Masters Degrees (Chemistry)

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Tricarbonyl rhenium(I) complexes with Schiff base chelates : bioinorganic chemistry to 186Re radiolabelling studies.
(2024) Mapapiro, Tariro Talent.; Booysen, Irvin Noel.
In the first experimental chapter, the synthetic approach of a lead rhenium metalcomplex, fac-[Re(CO)3(urbzt)Br] (urbzt = amino-5-((benzothiazol-2-ylmethylene)amino)-1,3-dimethyluracil) is reported from metallic rhenium and theurbzt free Schiff base. In addition, the formation of this metal complex was confirmedby LC-MS. This optimized synthetic approach was successfully mimicked in the 186Reradiolabelling experiments of urbzt and again confirmed by LC-MS. The second experimental chapter describes the synthesis and characterization of thenovel 2-aminoguanidine-derived Schiff base rhenium(I) compounds: fac-[Re(CO)3(Hguabs)Br]Br (1) (Hguabs· Cl = 2-((benzothiazole)methyleneamino)guanidine chloride) and fac-[Re(CO)3(guaquin)Br](2) (guaquin = 2-((quinolin-2 yl)methyleneamino)guanidine). Time-dependent UV-Visspectrophotometry show that these metal compounds undergoes sequential chloroligand substitution in DMSO followed by hydrolysis. The affinities of these metalcomplexes towards Calf Thymus Deoxyribonucleic acid (CT-DNA) and Bovine SerumAlbumin (BSA) were investigated. In particular, these metal compounds afforded highintrinsic DNA binding constants (Kb > 107 M-1) suggesting they both occupy within themajor and minor DNA grooves. Density functional theory calculations were conductedto rationalize the electronic transitions of the metal compounds. Molecular dockingsimulations were performed to probe the nature of the interaction between CT-DNAand the individual optimized conformers of the metal complexes. The in vitrocytotoxicity of 1 and 2 were assessed in various cancerous cell lines and benign cellswhere they showed activities in the low micromolar range.
Construction of functional and robust cobalt phthalocyanines modified electrodes for the electrocatalytic detection of metal-based and pharmaceutically derived pollutants.
(2024) Moodley, Danica.; Booysen, Irvin Noel.; Mambanda, Allen.
Water pollution has become a detrimental global concern in a world that continues to grow through industrialisation, population, and demand in sales from agricultural and pharmaceutical industries. It is therefore imperative for innovative methods of continuous water monitoring to be implemented to avoid the harsh effects that pollution poses to human, animal and environmental preservation. Advances from traditional analytical methods have been made to combat associated drawbacks such as tedious sample preparation, high maintenance costs, and lack of mobility. Electrochemical sensors can be used for the analysis of a vast range of water pollutants while offering on-site, simple analysis and inexpensive fabrication. Metallophthalocyanines have been utilised extensively as electrode modifiers due to their excellent redox properties and stability which can be fine-tuned by alteration of the metal centre and substituents. In addition, thes3e alterations improve selectivity, solubility and immobilisation onto electrode substrates. This research is aimed at the application of gold electrodes modified with CoPc-cou nanoconjugates and CoPc-cou electrospun nanofibers (ENFs) for the electrocatalytic detection of pollutants, paraquat and mercury, in real water samples. Experimental chapter one explores the optimization and application of a gold-modified electrode, CoPc-cou-f-MWCNTs/3-HT|Au, for the electrocatalytic detection of a water pollutant, paraquat (PQ). It was fabricated via a sequential modification procedure entailing the formation of self-assembled monolayers (SAMs) of a nanocomposite comprising of a coumarin tetra-substituted cobalt phthalocyanine (CoPc-cou) and carboxylic acid functionalized multiwalled carbon nanotubes (f-MWCNTs). This was followed by the in-situ immobilization of poly(3-hexylthiophene) ([3-HT]n) through electropolymerisation to render the chemically modified electrode (CME). Subsequently, the CME illustrated enhanced sensitivity towards PQ compared to the bare or CoPc-cou-f-MWCNTs modified electrodes. The CoPc-cou-f-MWCNTs/3-HT|Au electrode displayed a linear PQ detection range of 0.193 – 1000 μM with a limit of detection (LOD) and limit of quantification (LOQ) of 0.193 μM and 0.584 μM, respectively. Comparison between calibration curves for the modified electrode and HPLC-MS illustrates that the former method has a lower but comparable calibration sensitivity for PQ. In addition, this CME could electrocatalytically distinguish PQ within a real water sample collected from the Durban lagoon. Furthermore, the direct recovery of PQ in the lagoon water by the modified Au electrode was found to be 86%, which is lower than the calculated value of 97% obtained by HPLC-MS after rigorous solid-phase microextraction of the analyte. However, the lower percentage recovery could be rationalized by the interference studies. In experimental chapter two fabricated electrospun nanofibers containing CoPc-cou, polyaniline (PANI) and poly-vinyl alcohol (PVA) were used to modify a gold substrate which was subsequently immobilised using a 5% Nafion solution affording the CoPc-cou-ENFs-Nf|Au modified electrode. Comparison of the chemically modified electrode with the bare and other modified electrodes under optimised conditions displayed superior detection of mercury (Hg(II)) attaining a linear range of 10 – 3000 μM and an LOD and LOQ of 0.14 μM and 0.46 μM, respectively. This can be attributed to the affinity between Hg(II) and the mercaptocoumarin substituent (Hg-S) as well as the higher surface area occupied by the ENFs resulting in an increased number of active sites. Furthermore, the chemically modified electrode exhibit selectivity and sensitivity in an interference sample containing multiple heavy metals (Pb2+, Cd2+ and Hg2+). A good percentage recovery of 96% was attained when the CoPc-cou-ENFs-Nf|Au electrode was applied to a real water sample which was comparable to a percentage recovery of 98% which was attained using the ICP-OES to analyse the same water samples.
Removal of antiretrovirals using low-cost adsorbents: adsorption kinetics, adsorption isotherms and thermodynamics studies.
(2023) Simelane, Lindokuhle Anele.; Mahlambi, Precious Nokwethemba.; Rochat, Sebastien.; Baker, Ben.
