Zirconium diboride based coatings for thermal protection of re entry vehicles: Effect of MoSi2 addition

The possibility of improving the performances of new generation re-entry vehicles as well as reducing the mission costs is closely connected to the development of new thermal protection systems: innovative heat shield materials, able to withstand higher temperature during the lift re-entry manoeuvre, would permit to improve reusability and easiness of maintenance as well as to increase the manoeuvrability and the payload of vehicles. UHTC materials, and, among them, Zirconium diboride based materials, exhibit outstanding oxidation and thermal shock resistance, high emissivity and very high melting temperature: all these properties make ZrB₂ based materials possible candidates for thermal protection systems of atmospheric re-entry vehicles. The experimental activities described in the present work were focused on ZrB₂ based coatings deposited by plasma spray, containing SiC and MoSi₂ in different ratios: the work aimed at understanding the role of MoSi₂ addition in the oxidation resistance. XRD analysis was carried out on starting powders and plasma sprayed coatings, in order to evaluate the transformations of crystalline phases induced by thermal deposition and to analyse the possible presence of amorphous phase; XPS was also used to study the coatings surface composition and to obtain information about chemical bonds in the materials. Oxidation behaviour was studied carrying out exposure tests up to 1800 °C in static air to compare and model the oxidation kinetics of the different compositions: results show the better behaviour of materials containing MoSi₂. Microstructure of as sprayed and oxidised samples was observed by means of SEM and analysed by EDS to understand the link between dispersion of Mo containing phases and the improved oxidation resistance.     

Cumulative stabilizing effects of glycine to alanine substitutions in Bacillus subtilis neutral protease

Oligonucleotide-directed mutagenesis has been used to replaceglycine residues by alanine in neutral protease from Bacillussubtilis. One Gly to Ala substitution (G147A) was located ina helical region of the protein, while the other (G189A) wasin a loop. The effects of mutational substitutions on the functional,conformational and stability properties of the enzyme have beeninvestigated using enzymatic assays and spectroscopic measurements.Single substitutions of both G1y147 and Gly189 with Ala residuesaffect the enzyme kinetic properties using synthetic peptidesas substrates. When Gly replacements were concurrently introducedat both positions, the kinetic characteristics of the doublemutant were roughly intermediate between those of the two singlemutants, and similar to those of the wild-type protease. Bothmutants G147A and G189A were found to be more stable towardsirreversible thermal inactivation/unfolding than the wild-typespecies. Moreover, the stabilizing effect of the Gly to Alasubstitution was roughly additive in the double mutant G147A/G189A,which shows a 3.2°C increase in T_m with respect to the wild-typeprotein. These findings indicate that the Gly to Ala substitutioncan be used as a strategy to stabilize globular proteins. Thepossible mechanisms of protein stabilization are also discussed.     

Biosynthesis and spectroscopic characterization of 2-TM fragments encompassing the sequence of a human GPCR, the Y4 receptor

This paper presents a divide‐and‐conquer approach towards obtaining solution structures of G protein‐coupled receptors. The human Y4 receptor was dissected into two to three transmembrane helix fragments, which were individually studied by solution NMR. We systematically compared various biosynthetic routes for the expression of the fragments in Escherichia coli and discuss purification strategies. In particular, we have compared the production of transmembrane (TM) fragments as inclusion bodies by using the ΔTrp leader sequence, with membrane‐directed expression by using Mistic as the fusion partner, and developed methods for enzymatic cleavage. In addition, direct expression of two‐TM fragments into inclusion bodies is a successful route in some cases. With the exception of TM13, we could produce all fragments in isotope‐labeled form in quantities sufficient for NMR studies. Almost complete backbone resonance assignment was obtained for the first two helices, as well as for helices 5 and 7, and a high degree was obtained for TM6, while conformational exchange processes resulted in the disappearance of many signals from TM4. In addition, complete assignments were obtained for all residues of the N‐terminal domain, as well as the extracellular and cytosolic loops (with the exception of an undecapeptide segment in the second extracellular loop, EC2) and for the complete cytosolic C‐terminal tail. In total, backbone resonances of 78 % of all residues were assigned for the Y4 receptor. Predictions of secondary structure based on backbone chemical shifts indicate that most residues from the TM regions adopt helical conformations, with exception of those around polar residues or prolines. However, the domain boundaries differ slightly from those predicted for homology models. We suggest that the obtained chemical shifts might be useful in assigning the full‐length receptor.     

