Doctoral Degrees (Chemistry and Polymer Science)
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- ItemA 195Pt Nuclear magnetic resonance and molecular dynamics computer simulation study of the solvation of simple platinum (IV) chlorido complex anions in water and water-miscible solvent mixtures(2017-03) Engelbrecht, Leon de Villiers; Koch, Klaus R.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.ENGLISH ABSTRACT: A combined 195Pt NMR spectroscopy and Molecular Dynamics (MD) computer simulation study of the solvation of the octahedral Pt(IV) complex [PtCl6]2‒ in binary mixtures of water and the fully water-miscible organic solvents methanol, 2-methoxyethanol and 1,2-dimethoxyethane has been carried out. A recent 195Pt NMR chemical shift-trends study indicated a preferential solvation of the aforementioned platinum complex by the organic solvent component in such solvent mixtures. The solvent dependence of the intrinsic 1Δ195Pt(37/35Cl) NMR isotope shifts of [PtCl6]2‒ in pure solvents indicate a slight increase in magnitude ~7 ppb in the order water < methanol < 2-methoxyethanol < 1,2-dimethoxyethane. In selected equimolar binary mixtures of water and organic solvents, 1Δ195Pt(37/35Cl) is found to be similar in magnitude to that in the pure organic solvents, supporting the proposed preferential solvation of the platinum complex. 195Pt NMR T1 spin relaxation times and pulsed gradient spin echo (PGSE) translational diffusion measurements were performed for [PtCl6]2‒ in selected solvents and binary mixtures. The results were interpreted in the context of hydrodynamic continuum models of molecular diffusion; while these were found to be not strictly appropriate, the rotational and translational dynamics results appear to be notionally consistent with the preferential solvation phenomenon as indicated. A series of classical MD computer simulations were performed for [PtCl6]2‒ in these equimolar binary solvent mixtures, using a recently revised force field developed by Naidoo et al. The results using the standard force field indicate a strong preference for water in the primary solvation shell region of the complex in all solvent mixtures studied. A similar result is obtained for [PtCl4]2‒ in an equimolar water‒methanol mixture. Simulations were repeated with ionic charges scaled according to the recently developed Molecular Dynamics in Electronic Continuum (MDEC) theory, which is intended to account for the dielectric screening of charges in condensed phases. In these MDEC simulations, a significant reduction in the contribution of water to the primary solvation shells of both complexes is observed; this is particularly evident in the solvation shell of [PtCl6]2‒ in mixtures of water with 2-methoxyethanol and 1,2-dimethoxyethane, for which a strong preferential solvation by the organic components has been indicated. Dynamic properties were also computed from MD trajectories, and are qualitatively consistent with experimental trends, but deviate due to the solvent model combination. Finally, an interpretation of the fascinating 35/37Cl and 16/18O isotope-induced fine structure in the 195Pt NMR spectra of complexes of the type [PtCln(OH)6‒n]2‒, n = 0‒5, is presented, based on the expected trans-influence series of ligands for Pt(IV) OH‒ > Cl‒ > H2O in aqueous solution.
- Item59Co NMR, a tool for the study of the structure, reactivity and supramolecular chemistry of Co(III) complexes derived from a series of N-alkyl-N-alkyl(aryl)-N’-acyl(aroyl)thioureas(Stellenbosch : Stellenbosch University, 2020, 2020-03) Barnard, Ilse; Koch, Klaus Robert; Gerber, Wilhelmus Jacobus; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.ENGLISH ABSTRACT: A large library of Co(III) complexes, derived from selected monopodal and bipodal N,N- dialkyl-N’-acyl(aroyl)thioureas, have been synthesized and characterized. These ligands form tris(chelated) complexes in the thermodynamically more stable fac geometry with the cobalt metal. The monopodal ligands, and their corresponding Co(III) complexes, were separated into two groups based on the two R substituents of the C(S)-NRR’ function. The first group were named the symmetrically substituted acylthiourea derivatives, where R = R’. Due to the increased stability provided by the chelate effect as well as the selective population of the lower-energy d-orbitals, such complexes are generally expected to be kinetically stable in solution. Nonetheless, the ‘symmetrical’ Co(III) complexes were utilized for the study of the unexpected, slow and spontaneous ligand exchange reaction between non-identical homoleptic pairs of low-spin d⁶ octahedral fac-[Co(LA-κS,O)3] and fac-[Co(LB-κS,O)3] complexes in organic solvents. The exchange reaction between these complexes result in mixtures of their corresponding heteroleptic fac-[Co(LA-κS,O)2(LB-κS,O)] and fac-[Co(LA-κS,O)(LB-κS,O)2] complexes in solution. This discovery was followed by a quantitative determination of the ligand exchange equilibria as well as a relative rate study, as a function of temperature, of the reaction using rp-HPLC. It was then established that the high chemical shift sensitivity of ⁵⁹Co NMR is a powerful tool for the easy characterization of the Co(III) complexes derived from symmetrically substituted acylthioureas in solution. The utility of ⁵⁹Co NMR as a spectroscopic tool was exemplified after utilizing this technique for further additional studies of factors shown to dramatically affect the relative rate of ligand exchange in these complexes, specifically light and solvent. The second group of monopodal acylthiourea ligands investigated were named the asymmetrically substituted acylthioureas, where R ≠ R’. The partial double bond character in the C-N bond of the C(S)-NRR’ function results in the E,Z configurational isomerism of these ligands in solution. The isomerism is expressed as duplicate resonances in the ¹H NMR spectra of the uncoordinated ‘asymmetrical’ acylthiourea ligands. The Z to E isomer ratio varies depending on the two R and R’ substituents of the thiourea moiety. Notably, the isomerism in the uncoordinated ligands is passed on to the Co(III) chelates after coordination. As for the symmetrically substituted acylthiourea Co(III) complexes, we find that the ⁵⁹Co NMR chemical shift is very sensitive to the structure of the asymmetrical fac-[Co(Lⁿ-κS,O)3] complexes in solution. Significantly, the presence of the four, theoretically possible, fac- [Co(EEE-Lⁿ-κS,O)3], fac-[Co(EEZ-Lⁿ-κS,O)3], fac-[Co(EZZ-Lⁿ-κS,O)3] and fac-[Co(ZZZ-Lⁿ- κS,O)3] isomers is readily observable by means of ⁵⁹Co NMR, which shows four well-resolved resonances. From the relative peak integrals of the ⁵⁹Co NMR peaks, a semi-quantitative estimate of the relative amounts of the configurational isomers in solution was possible, although assignment of the isomers is not trivial. The assignment of the ⁵⁹Co peaks to each of the EEE, EEZ, EZZ and ZZZ configurational isomers of the asymmetrical fac-[Co(Lⁿ-κS,O)3] complexes were based on the relative E/Z ratio of the uncoordinated ligands, which was established from their ¹H NMR spectra. However, this assignment assumes that the relative E/Z ratio does not vary significantly during coordination to cobalt and remains therefore ambiguous. The distribution of the EEE, EEZ, EZZ and ZZZ configurational isomers was shown to be dependent on the solvent used during the analyses. Moreover, we evaluated the temperature needed to lift the barrier to rotation in the C-N bond of the coordinated ligands. Finally, the ⁵⁹Co NMR trends of several complexes isolated for single crystal X-ray diffraction analysis were complemented by DFT linear transit calculations of their configurational isomers. Finally, we investigate the interesting coordination chemistry of the well-ordered and multinuclear coordination systems of the bipodal acylthiourea analogues, namely aroylbis(N,N-dialkylthioureas). Two ligands were selected for this purpose, including isophthaloylbis(N,N-diethylthiourea) and a bipodal ligand with a modified aromatic spacer derived from catechol. These were utilized as pre-programmed chelating ligands to form metallamacrocyclic octahedral facial Cobalt(III) complexes via self-assembly. The latter catechol-spaced ligand was used for the synthesis of a number of oligometallic Co(III) complexes by one-pot reactions of the ligand, Co³⁺ and a variety of monovalent cations, to form metallacryptates of type {M⁺ ⊂ [Co2(L-κS,O))3]}(PF6), M⁺ = K⁺, Rb⁺, Cs⁺ and NH4+. A significant discovery, owing to the unique sensitivity of the ⁵⁹Co nucleus, was the instrumental role ⁵⁹Co NMR played in our attempt to characterize the metallacryptates and to determine their conditional stabilities in solution. This was achieved by way of several biphasic exchange experiments of cations between an aqueous and non-aqueous phase. This is a novel discovery that has allowed for the study of various factors shown to effect the stability of the metallacryptates in solution.
