Submitted in partial fulfilment of the requirements for the degree of B.A. (HONS) in the Department of African Languages at the University of Zululand, South Africa, 1990 . / The purpose of this research is to investigate why students at the" University of Zululand code-switch. The research focuses particularly on environmental and social factors which lead students to code-switch. It also looks into the functions of code switching. On the whole it is intended to contribute to the findings already made on code-switching. Many researchers have found out that people tend to code switch because it has become a habit to do so. Such a reason does not sound linguistically convincing to me. Others claim that people code-switch because they want to display their 'education' or that they come from urban areas or townships. This again is not convincing enough since code-switching is a reality and would seem to come naturally. It is not something one does when one feels like it is usually done unconsciously.

A number of amino acids containing terminal five- and six-membered heterocyclic substituents have been synthesised using a strategy in which a 6-oxopipecolate system is 'ring switched'. Functionalisation of the protected (2S)-6-oxopipecolate at C-5 with methyl formate, or with allyl bromide followed by ozonolysis, have provided the important 5-formyl-6-oxopipecolate and its' homologue as intermediates. Reaction of these intermediates with hydrazine gave a successful 'ring switching' reaction, yielding a pyrazolone and a pyridazin-6-one, which were deprotected to give the free amino acids. Alkylation of the homodehydro-6-oxopipecolate system at C-4 by Michael addition with allyl cuprate, followed by ozonolysis, provided the important 4-formyl-6- oxopipecolate intermediate. Reaction with hydrazine gave a successful 'ring switching' reaction yielding a pyridazin-6-one, which was deprotected to the free amino acid. Attempts to apply the 'ring switching' reaction to morpholinones, thiazanes, azepanes, thiazepines and were unsuccessful. The amino acids synthesised are currently being examined in a high throughput screen at Glaxo-Wellcome

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This dissertation explores the effects of two-photon absorption and color center induced absorption on all-optical switching devices. The amount of allowable two-photon absorption was quantified by the parameter T = 2βλ/n₂, where λ is the operating wavelength, β is the two-photon absorption coefficient, and n₂ is the nonlinear refractive index coefficient, the latter two being measured at λ. If the value of T exceeds unity, the operation of all-optical switching devices is in general degraded beyond usable regimes. This result was demonstrated by numerical experiments on systems of equations modelling a nonlinear directional coupler, a prototypical all-optical switching device. The value of T was measured in two fibers, one made of lead silicate glass, and one made of TiO₂-doped silica. We find the value of T to be greater than unity at a wavelength of 1.06 μm in both fibers. Significant color center formation was seen in the lead glass fiber. These color centers were created through two-photon absorption and destroyed through one-photon absorption. Color center induced absorption was seen to mimic two-photon absorption in certain regimes. The nonlinear optical response of semiconductor-doped glasses, an example of a one-photon resonant nonlinearity, was studied. A relaxation time which is dependent on the carrier density was found to be important when modelling the response of these glasses.

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by Siu Man Tsang. / Thesis (M.Ph.)--Chinese University of Hong Kong, 1987. / Bibliography: leaves 120-121.

Phase variation is mechanism of phenotypic switching used by many pathogenic bacterial species. This thesis describes work on three aspects of phase variation. Mathematical models are described which can be used to determine the rate of phase variation and subsequently the influence of variation rate and fitness differences associated with the altered phenotype on population structure. An approach to whole genome analysis has been developed which has been used to identify putative phase variable contingency genes in H. pylori, T. pallidum and N. meningitidis. This has identified many new contingency genes likely to be involved in host - bacterium and bacterium population interactions. Finally, a detailed molecular investigation of the promoter of the phase variable opc gene of N. meningitidis is presented. In this it is shown that the promoter located homopolymeric tract controls transcription by affecting the relative spacing and facing of promoter components, that this determines RNA polymerase binding to the promoter, and that this interaction involves direct contact of the a-subunit of RNA polymerase with the promoter. In addition it is shown that transcription is dependent upon an IHF consensus sequence in the opc promoter.