0 m s(-1)), (iii) covered with black patches at high speeds (about 4.0-6.0 m s(-1)) and (iv) completely black at very high speeds (about 7.0-12.0 m s(-1)). In the case of TZP, although black patches appear, transfer of TZP onto the disc surface and high wear of TZP occurs at 4.0 m s(-1). The order of the wear of the disc estimated from profilometric measurements is the same for all the ceramics. Except for Si3N4, the onset of wear of the ceramics is associated with the appearance of deep 'V' grooves on either side of the profile of the disc track. This can be explained on the basis of the thermal and hardness variations. Although other interaction products specific to the ceramic pin are present, the formation of iron oxides dominates the wear of the disc.

Spectroscopic Observation of Oscillations in the Corona During the Total Solar Eclipse of 22 July 2009

We performed high resolution spectroscopy of the solar corona during the total solar eclipse of 22 July 2009 in two emission lines: the green line at 5303 due to Fe xiv and the red line at 6374 due to Fe x, simultaneously from Anji (latitude 30A degrees 28.1' N; longitude 119A degrees 35.4' E; elevation 890 m), China. A two-mirror coelostat with 100 cm focal length lens produced a 9.2 mm image of the Sun. The spectrograph using 140 cm focal length lens in Littrow mode and a grating with 600 lines per millimeter blazed at 2 mu m provided a dispersion of 30 m and 43 m per pixel in the fourth order around the green line and third order around the red line, respectively. Two Peltier cooled 1k x 1k CCD cameras, with a pixel size of 13 mu m square and 14-bit readout at 10 MHz operated in frame transfer mode, were used to obtain the time sequence spectra in two emission lines simultaneously. The duration of totality was 341 s, but we could get spectra for 270 s after a trial exposure at an interval of 5 s. We report here on the detection of intensity, velocity, and line width oscillations with periodicity in the range of 25 -50 s. These oscillations can be interpreted in terms of the presence of fast magnetoacoustic waves or torsional Alfv,n waves. The intensity ratios of green to red emission lines indicate the temperature of the corona to be 1.65 MK in the equatorial region and 1.40 MK in the polar region, relatively higher than the expected temperature during the low activity period. The width variation of the emission lines in different coronal structures suggests different physical conditions in different structures.

Nucleation and growth characteristics of cavities during the early stages of tensile creep deformation in a superplastic zirconia-20 wt% alumina composite

Constant-stress tensile creep experiments on a superplastic 3-mol%-yttria-stabilized tetragonal zirconia composite with 20 wt% alumina revealed that cavities nucleate relatively early during tensile deformation. The number of cavities nucleated increases with increasing imposed stress. The cavities nucleate at triple points associated largely with an alumina grain, and then grow rapidly in a cracklike manner to attain dimensions on the order of the grain facet size. It is suggested that coarser-grained superplastic ceramics exhibit lower ductility due to the ease in formation of such grain boundary facet-cracks and their interlinkage to form a macroscopic crack of critical dimensions.

Application of an evolutionary fuzzy system for the estimation of workforce deployment and cross-training in an assembly environment

Owing to the increased customer demands for make-to-order products and smaller product life-cycles, today assembly lines are designed to ensure a quick switch-over from one product model to another for companies' survival in market place. The complexity associated with the decisions pertaining to the type of training and number of workers and their exposition to the different tasks especially in the current era of customized production is a serious problem that the managers and the HRD gurus are facing in industry. This paper aims to determine the amount of cross-training and dynamic deployment policy caused by workforce flexibility for a make-to-order assembly. The aforementioned issues have been dealt with by adopting the concept of evolutionary fuzzy system because of the linguistic nature of the attributes associated with product variety and task complexity. A fuzzy system-based methodology is proposed to determine the amount of cross-training and dynamic deployment policy. The proposed methodology is tested on 10 sample products of varying complexities and the results obtained are in line with the conclusions drawn by previous researchers.

Influence of HZSM-5 catalyst on the thermal degradation of poly(vinyl chloride) in solution

The kinetics of thermal degradation of poly(vinyl chloride) (PVC) in solution was investigated at various temperatures (210-250degreesC). The degradation rate coefficients were determined from the time evolution of the molecular weight distribution (MWD). The energy of activation, determined from the temperature dependence of the rate coefficient, was 26.6 kcal/mol. The degradation of PVC was also studied in the presence of a catalyst (HZSM-5 zeolite). The results indicated that increase of the degradation rate of PVC is first order with the HZSM-5 concentration up to 50 g/L and zero order at higher concentrations. The thermal degradation kinetics of PVC in the presence of 50 g/L of the catalyst was studied at various temperatures. The temperature dependency of the rate coefficient was used to calculate the activation energy (21.5 kcal/mol). This is consistent with the observation that the presence of a catalyst generally decreases the activation energy and promotes degradation. (C) 2002 John Wiley Sons, Inc.

Performance Enhancement of the Tunnel Field Effect Transistor using SiGe Source

Due to extremely low off state current (IOFF) and excellent sub-threshold characteristics, the tunnel field effect transistor (TFET) has attracted a lot of attention for low standby power applications. In this work, we aim to increase the on state current (ION) of the device. A novel device architecture with a SiGe source is proposed. The proposed structure shows an order of improvement in ION compared to the conventional Si structure. A process flow adaptable to conventional CMOS technology is also addressed.