The current study was directed to the synthesis and application of low-cost adsorbent for the removal of antiretroviral drugs (ARVDs) such as nevirapine, abacavir and efavirenz in wastewater samples. The study involved the modification and application of liquid chromatography photo diode array (LC-PDA) for the detection of the ARVDs of interest. The good separation of ARVDs was achieved using a gradient elution 50 % ACN: 50 % H2O (0-2 minutes), 70 %ACN: 30 % H2O (3-20 minutes).The macadamia nutshells, Platanus acerifolia leaves (London plane) were explored as easily accessible and eco-friendly adsorbents. The single synthetic route and high surface area of the polymer of intrinsic microporosity number-1(PIM-1) were the distinct properties that were ideal for exploring this adsorbent for removal of ARVDs in wastewater samples. The adsorbents were synthesized and characterized using Fourier transform infrared spectroscopy (FTIR),Scanning electron microscopy (SEM), Brunauer Emmett Teller (BET), Powder X-ray diffraction (PXRD). The results obtained from macadamia nutshell and Platanus acerifolia adsorbent for FTIR characterization were functional groups such O-H, C=O and C=C and many others whereas Platanus acerifolia adsorbent had additional N-H from in addition to one obtained in macadamia adsorbent. The PIM-1 showed CN, C=O and C-H functional groups. SEM showed rod-folded structure and flaky-folded structure for macadamia nutshell adsorbent and Platanus acerifolia leaves adsorbent respectively, whereas PIM-1 exhibited microporous to mesoporous pore on adsorbent surface. The BET showed a surface area, pore diameter and pore volume of 0.1180 m2/g, 27.98 nm, and 8.3×10-5 cm3/g for macadamia nutshells adsorbent and 1.14 m2/g, 0.0024 cm3/g and 4.09 nm for Platanus acerifolia leaves adsorbent. The PIM-1 had a surface of 557.39 cm2/g, pore volume 0.4123 cm3/g and pore diameter 2.96 nm. The PXRD of macadamia and Platanus acerifolia adsorbents had native crystalline cellulose structure whereas PIM-1 had an amorphous material. These characterization results indicated that the adsorbents have the potential to efficiently remove the ARVDs from in contaminated wastewater. Prior to the application of adsorbents, parameters such as adsorption time (5-240 minutes), solution pH (2-10), initial concentration (0.2-2 mg/L), adsorbent mass dosage (0.2-10 mg) and adsorption temperature (15-40°C) were investigated to access the removal efficiency of all the synthesized adsorbents on their ability to remove ARVDs in wastewater samples. Under optimum conditions the adsorption was conducted using 10 mg of the adsorbent in 10 mL wastewater sample spiked at a concentration 1.0 mg/L, at a pH of 7 and stirred at 150 rpm at 30°C. These conditions yielded a removal efficiency greater than 80 %, 90 % and 86 % using macadamia nutshells, Platanus acerifolia leaves and PIM-1 adsorbent, respectively in all ARVDs of interest. The study of adsorption kinetics, adsorption isotherms and thermodynamic model was essential for designing an efficient adsorption process to remove ARVDs which are pollutants of emerging concern. The results obtained showed that pseudo-second-order model well defined the kinetic data, and the adsorption isotherms was well fitted in Langmuir isotherm and adsorption process was exothermic in nature for macadamia nutshells and Platanus acerifolia leaves adsorbent. For PIM-1 adsorbent, the pseudo-second-order was dominant, and the adsorption isotherm was well defined by Freundlich model. Thermodynamic parameters showed that the adsorption was thermodynamically favored, spontaneous, and exothermic in nature. The adsorbents were then applied under optimum conditions and the amount adsorbed of ARVDs from wastewater samples were 94.41, 88.84 and 83.06 mg/g for nevirapine, abacavir and efavirenz respectively for macadamia nutshell adsorbent. For Platanus acerifolia leaves adsorbent, 97.56, 84.75 and 81.56 mg/g amount adsorbed of nevirapine, abacavir and efavirenz. On the other hand, PIM-1 adsorbent had an adsorption capacity of 83.65, 93.83 and 94.56 mg/g amount for nevirapine, abacavir and efavirenz, respectively. Overall, the macadamia nustshells, Platanus acerifolia leaves and PIM-1 adsorbents have illustrated to be efficient and cost-effective adsorbents for removal ARVDs in wastewater samples. However, the two agricultural adsorbents, macadamia nutshell and Platanus acerifolia leaves adsorbents could be highly recommended since their usage is able to reduce land and water pollution which is compromises water quantity which is already at stake across the globe.
Exploring the structure activity relationship of antiplasmodial compounds identified from the MMV Pathogen Box.
(2024) Mafuleka, Sean Manqoba.; Sithebe, Siphamandla.; Veale, Clinton Gareth Lancaster.
Over 200 million new infections are caused by malaria-causing plasmodium species. This results in over 500 000 annual deaths. These deaths are mostly young children under the age of five years. As there is an emergence of resistance to primitive first-line treatments, there is an increasing need for the development of new targets with novel scaffolds. For such advancements, we have to consider the structure-activity of antiplasmodial compounds. The Pathogen Box is a concept modelled on the Malaria Box, except the 400 drug-like compounds it contains are a diverse range of compounds which are active against numerous neglected diseases of interest, and is readily accessible. It unpacks 125 compounds of antiplasmodial activity, a lot of which have been identified from phenotypic screening of the GSK Tres Cantos Anti-Malarial Set (TCAMS). Upon request, select researchers around the globe receive a set of compounds from the Pathogen Box to help in the advancements towards neglected disease drug discovery. In turn, the researchers are requested to present, in the public domain, any data they will have generated in their work within two years. This presents an opportunity for a collaborative space for neglected disease drug research. In this project, compound MMV023227 was found to have promising antiplasmodial activity. We have therefore designed and synthesized some analogues of the hit compound with the purpose of identifying an SAR. We initiated the synthesis of the designed analogues of compound MMV023227 by successfully synthesising the three imidazole compounds that are 2-(3-bromophenyl)-4,5-dimethyl-1H-imidazole (3.1), 2-(3-bromophenyl)-4-methyl-1H-imidazole (3.2), and 2-(3-bromophenyl)-1H-imidazole (3.3) in yields between 33 – 42 %. We moved these compounds towards the desired final compounds through several stages, but we could only go as far as producing compounds N-(2-chlorobenzyl)-3-(1-(ethoxymethyl)-1H-imidazol-2-yl)aniline (3.9), N-(2 chlorobenzyl)-3-(1-(ethoxymethyl)-4-methyl-1H-imidazol-2-yl)aniline (3.10) and N-(2-chlorobenzyl)-3-(1-(ethoxymethyl)-5-methyl-1H-imidazol-2-yl)aniline (3.11) in yields between 7 – 41 %.
Identification of steroidal glycosides from South African Hyacinthaceae species.
(2022) Msomi, Nomzamo.; van Heerden, Fanie Retief.
Steroidal glycosides isolated from natural products have shown great bioactivity and anticancer potential. Previous studies showed that the Ornithogalum genus of Hyacinthaceae family is one of the sources of steroidal glycosides with vast structural diversity. Therefore, in search of cytotoxic steroidal glycosides from South African species, two Ornithogalum plant species were investigated. Phytochemical investigation of Ornithogalum saundersiae Baker bulbs afforded three undescribed steroidal glycosides namely 3-[(β-D-glucopyranosyl)oxy]-22-[α-L-rhamnopyranosyL-(1→2)-β-D-glucopyranosyl)oxy]-16,23-epoxy-23-(2-methyL-1-propenyl) cholestane triglycoside (170), (171), and (172). LC-MS-MS study conducted on bulb crude extract revealed a significant number of unidentified compounds. Liver cell viability assays on O. saunderisae crude extracts and isolated compounds reveal moderate cytotoxicity effects for 171 (IC50 = 25 µg/mL) and 172 (IC50 = 115 µg/mL). Negligible effects were shown for compound 170. Phytochemical investigation of Albuca batteniana Hilliard & B.L.Burtt. resulted in the isolation of one previously known compound named 3-O-β-D-glucopyranosyL-β-sitosterol (175) exhibiting some cytotoxic effects on liver cells (IC50 =275 µg/mL). All the structures of isolates were elucidated by extensive spectroscopic analysis.
