The ammonolysis of esters in liquid ammonia

The rates of ammonolysis of alkyl benzoate and phenylacetate esters in liquid ammonia increase with the acidity of the leaving group alcohol and show relatively large Brønsted βlg values of −1.18 and −1.34, respectively, when plotted against the aqueous pKa of the alcohol. The Brønsted βlg obtained using the pKa of the leaving group alcohol in liquid ammonia is significantly reduced to ~ −0.7, which indicates that the rate-limiting step involves a reaction of the tetrahedral intermediate with little C–OR bond fission in the transition state. The solvolysis reaction is subject to significant catalysis by ammonium ion, which, surprisingly, generates a similar Brønsted βlg indicating little interaction between the ammonium ion and the leaving group. It is concluded that the rate-limiting step for the ammonium-ion-catalysed solvolysis of alkyl esters in liquid ammonia is the diffusion-controlled protonation of the zwitterionic tetrahedral intermediate T+- to give T+, which is rapidly deprotonated to give T0 which is compatible with the rate-limiting step for the uncatalysed reaction being the formation of the neutral T0 by a ‘proton switc

Development of a thermal sensor for probing small volumes

This thesis presents the design, fabrication and characterisation of a thermal micro-sensor capable of operating on small scale samples. It was initially characterised by measuring the thermal conductivity of 100 μl liquid droplets and thin materials (sheets of paper) before being applied to the measurement of the water content of plant leaves. The device consists of a micro-heater and two thin-film thermocouples formed on a flexible polyimide substrate and used microfabrication techniques to precisely pattern the sensing elements. The heater was used to induce a temperature gradient within a sample in contact with the device, which was recorded as a difference in temperature (ΔT) between the two thermocouples. Various experiments have demonstrated that ∆T was dependent on the total thermal resistance (consisting of the bulk thermal conductivity and thermal contact resistance) of the sample under inspection. The device’s sensitivity to bulk thermal conductivity was shown by recording the variation in ∆T for 100 μl droplets of glycerine/water mixtures. Different compositions of the mixture served as thermal conductivity standards. The measurement of mixtures of propanol/water further demonstrated that the device could be used to indirectly monitor the composition of small volume binary solutions by measurement of the thermal conductivity. By monitoring the thermal conductivity of wetted paper, it was shown that the device was sensitive to the increase in the total thermal resistance as the paper dried out. Furthermore, theoretical and experimental drying times were in agreement and exhibited a similar dependence on the air temperature. This provided clear evidence of the device’s ability to monitor thermal conductivity of small samples. The final element of this work was the real-time monitoring of the water content of plant leaves. The device was clamped to an abscised leaf which was allowed to dry over a period of 6 hours. A comparison between the weight of the leaf and ∆T measurements showed a linear dependence. It was found that the changes in thermal properties were dominated by the water content of the leaf. The device was subsequently shown to be sensitive to changes in the water content of the leaves of plants subjected to water stress conditions, demonstrating its ability to monitor the real-time water content in-situ

An Imaging Fabry-Perot System for the Robert Stobie Spectrograph on the Southern African Large Telescope

We present the design of the Fabry-Perot system of the Robert Stobie Spectrograph on the 10-meter class Southern African Large Telescope and its characterization as measured in the laboratory. This system provides spectroscopic imaging at any desired wavelength spanning a bandpass 430 - 860 nm, at four different spectral resolving powers ranging from 300 to 9000. Our laboratory tests revealed a wavelength dependence of the etalon gap and parallelism with a maximum variation between 600 - 720 nm that arises because of the complex structure of the broadband multi-layer dielectric coatings. We also report an unanticipated optical effect of this multi-layer coating structure that produces a significant, and wavelength dependent, change in the apparent shape of the etalon plates. This change is caused by two effects: the physical non-uniformities or thickness variations in the coating layers, and the wavelength dependence of the phase change upon refection that can amplify these non-uniformities. We discuss the impact of these coating effects on the resolving power, finesse, and throughput of the system. This Fabry-Perot system will provide a powerful tool for imaging spectroscopy on one of the world's largest telescopes.Comment: 17 pages, 14 figures, accepted for publication in The Astronomical Journa