- ItemAdamantane copolymers(Stellenbosch : Stellenbosch University, 2001-12) Coetzee, Liezel; Van Reenen, A. J.; Stellenbosch University. Faculty of Science. Dept. of Chemistry & Polymer Science.ENGLISH ABSTRACT: This study concerns the incorporation of adamantane containing monomers 3-(1-adamantyl)-1-propene and 1-(1-adamantyl)-4-vinylbenzene into ethene, propene and higher a-olefins using different catalytic systems. The effect of the incorporation of the adamantane monomer on the physical and thermal properties of the polymers was investigated. A thorough study on the background of adamantane in general, as well as polymerization reactions involving the above-mentioned monomers and a-olefins was done. 3-(1-Adamantyl)-1-propene as well as 1,.(1-adamantyl)-4-vinylbenzene was sucessfully synthesized. The homo polymers of each monomer were made. The above-mentioned monomers were also polymerized: • . 3-( 1-adamantyl)-1-propene with ethene, propene and higher a-olefins, • 1-(1-adamantyl)-4-vinylbenzene with ethene and styrene. The copolymers of 3-(1-adamantyl)-propene as well as 1-(1-adamantyl)- 4-vinylbenzene were characterized as far as possible to show the influence of the incorporation of the adamantane group on the physical and chemical properties of the polymers. A series of 3-phenyl-1-propene copolymers with higher a-olefins were synthesized to compare the influence of the phenyl group to the adamantyl group on the relevant properties of the polymers.
- ItemAdvanced analytical methods for the analysis of complex polymers prepared by RAFT and RITP(Stellenbosch : Stellenbosch University, 2015-04) Wright, Trevor Gavin; Pasch, Harald; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.ENGLISH ABSTRACT: Synthetic polymers are complex compounds that have multiple distributions with regard to molar mass, chemical composition, functionality and molecular architecture. Therefore, the molecular complexity of these compounds can only be analysed using a combination of analytical techniques. Well-defined complex polymers can be prepared by different types of living radical polymerisation, including reversible addition–fragmentation chain transfer polymerisation (RAFT) and reverse iodine transfer polymerisation (RITP). Using these techniques, several different homopolymers and copolymers have been prepared. However, there is still space for some more extended research. Many different types of multifunctional RAFT agents have been reported in literature. A tetrafunctional RAFT agent was prepared in our laboratory and used for the first time in the polymerisation of styrene. The polymerisation reaction was followed using in situ 1H nuclear magnetic resonance (NMR) and the molar masses of the resultant polymers were determined using size exclusion chromatography (SEC). The molar masses of the star-shaped polystyrenes (PS) were found to be less than the theoretical molar masses. This was due to the fact that SEC was calibrated with linear PS standards, while the samples under investigation are branched. Linear and branched polymers have different hydrodynamic volumes at similar molar masses. In order to prove that the star-shaped polymers were in fact four-armed, the samples were cleaved by aminolysis to yield the linear PS arms. The molar masses of the arms were in agreement with the theoretical arm molar masses based in the fourarmed structure. RITP is a relatively new living radical polymerisation technique. Various monomers have been prepared using RITP, including acrylates, methacrylates and styrene. The polymers formed using this technique have been characterised by techniques such as SEC, NMR and mass spectrometry (MS). However, very little advanced characterisation work has been done on polymers synthesised via RITP. Polystyrene-block-poly(n-butyl acrylate) (PS-b-PBA) block copolymers were prepared via RITP and the microstructure analysed by in situ NMR and other advanced analytical techniques. The chromatograms from gradient HPLC of the PS-b-PBA block copolymers showed a separation based on chemical composition. The preparation of deuterated polymers via RITP has not been reported in literature. Hydrogenous-polystyrene-block-deuterated-polystyrene (hPS-b-dPS) was synthesised via RITP and analysed using liquid chromatography at critical conditions. An isotopic separation was achieved when critical conditions were established for hydrogenous PS (h-PS). A separation of the block copolymer from the first block was also achieved under chromatographic conditions where the block copolymer eluted in SEC mode while the first block eluted in LAC mode. The separation according to the block structure was confirmed by two-dimensional liquid chromatography.
