An overview of synchrotrons and synchrotron radiation is presented, along with the theory and practical considerations behind several types of X-ray spectroscopy. The theory and practical considerations of density functional theory are also given, with direct reference to some specific software packages.<p> Some synchrotron-excited X-ray spectroscopy measurements and density functional theory calculations of selenium and arsenic-doped selenium films are then outlined. The physical structure of crystalline and amorphous selenium and the electronic structure of amorphous selenium are discussed and comparison is made to the experimental results.<p> A weak feature in the conduction band is identified as a "fingerprint" of the degree of crystallization in amorphous selenium from X-ray absorption measurements. Similarly, a weak feature corresponding to lone-pairs in the valence band is identified as a "fingerprint" of the arsenic concentration from X-ray emission measurements.<p> Finally a detailed model of the structure of amorphous selenium is explained, and compared to experiment. This model is tested both by direct calculations and by a reverse Monte Carlo approach. The implications of this model with respect to the structure of amorphous and arsenic-doped amorphous selenium are discussed. Calculations suggest that simply randomizing the arrangement of "perfect" trigonal selenium is unable to reproduce the measurements of amorphous selenium; a moderate variation in the bond angle of "perfect" trigonal selenium is also necessary.

Chronic arsenic poisoning due to arsenic contamination of groundwater is a serious public health problem in Bangladesh and neighboring countries. Severe health effects associated with chronic exposure to arsenic include melanosis and several kinds of cancer. It is now generally agreed that the arsenic contamination of groundwater in Bangladesh is of geological origin. Arsenic naturally present in aquifers may be mobilized into drinking water by microbial action.<p> The formation of a novel arsenic-selenium compound: seleno-bis (S-glutathionyl) arsinium ion, [(GS)2AsSe]-, and its subsequent excretion in rabbit bile has been demonstrated previously. This molecular basis for the in vivo antagonism between arsenic and selenium was discovered using X-ray absorption spectroscopy. There is growing evidence that, in Bangladeshi people who are suffering long term chronic lowlevel arsenic poisoning, this antagonism is causing a selenium deficiency. Administering selenium supplements might provide a simple but highly effective treatment of the Bangladeshi arsenic poisoning.<p> In order to examine the disposition of [(GS)2AsSe]-, a set of rabbits were intravenously injected with selenite, arsenite or both. Whole blood, red blood cell and plasma samples were collected at different time intervals within 2hrs after injection and cecotrope samples 24hr after injection. Samples were examined using X-ray absorption spectroscopy and both arsenic and selenium K-near edge spectra were recorded.<p> iii Speciation of arsenic and selenium will be discussed in this thesis. Results indicate that [(GS)2AsSe]- is formed in blood very rapidly after injection of both arsenite and selenite, and then is removed from blood stream within 2hrs post injection. Results also show that [(GS)2AsSe]- is assembled in red blood cells, with no [(GS)2AsSe]- detected in plasma samples. [(GS)2AsSe]- is also found in cecotrope samples after injection of both arsenite and selenite.<p> The results of this study in rabbits will contribute to the understanding of chronic arsenic poisoning in humans.

No description available.

The RS 2400 irradiator has been looked to as a replacement for discontinued gamma irradiators. The RS 2400 has a cylindrical, rather than point, x-ray source, which yields higher dose rates. The irradiator unit allows the user to set the current, voltage, and time for which the sample is to be irradiated, but gives no conversion between these values and the dose delivered. Working with Mississippi State University’s Experimental Seafood Processing Laboratory (ESPL), the purpose of this research was to characterize the dose delivered by the RS 2400 for typical operating conditions. The RS 2400 exposure rate increases, as expected, as the current and voltage are increased. The x-ray beam is uniform within 10% at the surface of the x-ray tube over a wide range of voltages, with the exception of the leftmost 5 cm of the tube, where structural supports are located. At the maximum operating parameters (150 kV and 45 mA), the beam has a first half value layer (HVL1) of 13.66 mm aluminum, a homogeneity coefficient of 0.47, and equivalent photon energy (hveq) of 88.5 keV. This suggests a broad energy x-ray beam. The maximum deliverable dose rate to tissue at the surface of the x-ray tube is 65 Gy min-1 ± 3.1%, but it is unlikely that any sample will ever be irradiated this close to the x-ray tube. The standard sample canisters are 7.62 cm in diameter and the maximum deliverable dose rate to tissue at the canister location (with no canister present) is 37 Gy min-1 ± 3.1%. This is similar to the 45 Gy min-1 value that Rad Source Technologies, Inc. gives for the irradiator. Irradiation of live oysters is of primary interest to the ESPL. For irradiation, oysters will most likely be placed in the 10.2 cm diameter plastic canisters since the 7.62 cm diameter canisters are not wide enough to hold larger oysters. The oyster shells and increased distance from the x-ray source reduce the maximum deliverable dose rate to 14.1 Gy min-1 ± 6.5% for thin-shelled oysters and 12.3 Gy min-1 ± 6.2% for thick-shelled oysters.