Identification of alpha-glucosidase inhibitors in indigenous plants used for diabetes.
(2024) Ngcobo, Nkosinathi Khulekani.; van Heerden, Fanie Retief.; Rasalanavho, Muvhango.
Diabetes, a global health concern, is a metabolic disorder with a broad range of micro and macro complications. There is still a need to develop new diabetic therapies. Hence, there is a resurging interest in isolating bioactive compounds with antidiabetic activity. One of the therapeutic therapies is the inhibition of α-glucosidase, an enzyme located in the digestive tract that facilitates the hydrolysation of carbohydrates into glucose. Plant-derived compounds and their derivatives have played a significant role in developing numerous drugs, such as the antidiabetic drug metformin. With the intention to advance and explore new plant-derived compounds that can potentially be developed into new biologically active drugs that will be used to treat diabetes, several Indigenous South African species traditionally used to treat diabetes were selected for investigation. These plants were Bulbine frutescens (L.) Willd., Carpobrotus dimidiatus (Haw.) L.Bolus, Cassia abbreviata Oliv. subsp. beareana (Holmes) Brenan, Gomphocarpus tomentosus Burch. subsp. tomentosus, Maerua angolensis DC. subsp. angolensis, Merwilla plumbea (Lindl.) Speta, Pappea capensis Eckl. & Zeyh, Sclerocarya birrea (A.Rich.) Hochst. subsp. caffra (Sond.) Kokwaro, Senegalia mellifera (Vahl) Seigel & Ebinger subsp. detinens (Burch.) Kyal. & Boatwr., Senna italica Mill. subsp. arachoides (Burch.) Lock, Tabernaemontana elegans Stapf, and Ziziphus mucronata Willd. subsp. mucronata. Lead-like extracts were prepared for all the plant species, and the extracts were subjected to in vitro enzyme inhibition studies. The extract of S. birrea was selected for further investigation since it demonstrated the highest inhibitory activity against α-glucosidase. An active fraction of S. birrea stem bark was subjected to LC-MS analysis, and five compounds were identified in the extracts. Preparative chromatography was used to isolate four compounds: catechin, β-sitosterol, β-sitosterol glucopyranoside, and stigmasterol. Of the isolated compounds, only catechin showed reasonable inhibition of α-glucosidase.
Synthesis, structural characterization, and subsequent applications of salicyldiminato Pd (II) complexes as efficient catalysts for C-C bond-forming reactions.
(2024) Khoza, Khethukuthula Petronella Mallecia.; Sithebe, Siphamandla.
Palladium-catalysed cross-coupling reactions (i.e., Heck, Stille, Liebeskind Strogl, and Suzuki-Miyaura) are perceived as one of the most prominent, successful, and widely used strategies for the construction of complex organic compounds such as biaryls from simple precursors. Amongst the known carbon-carbon bond-forming reactions, the Suzuki-Miyaura (SM) cross-coupling reaction has emerged as one of the most efficient and powerful methodologies due to its high versatility, use of non-toxic reagents, mild reaction conditions, broad substrate scope, and functional group tolerance. The SM cross-coupling has not only greatly changed the landscape of the field of organic chemistry, but also profoundly impacted pharmaceuticals, agrochemicals, natural products, material science, and various related industries. Stable supporting ligands are key in the catalytic activities of Pd complexes. The design and development of supporting ligands that will result in stable Pd complexes while maintaining high catalytic activity and catalytic turnover through stereo-electronic effect alteration are highly desirable. With the aim of developing stable highly catalytic Pd complexes, our research focus is directed towards the synthesis, characterisation, and application of novel N, O-bis Schiff base chelated Pd (II) complexes (PdC1 - PdC7) from Schiff base ligands (L1 - L7). The synthesis of (E)-2-((phenylimino)methyl) phenol (L1), (E)-2-((4 tolylimino)methyl) phenol (L2), (E)-2-(((4-ethylphenyl)imino)methyl) phenol (L3), (E)-2-(((4-methoxyphenyl) imino)methyl) phenol (L4), (E)-2-(((4-fluorophenyl)imino)methyl) phenol (L5), (E)-2-(((4-bromophenyl)imino)methyl) phenol (L6), and (E)-4-((2-hydroxybenzylidene)amino) benzonitrile (L7) was achieved in excellent yields (80% - 94%) through a condensation reaction of salicyldehyde with various electron-withdrawing and donating para-substituted anilines. The reaction between L1 - L7 and Palladium acetate Pd (OAc)2) afforded PdC1 - PdC7 with yields ranging from 78% - 95%. The successful synthesis of L1 - L7 was verified by NMR, FTIR, and MS analyses, while all complexes (PdC1 - PdC7) were characterised using NMR, FTIR, MS, and Single X-ray crystallography. However, a comprehensive X-ray data report was only obtained for PdC2, as the X-ray crystallographic structures of PdC3 - PdC7 proved difficult to solve. The crystallographic analysis of PdC2 revealed that L2 is coordinated to the Pd metal center in a bidentate mode via the N-donor of the imine bond and the O-donor of the phenolic group. The complex displayed a distorted square planar geometry and crystallised in a monoclinic crystal system with a P21/c space group. All the complexes were tested for catalytic activity in the SM cross-coupling reaction between sodium (trihydroxy)phenylborate salt and 4-bromotoluene. The reactions were catalysed with a catalyst loading of 2 mol% for 24 hours in ethanol, yielding 18% - 32% of the desired biaryl product. Motivated by the successful cross-coupling reaction between sodium (trihydroxy)phenylborate salt and 4-bromotoluene, we opted to broaden the application scope of the complexes using the established reaction conditions in SM cross-coupling acylation reaction. We used the most active catalysts, PdC2 and PdC6 for constructing biaryl ketones through the reaction of benzoyl chloride with sodium (trihydroxy)phenylborate salt in toluene for 24 hours. The reactions yielded 15% and 30% of the desired products, respectively.
A phytochemical and elemental analysis of senecio serratuloides DC, and its antidiabetic potential.
(2024) Gumede, Andile.; Koorbanally, Neil Anthony.; Moodley, Roshila.; Mshengu, Bongiwe Pridesworth.
Senecio serratuloides DC, from the Asteraceae, has been widely used in South African traditional medicine for the treatment of various conditions such as swollen gums, chest pains, sores, cuts, burns and ulcers. This study was conducted to determine a chemical profile of the plant, both in terms of the secondary metabolites and essential and toxic elements contained in the plant. The plant extracts were further tested for their antibacterial (minimal inhibitory concentrations) and antidiabetic potential (α-amylase and α-glucosidase inhibitory activity). The phytochemical analysis of the leaves and stems resulted in the isolation of the sterols, β- sitosterol (S1) from the stems, and stigmasterol (S2) and stigmasterol glucoside (S3) from the leaves. A further rare sterol, 18α-ursa-12,20(30)-dien-3β-ol (S4) was also isolated from the leaves. Three flavonoids, quercetin (S5), quercetin-3-O-glucoside (S6) and hesperidine (S7) were also isolated from the leaves, along with an aromatic acid, caffeic acid (S8). Extracts of the plant showed good in vitro antidiabetic activity, with the methanol extract from the leaves exhibiting the highest activity against α-glucosidase and α-amylase. However, the extracts did not exhibit any significant antibacterial activity in the assays carried out. The elemental analysis of the plant indicated a decreasing order of Ca > Mg > Zn > Fe > Co > Cu > Cr > Mn > Ni > As > Se. The leaves also showed good quantities of vitamin C, with an appreciable amount being extracted during the cooking phase. The results also show that moderate consumption of the leaves pose no probable threats of metal poisoning. The extracts obtained from S. serratuloides DC were found to have no antibacterial activity. This is surprising, since some of the isolated compounds were reported to have shown some antibacterial properties, however, these may have been too low to have any significant effect in the crude extracts.
