Department of Chemistry
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Item Allelochemicals from Sorghum Bicolor that Stimulate Feeding by the Larvae of the Stem Borer Chilo Partellus(University of Ghana, 1988-08) Torto, B.Feeding bioassays with cellulose acetate discs impregnated with the hexane, ethyl acetate and methanol extracts of the leaf-whorls of field grown plants of sorghum cultivars IS 18363 (susceptible) and IS 2205 (resistant) showed that the methanol extracts were most stimulatory to the feeding of the third-ins tar larvae of Chilo partellus. Ethyl acetate extracts were intermediate in stimulatory activity whilst hexane extracts were the least stimulatory. Extracts of the more susceptible cultivar were more stimulatory than those of the more resistant cultivar and those of the whorls of the 3 week old plants were more stimulatory to larvae than those of the 6 week old plants. The phagostimulatory compounds in the ethyl acetate extracts were phenolic, p-hydroxybenzaldehyde and phydroxybenzoic acid being the major components and ferulic and caffeic acids being in minor amounts. p-Coumaric acid was also present in minor amounts but was non-stimulatory at all the doses tested. p-Hydroxybenzaldehyde was a more potent feeding stimulant for the larvae relative to some of its possible theoretical biogenetic analogues. Limited structure-activity studies with some hydroxybenzoic acids and their corresponding cinnamic acids showed that the former were more stimulatory to the feeding of the larvae than the latter and that oxygen substitution in the benzene ring was crucial for activity. The phagostimulatory compounds in the methanol extracts were phenolic, identical to those in the ethyl acetate extracts, and sugars. The sugars which were identified in the extracts comprised sucrose, fructose, glucose and xylose. The feeding response of larvae to these sugars followed the order sucrose » glucose fructosei xylose was non-stimulatory. Comparison of the activities of sucrose with mixtures of glucose and fructose showed that the high activity of the disaccharide is due to its total structure and not to a summation of its monosaccharide moieties. Sugars synergised with phenolics to give enhanced feeding response of the third-instar larvae. Chromatographic analyses of the extracts showed that stimulatory and non-stimulatory components in the extracts differed quantitatively rather than qualitatively in the whorls of the two cultivars at the two growth stages. This may have implication in resistance screening and breeding programmes.Item Synthesis and Characterisation of Novel Semiconducting Small Molecules for Applications in Organic Solar Cells.(University of Ghana, 2018-07) Antwi, B.Y.Five novel semiconducting organic small molecules have been synthesised, fully characterised, and their photovoltaic performance tested in fabricated organic solar cells. Three (molecules 5, 7 and 9) were acceptor-donor-acceptor (A-D-A) small molecules bearing an electron-rich (3,4-ethylenedioxythiophene) EDOT central core with different terminal groups. The additional two (molecules 20 and 21) were truxene derivatives which have been differentiated by the alkyl chain length on the molecular backbone. The physical properties of molecules 5, 7, 9, 20 and 21were determined by TGA, DSC, UV/Vis spectroscopy and cyclic voltammetry. Their optically measured HOMO-LUMO energy gaps in the solid state were in the range 1.57 – 2.33 eV, and in solution 1.88 - 2.15 eV. The electrochemical HOMO-LUMO energy gaps determined by cyclic voltammetry were in the range 1.97 - 2.5 eV. A conventional bulk-heterojunction device architecture was employed in the fabrication and testing of the photovoltaic performance for the electron donating molecules 7 and 9. The processing solvent and fabrication conditions were optimised along with the device morphology by addition of 1,8-Diiodooctane (DIO) solvent to the donor/acceptor (D/A) blend. The devices showed better performance when processed with chloroform compared to chlorobenzene. The optimised processing conditions were 1:3 weight ratio of the D/A blend for 7:PC71BM devices that was annealed at 90°C. For 9:PC71BM devices 1:4 D/A blend thermally annealed at 60°C was optimal. 1% DIO more than doubled the power conversion efficiency (PCE) of the best performing device for compound 7 (0.63 vs 1.36%), but only slightly improved that for compound 9 (1.03 vs 1.05%). Fabricated and tested devices for molecules 20 and 21 showed a bifunctional property of the two in bulk-heterojunction organic solar cells. The molecules with longer side chains, 21, displayed a better device performances compared to 20, even though they were not impressive.Item Exposure and Fate of Neonicotinoid Insecticides in Cocoa plantations in Ghana(University of Ghana, 2015-07) Dankyi, E.; Carboo, D.; Gordon, C.; Fomsgaard, I. S; University Of Ghana, College of Basic and Applied Sciences ,School of Physical and Mathematical Sciences, Department of ChemistryNeonicotinoids belong to the most important class of insecticides currently used in crop production and account for about a fourth of the insecticide market. They are considered highly effective against insect pests, safe to mammals, and possess multiple means of application for convenience and widespread usage in diverse crop protection. Neonicotinoids are systemic insecticides - they are absorbed into plants, travel through the vascular tissue, and help protect the plant from piercing and sucking insects. Their mode of action is through agonist activity at the postsynaptic nicotinic acetylcholine receptor (nAChRs) sites in the nervous system. In recent years, reports have emerged on possible harmful effects of neonicotinoid insecticides on bee health. Across Europe, America and