Short peptide alpha helices induced by multiple metal clips

Alpha helices are key structural components of proteins and important recognition motifs in biology. New techniques for stabilizing short peptide helices could be valuable for studying protein folding, modeling proteins, creating artificial proteins, and may aid the design of inhibitors or mimics of protein function.

Magnetic properties and Spin dynamics in magnetic Molecular Rings

Molecular nanomagnets are spin clusters whose topology and magnetic interactions can be modulated at the level of the chemical synthesis. They are formed by a small number of transition metal ions coupled by the Heisenberg's exchange interactions. Each cluster is magnetically isolated from its neighbors by organic ligands, making each unit not interacting with the others. Therefore, we can investigate the magnetic properties of an isolated molecular nanomagnet by bulk measurements. The present thesis has been mostly devoted to the experimental investigation of the magnetic properties and spin dynamics of different classes of antiferromagnetic (AF) molecular rings. This study has been exploiting various techniques of investigations, such as Nuclear Magnetic Resonance (NMR), muon spin relaxation (muSR) and SQUiD magnetometry. We investigate the magnetic properties and the phonon-induced relaxation dynamics of the first regular Cr9 antiferromagnetic (AF) ring, which represents a prototype frustrated AF ring. The magnetically-open AF rings like Cr8Cd are model systems for the study of the microscopic magnetic behaviour of finite AF Heisenberg chains. In this type of system the different magnetic behaviour depends length and on the parity of the chain (odd or even). In order to study the local spin densities on the Cr sites, the Cr-NMR spectra was collected at low temperature. The experimental result confirm the theoretical predictions for the spin configuration. Finally, the study of Dy6, the first rare-earth based ring that has been ever synthesized, has been performed by AC-SQuID and muSR measurements. We found that the dynamics is characterized by more than one characteristic correlation time, whose values depend strongly on the applied field.

The geochemistry of radioactive waste disposal

The present study attempted to identify the significant parameters which affect radionuclide migration from a low level radioactive waste disposal site located in a clay deposit. From initial sorption studies on smectite minerals, increased Kd with decreasing initial cation concentration was observed, and three sorption mechanisms were identified. The observation of anion dependent sorption was related to the existence of a mechanism in which an anion-cation pair are bound to the clay surface through the anion. The influence of competing cations, typical of inorganic groundwater constituents, depended on: (1) Ni/Co:Mn+(Mn+ = competing cation) ratio, (2) nature of M^n+, (3) total solution ionic strength. The presence of organic material in groundwater is well documented, but its effect on cation sorption has not been established. An initial qualitative investigation involving addition of simple organic ligands to Ni(Co)-hectorite samples demonstrated the formation of metal complexes in the clay interlayers, although some modified behaviour was observed. Further quantitative examination involving likely groundwater organic constituents and more comprehensive physical investigation confirmed this behaviour and enabled separation of the organic compounds used into two classes, according to their effect on cation sorption; (i) acids, (ii) amine compounds. X-ray photoelectron spectroscopy, scanning electron microscopy and Mossbauer spectroscopy were used to investigate the nature of transition metal ions sorbed onto montmorillonite and hectorite. Evidence strongly favoured the sorption of the hexaaquo cation, although a series of sorption sites of slightly different chemical characteristics were responsible for broadened peak widths observed in XPS and Mossbauer investigations. The surface sensitivity of XPS enabled recognition of the two surface sorption sites proposed in earlier sorption studies. Although thermal treatment of Fe^3+/Fe^2+-hectorite samples left iron atoms bonded to the silicate sheet structure, Mossbauer evidence indicated the presence of both ferric and ferrous iron in all samples.

Studies of clay minerals and their decomposition products

Mõssbauer spectroscopy and X-ray diffraction of five coals revealed the presence of pyrite, illite, kaolinite and Quartz, together with other minor phases. Analysis of the coal ashes indicated the formation of hematite and an Fe (3+) paramagnetic phase, the latter resulting from .the dehydroxylation of the clay minerals during ashing at 700 to 750 C. By using a combination of several physicochemical methods, different successive stages of dehydroxylation, structural consolidation, and recrystallisation of illite, montmorillonite and hectorite upon thermal treatment to 1300 C were investigated. Dehydroxylation of the clay minerals occurred between 450 and 750 C, the X-ray crysdallinity of illite and montmorillonite remaining until 800 C. Hectorite gradually recrystallises to enstatite at temperatures above 700°C. At 900 C the crystalline structure of all three clay minerals had totally collapsed. Solid state reactions occurred above 900 C producing such phases as spinel, hematite, enstatite, cristobalite and mullite. Illite and montmorillonite started to melt between 1200 and 1300°C, producing a silicate glass that contained Fe(3+) and Fe(2+) ions. Ortho-pnstatite, clino-enstatite and proto-enstatite were identified in the thermal products of hectorite, their relative proportions varying with temperature. Protoenstatite was stabilised with respect to metastable clinoenstatite upon cooling from 12000 C by the presence of exchanged transition metal cations. Solid state Nuclear Magnetic Resonance spectroscopy of thermally treated transition metal exchanged hectorite indicated the levels at which paramagnetic cations could be loaded on to the clay before spectral resolution is significantly diminished.