A DNA‐Encoded Library of Chemical Compounds Based on Common Scaffolding Structures Reveals the Impact of Ligand Geometry on Protein Recognition

A DNA‐encoded chemical library (DECL) with 1.2 million compounds was synthesized by combinatorial reaction of seven central scaffolds with two sets of 343×492 building blocks. Library screening by affinity capture revealed that for some target proteins, the chemical nature of building blocks dominated the selection results, whereas for other proteins, the central scaffold also crucially contributed to ligand affinity. Molecules based on a 3,5‐bis(aminomethyl)benzoic acid core structure were found to bind human serum albumin with a K_d value of 6 nm , while compounds with the same substituents on an equidistant but flexible l ‐lysine scaffold showed 140‐fold lower affinity. A 18 nm tankyrase‐1 binder featured l ‐lysine as linking moiety, while molecules based on d ‐Lysine or (2S,4S)‐amino‐l ‐proline showed no detectable binding to the target. This work suggests that central scaffolds which predispose the orientation of chemical building blocks toward the protein target may enhance the screening productivity of encoded libraries.     

The infusion of calcium gluconate in coronary disease patients induces myocardial ischemia via a possible adenosine-mediated mechanism

The development of spontaneous mammary tumors was observed for about 2 years in a group of 25 female Sprague-Dawley rats aged over 1 year at the beginning of the study. All younger females in our animal facility were similarly monitored. In old females, the incidence of spontaneous mammary tumors was 64%. The parity of rats did not protect them from tumorigenesis, but the proportion of malignant tumors was higher in virgin (57%) than in parous (13%) rats. Activities of cytochrome P450IA1-dependent enzyme (aryl hydrocarbon hydroxylase, AHH) and NADPH-cytochrome P450 reductase (NCR) were determined in microsome fractions isolated from livers, lungs, uteri and tumors of rats. AHH and NCR activities in tumors and uteri were low compared to those in livers or lungs. In tumors, the activity distributions were wide, even in different tumors of the same animal the AHH activities varied as widely as between different animals. The activities in benign and malignant tumors were not statistically significantly different. No correlation with liver, lung or uterine activities was found either. With ageing of the rat, the AHH activities in tumors, liver and lungs decreased. The behavior of AHH in spontaneous mammary tumors in rats seems to be similar to that found in chemically induced tumors and seems to show individual regulation, possibly altered by tumorigenesis in each individual tumor.     

Ripples and layers in ultrathin MoS2 membranes

Single-layer molybdenum disulfide (MoS2) is a newly emerging two-dimensional semiconductor with a potentially wide range of applications in the fields of nanoelectronics and energy harvesting. The fact that it can be exfoliated down to single-layer thickness makes MoS2 interesting both for practical applications and for fundamental research, where the structure and crystalline order of ultrathin MoS2 will have a strong influence on electronic, mechanical, and other properties. Here, we report on the transmission electron microscopy study of suspended single- and few-layer MoS2 membranes with thicknesses previously determined using both optical identification and atomic force microscopy. Electron microscopy shows that monolayer MoS2 displays long-range crystalline order, although surface roughening has been observed with ripples which can reach 1 nm in height, just as in the case of graphene, implying that similar mechanisms are responsible for the stability of both two-dimensional materials. The observed ripples could explain the degradation of mobility in MoS2 due to exfoliation. We also find that symmetry breaking due to the reduction of the number of layers results in distinctive features in electron-beam diffraction patterns of single- and multilayer MoS2, which could be used as a method for identifying single layers using only electron microscopy. The isolation of suspended single-layer MoS2 membranes will improve our understanding of two-dimensional systems, their stability, and the interplay between their structures, morphologies, and electrical and mechanical properties.     

Infrared thermometry study of nanofluid pool boiling phenomena

Infrared thermometry was used to obtain first-of-a-kind, time- and space-resolved data for pool boiling phenomena in water-based nanofluids with diamond and silica nanoparticles at low concentration (<0.1 vol.%). In addition to macroscopic parameters like the average heat transfer coefficient and critical heat flux [CHF] value, more fundamental parameters such as the bubble departure diameter and frequency, growth and wait times, and nucleation site density [NSD] were directly measured for a thin, resistively heated, indium-tin-oxide surface deposited onto a sapphire substrate. Consistent with other nanofluid studies, the nanoparticles caused deterioration in the nucleate boiling heat transfer (by as much as 50%) and an increase in the CHF (by as much as 100%). The bubble departure frequency and NSD were found to be lower in nanofluids compared with water for the same wall superheat. Furthermore, it was found that a porous layer of nanoparticles built up on the heater surface during nucleate boiling, which improved surface wettability compared with the water-boiled surfaces. Using the prevalent nucleate boiling models, it was possible to correlate this improved surface wettability to the experimentally observed reductions in the bubble departure frequency, NSD, and ultimately to the deterioration in the nucleate boiling heat transfer and the CHF enhancement.