On decays of light unflavoured pseudoscalar mesons

The ongoing and planned experimental activities with direct reference to light unflavoured pseudoscalar mesons motivate a new theoretical study regarding their properties. An overview including details on new precise calculations is presented.Comment: 9 pages, 1 fugre, presented at Physics of Fundamental Symmetries and Interactions - PSI201

Conserved mechanisms of microtubule-stimulated ADP release, ATP binding, and force generation in transport kinesins

Kinesins are a superfamily of microtubule-based ATP-powered motors, important for multiple, essential cellular functions. How microtubule binding stimulates their ATPase and controls force generation is not understood. To address this fundamental question, we visualized microtubule-bound kinesin-1 and kinesin-3 motor domains at multiple steps in their ATPase cycles - including their nucleotide-free states - at ~7Å resolution using cryo-electron microscopy. In both motors, microtubule binding promotes ordered conformations of conserved loops that stimulate ADP release, enhance microtubule affinity and prime the catalytic site for ATP binding. ATP binding causes only small shifts of these nucleotide-coordinating loops but induces large conformational changes elsewhere that allow force generation and neck linker docking towards the microtubule plus end. Family-specific differences across the kinesin-microtubule interface account for the distinctive properties of each motor. Our data thus provide evidence for a conserved ATP-driven mechanism for kinesins and reveal the critical mechanistic contribution of the microtubule interface

Sketch-A-Shape: Zero-Shot Sketch-to-3D Shape Generation

Significant progress has recently been made in creative applications of large pre-trained models for downstream tasks in 3D vision, such as text-to-shape generation. This motivates our investigation of how these pre-trained models can be used effectively to generate 3D shapes from sketches, which has largely remained an open challenge due to the limited sketch-shape paired datasets and the varying level of abstraction in the sketches. We discover that conditioning a 3D generative model on the features (obtained from a frozen large pre-trained vision model) of synthetic renderings during training enables us to effectively generate 3D shapes from sketches at inference time. This suggests that the large pre-trained vision model features carry semantic signals that are resilient to domain shifts, i.e., allowing us to use only RGB renderings, but generalizing to sketches at inference time. We conduct a comprehensive set of experiments investigating different design factors and demonstrate the effectiveness of our straightforward approach for generation of multiple 3D shapes per each input sketch regardless of their level of abstraction without requiring any paired datasets during training

Plastron respiration using commercial fabrics

A variety of insect and arachnid species are able to remain submerged in water indefinitely using plastron respiration. A plastron is a surface-retained film of air produced by surface morphology that acts as an oxygen-carbon dioxide exchange surface. Many highly water repellent and hydrophobic surfaces when placed in water exhibit a silvery sheen which is characteristic of a plastron. In this article, the hydrophobicity of a range of commercially available water repellent fabrics and polymer membranes is investigated, and how the surface of the materials mimics this mechanism of underwater respiration is demonstrated allowing direct extraction of oxygen from oxygenated water. The coverage of the surface with the plastron air layer was measured using confocal microscopy. A zinc/oxygen cell is used to consume oxygen within containers constructed from the different membranes, and the oxygen consumed by the cell is compared to the change in oxygen concentration as measured by an oxygen probe. By comparing the membranes to an air-tight reference sample, it was found that the membranes facilitated oxygen transfer from the water into the container, with the most successful membrane showing a 1.90:1 ratio between the cell oxygen consumption and the change in concentration within the container

An overview of technical considerations for Western blotting applications to physiological research

The applications of Western/immuno-blotting (WB) techniques have reached multiple layers of the scientific community and are now considered routine procedures in the field of physiology. This is none more so than in relation to skeletal muscle physiology (i.e. resolving the mechanisms underpinning adaptations to exercise). Indeed, the inclusion of WB data is now considered an essential aspect of many such physiological publications to provide mechanistic insight into regulatory processes. Despite this popularity, and due to the ubiquitous and relatively inexpensive availability of WB equipment, the quality of WB in publications and subsequent analysis and interpretation of the data can be variable, perhaps resulting in spurious conclusions. This may be due to poor laboratory technique and/or lack of comprehension of the critical steps involved in WB and what quality control procedures should be in place to ensure robust data generation. The present review aims to provide a detailed description and critique of WB procedures and technicalities, from sample collection through preparation, blotting and detection to analysis of the data collected. We aim to provide the reader with improved expertise to critically conduct, evaluate and troubleshoot the WB process, to produce reproducible and reliable blots