- ItemAlternating hetero-arm copolymer molecular brushes as scaffolds for inorganic nano-wires(Stellenbosch : Stellenbosch University, 2013-03) Hadasha, Waled Ajili; Klumperman, Bert; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.ENGLISH ABSTRACT: This study describes the synthesis and self-assembly of hetero-arm molecular brushes (hetero-arm MBs). These MBs consist of two polymeric side chains (SCs) of different natures, alternatingly distributed along the main chain (backbone). Two different types of hetero-arm MBs were prepared: first, alternating amphiphlic hetero-arm MBs (AMBs), and second, alternating hetero-arm MBs (AHMBs). Hetero-arm AMBs were synthesized via two strategies: (I) via a combination of “grafting through” and “grafting onto”, and (II) via “grafting through”. In approach (I), poly[vinyl benzyl (polyethylene glycol)-alt-N-alkyl-maleimide)] (poly[VB-(PEG12)-alt-N-(CnH2n+1)-MI]) was prepared via radical copolymerization of vinyl benzyl-terminated polyethylene glycol (VB-PEG12) with maleic anhydride (MAnh) (grafting through), which produces graft copolymers with PEG SCs and reactive succinic anhydride repeat units alternatingly distributed along the backbone. These graft copolymers were then modified by nucleophilic substitution (imidization) with alkyl amines (CnH2n+1-NH2) on the succinic anhydride residues (grafting onto). Three different primary amines possessing different alkyl chain lengths (n = 4, 12 and 16) were used in the modification process. In this way, hetero-arm AMBs with different hydrophilic to hydrophobic ratios were obtained. In approach (II), similar hetero-arm AMBs were prepared in a one-step grafting through approach. In this case, poly[VB-(PEG17)-alt-N-(CnH2n+1)-MI] (n = 10, 16 and 20) was prepared via radical copolymerization of VB-PEG17 with N-dodecylmaleimide, N-hexadecylmaleimide and N-icosylmaleimide. Following the synthesis step, self-assembly of these hetero-arm AMBs in arm-selective solvents was investigated in relation to the alkyl chain length. The morphology of the obtained assemblies was characterized by Field Emission gun-Scanning Electron Microscopy (FE-SEM), Transmission Electron Microscopy (TEM) and Fluorescence Microscopy (a fluorescent dye was encapsulated). Cylindrical-like aggregates, length 10 μm and diameter ~ 250 nm, were obtained upon hetero-arm AMBs self-assembly. The second type of hetero-arm MBs was hetero-arm AHMBs, in which the SCs consist of PEG and poly(N,N-dimethylamino-ethyl methacrylate) (PDMAEMA). These hetero-arm AHMBs were prepared via a combination of grafting through and grafting from approaches. In this case, poly[vinyl benzyl-(polyethylene glycol)-alt-N-(poly(N,N-dimethylamino-ethyl methacrylate) maleimide] (poly[VB-(PEG17)-alt-N-(PDMAEMA)-MI) was prepared in the following steps: (1) alternating poly[vinyl benzyl-(polyethylene glycol)-alt-N-(4-hydroxyphenyl) maleimide] (poly[VB-(PEG17)-alt-N-(HPh)-MI]) was synthesized via radical copolymerization of VB-PEG17 with N-(4-hydroxyphenyl) maleimide (N-HPhMI). (grafting through), (2) the hydroxyl sites were esterified with 2-bromoisobutyryl bromide to afford poly [vinyl benzyl-(polyethylene glycol)-alt-N-(4-(2-bromobutyryloxy)phenyl) maleimide] (poly[VB-(PEG17)-alt-N-(BrPh)-MI]) (macroinitiator) and (3) an atom transfer radical polymerization (ATRP) reaction of 2-(N,N-dimethylamino)ethyl methacrylate (DMAEMA) was initiated from the obtained macroinitiator. This approach afforded poly[VB-(PEG17)-alt-N-(PDMAEMA)-MI] hetero-arm AHMBs with two water soluble SCs; however, one is water soluble at all pHs and temperatures (i.e. PEG), while the other is a pH- and temperature-sensitive polymer (i.e. PDMAEMA). Initial attempts were made to fabricate cylindrical organo/silica hybrid materials based on these AMHBs as the organic template and tetra-ethylorthosilicate as the silica precursor. Preliminary results indicate the formation of silica nano-wires, ~ 8 μm in length and ~45 nm in diameter. The self-assembly behavior of these AHMBs in water at a temperature above the lower critical solution temperature of PDMAEMA (> 55 °C) was also investigated. Fibril morphology (~ 30 nm in diameter) was observed. This study addresses initial attempts to fabricate organic/inorganic hybrid materials with controlled size and morphologies via densely grafted hetero-arm molecular brushes.
- ItemAmine end-functional poly(N-vinylpyrrolidone) as amMacroinitiator for L-lysine N-carboxyanhydride polymerization - towards the preparation of pH-responsive micelles for drug delivery(Stellenbosch : Stellenbosch University, 2015-03) Ilchev, Alexander; Klumperman, Bert; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.ENGLISH ABSTRACT: Cancer is a notorious affliction that knows no age, gender, ethnic, racial, or species bounds and is responsible for over 14% of annual worldwide human deaths. There is no universal cure and the treatments that exist have poor probabilities of success. Chemotherapy is often considered the staple for cancer treatment as it can enter areas of the body that are unsafe for surgery and can treat tumors that are too small to be detected, even with modern imaging techniques. However, chemotherapy can induce many harmful and fatal side-effects. It can also lose its therapeutic effect if the cancer mutates to become multi-drug resistant. These shortcomings can be linked to the poor selectivity and pharmacokinetics of conventional chemotherapy drugs. Modern research focusses on improving these aspects of existing chemotherapy regimens through the incorporation of drug delivery principles. This dissertation focusses on the development of a novel, polymeric, pHresponsive drug delivery system for chemotherapy that incorporates the chemotherapeutic drug as well as a cell-penetrating peptide in a prodrug formulation. The system was designed to inhibit the release of its components into healthy tissues while selectively accumulating, through the enhanced permeability and retention effect, and releasing its payload, through reversible hydrolysis of imine bonds, within tumor tissues. Poly(L-lysine) was chosen as the cell-penetrating peptide since it is able to form imine bonds through its ε-amine functional groups on its residues. It was prepared by the primary amine-initiated ring-opening polymerization of Nε-(benzyloxycarbonyl)-L-lysine Ncarboxyanhydride at 0 °C and pressures lower than 1 mbar as these conditions allow for a controlled, living polymerization to occur. The benzyloxycarbonyl end-group was removed by acidolysis with HBr in a mixture of dichloromethane and 1,4-dioxane at 0 °C. The initiator used for the ring-opening polymerization was poly(N-vinylpyrrolidone) with a primary amine end-group, prepared by RAFT-mediated polymerization with O-ethyl-S-(phthalimidylmethyl)xanthate. This RAFT agent was shown to display slow pre-equilibrium kinetics which was linked to the lower relative stability of the phthalimidylmethyl radical compared to the poly(N-vinylpyrrolidone) propagating radical. The pre-equilibrium and main equilibrium kinetics of the RAFT polymerization were optimized by performing the polymerization in semi-batch mode. During the semi-batch polymerization, the monomer to RAFT agent ratio could be controlled by adjusting the monomer feed. This ratio was shown to be inversely proportional to the probability of radical transfer from the propagating radical to the phthalimidylmethyl radical. The phthalimide end-group could be converted to a primary amine both by reacting with hydrazine in methanol as well as reduction with sodium borohydride in water followed by hydrolysis in 1 M HCl at 60 °C. Doxorubicin and benzaldehyde could be conjugated, via imine bonds, to the poly(N-vinylpyrrolidone-block-L-lysine) copolymer spontaneously in methanol. Both types of conjugates would self-assemble into micelles when dispersed in water. However, the Doxorubicin conjugates were unstable, precipitating out of solution within 24 hours. The benzaldehyde conjugates were stable in water for over 24 hours. This suggested that a formulation of Doxorubicin and benzaldehyde conjugated to the block copolymer may be sufficiently stable under the physiological conditions of blood plasma.