This thesis presents a lithic attribute and geochemical analysis of the lithic material recovered from coastal site of Fatumafuti, on Tutuila Island, in the Samoan archipelago during 1050-520 BP. The goal of this thesis is to clarify the nature of stone tool production and to add to our current understanding of the cultural transformations from Lapita to a Polynesian identity. To complete this goal four research questions are addressed. What is the stage of reduction (cha ne operatoire) at Fatumafuti? Does the assemblage vary over space and time? Where did the source material come from? And, what was the organization of lithic craft production? Specifically, is there evidence for specialization? The lithics at Fatumafuti contain multiple segments in the technical sequence of tool manufacture (cha ne operatoire). The two major segments are middle stage and late stage reduction, and two minor segments are early stage reduction and tool rejuvenation. Expedient tools found on site indicate that prehistoric groups did not rely on a completely curated technology. Tool manufacture was geared toward producing a variety of tools, as opposed to a specific product. Production was most intense towards the coastal portion of the site during the earlier cultural component and then shifted towards the talus base during the later cultural component. Using non-destructive Energy Dispersive X-Ray Fluorescence (EDXRF), elemental concentrations were analyzed and compared to those of Tataga-matau, Lau?agae, Asiapa and Alega. One, possibly two, sources were utilized at this site; however, they are not chemically similar to Tatagamatau, Lau'agae, Asiapa and Alega. I conclude that people of Fatumafuti practiced independent household production at the end of the Aceramic and beginning of the Recent period. Either the intensification of lithic craft production that is seen during the height of complex chiefdoms is not seen at Fatumafuti, or these social transformations had not yet taken hold. With more cases that date to this time, we may find that Samoan chiefdoms had not attained full complexity at this point.

Malaria is a disease that causes more than 1 million deaths per year world wide and more than 400 million clinical cases. Due to the acquired resistance of Plasmodium falciparum to the drugs used to control the infection, searching for new anti-malaria drugs is necessary in modern days. Recent studies have shown that the parasite synthesizes fatty acids using a fatty acid synthase type II (FAS-II) instead of a type-I fatty acid synthase (FAS-I) that is present in other eukaryotes. Plasmodium falciparum enoyl reductase (PfENR) is responsible for the last step of fatty acid biosynthesis in the parasite. This enzyme is located within the apicoplast, a plastid-like organelle that is responsible for several important metabolic pathways, including fatty acid biosynthesis. It is known that triclosan is an inhibitor of ENR in bacteria and we and others have shown that it is also effective against ENR in apicomplexan organisms such as P. falciparum. However triclosan cannot be used to treat malaria in humans because it has metabolic liability (glucoronidation) which limits its inhibitory potency. We have used X-ray crystallography and a Structural Activity Relationship (SAR) strategy to design and cocrystallize a tertiary complex of PfENR with NAD+ and triclosan derivatives to improve their properties as drugs to treat malaria. More than five hundred triclosan derivatives were synthesized, and their in vitro and in vivo inhibitory activity evaluated. Furthermore, structural studies were made of their affinity to interact with residues in PfENR active sites, as well as with the cofactor NAD+. A total of six PfENR-NAD+-triclosan analog/complexes structures were determined. Analogs which had replacements of chloride groups at position 5 of ring A and 4' of ring B were determined, allowing the structural analysis of the binding of these triclosan analogs to PfENR. In addition, the urea derivatives (modification at position 1) as well as phenylsulphonamides (modification at position 2') have shown to be more potent inhibitors than triclosan in the in vivo assay. The analysis of the inhibitory properties and the structure of these analogs bound to PfENR will provide novel compounds in the search for new anti-malarial drugs.

Thesis (Ph. D.)--Open University. BLDSC no. DXN041011.

Thesis (Ph. D.)--University of Canterbury, 2008. / Typescript (photocopy). Includes bibliographical references (p. 126-133).

Thesis (M. Phil.)--University of Hong Kong, 2001. / Includes bibliographical references (leaves [86]-88).

Thesis (Ph. D.)--University of Queensland, 2002. / Includes bibliographical references.