Nucleotide-induced conformational changes in the ATPase and substrate binding domains of the DnaK chaperone provide evidence for interdomain communication

Interactions of the DnaK (Hsp70) chaperone from Escherichia coli with substrates are controlled by ATP. Nucleotide-induced changes in DnaK conformation were investigated by monitoring changes in tryptic digestion pattern and tryptophan fluorescence. Using nucleotide-free DnaK preparations, not only the known ATP-induced major changes in kinetics and pattern of proteolysis but also minor ADP-induced changes were detected. Similar ATP-induced conformational changes occurred in the DnaK-T199A mutant protein defective in ATPase activity, demonstrating that they result from binding, not hydrolysis, of ATP. N-terminal sequencing and immunological mapping of tryptic fragments of DnaK identified cleavage sites that, upon ATP addition, appeared within the proposed C-terminal substrate binding region and disappeared in the N-terminal ATPase domain. They hence reflect structural alterations in DnaK correlated to substrate release and indicate ATP-dependent domain interactions. Domain interactions are a prerequisite for efficient tryptic degradation as fragments of DnaK comprising the ATPase and C-terminal domains were highly protease-resistant. Fluorescence analysis of the N-terminally located single tryptophan residue of DnaK revealed that the known ATP-induced alteration of the emission spectrum, proposed to result directly from conformational changes in the ATPase domain, requires the presence of the C-terminal domain and therefore mainly results from altered domain interaction. Analyses of the C-terminally truncated DnaK163 mutant protein revealed that nucleotide-dependent interdomain communication requires a 15-kDa segment assumed to constitute the substrate binding site.     

CSimMDMV: A parallel program for stochastic characterization of multi-dimensional, multi-variant, and multi-scale distribution of heterogeneous reservoir rock properties from well log data

We present CSimMDMV, a software package to simulate two- and three-dimensional, multi-variant heterogeneous reservoir models from well logs at different characteristic scales. Based on multi-variant conditional stochastic simulation, this software is able to parameterize multi-dimensional heterogeneities and to construct heterogeneous reservoir models with multiple rock properties. The models match the well logs at borehole locations, simulate heterogeneities at the level of detail provided by well logging data elsewhere in the model space, and simultaneously honor the correlations present in various rock properties. It provides a versatile environment in which a variety of geophysical experiments can be performed. This includes the estimation of petrophysical properties and the study of geophysical response to the heterogeneities. This paper describes the theoretical basis of the approach and provides the details of the parallel implementation on a Linux cluster. A case study on the assessment of natural gas hydrate amount in Northwest Territories, Canada is provided. We show that the combination of rock physics theory with multiple realizations of three-dimensional and three-variant (3D-3V) gas hydrate reservoir petrophysical models enable us to estimate the average amount of gas hydrate and associated uncertainties using Monte Carlo method.     

In situ high-energy X-ray diffraction of precipitation and dissolution reactions during heating of Al alloys

Journal of Materials Science - During heating of Al alloys, typically a sequence of precipitation and dissolution reactions occurs and the single (partly opposing) reactions superimpose....     

On the extraordinary low quench sensitivity of an AlZnMg alloy

The scope of this work was to investigate the quench sensitivity of a high-purity wrought aluminum alloy Al6Zn0.75 Mg (in this work called 7003_pure). This is compared to a similar alloy with the additions of Fe, Si, and Zr at a sum less than 0.3 at.% (in this work called 7003_Fe,Si,Zr). Differential scanning calorimetry (DSC) was used for an in situ analysis of quench induced precipitation in a wide range of cooling rates varying between 0.0003 and 3 K/s. In 7003_pure, three main precipitation reactions were observed during cooling, a medium temperature reaction with a distinct double peak between 325 and 175 °C and a very low temperature reaction starting at about 100 °C. An additional high temperature reaction related to the precipitation of Mg₂Si starting at 425 °C has been observed for 7003_Fe,Si,Zr. In terms of hardness after natural as well as artificial aging, alloy 7003_pure shows a very low quench sensitivity. Hardness values on the saturation level of about 120 HV1 are seen down to cooling rates of 0.003 K/s. The as-quenched hardness (5 min of natural aging) shows a maximum at a cooling rate of 0.003 K/s, while slower and faster cooling results in a lower hardness. In terms of hardness after aging, 0.003 K/s could be defined as the technological critical cooling rate, which is much higher for 7003_Fe,Si,Zr (0.3–1 K/s). The physical critical cooling rates for the suppression of any precipitation during cooling were found to be about 10 K/s for both variants.

     

Nucleation behaviour and microstructure of single Al-Si12 powder particles rapidly solidified in a fast scanning calorimeter

The understanding of rapid solidification behaviour, e.g. the undercooling versus growth velocity relationship, is crucial for tailoring microstructures and properties in metal alloys. In most rapid solidification processes, such as additive manufacturing (AM), in situ investigation of rapid solidification behaviour is missing because of the lack of accurate measurement of the cooling rate and nucleation undercooling. In the present study, rapid solidification of single micro-sized Al-Si12 (mass%) particles of various diameters has been investigated via differential fast scanning calorimetry employing controllable cooling rates from 100 to 90,000 K s^?1 relevant for AM. Based on nucleation undercooling and on microstructure analysis of rapidly solidified single powder particles under controlled cooling rates, two different heterogeneous nucleation mechanisms of the primary α-Al phase are proposed. Surface heterogeneous nucleation dominates for particles with diameter smaller than 23 μm. For particles with diameter larger than 23 μm, the nucleation of the primary α-Al phase changes from surface to bulk heterogeneous nucleation with increasing cooling rate. The results indicate that at large undercoolings (>?95 K) and high cooling rates (>?10,000 K s^?1), rapid solidification of single particle can yield a microstructure similar to that formed in AM. The present work not only proposes new insight into rapid solidification processes, but also provides a theoretical foundation for further understanding of microstructures and properties in additively manufactured materials.