- ItemAnalysis of complex tannins by multidimensional techniques(Stellenbosch : Stellenbosch University, 2019-03) Venter, Pieter; De Villiers, Andre J.; Pasch, Harald; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.ENGLISH ABSTRACT: Hydrolysable tannins are plant derived (poly)phenolics which for centuries have been used in tanning, the transformation of animal hides into leather. In addition to their industrial applications, epidemiological evidence suggests that the consumption of hydrolysable tannins can be linked to disease prevention and treatment, although their mode of action at a molecular level is still unknown. In order to investigate their biological and chemical properties, the determination of the molecular composition of hydrolysable tannins in plant material is a critical step. Liquid chromatography hyphenated to mass spectrometry (LC-MS) is the preferred analytical approach to obtain this information, although conventional methods often provide insufficient performance for highly complex mixtures of hydrolysable tannins. The goal of the work reported in this thesis was, therefore, to develop improved multidimensional methods for the analysis of hydrolysable tannins. In the first part of the study, a methodology for the comprehensive analysis of ellagitannins and gallotannins in chestnut and tara using reversed phase liquid chromatography (RPLC) and hydrophilic interaction chromatography (HILIC) combined with ultraviolet (UV), ion mobility (IM) spectrometry and high-resolution mass spectrometry (HR-MS) detection is reported. Good chromatographic performance was achieved using both HILIC and RP-LC for chestnut tannins, with the complementary separation mechanisms proving useful for particular compound classes also pointing to the potential of the combination of HILIC and RP-LC in a comprehensive two-dimensional separation. In contrast, RP-LC provided much better separation of tara gallotannins than HILIC. Incorporation of IM into the LC-MS workflows offered several advantages for such non-targeted screening analyses, including improved mass spectral quality by filtering MS data according to IM arrival time, the availability of an additional identification criterion in the form of collisional cross section (CCS) values obtained from arrival times, and an improved MS duty cycle and, therefore improved sensitivity. Furthermore, additional isomeric species were resolved by IM and, in combination with UV spectral data, IM provided a simple methodology to differentiate between positional isomers. In the second part of the study, an on-line comprehensive two-dimensional HILIC×RPLC method was developed for the analysis of hydrolysable tannins by kinetic optimisation of the relevant experimental parameters. The use of a weak make-up flow to dilute the HILIC effluent and avoid second dimension injection band broadening resulted in excellent performance, with practical peak capacities in excess of 1000 determined for both samples. Despite on-column dilution and the short second dimension analysis times ultimately limiting the number of compounds detected in chestnut and tara, the group-type separations obtained show promise for the fast fingerprint analysis of hydrolysable tannins. Building on previous findings, the final part of the work evaluated the potential of IM spectrometry as a third dimension in a comprehensive three-dimensional HILIC×RPLC×IM configuration. Apart from the inherent advantages of IM alluded to above, the additional separation step offered by IM contributed to an increase in practical peak capacity by a factor of 13 when integrated into the LC×LC workflow. Certain instrumental constraints which limit the ultimate performance of LC×LC×IM-HR-MS are highlighted. Despite these, this approach shows significant promise for the high resolution separation of highly complex mixtures.
- ItemAnalysis of vegetable oils, seeds and beans by TGA and NMR spectroscopy(Stellenbosch : University of Stellenbosch, 2011-03) Retief, Liezel; Koch, Klaus R.; McKenzie, Jean; University of Stellenbosch. Faculty of Sciences. Dept. of Chemistry and Polymer Science.ENGLISH ABSTRACT: Due to the commercial, nutritional and health value of vegetable oils, seeds and beans, the analysis of their components is of much interest. In this dissertation oil-containing food products, specifically vegetable oils, seeds and beans, were investigated. Selected minor components of three locally produced vegetable oils, namely apricot kernel, avocado pear and macadamia nut oils were investigated using 31P NMR spectroscopy. These minor components, including 1,2 diacylglycerols, 1,3 diacylglycerols and free fatty acids, were identified in the 31P NMR spectra of each of the three vegetable oils for the first time. Two approaches were used for the quantification of the minor components present in the spectra. A calibration curve approach used known concentrations of standard minor components to establish calibration curves while a direct correlation approach calculated the unknown concentration of minor components in the vegetable oils using a known amount of standard compound within the analysis solution. These approaches aided in determining the concentration of minor components during storage studies in which vegetable oils were stored in five different ways: exposed to light, in a cupboard, in a cupboard wrapped in tin foil, at -8 °C and at 5 °C. It was found that determining the best storage condition for each oil was difficult since individual minor components were affected differently by the various storage conditions. However, in general the best storage conditions appeared to be 5 °C and -8 °C. The oil, protein and carbohydrate contents of sesame, sunflower, poppy, and pumpkin seeds, and soy, mung, black and kidney beans were analysed by thermogravimetric analysis and 13C NMR solid state NMR spectroscopy. It was shown that the first derivative of TGA data for seeds and beans can give valuable information about the carbohydrate, moisture, protein and fat content. This has not been previously demonstrated. For the seeds, the integration of a region between 270–480 ºC was equal to the sum of the oil and protein content and compared well to quantitative results obtained by other conventional methods. For beans the integration of a region between 180-590 ºC, gave a value which represented the sum of the oil, protein and carbohydrate content. 13C solid state NMR spectroscopy, including SPE-MAS, CP-MAS and variable contact time experiments, was carried out on these seeds and beans and gave valuable information on the solid-like and liquid-like components. To our knowledge these seeds and beans have never been previously analysed using this technique. 13C SPE-MAS NMR spectroscopy indicated that the seeds contained more liquid-like components than the beans. In turn the 13C CP-MAS NMR spectra indicated that beans had higher levels of solid-like components than the seeds. These conclusions correlated well with the quantities of liquid-like components and solid-like components that were determined by conventional methods and TGA. Preliminary studies using T1pH experiments on the components present in the seeds and beans led to a few observations. Most interesting is that a model using a two- phase fit in order to determine T1pH values appears to be more accurate than a one-phase model.