A highly efficient, low-cost, and sustainable method of water purification and desalination using solar-driven interfacial evaporation.
(2024) Nnaeme, Esther Uchechukwu.; Van Zyl, Werner Ewald.; Bissessur, Ajay Bissessur.
Water scarcity has become one of the most daunting global challenges, and as a result, a continuous supply of potable water has become a bane to most societies. Techniques such as distillation and reverse osmosis have been adopted in the production of potable water but these processes are energy-consuming and highly expensive making them less attractive to many households. A viable economical technique is the removal of salt from seawater or brackish water through a solar distiller. This research was based on the design of a low-cost and new improved solar distiller which was made up of a wooden basin and an inclined glass cover. In the basin is contained sea or brackish water and photothermal materials which include recycled materials that act as an insulation material, an evaporation structure, and a solar absorber. These photothermal materials were designed and fabricated to meet with the current state-of-the-art method of evaporation which is solar interfacial evaporation. The fabricated materials were characterized using scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FTIR). The solar distiller and photo thermal materials were evaluated for their efficiencies via real-time outdoor experiments using solar energy. The rate of evaporation was calculated, while parameters such as pH, conductivity, Total dissolved solids (TDS) and salinity were analyzed on the freshwater collected and compared with the standard of drinking water by the World Health Organisation (WHO) and South African National Standard (SANS241). Heavy metal concentration in the water samples and remediated water collected were analyzed using inductively coupled plasma optical emission spectroscopy (ICP-OES) and compared with the WHO and SANS241 standards as well. The findings could provide adequate and affordable potable water to all households irrespective of societal status and it will reduce the cost of health management, as many diseases associated with consumption of untreated water can be drastically reduced. This is in-line with South Africa’s National Development Plan (NDP) 2030 and the United Nations’ Sustainable Development Goals (SDGs).
Determination of neonicotinoid insecticides in water, soil and sediment samples: acute and chronic risk assessment.
(2022) Ngomane, Nkosinathi Chris.; Mahlambi, Precious Nokwethemba.
Neonicotinoids are a type of insecticides pesticides widely used worldwide as a result of their low vertebrates toxicity, relative environmental stabilities, good bioavailability and high level of selectiveness. These insecticides are commonly employed in agricultural activities, in grass management and horticulture as well as in households to control domestic pet flea. Due to neonicotinoids intensive usage, they are continuously introduced to the water bodies where they can adversely affect the aquatic life and accumulate in sediments. Moreover, they can end up in drinking and unintentionally consumed by human beings resulting to health effects. With this regard, this work reports for the first time on the occurrence of neonicotinoids in sediment, soil tap, sludge, wastewater and river water samples from the province of KwaZulu-Natal. Also, the ecological risk of neonicotinoids in water sources was also assessed for the first time in the samples from this province.The liquid chromatography coupled with a photo-diode array detector (LC-PDA) method was modified and applied for the simultaneous detection of neonicotinoids (clothianidin, thiamethoxam and imidacloprid). Ultrasonic extraction (UE), soxhlet extraction (SE) and solid-phase extraction (SPE) methods were developed and applied for the extraction of nitro-guanidine neonicotinoids in water, soil and sediment samples. The SPE, SE, and UE parameters that influence the recoveries of the analytes were first optimized before application to real samples for the analytes recovery improvement. The SPE was used for the extraction of neonicotinoids in sludge and water samples, while SE and UE were both used to extract soil and sediment samples. The extraction conditions optimized for SPE were conditioning solvent and sample volume. While for the UE were extraction time, extraction solvent, and the solvent volume. And for SE method, extraction solvent and the extraction solvent volume were optimized. The LC-PDA method used for detection was also first optimized to improve peak separation, retention times, detection limits and quantification limits. The optimized parameters for the LC-PDA method were the mobile phase, flow rate, and the PDA detection wavelength. Optimum water recoveries of the neonicotinoids ranged from 79 to 112%. The detection and quantification limits of the analytes in water samples were 0.013 - 0.031 μg/L and 0.041 - 0.099 μg/L, respectively. The obtained analytes concentration ranged from 0.061 - 0.10 μg/L, 0.077- 3.76 μg/L and 0.99 - 15 μg/L in tap, river and wastewater, respectively. Analyte recoveries ranged from 85 - 102% in soil and 92 - 103% in sediment for the ultrasonic extraction method. The neonicotinoid recoveries ranged from 83 to 109% in soil and between 84 to 94% in sediment samples for the Soxhlet extraction method. The method’s detection limits and quantification limits in solid samples ranged from 40 - 80 μg/kg and 140 - 270 μg/kg, respectively. The relative standard deviation was less than 4%. The concentration determined in real environmental samples were 47 to 410 μg/kg in soil and 25 to 410 in sediment. The toxicity studies showed that clothianadin pose a high risk towards daphnia species in the river. Imidacloprid, clothianidin and thiamethoxam posed medium risk against algae, daphnia and fish species in the effluent receiving water bodies. These results imply the necessity to continuously monitor these neonicotinoids in the water sources. In South Africa there is limited data concerning the environmental occurrence of neonicotinoids, therefore this work will contribute towards the information available for the analysis of neonicotinoids. This will assist the policy makers to establish the MRL values that are precise for the African continent.
Syntheses of (pyridyl) pyrazine carboxamide palladium(II) complexes, their DNA/BSA interactions, and cytotoxicity studies.
(2023) Mvelase, Sabathile Thandeka.; Ojwach, Stephen Otieno.