Effect of irradiation on the microstructure and mechanical behavior of nanocrystalline nickel

The effect of 4.0 MeV proton irradiation on the microstructure and mechanical properties of nanocrystalline (nc) nickel was investigated. The irradiation damage induced in the sample was of the order of 0.004 dpa. Transmission electron microscopy of irradiated samples indicated the presence of dislocation loops within the grains. An increase in hardness and strain-rate sensitivity (m) of nc-Ni with irradiation was noted. The rate-controlling deformation mechanism in irradiated nc-Ni was identified to be interaction of dislocations with irradiation-induced defects. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Influence of Tempered Microstructures on the Transformation Behaviour of Cold Deformed and Intercritically Annealed Medium Carbon Low Alloy Steel

This research is focused on understanding the role of microstructural variables and processing parameters in obtaining optimised dual phase structures in medium carbon low alloy steels. Tempered Martensite structures produced at 300, 500, and 650 degrees C, were cold rolled to varied degrees ranging from 20 to 80% deformation. Intercritical annealing was then performed at 740, 760, and 780 degrees C for various time duration ranging from 60 seconds to 60 minutes before quenching in water. The transformation behaviour was studied with the aid of optical microscopy and hardness curves. From the results, it is observed that microstructural condition, deformation, and intercritical temperatures influenced the chronological order of the competing stress relaxation and decomposition phase reactions which interfered with the rate of the expected alpha -> gamma transformation. The three unique transformation trends observed are systematically analyzed. It was also observed that the 300 and 500 degrees C tempered initial microstructures were unsuitable for the production of dual structures with optimized strength characteristics.

The interaction of halogen molecules with SWNTs and graphene

The interaction of halogen molecules of varying electron affinity, such as iodine monochloride (ICl), bromine (Br(2)), iodine monobromide (IBr) and iodine (I(2)) with single-walled carbon nanotubes (SWNTs) and graphene has been investigated in detail. Halogen doping of the two nanocarbons has been examined using Raman spectroscopy in conjunction with electronic absorption spectroscopy and extensive theoretical calculations. The halogen molecules, being electron withdrawing in nature, induce distinct changes in the electronic states of both the SWNTs and graphene, which manifests with a change in the spectroscopic signatures. Stiffening of the Raman G-bands of the nanocarbons upon treatment with the different halogen molecules and the emergence of new bands in the electronic absorption spectra, both point to the fact that the halogen molecules are involved in molecular charge-transfer with the nanocarbons. The experimental findings have been explained through density functional theory (DFT) calculations, which suggest that the extent of charge-transfer depends on the electron affinities of the different halogens, which determines the overall spectroscopic properties. The magnitude of the molecular charge-transfer between the halogens and the nanocarbons generally varies in the order ICl > Br(2) > IBr > I(2), which is consistent with the expected order of electron affinities.

The study of pressure induced structural phase transition in spin-frustrated Yb2Ti2O7 pyrochlore

Our in situ x-ray diffraction and Raman measurements of Yb2Ti2O7 pyrochlore show that it undergoes a reversible structural phase transition from cubic pyrochlore to a monoclinic phase at similar to 28.6 GPa. Analysis of the x-ray data shows the transition to be thermodynamically first order and the high pressure phase to be substitutionally disordered. These experimental results are supported by our first principles calculations. (C) 2012 American Institute of Physics. [doi:10.1063/1.3681300]

PAMAM Dendrimer-Drug Interactions: Effect of pH on the Binding and Release Pattern

Understanding the dendrimer-drug interaction is of great importance to design and optimize the dendrimer-based drug delivery system. Using atomistic molecular dynamics (MD) simulations, we have analyzed the release pattern of four ligands (two soluble drugs, namely, salicylic acid (Sal), L-alanine (Ala), and two insoluble drugs, namely, phenylbutazone (Pbz) and primidone (Prim)), which were initially encapsulated inside the ethylenediamine (EDA) cored polyamidoamine (PAMAM) dendrimer using the docking method. We have computed the potential of mean force (PMF) variation with generation 5 (G5)-PAMAM dendrimer complexed with drug molecules using umbrella sampling. From our calculated PMF values, we observe that soluble drugs (Sal and Ala) have lower energy barriers than insoluble drugs (Pbz and Prim). The order of ease of release pattern for these drugs from G5 protonated PAMAM dendrimer was found to be Ala > Sal > Prim > Pbz. In the case of insoluble drugs (Prim and Pbz), because of larger size, we observe much nonpolar contribution, and thus, their larger energy barriers can be reasoned to van der Waals contribution. From the hydrogen bonding analysis of the four PAMAM drug complexes under study, we found intermolecular hydrogen bonding to show less significant contribution to the free energy barrier. Another interesting feature appears while calculating the PMF profile of G5NP (nonprotonated)-PAMAM Pbz and G5NP (nonprotonated)-PAMAM-Sal complex. The PMF was found to be less when the drug is bound to nonprotonated dendrimer compared to the protonated dendrimer. Our results suggest that encapsulation of the drug molecule into the host PAMAM dendrimer should be carried out at higher pH values (near pH 10). When such complex enters the human body, the pH is around 7.4 and at that physiological pH, the dendrimer holds the drug tightly. Hence the release of drug can occur at a controlled rate into the bloodstream. Thus, our findings provide a microscopic picture of the encapsulation and controlled release of drugs in the case of dendrimer-based host-guest systems.