- ItemApplicatins of liquid chromatography-tandem mass spectrometry to wine analysis : targeted analysis and compound identification(Stellenbosch : Stellenbosch University, 2012-12) Alberts, P.; Stander, M. A.; De Villiers, A. J.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.ENGLISH ABSTRACT: The wine industry is an important sector of agriculture and wine analysis forms the basis of assessing compliance of its commodities with regulatory standards and research in this field. Liquid chromatography (LC) is extensively used for the determination of a wide range of nonvolatile wine components, but conventional detectors impose performance limitations on the technique that prevents its application to sophisticated analytical problems. In particular, conventional detectors for LC often lack the sensitivity and specificity for the determination of many wine compounds, especially trace level analytes, and furthermore, do not possess spectral capabilities for compound identification or structure elucidation. The hyphenation of mass spectrometry (MS) to LC has led to the introduction of a range of detectors that confers high levels of sensitivity and selectivity to the technique. In addition, a wide variety of MS architectures are available that are inherently suited for targeted analysis or structure elucidation studies. In this dissertation, the potential benefits of liquid chromatography – tandem quadrupole mass spectrometry (LC-MS/MS) to solve analytical problems relevant to the wine industry are explored. LC-MS/MS is a particularly versatile analytical technique because both mass analysers can be operated in full-spectrum mode or selected-ion monitoring, which, together with optional fragmentation, gives rise to four modes of operation that may be used for highly specific and sensitive targeted analysis or spectral investigations. In multiple reaction monitoring (MRM) mode, both analysers are set at single ion frequencies specific for the compound under investigation and one or more of its product fragments, respectively. MRM mode is ideally suited for trace level analysis in complex mixtures, even in cases where the target components are not resolved from interferences. In this study, MRM detection was used to solve challenges relevant to the wine industry for the selective quantitation of target analytes that could not be analysed by conventional LC methods. The application of this approach for the analysis of natamycin, ethyl carbamate (EC) and 3-alkyl-2- methoxypyrazines (MPs) in wine is demonstrated. Natamycin is an antimicrobial preservative that is not permitted in wine in the European Union. A rapid and sensitive method for the determination of natamycin was developed, and has been used since 2009 to regulate this vitally important sector of the South African wine export industry. EC is a natural carcinogen that occurs at trace level amounts in alcoholic products. It also has the potential to accumulate in wines and can occur in very high concentrations in some fruit brandies. The determination of EC is complicated by its physicochemical properties, and available analytical methods suffer from drawbacks such as the requirement for elaborate extraction procedures and high solvent consumption. A novel method for the determination of EC in wines, fortified wines and spirits is described and it was applied to perform an audit of the South African industry as well as to investigate factors responsible for its accumulation in alcoholic beverages. This work forms an integral part of the food safety mandate of the State and it ensures that export products comply with international norms for trade. MPs are ultra-trace-level aroma compounds that contribute to the varietal character of Sauvignon blanc wines. Their analytical determination is challenging due to their low levels of occurrence. The loading capacity of LC combined with the sensitivity and resolving power of MS was exploited to analyse concentrated extracts, in order to achieve very low limits of detection. The performance of the LC-MS/MS method enabled the quantitation of these compounds at their natural levels of occurrence, including the first quantitation and spectral confirmation of 3- ethyl-2-methoxypyrazine in wine. Extensive data pertaining to South African Sauvignon blanc wines are reported and statistical analysis is performed, reporting the correlation of variables such as vintage and origin as well as wine parameters such as malic acid with wine MPs. Furthermore, the application of LC-MS/MS for structural elucidation and screening of target classes of analytes was demonstrated for the analysis of red wine anthocyanins. The anthocyanidin-glycosides are responsible for the colour of red grapes and wine, contribute to the sensory properties of wine, and are also of interest due to their beneficial biological properties. Their determination is complicated by their large numbers and structural diversity, further exacerbated by diverse reactions during wine ageing as well as the lack of reference standards for most members of this class of compounds. Tandem MS in scan mode was used for the highly selective detection of glycosylated anthocyanins and derivatives, exploiting the predictable elimination of the sugar moiety in neutral loss mode. Concurrent survey scan experiments were used to unambiguously identify neutral loss detected compounds. The method therefore follows a simplified and structured approach for unambiguous peak identification based on elution order and mass spectral information to impart a high level of certainty in compound identification. In summary, the work presented in this dissertation demonstrates that LC-MS/MS is a versatile and powerful analytical approach for the analysis of diverse compounds of relevance to the wine industry. The sensitivity and specificity of MRM mode, and the selectivity and spectral capabilities of neutral loss and survey scan modes of MS/MS detection, is amply demonstrated by the applications presented in the dissertation.
- ItemApplication of modern analytical techniques and chemometric methods to the chemical characterisation of South African wines : determination of non-volatiles(Stellenbosch : Stellenbosch University, 2004-12) De Villiers, A. J. (Andre Joubert); Sandra, P. J. F.; Crouch, A. M.; Stellenbosch University. Faculty of Science . Dept. of Chemistry & Polymer Science.ENGLISH ABSTRACT: The present study deals in the first instance with the improvement of current analytical techniques for the analysis of the non-volatile content of wines. An improved sample preparation method, using solid phase extraction (SPE), was initially developed for the analysis of organic acids, sugars and phenolic compounds. Consequently, modem analytical methodologies were assessed to obtain optimal techniques for the separation of various non-volatile compounds. A capillary electrophoresis (CE) method, demonstrably more reliable than currently used high performance liquid chromatography (HPLC) and CE methods, is proposed for the analysis of organic acids. HPLC with refractive index (RI) or evaporative light scattering detection (ELSD) proved more suitable than CE for the analysis of sugars in dry wines. Liquid-chromatography-mass spectroscopy (LC-MS) offered superior sensitivity and resolution compared to the relatively new technique of CE-MS for the analysis of wine phenolics. LC-MS was further applied for the efficient and sensitive analysis of non-coloured phenolics and anthocyanins in wine. Negative- and positive electrospray ionisation, respectively, were used in conjunction with an ion-trap mass analyzer, for the identification of 34 phenolics and 31 anthocyanins in red wine samples. Complementary CE and LC methods were developed to allow the identification of artificial dyes in red wines, added illegally to improve their colour. Also, the application of stir bar sorptive extraction (SBSE) with liquid desorption and micellar electrokinetic chromatography (MEKC) for the analysis of bitter acids in beer is reported. In the second part of the thesis, the analytical results obtained for South African red and white wines were evaluated. Following comparison of the results with literature reports, several pattern recognition techniques were employed. A classification function obtained by linear discriminant analysis (LDA) was used to classify both red and white wines according to variety, based on their chemical composition. This classification is achieved independent of the factors of wine age or geographical origin, making it useful for authenticity evaluation.
