6 research outputs found
Mechanical behaviour of laminated hybrid adhesive point connections when exposed to humidity conditions
No full textThe use of structural glass has increased lately, due to the higher demands for transparency in modern architecture. Therefore, the role of glass changes from a mere transparent function (used in windows only) to a function in which the glass is contributing to the bearing structure as well. Because of the fragile nature of glass, it is important to understand its behaviour. Due to the manufacturing process, the strength of glass is decreased severely by small cracks that are present on the surface. Also, glass cannot redistribute stress peaks very well. Stress peaks tend to occur near the connection between two (glass) components, making this a critical part in glass design. A common way to joint glass elements is by a bolted connection. However, this type of connection does not seem suitable for glass design. Due to the drilling process extra cracks will occur, causing a reduced material resistance, and at the edge of the holes large stress intensifications arise. Adhesive connections are more in line with the above stated problems of glass design. No drilling is needed, so no extra cracks will occur. Also, forces are transferred over the entire surface of the connection, leading to smaller stress peaks in the glass. However, adhesive connections know risks of their own. Problems are (1) the sensitivity of some adhesives to aging affects like humidity, (2) the strongly non-linear behaviour of the adhesive which causes stress peaks in the connection, and (3) the complex manufacturing process of the joint, because some adhesives will liquefy and squeeze out during the lamination process. In this thesis a concept for an adhesive connection is proposed that addresses these issues. The concept proposes the use of two adhesives in order to reach the following effect. The first adhesive is a rigid ionomer adhesive (SG) that is very sensitive for humidity and the second adhesive is a flexible silicone adhesive (TSSA) which is not sensitive to aging effects. The first adhesive is used in the centre of the connection and the second adhesive forms an external ring to protect the SG against moisture exposure. Also, the non-linear behaviour of the rigid adhesive is leading to stress peaks at the edge of the connection. By placing a less rigid adhesive at the edge, the overall stress distribution is more evenly divided. The manufacturing process is enhanced because the TSSA remains solid during the lamination process, so it can be used as a mould for the SG that will liquefy. The concept is called a hybrid adhesive connection. The concept is tested for a circular point connection exposed to tensile loading. This is done by means of an experimental and numerical analysis. The experimental analysis focusses on the deterioration of the hybrid circular connection after exposure to either immersion or 100% relative humidity. The numerical analysis concentrates on the stress distribution of the proposed connection. The following conclusions can be drawn from this investigation. In the manufacturing process of the experimental analysis, also the TSSA is squeezed out due to its softness. Therefore, extra measures should be taken to prevent this, which means that the manufacturing process of hybrid connections is not improved in comparison to SG connections. Deterioration of the connection due to humidity is still visible for a hybrid connection containing a ring of TSSA with a width of 5mm. This effect is less visible when a width of 10mm TSSA is used. Unfortunately, due to lamination problems, the samples suffered from poor SG bonding, which makes it difficult to give substantial conclusions on the matter. The numerical analysis gave more clear results. The E-modulus of TSSA is very low in comparison to that of SG. Therefore, the contribution of the TSSA to the bearing capacity of the connection is negligible, so the stress distribution is not enhanced. On the other hand, due to the external ring of TSSA, the stress peaks in the connection are moved inwards, from the perimeter of the connection to the perimeter of the SG centre. This is a positive effect with regard to humidity exposure. The strength of the hybrid circular point connection in comparison to the TSSA connections, is only slightly increased, but the stiffness of the hybrid connection surely is enhanced. Treats of the hybrid connection might be the risk of air bubbles in between the SG and TSSA and the absence of the whitening effect in the TSSA.Civil Engineering and GeosciencesStructural EngineeringBuilding Engineerin
Calibrating the Bacterial Growth Rate Speedometer: A Re-evaluation of the Relationship Between Basal ppGpp, Growth, and RNA Synthesis in Escherichia coli
No full textThe molecule guanosine tetraphophosphate (ppGpp) is most commonly considered an alarmone produced during acute stress. However, ppGpp is also present at low concentrations during steady-state growth. Whether ppGpp controls the same cellular targets at both low and high concentrations remains an open question and is vital for understanding growth rate regulation. It is widely assumed that basal ppGpp concentrations vary inversely with growth rate, and that the main function of basal ppGpp is to regulate transcription of ribosomal RNA in response to environmental conditions. Unfortunately, studies to confirm this relationship and to define regulatory targets of basal ppGpp are limited by difficulties in quantifying basal ppGpp. In this Perspective we compare reported concentrations of basal ppGpp in E. coli and quantify ppGpp within several strains using a recently developed analytical method. We find that although the inverse correlation between ppGpp and growth rate is robust across strains and analytical methods, absolute ppGpp concentrations do not absolutely determine RNA synthesis rates. In addition, we investigated the consequences of two separate RNA polymerase mutations that each individually reduce (but do not abolish) sensitivity to ppGpp and find that the relationship between ppGpp, growth rate, and RNA content of single-site mutants remains unaffected. Both literature and our new data suggest that environmental conditions may be communicated to RNA polymerase via an additional regulator. We conclude that basal ppGpp is one of potentially several agents controlling ribosome abundance and DNA replication initiation, but that evidence for additional roles in controlling macromolecular synthesis requires further study.BN/Greg Bokinsky La