Effect of base dissipation on the granular flow down an inclined plane

The effect of base dissipation on the granular flow down an inclined plane is examined by altering the coefficient of restitution between the moving and base particles in discrete element (DE) simulations. The interaction laws between two moving particles are kept fixed, and the coefficient of restitution (damping constant in the DE simulations) between the base and moving particles are altered to reduce dissipation, and inject energy from the base. The energy injection does result in an increase in the strain rate by up to an order of magnitude, and the temperature by up to two orders of magnitude at the base. However, the volume fraction, strain rate and temperature profiles in the bulk (above about 15 particle diameters from the base) are altered very little by the energy injection at the base. We also examine the variation of h(stop), the minimum height at the cessation of flow, with energy injection from the base. It is found that at a fixed angle of inclination, h(stop) decreases as the energy dissipation at the base decreases.

Structural and mechanistic investigations on Salmonella typhimurium acetate kinase (AckA): identification of a putative ligand binding pocket at the dimeric interface

Background: Bacteria such as Escherichia coli and Salmonella typhimurium can utilize acetate as the sole source of carbon and energy. Acetate kinase (AckA) and phosphotransacetylase (Pta), key enzymes of acetate utilization pathway, regulate flux of metabolites in glycolysis, gluconeogenesis, TCA cycle, glyoxylate bypass and fatty acid metabolism. Results: Here we report kinetic characterization of S. typhimurium AckA (StAckA) and structures of its unliganded (Form-I, 2.70 angstrom resolution) and citrate-bound (Form-II, 1.90 angstrom resolution) forms. The enzyme showed broad substrate specificity with k(cat)/K-m in the order of acetate > propionate > formate. Further, the K-m for acetyl-phosphate was significantly lower than for acetate and the enzyme could catalyze the reverse reaction (i.e. ATP synthesis) more efficiently. ATP and Mg2+ could be substituted by other nucleoside 5'-triphosphates (GTP, UTP and CTP) and divalent cations (Mn2+ and Co2+), respectively. Form-I StAckA represents the first structural report of an unliganded AckA. StAckA protomer consists of two domains with characteristic beta beta beta alpha beta alpha beta alpha topology of ASKHA superfamily of proteins. These domains adopt an intermediate conformation compared to that of open and closed forms of ligand-bound Methanosarcina thermophila AckA (MtAckA). Spectroscopic and structural analyses of StAckA further suggested occurrence of inter-domain motion upon ligand-binding. Unexpectedly, Form-II StAckA structure showed a drastic change in the conformation of residues 230-300 compared to that of Form-I. Further investigation revealed electron density corresponding to a citrate molecule in a pocket located at the dimeric interface of Form-II StAckA. Interestingly, a similar dimeric interface pocket lined with largely conserved residues could be identified in Form-I StAckA as well as in other enzymes homologous to AckA suggesting that ligand binding at this pocket may influence the function of these enzymes. Conclusions: The biochemical and structural characterization of StAckA reported here provides insights into the biochemical specificity, overall fold, thermal stability, molecular basis of ligand binding and inter-domain motion in AckA family of enzymes. Dramatic conformational differences observed between unliganded and citrate-bound forms of StAckA led to identification of a putative ligand-binding pocket at the dimeric interface of StAckA with implications for enzymatic function.

Analysis of large-amplitude stratospheric mountain wave event observed from the AIRS and MLS sounders over the western Himalayan region

Mountain waves in the stratosphere have been observed over elevated topographies using both nadir-looking and limb-viewing satellites. However, the characteristics of mountain waves generated over the Himalayan Mountain range and the adjacent Tibetan Plateau are relatively less explored. The present study reports on three-dimensional (3-D) properties of a mountain wave event that occurred over the western Himalayan region on 9 December 2008. Observations made by the Atmospheric Infrared Sounder on board the Aqua and Microwave Limb Sounder on board the Aura satellites are used to delineate the wave properties. The observed wave properties such as horizontal (lambda(x), lambda(y)) and vertical (lambda(z)) wavelengths are 276 km (zonal), 289 km (meridional), and 25 km, respectively. A good agreement is found between the observed and modeled/analyzed vertical wavelength for a stationary gravity wave determined using the Modern Era Retrospective Analysis for Research and Applications (MERRA) reanalysis winds. The analysis of both the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis and MERRA winds shows that the waves are primarily forced by strong flow across the topography. Using the 3-D properties of waves and the corrected temperature amplitudes, we estimated wave momentum fluxes of the order of similar to 0.05 Pa, which is in agreement with large-amplitude mountain wave events reported elsewhere. In this regard, the present study is considered to be very much informative to the gravity wave drag schemes employed in current general circulation models for this region.