- ItemApplication of modern chromatographic technologies for the analysis of volatile compounds in South African wines(2009-12) Weldegergis, Berhane Tekle; Crouch, A. M.; De Villiers, A.; University of Stellenbosch. Faculty of Science. Dept. of Chemistry and Polymer Science.ENGLISH ABSTRACT: The present study was initiated by the wine industry of South Africa to overcome the lack of available information on the flavor and aroma of South African wines. The aim was to develop new analytical methods and improve existing ones for the analysis of volatile compounds in the South African wines. Initially a new analytical method based on stir bar sorptive extraction (SBSE) in the headspace mode for the analysis of 37 pre-selected volatile compounds was developed and validated. Consequently, the method was improved by making important modifications and increasing the number of compounds analyzed to 39. This method was successfully applied to a large number of Pinotage wines of vintages 2005 and 2006. The quantitative data of these wines were subjected to chemometric analysis in order to investigate possible co- /variances. A clear distinction was observed between the two vintages, where the 2005 wines were more characterized by wood-related compounds and the 2006 wines by the fermentation compounds. The developed method was further applied to other cultivars of vintage 2005, including two white (Sauvignon Blanc and Chardonnay) and three red (Shiraz, Cabernet Sauvignon and Merlot) cultivars. In a similar fashion, the quantitative data of the six cultivars of vintage of 2005 were analysed by chemometric methods. Significant differences were observed between the two white cultivars and among the four red cultivars. It was shown that among these cultivars, the major role-players were the wood and fermentation related volatiles. A striking observation was the confirmation of the unique character of the Pinotage wines compared to the other red cultivars, mainly influenced by the high level of isoamyl acetate and low level of isoamyl alcohol, the former being categorized as a varietal compound for Pinotage expressed by a fruity (banana) odor. In addition, advanced chromatographic technology in the form of comprehensive twodimensional gas chromatography (GC × GC) coupled to time-of-flight mass spectrometry (TOFMS) was investigated for the detailed analysis of volatile compounds in young South African wines. This work focused primarily on Pinotage wines. In the first instance, solid phase micro extraction (SPME) in the headspace mode in combination with GC × GC-TOFMS was used. Due to the high resolution and large peak capacity of GC × GC, more than 200 compounds previously reported as wine components were identified. These compounds were dominated by the highly volatile and less polar compounds, mainly due to the characteristics of SPME. In an attempt to further extend these results, another selective extraction method, solid phase extraction (SPE) was used in combination with GC × GC-TOFMS analysis. Using this technique, more than 275 compounds, most of them unidentified using the previous method, were detected. These groups of compounds include volatile phenols, lactones as well as mostly aromatic esters and norisoprenoids, which can potentially influence the aroma and flavor of wine. The techniques developed as part of this study have extended our knowledge of the volatile composition of South African wines.
- ItemApplications of superparamagnetic nanoparticles for the separation and recovery of PGM metals from acidic wastewater solutions(Stellenbosch : Stellenbosch University, 2018-12) Khutlane, Joyce Tsepiso; Malgas-Enus, Rehana; Koch, Klaus R.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.ENGLISH ABSTRACT: South Africa is one of the leading countries in the production of precious metals (Au and PGMs). The refinery final stages for recovering precious metals makes use of two hydrometallurgical processes: solvent extraction and extraction with ion exchangers. Even though both methods improved the recovery of these metals from the ore, they still suffer from setbacks ranging from large amounts of secondary waste solutions, expensive solvents and resins, and filtration in terms of the use of ion exchange resins for extraction. Therefore, a need for the development of simple and environmentally friendly refining processes is required to minimize the recurrence of the abovementioned setbacks. Naked magnetic iron oxide nanoparticles (MIONs) are promising materials for adsorption studies of metal ions/complexes from wastewater due to their versatile magnetic properties, which allow a facile remote control, separation and analyte recovery from solution. This work makes use of magnetite nanoparticles (MIONs) which are superparamagnetic and less expensive compared to adsorbents used by the South African refineries for extraction of Au(III)-Cl, Pd(II)-Cl and Pt(IV)-Cl complexes from acidic aqueous solutions. Synthesis of naked MIONs was carried out by using the chemical co-precipitation method. Naked MIONs successfully sorbed Au(III), Pd(II) and Pt(IV) complexes from acidic aqueous solutions, showing a higher adsorption affinity for Au(III)-Cl species compared to Pd(II) and Pt(IV) species. The adsorption kinetics of the three metal complexes using naked MIONs followed a pseudo-second-order kinetic mode, indicating that the chemical adsorption was the rate-limiting step. The equilibrium adsorption of Au(III)-Cl species onto naked MIONs at pH 1.0, pH 3.0 and pH 5.0, was fitted with Langmuir adsorption isotherms. It was found that the experimental data was in reasonably good agreement with the Langmuir model, suggesting that Au forms a monolayer coverage on the surface of naked MIONs. The adsorption capacities were as follows: 10.44 mg.g-1, 18.98 mg.g-1, and 27.25 mg.g-1 respectively. The proposed mechanism responsible for adsorption of Au(III), Pd(II) and Pt(IV) complexes onto naked MIONs was governed by the electrostatic attractions and metal reduction. These naked MIONs were found to be unstable at pH1, which is usually the conditions used for extraction in the mining industry. To circumvent this problem, the surface of MIONs was functionalised with aliphatic carboxylic acids, dendrimer micelles and then with both carboxylic acids and dendrimer micelles. Characterisation of naked MIONs and functionalised MIONs was carried out by IR, PXRD, FESEM, HRTEM and ICP-OES. Competitive adsorption of Au(III), Pd(II), and Pt(IV) complexes onto naked MIONs and functionalised MIONs was explored by varying solution pH and the contact time. The dendrimer micelles played a vital role in adsorption of Pd(II) and Pt(IV) complexes. Adsorption kinetics followed a pseudo-second-order kinetics model. The adsorption isotherms all obeyed the Langmuir model in the case of Au(III), Pd(II), and Pt(IV) complexes by naked MIONs and, adsorption isotherm for Au(III) and Pd(II) complexes using modified MIONs obeyed Langmuir, while adsorption of Pt(IV) species followed the Freundlich model. The desorption studies showed that the best desorption reagents were 1.0 M HNO3 and 1.0 M HNO3/0.5 M thiourea. The modified MIONs were stable at low pH, even at pH1, and the extraction potential of the modified MIONs were comparable to that of the naked MIONs.
- ItemBiodegradable polymeric prodrugs for the delivery of antimalarial combination therapy(Stellenbosch : Stellenbosch University, 2019-12) Fortuin, Lisa; Klumperman, Lubertus; Pfukwa, Rueben; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.ENGLISH ABSTRACT: The design and synthesis of a novel polymeric prodrug was achieved for the synergistic antimalarial drug combination of artemether and lumefantrine. To this end, a lumefantrine-conjugated amphiphilic block copolymer (BCP) was self-assembled into micelles with artemether entrapped within the hydrophobic core of the micelles. To develop this, different acid-labile linkages were investigated for their ability to selectively cleave under the low pH conditions of the parasitophorous food vacuole within a Plasmodium-infected red blood cell and remain stable in the neutral pH conditions of serum. The β-thiopropionate ester, amide and maleimide linkages as well as the acetal linkage were studied using 1H NMR spectroscopy-based kinetic analysis, by synthesising seven model drug compounds, where the model drug (aromatic group) was linked to RAFT-made poly(N-vinylpyrrolidone) (PVP). The model system based on the acetal linkage had a t12⁄ of 107 h at pH 5.0, and remained stable at pH 7.4, providing the impetus for its use in the synthesis of the antimalarial polymeric prodrug. The parent BCP of PVP-block-poly(δ-valerolactone) (PVP-b-PVL) was explored and the synthesis optimised to aid in the development of PVP-block-poly(α-allylvalerolactone) (PVP-b-PAVL), used as the polymer carrier in the drug delivery system. To achieve this, hydroxy-functional PVP was applied as the macroinitiator in the ring-opening polymerisation of δ-valerolactone or its allylated derivative, where triazabicyclodecene was found to be the more effective organocatalyst compared with diazabicycloundecene and cocatalyst thiourea. The crystallisation-driven self-assembly (CDSA) behaviour of PVP-b-PVL was studied in detail where spherical micelles, cylindrical micelles and lamellar micelles were observed using TEM analysis. A novel method to induce micellar morphological transitions was developed in the form of the freeze-thaw process. Lumefantrine was conjugated to the BCP in a ‘click’-type reaction via an acetal linkage, after PVP-b-PAVL was carboxylated and functionalised with EGVE. Artemether was physically entrapped within the core of spherical micelles (114 nm in diameter from DLS) using the cosolvent technique with THF as the non-selective solvent and water as the selective solvent for the hydrophilic block, PVP. The targeting ligand, a low molecular weight peptide with sequence ‘GSRSKGT’ was modified to bear thiol groups using succinimidyl 3-(2-pyridyldithio)propionate (SPDP) modification (quantitative) and it was bioconjugated to the BCP prodrug micelles using disulfide exchange (12 ± 3 mol%). The prodrug micelles had a drug loading content of 20.5 wt.%, a critical micelle concentration of 2.0 10-3 mg·mL-1 indicating thermodynamic stability, and remained stable in a simulated physiological environment for up to four days. Negligible haemolysis to RBCs and non-cytotoxicity to HepG2 cells were found. The prodrug micelles can be classified as having good activity towards P. falciparum parasites; however, they exhibit a higher IC50 value of 1021.8 ± 270.9 nM compared with the free drug combination. The developed drug delivery system is robust, tuneable and non-toxic to healthy cells, indicating that it holds potential for its application and further development in therapeutic nanosystems.