Posttranslational control of plsb is sufficient to coordinate membrane synthesis with growth in escherichia coli
No full textEvery cell must produce enough membrane to contain itself. However, the mechanisms by which the rate of membrane synthesis is coupled with the rate of cell growth remain unresolved. By comparing substrate and enzyme concentrations of the fatty acid and phospholipid synthesis pathways of Escherichia coli across a 3-fold range of carbon-limited growth rates, we show that the rate of membrane phospholipid synthesis during steady-state growth is determined principally through allosteric control of a single enzyme, PlsB. Due to feedback regulation of the fatty acid pathway, PlsB activity also indirectly controls synthesis of lipopolysaccharide, a major component of the outer membrane synthesized from a fatty acid synthesis intermediate. Surprisingly, concentrations of the enzyme that catalyzes the committed step of lipopolysaccharide synthesis (LpxC) do not differ across steady-state growth conditions, suggesting that steady-state lipopolysaccharide synthesis is modulated primarily via indirect control by PlsB. In contrast to steady-state regulation, we found that responses to environmental perturbations are triggered directly via changes in acetyl coenzyme A (acetyl-CoA) concentrations, which enable rapid adaptation. Adaptations are further modulated by ppGpp, which regulates PlsB activity during slow growth and growth arrest. The strong reliance of the membrane synthesis pathway upon posttranslational regulation ensures both the reliability and the responsiveness of membrane synthesis. IMPORTANCE How do bacterial cells grow without breaking their membranes? Although the biochemistry of fatty acid and membrane synthesis is well known, how membrane synthesis is balanced with growth and metabolism has remained unclear. This is partly due to the many control points that have been discovered within the membrane synthesis pathways. By precisely establishing the contributions of individual pathway enzymes, our results simplify the model of membrane biogenesis in the model bacterial species Escherichia coli. Specifically, we found that allosteric control of a single enzyme, PlsB, is sufficient to balance growth with membrane synthesis and to ensure that growing E. coli cells produce sufficient membrane. Identifying the signals that activate and deactivate PlsB will resolve the issue of how membrane synthesis is synchronized with growth.BN/Greg Bokinsky La
Cellular assays identify barriers impeding iron-sulfur enzyme activity in a non-native prokaryotic host
No full textIron-sulfur (Fe-S) clusters are ancient and ubiquitous protein cofactors and play irreplaceable roles in many metabolic and regulatory processes. Fe-S clusters are built and distributed to Fe-S enzymes by dedicated protein networks. The core components of these networks are widely conserved and highly versatile. However, Fe-S proteins and enzymes are often inactive outside their native host species. We sought to systematically investigate the compatibility of Fe-S networks with non-native Fe-S enzymes. By using collections of Fe-S enzyme orthologs representative of the entire range of prokaryotic diversity, we uncovered a striking correlation between phylogenetic distance and probability of functional expression. Moreover, coexpression of a heterologous Fe-S biogenesis pathway increases the phylogenetic range of orthologs that can be supported by the foreign host. We also find that Fe-S enzymes that require specific electron carrier proteins are rarely functionally expressed unless their taxon-specific reducing partners are identified and co-expressed. We demonstrate how these principles can be applied to improve the activity of a radical S-adenosyl methionine(rSAM) enzyme from a Streptomyces antibiotic biosynthesis pathway in Escherichia coli. Our results clarify how oxygen sensitivity and incompatibilities with foreign Fe-S and electron transfer networks each impede heterologous activity. In particular, identifying compatible electron transfer proteins and heterologous Fe-S biogenesis pathways may prove essential for engineering functional Fe-S enzyme-dependent pathways.BN/Greg Bokinsky LabBT/Environmental Biotechnolog
An Idealized Meteorological-Hydrodynamic Model for Exploring Extreme Storm Surge Statistics in the North Sea
No full textThis paper explores an alternative method to determine extreme surge levels at the Dutch Coast. For this exploration, specific focus is on the extreme water level at Hoek van Holland, The Netherlands. The alternative method has been based on a joint probability model of the storm characteristics at the North Sea. The intent of this method is to provide a better physical and statistical insight into the effects of meteorological characteristics on surge levels and surge duration, especially for surges of more extreme storms currently not captured in existing water level measurement records. The meteorological part is an analytical parametrical model based on the Holland model for hurricanes, which results in time- and space-varying wind and pressure fields of North Sea storms. The wind and pressure forcing is then applied in the hydrodynamic model which numerically solves the nonlinear depth-averaged shallow water equations in a one-dimensional domain from the edge of the continental shelf between Scotland and Norway to Hoek van Holland. Validation against wind observations from historical storms at one location in the entire domain shows good results. Results of the calibrated surge level model are reasonable if peak surge levels are considered. The surge duration, however, is underestimated by the model. Next, the model has been applied to define extreme surge levels using Monte Carlo Analysis. Probability density distributions for the storm parameters based on historical data have been used as input. The computed surge level (including tide) with a statistical return period of 10,000 years appears to be close to the value from statistical extrapolation of surge levels. The output also indicates that the average duration of computed surges with a return period of 10,000 year is roughly two hours longer than the storm duration currently adopted.Hydraulic EngineeringCivil Engineering and Geoscience