The compressive creep and load relaxation properties of a series of high aluminium zinc-based alloys

A new family of commercial zinc alloys designated as ZA8, ZA12, and ZA27 and high damping capacity alloys including Cosmal and Supercosmal and aluminium alloy LM25 were investigated for compressive creep and load relaxation behaviour under a series of temperatures and stresses. A compressive creep machine was designed to test the sand cast hollow cylindrical test specimens of these alloys. For each compressive creep experiment the variation of creep strain was presented in the form of graphs plotted as percentage of creep strain () versus time in seconds (s). In all cases, the curves showed the same general form of the creep curve, i.e. a primary creep stage, followed by a linear steady-state region (secondary creep). In general, it was observed that alloy ZA8 had the least primary creep among the commercial zinc-based alloys and ZA27 the greatest. The extent of primary creep increased with aluminium content to that of ZA27 then declined to Supercosmal. The overall creep strength of ZA27 was generally less than ZA8 and ZA12 but it showed better creep strength than ZA8 and ZA12 at high temperature and high stress. In high damping capacity alloys, Supercosmal had less primary creep and longer secondary creep regions and also had the lowest minimum creep rate among all the tested alloys. LM25 exhibited almost no creep at maximum temperature and stress used in this research work. Total creep elongation was shown to be well correlated using an empirical equation. Stress exponent and activation energies were calculated and found to be consistent with the creep mechanism of dislocation climb. The primary α and β phases in the as-cast structures decomposed to lamellar phases on cooling, with some particulates at dendrite edges and grain boundaries. Further breakdown into particulate bodies occurred during creep testing, and zinc bands developed at the highest test temperature of 160°C. The results of load relaxation testing showed that initially load loss proceeded rapidly and then deminished gradually with time. Load loss increased with temperature and almost all the curves approximated to a logarithmic decay of preload with time. ZA alloys exhibited almost the same load loss at lower temperature, but at 120°C ZA27 improved its relative performance with the passage of time. High damping capacity alloys and LM25 had much better resistance to load loss than ZA alloys and LM25 was found to be the best against load loss among these alloys. A preliminary equation was derived to correlate the retained load with time and temperature.

Melting and solidification of Zinc-Aluminium alloys

Following a scene-setting introduction are detailed reviews of the relevant scientific principles, thermal analysis as a research tool and the development of the zinc-aluminium family of alloys. A recently introduced simultaneous thermal analyser, the STA 1500, its use for differential thermal analysis (DTA) being central to the investigation, is described, together with the sources of support information, chemical analysis, scanning electron microscopy, ingot cooling curves and fluidity spiral castings. The compositions of alloys tested were from the binary zinc-aluminium system, the ternary zinc-aluminium-silicon system at 30%, 50% and 70% aluminium levels, binary and ternary alloys with additions of copper and magnesium to simulate commercial alloys and five widely used commercial alloys. Each alloy was shotted to provide the smaller, 100mg, representative sample required for DTA. The STA 1500 was characterised and calibrated with commercially pure zinc, and an experimental procedure established for the determination of DTA heating curves at 10°C per minute and cooling curves at 2°C per minute. Phase change temperatures were taken from DTA traces, most importantly, liquidus from a cooling curve and solidus from both heating and cooling curves. The accepted zinc-aluminium binary phase diagram was endorsed with the added detail that the eutectic is at 5.2% aluminium rather than 5.0%. The ternary eutectic trough was found to run through the points, 70% Al, 7.1% Si, 545°C; 50% Al, 3.9% Si, 520°C; 30% Al, 1.4% Si, 482°C. The dendrite arm spacing in samples after DTA increased with increasing aluminium content from 130m at 30% to 220m at 70%. The smallest dendrite arm spacing of 60m was in the 30% aluminium 2% silicon alloy. A 1kg ingot of the 10% aluminium binary alloy, insulated with Kaowool, solidified at the same 2°C per minute rate as the DTA samples. A similar sized sand casting was solidified at 3°C per minute and a chill casting at 27°C per minute. During metallographic examination the following features were observed: heavily cored phase which decomposed into ' and '' on cooling; needles of the intermetallic phase FeAl4; copper containing ternary eutectic and copper rich T phase.

