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Magister Scientiae - MSc / This work examined the systematic approach to protein crystallization, exploring some of the techniques that have been developed to enhance the success rate of crystallization. The work was centered on two proteins; namely Vaccinia virus complement control protein (VCP) and glutamate dehydrogenase (GDH) from Bacteriodes fragilis. The crystal structures of the full lengthe native VCP and VCP bound to heparin had already been determined. In the same way, the structure of GDH from Bacteriodes fragilis is not known, even though structures of other GDHs from different organisms have been determined. / South Africa

In 1970, Freiser made the theoretical prediction that a biaxial nematic phase could exist. In auniaxial nematic, only one particle axis is aligned but with a biaxial nematic, all three molecular axes are aligned. This phase is expected to occur for particle whose shape is intermediate between that of a disc and a rod. Board and bent-core particles are examples of such structures. Despite extensive experimental investigation, however, very few biaxial nematic systems have been found. Yu and Saupe [21] have shown the occurrence of a biaxial nematic phase generated for a lyotropic system. Similarly van der Pol observed a biaxial nematic phase in a colloidal suspension of board-like goethite particles [22]. For thermotropic, molecular systems, however, the situation is less clear-cut. Merkel et al., [23] and Figueirinhas et al., [24] claim that Tetrapodes have can exhibit a biaxial nematic phase, whilst Acharya et al., [25] and Prasad et al., [26] have also suggested the occurrence of this same phase with bent-core molecules, though experimental uncertainty still exist. With regards to theoretical predictions of the biaxial nematic transition, one notes in particular the work of Taylor and Herzfeld [13] on hard sphero-platelets, which predicts a rich phase diagram, notably containing an unusual discotic smectic phase. To date there are few simulations of board-like models, such as Vanakaras et al., [27] being arecent exception, developing a phase diagram for hard board-like colloids. Similarly, Escobedo[28] has produced a phase diagram of hard cuboids. We present molecular dynamics simulation results on a short range repulsive fused-hexagonmodel, somewhat resembling hard boards. Depending on the geometry of the board, we observe uniaxial and biaxial nematics, smectics A and C, a biaxial smectic phase and a columnar phase. Possibly the most interesting result is the existence of the theoretically predicted discoticsmectic. We further investigated the effect of applying both external fields and shears to several of the structures. The former, among other things, aided the alignment of the particles in the phase, removing dislocations. The shear was also seen to increase biaxial ordering, however, it also demonstrates an ability to introduce clear layer fractures, seen when shearing forces became overly dominant. An applied electrical field was able to induce isotropic!biaxial nematic and biaxial smectic switching. Finally we consider briefly less-symmetric arrangements of fused hexagons, including chiral particles. These systems proved hard to equilibrate but discotic nematic phases were observed for certain structures. Chiral clusters were also observed, however, no globally chiral phase was found. Columnar structures were also seen, but showing a weak overall alignment as columnstended to point along several directions.

No description available.

The magnetic properties of some rare earth based Heusler alloys have been investigated. Rare earth Heusler alloys of the form Pd2Reln exhibit magnetic behaviour characterised by magnetic moments localised on the rare earth atoms. X-ray and neutron diffraction investigation show that all alloys crystallise in the cubic L21 Heusler structure with space group Fm3m.

The motions of particles dispersed in liquid crystals can be influenced by the application of an electric field, the effect depending on the field frequency and field amplitude. Sandwich cells under the application of electric field are widely used as the tool in order to investigate the fundamental research relating to electro-optic display technology. Therefore, the aim of this experimental work is to find and investigate novel motion of the particles dispersed in the liquid crystal phases, held within a sandwich cell. For the liquid crystal–particle systems in the sandwich cells in this thesis, the particle shapes, temperature and cell geometry are all shown to have an influence on the regime of the particle’s motions, with different phenomena observed using three different phases of liquid crystals. The experiments are designed to find and investigate the novel motion of the micron sized silica particles in the liquid crystal phases. In the chiral nematic phase, spherical particles are shown to exhibit linear motion, which is related to the electrophoretic mobility. Such spherical particles are also observed to show circular motion which is found to have a field dependency that can be related to Quincke rotation. A maximum frequency for motion occurs which is found to possibly be related to the effect of the ion diffusion in the liquid crystal-particle composite system. The direction of the circular motion is found to be independent of the handedness of the chiral nematic material. In the isotropic phase of a chiral nematic liquid crystal, the spherical particles do not exhibit any linear motion, which shows the system does not follow the traditional electrophoresis observed in normal isotropic liquids. The circular motion of the spherical particle that is observed in the isotropic phase is analysed in terms of the Quincke rotation and again shows the Maxwell relaxation time. The electric-field induced motion of elongated particles in four different nematic systems is examined. In this case of planar aligned systems, linear motion is observed, in which the velocity shows a minimum for particles of the same length as the cell gap. A novel field-induced defect texture appears in the homeotropic device containing a nematic liquid crystal of negative dielectric anisotropy. Interestingly, the motion of the particle is found to be strongly coupled with the defects formed.

In iron-doped lithium niobate and other similar crystals, exposure to light of appropriate wavelength induces small changes in the refractive index. This phenomenon is called the photorefractive effect. It allows phase holograms to be stored in these crystals. The work to be described was undertaken to obtain a better understanding of the mechanisms of the photorefractive effect and to investigate possible engineering applications. The photorefractive effect is believed to involve the spatial redistribution of photoexcited electrons among traps. This causes a space charge field to develop-which modulates the refractive index via the linear electro-optic effect. A new bulk photovoltaic effect special to ferroelectric crystals, first recognized by Glass et al., is important in the photorefractive effect in these crystals. It is shown that the finite electron transport length in this effect makes the photovoltaic current distribution spatially shifted from the light intensity pattern that causes it. Moreover, it is shown that the spatially varying photovoltaic current component which is responsible for the hologram formation decreases as the spatial frequency of the light interference pattern increases. Hologram writing by the photorefractive effect is modelled for arbitrary electron transport length. The treatment allows for the feedback effect of the space charge field and for the dark conductivity of the crystal. The model applies to uniform illumination and constant applied voltage conditions. It is shown that except in crystals where diffusion dominates the .hologram is spatially shifted from the light intensity pattern that caused it because of the finite electron transport length associated with the bulk photovoltaic effect. Experimental results which bear upon the bulk photovoltaic effect and the associated electron transport length are reported. Hologram writing with an arbitrary one-dimensional light intensity distribution is modelled allowing for the feedback effect of the space charge field at all writing times, A large scale space charge field associated with the envelope of the light is shown to affect the writing process. It is found that for any type of intensity distribution an increase in the fraction of the crystal which is illuminated improves the efficiency of the hologram writing process. Also for partially illuminated crystals the storage efficiency improves as the photoconductivity approaches the dark conductivity value from above. For a fully illuminated crystal the storage efficiency improves as the ratio of the photoconductivity to dark conductivity increases. Experimental observations of the effect of the large scale field on hologram storage are reported. Beam distortion and optically induced scattering are two problems encountered while storing holograms in lithium hipbate. We report experimental observations and theoretical models for these phenomena. It is shown that beam distortion is due to the defocusing action of the large scale refractive index change due to the envelope of light. Light scattering is suggested to be due to the lens action of the index variations due to laser speckles inside the crystal. A theoretical treatment of the spatial filtering properties of volume holograms is presented. Practical applications of volume holograms in the fields of interferometric testing and optical. communications are also discussed. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Unknown