Reaction of pyrazine-2,3-dicarboxylic acid with a respective amine in the presence of triphenyl phosphite afforded the corresponding carboxamide ligands: [N2, N3-bis(pyridin 2-yl)pyrazine2,3-dicarboxamide] (L1), [N2, N3-bis(6-methylpyridin-2-yl)pyrazine-2,3-dicarboxamide] (L2), [N2,N3-bis(4-methylpyridin-2-yl)pyrazine-2,3-dicarboxamide] (L3), and [N2, N3bis(quinoline-8-yl)pyrazine-2,3-dicarboxamide] (L4). Treatments of the corresponding (pyridyl)pyrazine carboxamide ligands with [PdCl2(NCCH3)2 afforded new mononuclear and dinuclear palladium(II) complexes with a general formula, [Pd2(L1)2Cl2] (Pd1), [Pd2(L2)2Cl2] (Pd2), [Pd2(L3)2Cl2] (Pd3) and [Pd(L4)Cl] (Pd4). The identities and coordination nature of the palladium(II) complexes were established through a combination of characterization techniques such as NMR, FT-IR spectroscopy, mass spectrometry, elemental analysis, and single X-ray crystallography. The molecular structures of the dinuclear Pd1 and Pd3 complexes reveal that the two (pyridyl) pyrazine carboxamide ligands coordinate with the palladium atom via one arm, while the other arm remains non-coordinating. The ligands are bridged by two palladium atoms to form dinuclear palladium(II) complexes. While one ligand coordinates to the palladium in a bidentate fashion via the nitrogen atoms of the pyrazine and amide groups, the other ligand unit coordinates to the palladium through the pyridine nitrogen atom to give two ligand units and two palladium atoms in the complex coordination sphere. On the other hand, ligand L4 binds to palladium atom in a tridentate fashion via the pyrazine, amide, and pyridine nitrogen atoms to give complex Pd4 as a mononuclear species. The interaction of the palladium complexes (Pd1-Pd4) with calf thymus DNA (CT-DNA) was monitored using UV-Vis, and fluorescence spectroscopy. Absorption spectroscopic studies revealed that complexes Pd1-Pd4 interact with CT-DNA via intercalative mode, and the computed intrinsic binding constant (Kb) values range from (4.28-13.12) x 106 M-1. In addition, Ksv values of (1.82-28.41) x 106 M-1 and KF values of (1.01-53.44) x 104 M-1 determined in competitive binding studies confirmed the intercalative binding mode. The interaction of the complexes with CT-DNA decrease in the order of Pd3 > Pd2 > Pd1 > Pd4. Furthermore, bovine serum albumin (BSA) interaction was evaluated using fluorescence studies and the results revealed the existence of a static quenching mechanism with bimolecular constant, kq range of (0.66-13.99) x 1014 M-1 s-1. The Ksv values of (1.48-29.67) x 106 M-1 and KF values of (0.10-16.10) x 105 M-1 confirmed the interaction between BSA and palladium complexes. The interaction follows this order Pd2 > Pd1 > Pd3 > Pd4, which is inconsistent with the CT-DNA trend. In general, ligands bearing electron-donating methyl groups (L2 and L3) contributed to higher binding constants in their respective complexes Pd2 and Pd3 compared to the unsubstituted complex Pd1. In addition, the mononuclear complex Pd4 showed the weakest interactions with both the DNA and BSA, pointing to some beneficial effects of increased metal atoms in the complexes. The cytotoxic effect of the ligands L1-L3 and complexes Pd1-Pd4 were examined against a human breast cancer cell line (MCF-7) using 3-(4,5-Dimethyl-2thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assay. In general, the ligands displayed poor cytotoxicity L1 (IC50 > 400 μM), L2 (IC50 = 182.4 μM), L3 (IC50 = 80.2 μM), when compared to their respective palladium(II) complexes Pd2 (IC50 = 154.9 μM), Pd3 (IC50 = 230.1 μM). Complexes Pd1 (IC50 = 11.4 μM), and Pd4 (IC50 = 61.5 μM) displayed high and moderate cytotoxic activity which was attributed to the planarity of the complexes.
Vinyl-addition polymerization of norbornene catalyzed by (pyridyl)imine Ni(II), Pd(II), Co(II), and Fe(II) complexes.
(2023) van der Westhuizen, Barend Danielle.; Ojwach, Stephen Otieno.
The thesis reports the syntheses, and structural characterization of (pyridyl)imine transition metal complexes and their applications as pre-catalysts in the vinyl-addition polymerization of norbornene. The bidentate ligand (E)-N-(1 phenylethyl)-1-(pyridin-2-yl)methanimine (L1) was synthesized by reactions of 2-pyridine carboxaldehyde with (R)-(+)-α-methyl benzylamine in the presence of p-TsOH and obtained in moderate yields of 64%. On the other hand, the tridentate ligand (E)-1-(pyridin-2-yl)-N-(pyridin-2-ylmethyl)methanimine (L2) was obtained by reacting 2-pyridine carboxaldehyde with 2-picolylamine in the presence of p-TsOH in high yields of 90%. Reactions of bidentate ligand L1 and tridentate ligand L2 with NiCl2, [Pd(COD)Cl2], FeCl2, and CoCl2 metal salts gave the corresponding complexes [Ni(L1)2Cl2] (Ni1), [Ni(L2)Cl2] (Ni2), [Pd(L1)Cl2] (Pd1), [Co(L1)3][2PF6] (Co1), [Co(L2)Cl2] (Co2), [Fe(L1)3][2PF6] (Fe1), [Fe(L2)Cl2] (Fe2) in low to moderate yields (18% - 60%). The identities of the isolated complexes were confirmed by characterization with FT-IR spectroscopy, mass spectrometry, elemental analysis, single crystal X-ray crystallography for Co1, and nuclear magnetic resonance where applicable. The solid state structure of complex Co1 was established as tris-chelated, containing three ligands to give an octahedral coordination environment. The metal complexes were evaluated as pre-catalysts in the vinyl-addition polymerization of norbornene to produce poly(2,3-bicyclo[2.2.1]heptene) using modified methyl aluminoxane (MMAO) as the co-catalyst. Complex Ni1 was the most active with catalytic activity of 22.7 g×10³(PNBE).mol(M)-¹.h-¹ followed by complex Pd1 which showed catalytic activity of 17.6 g×10³(PNBE).mol(M)-¹.h-¹ whereas complex Co1 showed 0.7 g×10³(PNBE).mol(M)-¹.h-¹ and complex Fe1 showed catalytic activity of 0.3 g×10³(PNBE).mol(M)-¹.h-¹ concluding that the choice of metal center is of absolute importance to achieve high catalytic activity. The number of electron donor atoms in the ligand structure influenced catalytic activity as bidentately chelated complex Ni1 showed catalytic activity of 22.7 g×10³(PNBE).mol(M)-¹.h-¹ whereas the tridentately chelated Ni2 showed catalytic activity of 81.9 g×10³(PNBE).mol(M)-¹.h-¹. The influence of reaction parameters were investigated using Ni1 and Ni2 as pre-catalysts and it was concluded that monomer/metal ratios, co-catalyst/metal ratios, reaction temperatures, reaction times, and solvent choice influenced catalytic activity. A higher monomer/metal ratio of 1250 resulted in catalytic activity of 17.5 g×10³(PNBE).mol(M)-¹.h-¹ compared to the value of 6.8 g×10³(PNBE).mol(M)-¹.h-¹ obtained from a lower monomer/metal ratio of 625. An optimum co-catalyst/metal ratio of 1500 was established and recorded catalytic activity of 33.7 g×10³(PNBE).mol(M)-¹.h-¹. Polymerization reactions at room temperature gave higher monomer conversions of 70% as opposed to lower conversions of 17% obtained at 50 °C. The choice of solvent influenced catalytic activities of the complexes, with the more polar o-chlorobenzene solvent giving the highest monomer conversion of 70% in comparison to 31% obtained in toluene solvent. Polymers formed from all complexes were of the vinyl type with possible ring-opening metathesis polymeric inserts present in the polymer backbone. Thermal gravimetric analysis and differential scanning calorimetry of the formed polymers demonstrated that polymers formed from complex Co2 and complex Fe2 displayed degradation temperatures of 492 °C and 478 °C respectively opposed to polymers formed from complex Ni2 and complex Pd2 which gave values of 478 °C and 462 °C.