Effect of aspect ratio on thermal convection in open and closed systems

Internally heated fluids are found across the nuclear fuel cycle. In certain situations the motion of the fluid is driven by the decay heat (i.e. corium melt pools in severe accidents, the shutdown of liquid metal reactors, molten salt and the passive control of light water reactors) as well as normal operation (i.e. intermediate waste storage and generation IV reactor designs). This can in the long-term affect reactor vessel integrity or lead to localized hot spots and accumulation of solid wastes that may prompt local increases in activity. Two approaches to the modeling of internally heated convection are presented here. These are based on numerical analysis using codes developed in-house and simulations using widely available computational fluid dynamics solvers. Open and closed fluid layers at around the transition between conduction and convection of various aspect ratios are considered. We determine optimum domain aspect ratio (1:7:7 up to 1:24:24 for open systems and 5:5:1, 1:10:10 and 1:20:20 for closed systems), mesh resolutions and turbulence models required to accurately and efficiently capture the convection structures that evolve when perturbing the conductive state of the fluid layer. Note that the open and closed fluid layers we study here are bounded by a conducting surface over an insulating surface. Conclusions will be drawn on the influence of the periodic boundary conditions on the flow patterns observed. We have also examined the stability of the nonlinear solutions that we found with the aim of identifying the bifurcation sequence of these solutions en route to turbulence.

The deoxidation of steel with simple and complex deoxidisers

The deoxidation of steel with complex deoxidisers was studied at 1550°C and compared with silicon, aluminium and silicon/aluminium alloys as standards. The deoxidation alloy systems, Ca/Si/Al, Mg/Si/Al and Mn/Si/Al, were chosen for the low liquidus temperatures of many of their oxide mixtures and the potential deoxidising power of their constituent elements. Product separation rates and compositional relationships following deoxidation were examined. Silicon/aluminium alloy deoxidation resulted in the product compositions and residual oxygen contents expected from equilibrium and stoichiometric considerations, but with the Ca/Si/Al and Mg/Si/Al alloys the volatility of calcium and magnesium prevented them participating in the final solute equilibrium, despite their reported solubility in liquid iron. Electron-probe microanalysis of the products showed various concentrations of lime and magnesia, possibly resulting from reaction between the metal vapours and dissolved oxygen.The consequent reduction of silica activity in the products due to the presence of CaO and hgO produced an indirect effect of calcium and magnesium on the residual oxygen content. Product separation rates, indicated by vacuum fusion analyses, were not significantly influenced by calcium and magnesium but the rapid separation of products having a high Al2O3Si02 ratio was confirmed. Manganese participated in deoxidation, when present either as an alloying element in the steel or as a deoxidation alloy constituent. The compositions of initial oxide products were related to deoxidation alloy compositions. Separated products which were not alumina saturated, dissolved crucible material to achieve saturation. The melt equilibrated with this slag and crucible by diffusion to determine the residual oxygen content. MnO and SiO2 activities were calculated, and the approximate values of MnO deduced for the compositions obtained. Separation rates were greater for products of high interfacial tension. The rates calculated from a model based on Stoke's Law, showed qualitative agreement with experimental data when corrected for coalescence effects.

Recent advances in spin crossover research

Thermal spin transition (spin crossover), one of the most fascinating dynamic electronic structure phenomena occurring in coordination compounds of third row transition metal ions, mostly of iron(II), iron(III) and cobalt(II) with critical ligand field strengths competing with the spin pairing energy, has attracted increasing attention by many research groups. One of the reasons is the promising potential for practical applications. In this chapter we intend to cover essential recent work, primarily accomplished within the European research network on “Thermal and Optical Switching of Molecular Spin States (TOSS)”. New spin crossover compounds and their thermal spin transition behaviour, also under applied pressure, novel effects observed by irradiation and magnetic field, will be discussed. Progress in theoretical treatments of spin crossover phenomena, particularly cooperativity, will be briefly outlined. The chapter concludes with a summary of research highlights published by the partner laboratories of the TMR network TOSS.