Australia, there has been calls for ban and reevaluation of the use of neonicotinoid insecticides. In Europe, a ban has been in place for almost two years following a scientific report by the European Food Safety Authority (EFSA) which identified “high acute risks” for bees. However, the availability of conflicting reports on the effects of neonicotinoids on bees has prompted intense research on these class of insecticides. Quite clearly, understanding the effects of neonicotinoids on organisms in the ecosystem will rely on accurate knowledge of their exposure in the environment. In Ghana, neonicotinoids are one of the most widely used class of insecticides particularly in cocoa production, where they are important for the control of mirids. Insect pests are a major concern in crop production in tropical conditions, due to the prevailing conducive environment for their growth. In cocoa production, insect pests such as mirids contribute to significant loses in yield. To address this concern, the government of Ghana introduced a free mass application policy on insecticides in cocoa farming, which has contributed to substantial improvements in yields of cocoa beans. Under the program, neonicotinoids are the major class of insecticides used. In spite of the remarkable contribution of neonicotinoids to cocoa production, concerns about the environment and food safety have arisen due to widespread and intensive use of these chemicals in cocoa farms. Addressing these concerns requires accurate knowledge of the behavior and fate of these class of insecticides in the Ghanaian environment. The primary goal of this work was to assess the extent of exposure, behavior and fate of neonicotinoids in the Ghanaian environment, particularly in cocoa plantations where they are extensively used. To achieve this, concentrations of neonicotinoid residues in soils across all the cocoa producing regions of the country were studied to examine their environmental exposure. Their fate in soils was studied by investigating their dissipation and sorption behavior using established kinetic models and isotherms. Finally, the exposure to cocoa beans (food) was studied to ascertain concerns for food safety. Analytical methods and instrumentation was an important aspect of this work to ensure accurate and reliable data. To this end, the Quick, Easy, Cheap, Effective, Rugged and Safe (QuEChERS) procedure was optimised and used for the diverse matrixes understudy. Quantification of analytes involved the use of liquid chromatography- tandem mass spectrometry (LC-MS/MS) for high sensitivity and low detection limits in the complex soil and cocoa matrixes used. The findings from the study suggest that, neonicotinoids are persistent in the Ghanaian soils studied and may be found in soils several months to years after application. Sorption studies revealed that, sorption coefficients of neonicotinoids are generally low, with a high potential for leaching into surface and underground water systems. Due perhaps to their systemic nature and high application rates, neonicotinoids may accumulate to high levels in cocoa beans, particularly in cocoa shells. The findings from this study has implications for pesticide application and current policy on pesticide usage, and reveal that, efficient application regimes are needed in cocoa production to ensure food and environmental safety.Item Targeting of Tlck-Coupled Liposome to Simian Virus 40 Transformed Mouse Fibroblasts(University of Ghana, 1984-08) Ankrah, N.A.; Wei R.; Flechtner W.T.; University of Ghana, College of Basic and Applied Sciences, School of Physical and Mathematical Sciences, Department of Chemistry.N-a-tosyl-L-lysyl-chloromethyl ketone (TLCK) has been coupled to ara-C or colchicine encapsulated liposomes via glutaraldehyde treated liposomal phosphatidylethanolamine (Lp-G-TLCK) or sodium metaperiodate oxidized oligosaccharide side chain of liposomal digoxin (Lp-D-TLCK). TLCK has been shown to react with specific histidine residue at the active site of such protein as trypsin and has high binding affinity for simian virus 40 and polyoma virus transformed fibroblasts. A 20-minute exposure of cells to Lp-G-TLCK containing ara-C impaired the growth of tumor cells (SVT2 ) for up to two days, but did not impair the growth of normal cells (Balb/C 3T3). However, at 50-minute exposure, the Lp-G-TLCK containing ara-C impaired the growth of Balb/C 3T3 cells at day one but recovered by the second day. The SVT2 cells, on the other hand, did not recover from the cytotoxic effect of Lp-GTLCK, containing ara-C during the second day. The underivatized liposome (Lp-PE) containing ara-C was cytotoxic to both Balb/C 3T3 and SVT2 cells at 20 or 50 minute exposure periods. The Lp-D containing ara-C, at 20 to 60-minute exposure, slightly impaired the growth of SVT2 and Balb/C 3T3 cells at day one. Such cytotoxic effect was, however, not observed by the second and third days of growth. In contrast, Lp-D-TLCK containing ara-C impaired the growth of SVT^ cells while the growth of Balb/C 3T3 cells was nearly indistinguishable from control cells over a threeday period. Thus TLCK selectively enhanced ara-C encapsulated Lp-D-TLCK SVT2 cell cytotoxicity. Lp-D-TLCK containing colchicine, while impairing the growth of Balb/C 3T3 cells, was particularly cytotoxic to SVT2 cells, indicating that colchicine may be an unsuitable anti-tumor drug for use in a culture system with normal cells in their exponential growth phase. The uptake of Lp—D—TLCK by SVT2 cells was higher than the uptake by Balb/C 3T3 cells. With respect to underivatized liposome (Lp-D) the uptake by Balb/C 3T3 cells was higher than the uptake by SVT2 cells, indicating that TLCK enhanced liposome reaction with SVT2 cells and may retard reaction with the Balb/C 3T3 cells.