Synthesis of ketones through aerobic transition metal-catalyzed cross-coupling of thioesters.
(2021) Mthombothi, Zabenguni Zakithi.; Sithebe, Siphamandla.
Asymmetric diaryl ketones are an important class of compounds in organic chemistry due to their presence in natural products, synthesis, cosmetics as well as in biological active compounds. The aim of this project was to expand the scope of thiophilic metal carboxylates catalysts that can be applied to the aerobic Liebeskind-Srogl cross coupling reaction between various thioesters and phenylboronic acids. Thioesters bearing electron neutral, withdrawing and donating groups were successfully synthesized in yields ranging from 35 to 54%Xcv. The electron neutral thioester was used in the optimization of the aerobic Liebeskind-Srogl reaction and CuMeSal proved to be the most effective catalyst in this protocol. CuMeSal was applied in the aerobic synthesis of the asymmetric diaryl ketones bearing a wide range of functional groups including CF3, SMe, OMe and F yielding up to 65% in 24hrs. Aerobic Liebeskind-Srogl cross-coupling reaction was applied in the successful synthesis of chalcones, through the coupling of commercially available phenylboronic acids with previously synthesised thioesters catalysed by CuMeSal in DMF for 24hrs,furnishing desired products in poor to excellent yields 26-89%.
Synthesis, characterization and substitution reaction studies of pyridyl N,N’-bidentate palladium(II) complexes. A kinetic and mechanistic study.
(2023) Mjwara, Pinky Ncomela.; Sithebe, Siphamandla.; Papo, Tshephiso Rose.
The influence of structural as well as electronic properties of bidentate N,N chelates with different substituents on the mononuclear Pd(II) complexes were investigated. The complexes were synthesized and characterized by various spectroscopic methods such as 1H & 13C NMR, FT-IR, LC-MS, CHN and single x-ray crystallography. For the first set of complexes (Chapter 3), we studied the unexplored kinetics and mechanistic behaviour of N,N’-pyridyl Pd(II) complexes, viz. dichloro-(N-((pyridin-2-yl)methyl)aniline)palladium(II) (PdL1), dichloro-(4-fluoro-N-((pyridin-2-yl)methyl)aniline)-palladium(II) (PdL2), dichloro-(4-bromo-N-((pyridin-2-yl)methyl)aniline)-palladium(II) (PdL3), dichloro-(4-methoxy-N-((pyridin-2-yl)methyl)aniline)-palladium(II) (PdL4) and dichloro-(4-ethyl-N-((pyridin-2-yl)methyl)aniline)-palladium(II) (PdL5). The substitution behaviour of coordinated chloride atoms by three bio-relevant thiourea nucleophiles, viz. thiourea (TU), N,N’-dimethylthiourea (DMTU) and N,N,N’,N’-tetramethylthiourea (TMTU), of different steric demands was studied in a 0.1 M solution of ultra-pure water under pseudo-first order conditions. The reactions were studied as a function of concentration and temperature using standard Stopped-Flow and UV-Vis spectrophotometric technique. The substitution of the chloride atoms from the Pd metal by thiourea nucleophiles was a two-step reaction where the chloride trans to the pyridine ligand was substituted first, since the pyridine has a stronger trans effect compared to the amine group. The reactivity of mononuclear Pd(II) complexes containing bidentate N,N’-donor ligands with different substituents depends on the electronic effects of the complexes. The reactivity of the complexes increased with the presence of electron withdrawing substituents and decreased when an electron donating group was attached on the para position of the aniline moiety. The electron withdrawing groups influence the pull of electrons from the electron deficient amine that is coordinated to the metal center which results in the loss of electron density from the ligand moiety and increases the electrophilicity of the metal center and thus the substitution reaction. The reactivity of the nucleophiles depends on steric effects, with the bulky TMTU being the least reactive. The negative entropies and second order kinetics for all the substitution reactions support an associative mode of substitution mechanism. DFT calculations were performed to account for the observed reactivity of all the complexes studied. For the second set of novel Pd(II) complexes (Chapter 4), viz. bis[N-(4-bromophenyl)pyridine-2-carboxamidato] Palladium (Pd1) and Palladium(II) [N-(4-bromophenyl)-2-pyridinecarboxamide), pyridine chloride (Pd2), crystals were obtained and the structures were studied. Pd1 crystallizes in the monoclinic crystal system and in the P21/c space group, and Pd2 crystallizes in the orthorhombic system, with the space group Pbca.
Synthesis and the application of molecularly imprinted polymers as solid-phase extraction and dispersive solid-phase extraction sorbents in the extraction of antiretroviral drugs in water.
(2022) Xolo, Thabiso.; Mahlambi, Precious Nokwethemba.
A multi-template molecularly imprinted polymer was synthesized, characterized, and applied to real water samples for the specific extraction of antiretroviral drugs (ARVDs), abacavir, nevirapine, and efavirenz. A MIP was synthesized by bulk polymerization at 60 ºC using abacavir, nevirapine and efavirenz (templates), 2-vinylpyridine (functional monomer), 1,1’- azobis-(cyclohexanecarbonitrile) (initiator), ethylene glycol dimethacrylate (cross-linker) and toluene: acetonitrile (9:1, v/v) (porogenic solvent mixture) for 16 hours. The temperature was then increased to 80 ℃ for 24 hours to ensure complete polymerization. The synthesized MIP was washed with acetic acid: acetonitrile (1:9, v/v) via soxhlet extraction until all three ARVDs were undetected in the washing solvent using high performance liquid chromatography coupled with a photo-diode array detector. A non-imprinted polymer (NIP) was synthesised using the same reagents and quantities except for the templates. Both MIP and NIP were characterized using Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), scanning electron microscope (SEM), N2 physisorption analysis, and elemental analysis. The FTIR results showed that the polymers were similar in structure and BET showed that they were mesoporous. The SEM showed that the MIP surface was rougher when compared to the NIP and characterization with TGA showed that they were both thermally stable. The synthesized MIP was used to study its adsorption kinetics and isotherms. Kinetics modelling revealed that the Pseudo-second-rate order kinetics was the best fitting model with correlation coefficient of 1 compared to Pseudo-first-rate order kinetics which had a correlation efficient of 0.81-0.983 for all target analytes. The best fitting adsorption isotherm was the Freundlich model with a correlation range of 0.9451-0.986 for compared to the Langmuir model which had correlation efficient of 0.6692-0.93390.0198-0.6782 for all target analytes. The traditional solid phase extraction and the MIP-based solid phase extraction methods were applied in distilled water samples spiked with 1 mg.L-1 of ARVDs and recoveries obtained were ranging from 91.68-94.59% and 97.20 to 99.68%, respectively. The MIP-based dispersive solid phase extraction method was successfully optimized with recoveries ranging from 100.28% to 102.60% for all three analytes. Selectivity studies were conducted using both the NIP and MIP with lamivudine and diclofenac as competitors. The recoveries obtained for the MIP ranged between 92% to 98% for the target analytes while they were 63% to 79% for competitors. These results showed good selectivity and strong affinity of the polymer towards the target analytes than the competitors. This is justified by the presence of imprinted recognition sites that have the same functional groups, size, and shape as the target analytes/templates hence recoveries were low for competitors. The MIP was more selective towards analytes of interest compared to the NIP (recoveries ranged from 87.9% to 91%) for the analytes of interest which indicates successful imprinting on the MIP. Reusability studies showed that the MIP can be reused for up to 8 cycles with recoveries above 92% for all target analytes. The developed, adopted, and validated methods were then applied to wastewater, tap water and river water samples from around KwaZulu-Natal. The concentrations obtained for abacavir, nevirapine and efavirenz were 10.65-295.90 μg.L-1 in wastewater, 1.95-13.15 μg.L-1 in river water, and 2.17-6.27 μg.L-1 in tap water. Efavirenz was the most dominant and consistently detected ARVD in all samples. The MIP-DSPE was the most sensitive and selective extraction technique compared to SPE and MIP-SPE. Umhlathuzana and Amanzimtoti were the most ARVD’s polluted wastewater treatment plants, whilst Northern wastewater water works was the least polluted. Camps Drift was the most ARVD’s polluted sampling point in Msunduzi river. Napierville and Scottsville showed the most contaminated tap water samples from suburbs around Pietermaritzburg.
