Effects of introduced aspartic and glutamic acid residues on the P'1 substrate specificity, pH dependence and stability of carboxypeptidase Y

Carboxypeptidase Y is a serine carboxypeptidase isolated from Saccharomyces cerevisiae with a preference for C-terminal hydrophobic amino acid residues. In order to alter the inherent substrate specificity of CPD-Y into one for basic amino acid residues in P'1, we have introduced Asp and/or Glu residues at a number of selected positions within the S'1 binding site. The effects of these substitutions on the substrate specificity, pH dependence and protein stability have been evaluated. The results presented here demonstrate that it is possible to obtain significant changes in the substrate preference by introducing charged amino acids into the framework provided by an enzyme with a quite different specificity. The introduced acidic amino acid residues provide a marked pH dependence of the (kcat/Km)FA-A-R-OH/(kcat/Km)FA-A-L-OH ratio. The change in stability upon introduction of Asp/Glu residues can be correlated to the difference in the mean buried surface area between the substituted and the substituting amino acid. Thus, the effects of acidic amino acid residues on the protein stability depend upon whether the introduced amino acid protrudes from the solvent accessible surface as defined by the surrounding residues in the wild type enzyme or is submerged below.     

Solid state transformations of Au-Cu-Pt alloys studied by in-situ X-ray synchrotron radiation and DSC

Seven alloys of the ternary Au-Cu-Pt system, containing 75 or 76.5 wt.% Au with 0–15 wt.% Pt and heat-treated to a fully ordered condition, are examined using in-situ synchrotron radiation to determine transformation temperatures and phases formed, and to investigate mechanisms of ordering and disordering. These experiments are complemented by differential scanning calorimetry. Diffraction data show that the alloys can be separated into three groups according to their stable low-temperature ordered phase(s), namely L1₀, L1₂, or the two combined. Platinum increases transformation temperatures in comparison with binary AuCu, two-phase ordered alloys showing the highest transformation temperatures. Details of the evolution of the X-ray diffraction peak structures upon heating and cooling point to significant differences between mechanisms of disordering and ordering: whereas ordering visibly proceeds at significant undercooling by nucleation and growth, disordering appears, from the visible shifts in peak position, to progress in more homogeneous fashion within the alloy.     

Chemical stability of zirconia-stabilized alumina fibers during pressure infiltration by aluminum

Metal matrix composites composed of high-purity aluminum and Du Pont PRD-166 continuous zirconia-stabilized polycrystalline alumina fibers are fabricated by liquid metal infiltration using three different casting procedures. The microstructure of the composites is analyzed using optical and electron microscopy, including analytical electron microscopy. It is found that discrete faceted particles of ZrAl₃ form at the interface and grow into the matrix of samples processed above the melting point of the matrix for 13 minutes or more. The formation of this compound is in agreement with thermodynamic stability calculations. It is also found that there is a reaction between solid aluminum and the fibers at 913 K, yielding a reaction product which has the same morphology as that observed with molten aluminum. When the fibers are infiltrated with an initial preform temperature below the metal melting point and a solidifination time below 1 minute, no reaction products were visible in the composite using the scanning electron microscope (SEM). This leads to the conclusion that aluminum matrix composites can be cast with no apparent interfacial reaction product using these fibers provided that adequate processing parameters are chosen.     

Nitrate leaching during establishment of willow (Salix viminalis) on two soil types and at two fertilization levels

The production of willow in short rotation is expected to result in low nitrate leaching losses as is the case with other permanent crops. However, there is a risk of leaching of nitrate during establishment when the plant cover is limited. Nitrate leaching was followed for three years from the establishment in 1993 of willow (Salix viminalis) at two sites, a coarse sand at Jyndevad and a loamy sand at Foulum. Two levels of nitrogen, 0 or 75 kg N/ha in NPK, were applied annually, though only 38 N/ha was applied in the first year.

Leaching was calculated from usually weekly measurements of the nitrate concentration in soil solution and relating with the amount of percolation based on measured precipitation and changes in the soil water content over the sampling periods. Mean leaching for the three periods 1993–94, 1994–95 and 1995–96 amounted to respectively 142, 61 and 0 kg N/ha at Foulum and to 130, 9 and 4 kg N/ha at Jyndevad. The high leaching in the first year was caused by an unusually high content of mineral N in the soil at the start of the experiment. The absence of any leaching 1995–96 at Foulum was due to very low precipitation and a consequent lack of percolation. For the first period (1993–94) the fertilized treatment increased leaching by 32 kg N/ha compared to no fertilizer as a mean of the two sites, while the difference for the following two periods was reduced to 1–2 kg N/ha. It is concluded that application of nitrogen should be avoided in the year of planting of willows, while in the following years75 kg N/ha can be given without risk of increased leaching.     

Vapor Annealing as a Post-Processing Technique to Control Carrier Concentrations of Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> Thin Films

This article demonstrates that carrier concentrations in bismuth telluride films can be controlled through annealing in controlled vapor pressures of tellurium. For the bismuth telluride source with a small excess of tellurium, all the films reached a steady state carrier concentration of 4 × 10¹⁹ carriers/cm³ with Seebeck coefficients of −170 μV K⁻¹. For temperatures below 300°C and for film thicknesses of 0.4 μm or less, the rate-limiting step in reaching a steady state for the carrier concentration appeared to be the mass transport of tellurium through the gas phase. At higher temperatures, with the resulting higher pressures of tellurium or for thicker films, it was expected that mass transport through the solid would become rate limiting. The mobility also changed with annealing, but at a rate different from that of the carrier concentration, perhaps as a consequence of the non-equilibrium concentration of defects trapped in the films studied by the low temperature synthesis approach.     

Young’s modulus of ceramic particle reinforced aluminium: Measurement by the Impulse Excitation Technique and confrontation with analytical models

Young’s modulus of particulate metal matrix composites with volume fractions of reinforcement ranging from 40% to 60% is measured with the Impulse Excitation Technique. Results are compared with predictions of four common micromechanical mean-field models. Good agreement is obtained with the Torquato identical hard spheres (TIHS) and the generalized self-consistent (GSC) model.     

The effect of gravity on solution-reprecipitation during liquid phase sintering

Solution-reprecipitation during liquid phase sintering can lead to gravity-aligned gradients in the amount of refractory phase as a result of the interaction between gravitational forces and capillary forces. We provide an anlaysis of this mechanism for gradient formation and show that for most important engineering materials, solution-reprecipitation does not cause substantial gravity-induced gradients. This conclusion is in agreement with published data for tungsten heavy alloy materials containing volume fractions of solids greater than about 0.7 at the sintering temperature. Macrostructural gradients in liquid phase sintered materials have been reported in the literature; however, these materials contain sufficient liquid at the sintering temperature that solid grains settle within the liquid, perhaps contributing to the observed gradients.     

Infiltration of fibrous preforms by a pure metal: Part V. Influence of preform compressibility

Infiltration by a pure matrix in the presence of preform deformation and partial matrix solidification is analyzed using a bounding approach for the preform rheology where solid metal is present. It is found, using parameters for the infiltration of short alumina fiber preforms by aluminum, that the two bounds are close in comparison to other factors of uncertainty in the prediction of infiltration rate. Using this approach, preform compression is shown to exert a significant influence on the infiltration rate for the system explored; in particular, the analysis shows the existence of an optimal value of applied pressure. Simplifications in the analysis are also presented, which yield fairly accurate results while easing their computation significantly.