- ItemBlock and graft copolymers as flocculants for wastewater treatment(Stellenbosch : Stellenbosch University, 2018-03) Narsingh, Udi; Klumperman, Lubertus; Pfukwa, Rueben; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.ENGLISH ABSTRACT: Water pollution, due to toxic metals and organic compounds, remains a serious environmental problem. Many of the contaminants found in industrial and municipal wastewaters are known to be toxic or carcinogenic. Heavy metals are not biodegradable and tend to accumulate in living organisms, causing various diseases. The treatment of these wastewaters involves, among other steps, flocculation to remove, not only the suspended, colloidal particles but also heavy metals and other contaminants that get adsorbed on them. In this research, graft and triblock copolymer flocculants were synthesized to flocculate the suspended solids. For graft copolymerized flocculants, acrylamide was grafted onto cellulose. Cellulose is inexpensive and most abundantly available from plant and microbial resources. Polyacrylamide is a synthetic polymer that is polar and widely used for the treatment of wastewaters. Acrylamide was grafted onto cellulose using a conventional chemical free-radical initiation, as well as microwave-assisted and microwave- initiated methods. The synthesis procedure for each of the methods was optimized for percentage grafting using experimental design with response surface methodology. The optimally synthesized graft copolymer, Cell-g-PAM, from each of the synthesis methods was compared with highly recommended polyacrylamide based commercial flocculants for the flocculation of a model kaolinite suspension. The flocculant synthesized via the microwave-assisted method outperformed all flocculants tested. For triblock flocculants, two ABA type triblock copolymers were synthesized using single electron transfer living radical polymerization (SET-LRP). For both copolymers, the central block was poly(ethylene glycol) (PEG). PEG is a hydrophobic, pH insensitive polymer that adsorbs strongly onto colloidal particles. The first outer block was poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA). PDMAEMA is temperature and pH responsive and known to interact strongly with anionic colloidal particles and heavy metals. The second outer block was polyacrylamide. Various molecular weight triblock copolymers were synthesized and used for the flocculation of a kaolinite suspension. The mechanism of flocculation changed from patch to bridging as the length of the flocculants increased. The graft copolymer flocculants performed better than the block copolymer flocculants in the flocculation of kaolinite.
- ItemBonding interactions in congested molecules: a study of the interatomic forces and the molecular electrostatic potential(Stellenbosch : Stellenbosch University, 2022-09) von Berg, Stuart Raymond Colenzo; Dillen, Jan; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.ENGLISH ABSTRACT: The purpose of this research is to apply the quantum theory of atoms in molecules (QTAIM) to the molecular electrostatic potential (MEP) field and use the topology of the MEP to determine whether a stabilising interaction occurs between two hydrogens in a congested molecule. A method for comparing bond strength using the ratio of the nuclear and electronic components of the MEP is developed and applied to the congested molecules. The MEP ratio was used to associate the bond strength of the hydrogen-hydrogen interaction in congested molecules to that of a hydrogen bond between water molecules. Despite this result, analysis of the electron density, laplacian and kinetic energy created an equally compelling argument against the interaction being stabilising.
- ItemCarbene and coordination complexes of gold, palladium, platinum and manganese derived from N-containing heterocycles(Stellenbosch : Stellenbosch University, 2001-12) Deetlefs, Maggel; Raubenheimer, H. G.; Stellenbosch University. Faculty of Science. Dept. of Chemistry & Polymer Science.ENGLISH ABSTRACT: Please refer to fulltext for abstract
- ItemCation-π induced association and nano-structured aggregate formation of water-soluble [PtII(diimine)(Ln-S,O)]+ complexes examined with high-resolution 1H and DOSY NMR : towards understanding their potential antimalarial activity(Stellenbosch : Stellenbosch University, 2013-12) Kotze, Izak Aldert; Koch, Klaus R.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.ENGLISH ABSTRACT: Please refer to full text for abstract
- ItemCharacterization of glycopolymers by asymmetric flow field-flow fractionation(Stellenbosch : Stellenbosch University, 2020-12) Williams, Chelsea Danielle; Pasch, Harald; Lederer, Albena; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.ENGLISH ABSTRACT: An alternative route for studying the interactions between human serum albumin (HSA) and dendronized glycopolymers was implemented by applying a versatile asymmetric flow field-flow fractionation (AF4) technique. Dendronized polymers (DenPols) decorated with maltose moieties, have an abundance of hydrogen bonding sites that isdesirable for interactions with HSA. The difficulty with the characterization of lysine maleimide DenPols and poly(ethylene glycol) (PEG) maleimide DenPols arethe ultrahigh molar masses and the molecular heterogeneity. Here, we aimed to apply the gentle AF4 technique in combination with a refractive index and light scattering detectorsto comprehensively characterize the lysine maleimide DenPols decorated with a maltose shell (MI-G0-MAL –MI-G3-MAL), PEG maleimide DenPol (MI-G1-PEG-MAL), human serum albumin (HSA) and complexes formed by the interaction of DenPols andHSA. An in-depthanalysis of the DenPols,regarding theirmolar mass distribution, radiusof gyration,hydrodynamic radius, dispersity,and molecular architecturewas conducted. The determination of these properties wasexamined using batch mode dynamic light scattering (DLS), size exclusion chromatography (SEC) and AF4. Bothseparation techniques explicitly showed the pronounced aggregation of the DenPols. AF4 showed the DenPols being present assingle macromolecules with arandom coil conformationand aggregatesas elongated (rod-like) or spherical conformations. Complexes formed between DenPols and HSA showed a deviation in the aggregation mechanism compared to the individual DenPols, as the conformation of the non-aggregated and aggregated structures were different. The complexation behaviour between DenPols and HSA displayedaggregated structures of hard dense spherical and swollen moleculararchitectures. MI-G3-MAL with more available hydrogen bonding sites showed significant changes in the conformation when interacting with HSA. The study demonstrated that the multivalentinteractions of DenPols with HSA indicate the tunable aggregation and conformation of DenPols.