One-pot, multicomponent oxidative synthesis of 2,4,5-trisubstituted imidazoles from internal alkenes using an I2/DMSO system.
(2023) Majola, Nonhlelo.; Jeena, Vineet.
Imidazoles are vital heterocyclic compounds usually incorporated in natural products such as biotin, vitamin B12, histamine, and histidine. 2,4,5-trisubstituted imidazoles, in particular, possess versatile biological and pharmaceutical activities such as antidiabetic, antimalarial, and analgesic properties. A traditional procedure for the synthesis of these elegant compounds involves the cyclocondensation reaction between a 1,2-diketone, an aldehyde, and ammonia in the presence of an acid or metal catalyst. However, this methodology suffers from various shortcomings such as the use of acid or metal catalysts, tedious work-up procedures, use of toxic reagents, and substrate scope limitations. Hence, the development of new methods to synthesize 2,4,5-trisubstituted imidazoles is of vital importance. This study describes the preparation of 2,4,5-trisubstituted imidazoles from alkenes using an environmentally benign iodine/DMSO system. This novel methodology was applied to a broad substrate scope such as substituted benzaldehydes, heterocyclic aldehydes, bulkier aldehydes, and substituted stilbenes, and afforded the target compounds in moderate to high yields under mild reaction conditions. Preliminary mechanistic studies revealed that 1,2-diketone is a key intermediate and that the mechanism is not radical-mediated. It also revealed that the oxygen source is DMSO and that the coupling step is catalyzed by iodine coordination and hydrogen bonding from the solvent. Based on the results obtained from the preliminary mechanistic investigations, a reasonable mechanism is proposed.
Synthesis of non-natural amino acids as covalent inhibitors for protein-protein interactions.
(2023) Dladla, Siphamandla Austen.; Sithebe, Siphamandla.; Veale, Clinton Gareth Lancaster.
There is still a need to develop new cancer therapies for troubling cancers. Hence, a resurging interest in compounds that engage their target through covalent interactions. Lysine’s amine can be engaged covalently with a weak electrophile (SO2F) extending the potential of covalent inhibitors. Herein, we were prompted to investigate the synthesis of non-natural amino acids, modified to include weakly electrophilic warheads, which could potentially target specific lysine residues. Three new non-natural amino acids were successfully synthesized, methyl (S)-2-((tert-butoxycarbonyl)amino)-3-(4-((fluorosulfonyl)oxy)phenyl)propanoate, 3.5, methyl (S)-2-((tert-butoxycarbonyl)amino)-2-(4-((fluorosulfonyl)oxy)phenyl)acetate, 3.9, and methyl (S)-2-((tert-butoxycarbonyl)phenyl)propanoate, 3.35, in 85%, 89%, and 63.7% yield, respectively. Our study explored the synthetic pathway of a three-step procedure toward the target compounds, with the initial esterification of the carboxylic acid group, followed by the N-Boc protection of the amine group. Finally, the key sulfonation of the N-Boc protected amino methyl ester, where for 3.5 and 3.9, was performed through ex-situ generation of sulfuryl fluoride, which was installed following the substitution of the hydrogen on the hydroxyl group by SO2F. For 3.35, it was achieved through a palladium-catalyzed system and an in-situ fluorine introduction, where para iodine was substituted by the SO2 generated from DABSO. Under physiological conditions, compound 3.5 was assessed for possible interaction through its electrophilic warhead, with nucleophilic N-Boc-lysine side chain. The LCMS and NMR buffered assays were conducted, and in both these studies, the characteristics of a possible binding happening can be observed, hence an adduct N2-(tert-butoxycarbonyl)-N6-((4-((S)-2-((tert-butoxycarbonyl)amino)-3-methoxy-3-oxopropyl)phenoxy)sulfonyl)-L-lysine 3.5a formation.
Oxidative dehydrogenation of n-octane using morphologically different VMgO catalysts.
(2017) Magwenyane, Ayanda Mandlenkosi.; Friedrich, Holger.
ABSTRACT Gas phase paraffin oxidative dehydrogenation (ODH) has been explored extensively using medium chain paraffins such as n-hexane and n-octane as feedstock over various catalytic systems. Among these systems, the activity and stability of magnesium vanadates have been studied during the ODH of these paraffins. In most cases, it was found that 15 wt% vanadium was the optimum loading on the support, i.e. magnesium oxide. In addition, it was found that the morphology of the magnesium oxide influenced the performance of the catalyst. However, this has not been explored in much detail. In this work, magnesium oxide was synthesized using several methods to produce surfaces that differed morphologically, such as materials with a cubic morphology, denoted MgO-A, spherical morphology (MgO-B) and MgO with a nano-sheet type morphology (MgO-C). These supports were then loaded with 15 wt% vanadium by wet impregnation method using different synthesis methods. They were characterized using X-ray diffraction (XRD), Brunauer-Emmet-Teller technique, inductively-coupled plasma-optical emission spectroscopy, Raman spectroscopy, scanning and transmission electronic microscopy, temperature programed reduction (TPR) and temperature programed desorption (TPD). Electron micrographs generated for all the catalysts confirmed their respective morphologies. The XRD and Raman results showed the formation of magnesium orthovanadate and pyrovanadate phases for the cubic MgO. Only the orthovanadate phase was present for the other VMgO catalysts. This was confirmed by TPR which showed two reduction peaks for VMgO-A and one reduction peak for the other catalysts. The catalysts were tested for the ODH of n-octane at 450 °C using air as an oxidant. At an iso-conversion of 14 ± 1 %, VMgO-B (water), whose support had a spherical morphology was more selective towards ODH products, whereas VMgO-A (oxalic acid) where a cubic morphology was observed for the support, carbon oxides (COx) were the dominant products and this correlated well with the ammonium-TPD results which showed that catalysts with weak and medium acidic sites promoted the formation of ODH products, whereas catalysts with strong acidic site were more selective towards COx, in agreement with published results. Scanning electron microscopy images of the used catalyst showed agglomeration of particles while XRD showed a peak for the reduced phase of magnesium orthovanadate. This study has shown that catalysts with the same formulation but with different morphologies synthesised by methods modified to suit its physical and chemical properties, influenced the variance in the catalysis.
Plant-mediated synthesis of zinc oxide nanoparticles using extracts and isolated compounds from senecio serratuloides and their biological activity.
(2018) Joyisa, Nonhlanhla.; Moodley, Roshila.; Pruessner, Karin.
Senecio serratuloides DC of the Asteraceae family is a medicinal plant used in South African traditional medicine for the treatment of skin diseases, sexually transmitted infections (STIs) and wounds. Despite the ethnomedicinal significance of the plant, a phytochemical investigation to determine the active components for future pharmacological developments has not been conducted. Nanotechnology is a promising field in the development of biocompatible metal nanoparticles from bio-resources. Zinc oxide nanoparticles (ZnONPs) are of great interest due to their wide range of applications in the field of biomedical sciences. Research on the exploitation of plant materials for the green synthesis of nanoparticles is increasing rapidly. The aim of this study was therefore to extract, isolate and identify secondary metabolites from S.serratuloides, to use the extracts and isolates as reducing agents in the synthesis of zinc oxidenanoparticles, and to compare the antioxidant, antibacterial and anti-quorum sensing activities of extracts, isolated compounds, freestanding and plant-derived zinc oxide nanoparticles. The phytochemical investigation yielded one sesquiterpene (farnesylamine), five tripernoids (sitosterol, α- and β-amyrin, stigmasterol and taraxerone) and two jacaranones (jacaranone and a mixture of jacaranone and methyl-2-(1-hydroxy-4-oxocyclohexyl)acetate). The synthesised zinc oxide nanoparticles were characterised using spectroscopic and microscopic techniques. Spherical zinc oxide nanoparticles were successfully synthesised but exhibited a wide size range. ZnONPs synthesised using jacaranones showed good antioxidant activity whilst ZnONPs synthesised using extracts of S. serratuloides showed moderate antioxidant activity. The extracts, phytocompounds and nanoparticles were tested for antibacterial activity against three Gram-positive bacteria (Pseudomonas aeruginosa, Staphylococcus aureus and Enterococcus faecalic) and two Gram-negative bacteria (Escherichia coli and Chromobacterium violaceum). The extracts demonstrated promising antibacterial activityagainst Chromobacterium violaceum. Amongst the isolated phytocompounds, jacaranones showed promising antibacterial activity against two Gram-positive bacteria, Staphylococcus aureus and Enterococcus faecalic, with good antibacterial activity against Chromobacterium violaceum. Nanoparticles did not possess antibacterial activity. The isolated jacaranone, extracts and nanoparticles were further evaluated for quorum sensing inhibitory activity using a qualitative agar-overlay assay. The extracts showed promising anti-quorum sensing activity whilst the jacaranone showed good anti-quorum sensing activity. Nanoparticles did not show anti-quorum sensing activity. This can be attributed to freestanding nanoparticles not possessing activity against the bacterial strains tested and in plant-derived nanoparticles, the amount of the active compound capping particles could be too little to impart activity.
Influence of different fuels on the properties of solution-combustion synthesized palladium/ceria catalysts for low-temperature methane combustion.
(2018) Mkhwanazi, Thabiso Perfect Oscar.; Friedrich, Holger Bernhard.; Singh, Sooboo.; Mahomed, Abdul Samad.
Catalytic combustion of methane is a leading technology in energy production, emission prevention and gas clean-up. Its main advantage over traditional flame combustion is to carry out complete oxidation of fuel at low temperatures. Noble metals supported over high surface area supports are known to combust methane at low-temperatures. However, noble metals supported on oxide supports has been observed to result in high methane combustion activity at low temperatures. In recent studies on methane combustion, it was observed that the use of ceria as a support can significantly improve the catalyst activity. PdO supported on ceria is known to be a very active catalyst for methane combustion. However, this catalyst still suffers from poor activity at low temperature (below 673 K) and deactivation at high temperature (above 973 K) owing to the formation of metallic Pd from PdO particles. In this study, a comparison between solution combustion synthesis (SCS) and conventional incipient wet-impregnation catalysts was made and discussed for low temperature methane combustion. The PdO/CeO2 catalysts was prepared by the solution combustion synthesis method (SCS) with different fuels including oxalylhydrazide (ODH), citric acid monohydrate, urea, β-alanine, and tartaric acid were subsequently evaluated for low-temperature methane combustion. Each fuel is known to affects the physical and chemical properties of the catalyst which further influences the catalytic performances. To the best of our knowledge, the effect of fuels on the properties of SCS synthesised PdO/CeO2 catalysts, for low temperature methane combustion has not been reported. To evaluate these effects, several fuels such as oxalylhydrazide (ODH), citric acid monohydrate, urea, β-alanine, and tartaric acid were used. Furthermore, all prepared catalysts were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray spectrometer (EDX), laser Raman spectroscopy (LRS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), N2-physisorption analysis and X-ray photoelectron spectroscopy (XPS). Furthermore, all prepared catalysts were tested for methane combustion activity at 4 vol% methane in air and GHSV of 15 000 h-1. The N2-Physisorption analysis revealed that SCS ceria had a high surface area and a smaller crystallite size when compared to the commercial ceria. From Raman spectroscopy, the SCS ceria was established to contain more defects and thus, contained a higher amount of lattice oxygen incomparison to the commercial ceria. The SCS ceria provided almost 2.5-fold higher methane conversion than the commercial ceria at 600 ̊C. When the 2 wt.% PdO/CeO2 catalysts were compared, it was revealed that the SCS synthesized catalysts had a higher surface area and more oxygen vacancies and thus, higher catalytic activity in comparison to the catalysts prepared using wet impregnation. The study of the effect of fuels revealed, that different fuels result in catalysts with different physical and chemical properties. The surface areas of the prepared catalysts were observed to decrease according to the following trend, Pd0.03Ce0.97O2-δ-urea > Pd0.03Ce0.97O2-δ-tartaric acid > Pd0.03Ce0.97O2-δ-alanine > Pd0.03Ce0.97O2-δ-citric acid > Pd0.03Ce0.97O2-δ-ODH. Raman spectroscopy, revealed that the amount of oxygen vacancies decreased in the order of Pd0.03Ce0.97O2-δ-citric acid > Pd0.03Ce0.97O2-δ-ODH > Pd0.03Ce0.97O2-δ-alanine > Pd0.03Ce0.97O2-δ-tartaric acid > Pd0.03Ce0.97O2-δ-δurea. The H2-TPR and XPS studies, revealed that the citric acid and ODH synthesized catalysts contained supported PdO in comparison to the other SCS synthesized catalysts. The T50 was observed to decrease in the following order Pd0.03Ce0.97O2-δ-citric acid > Pd0.03Ce0.97O2-δ-ODH > Pd0.03Ce0.97O2-δ-urea > Pd0.03Ce0.97O2-δ-alanine > Pd0.03Ce0.97O2-δ-tartaric acid. However, the T100 was observed to decrease in following order Pd0.03Ce0.97O2-δ-citric acid > Pd0.03Ce0.97O2-δ-urea > Pd0.03Ce0.97O2-δ-ODH > Pd0.03Ce0.97O2-δ-alanine > Pd0.03Ce0.97O2-δ-tartaric acid.

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