- ItemChemical characterisation of the aroma of honeybush (Cyclopia) species(Stellenbosch : University of Stellenbosch, 2010-12) Cronje, Joan Christel; Le Roux, Maritha; Burger, B. V.; University of Stellenbosch. Faculty of Science. Dept. of Chemistry and Polymer Science.ENGLISH ABSTRACT: Honeybush tea, also known as “South Africa’s sweetest tea”, is a herbal tea made from the leaves and twigs of Cyclopia spp., indigenous to the fynbos biome in the Western and Eastern Cape provinces of South Africa. The pleasant sweet aroma and taste of fermented honeybush, its low tannin content and the absence of caffeine have led to widespread interest in the commercial cultivation and processing of honeybush tea since the mid-1990s. Although more than 20 species of honeybush grow in the wild, only a few species are commercially exploited for the manufacture of tea. Currently the more prominent species are C. intermedia, C. subternata, C. genistoides, and C. sessiliflora. The present research contributes to a comprehensive honeybush research programme being conducted at the Agricultural Research Council (ARC) Infruitec-Nietvoorbij in South Africa. The first phase of the present study, using C. genistoides as representative species, was aimed at developing the necessary methodology for the analysis of extremely low concentrations of honeybush volatiles. A high-capacity headspace sample enrichment probe was applied successfully in conjunction with gas chromatography-mass spectrometry (GC-MS) to analyse the volatile organic compounds present in dry or infused unfermented and fermented honeybush. A total number of 255 volatile compounds were identified in unfermented and fermented honeybush, the majority of which are terpenoids (138; 54%) comprising mostly terpenes, terpene ketones, terpene alcohols and terpene ethers. Of the other compound classes, the aldehydes are the largest group, followed by esters, hydrocarbons and ketones. The stereochemistry of the identified compounds was determined whenever possible. This is the most comprehensive chemical characterisation of the volatile compounds in a South African herbal plant reported to date. A comparative study of green and fermented honeybush showed that the same compounds are, to a large extent, present in both, albeit in different relative concentrations. Not all of the identified honeybush volatiles are necessarily odour-active compounds contributing to the overall typical honeybush aroma. An important aspect of this research was thus the identification of the 46 odour-active compounds in fermented honeybush by means of gas chromatography-olfactometry (GC-O), using detection frequency and aroma extract dilution analysis methods. Fifteen of these compounds, mainly terpenoids, were singled out as the most intense individual contributors to the honeybush aroma based on consideration of all the relevant GC-O data. The odours of certain compounds, i.e. (6E,8Z)-megastigma-4,6,8-trien-3-one, (6E,8E)-megastigma- 4,6,8-trien-3-one, (7E)-megastigma-5,7,9-trien-4-one, 10-epi- -eudesmol, epi- -muurolol and epi- - cadinol, were perceived by GC-O assessors as typically honeybush-like. The quantitative GC-MS data of seven different Cyclopia samples (including four different species and variants thereof) were compared with respect to all the volatile components and particularly with respect to the odour-active compounds. Interesting variations were found in the concentrations of certain odour-active compounds in the various samples. The quantitative data obtained for the odour-active honeybush volatiles and data obtained from the sensory analysis of eight Cyclopia samples (including four different species and variants thereof) were subjected to statistical analysis and interesting associations between compounds with certain sensory aroma attributes were established. The present study has made a major contribution to the scientific knowledge regarding one of South Africa’s most popular indigenous herbal teas.
- ItemChemical speciation of RhIII complexes in acidic, halide-rich media by means of 103Rh NMR spectroscopy : the importance of speciation in the selective separation and recovery of rhodium(Stellenbosch : Stellenbosch University, 2013-12) Geswindt, Theodor Earl; Koch, Klaus R.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.ENGLISH ABSTRACT: In this thesis, the recovery of RhIII from both synthetically prepared and authentic industrial PGM-containing solutions was systematically investigated via organic precipitation methods using several commercially available, N-containing organic receptors including (amongst others) diethylenetriamine (Deta), triethylenetetramine (Teta), tetraethylenepentamine (Tepa) and tris(2-aminoethyl)amine (Tren). These organic receptors act as precipitating agents in the presence of an appropriate protonating agent (HCl) by lowering the solubility of the PGM chlorido-anions through an ion-pairing mechanism. The recovery of RhIII from synthetically prepared PGM (RhIII and PtIV) containing solutions using these precipitants was excellent, while poor Rh recovery from authentic industrial process solutions was achieved. The poor Rh recovery from these process solutions was ascribed to the species distribution of the [RhCln(H2O)6-n]3-n complexes. In order to validate the proposition that RhIII speciation effects are responsible for the poor Rh recovery observed during the precipitation studies, attempt were made to describe the species distribution of the [RhCln(H2O)6-n]3-n (n=3-6) by means of high-resolution 103Rh NMR spectroscopy. A detailed high-resolution 103Rh NMR spectroscopic study of the series of [RhCln(H2O)6-n]3-n (n=3-6) complexes was conducted. During this study, all six RhIII aqua chlorido-complexes have unambiguously been characterized by means of high-resolution 103Rh NMR spectroscopy, proving the powerful analytical capability of this technique. Characterization of these complexes is based on the detailed analysis of the 35Cl/37Cl isotope effects which is observed in the 19.11 MHz 103Rh NMR resonances of the [RhCln(H2O)6-n]3-n (n=3-6) complexes in aqueous HCl solutions at 292 K. These resonances show that the “finestructure” of each of the 103Rh resonances may be understood in terms of its unique isotopologue, and in certain cases, the isotopomer distribution of each complex, which manifests as a result of its statistically expected 35Cl/37Cl isotopologue and isotopomer distributions. As a result, the 103Rh NMR resonance structure serves as a unique “NMRfingerprint”, which allows for the unambiguous assignment of [RhCln(H2O)6-n]3-n (n=3-6) complexes, without the reliance on accurate δ(103Rh) chemical shifts. Furthermore, this study reports the first direct species distribution diagram for the [RhCln(H2O)6-n]3-n (n=3-6) series of complexes (in aqueous HCl solutions at 292 K) as a function of the “free” (unbound) chloride concentration, constructed from 103Rh NMR measurements. The need for a revised speciation diagram of [RhCln(H2O)6-n]3-n (n=3-6) complexes is clearly reflected by the vast differences observed in the literature reported species distribution diagrams, which makes it difficult to decide which set of experimental conditions (if any) is required for the quantitative and “selective” recovery of RhIII from aqueous HCl solutions containing associated PGMs (Pt, Pd, Ir, Ru) as well as other transition metals. The documented species distribution diagrams for RhIII have been generally constructed via data from indirect (kinetic and spectrophotometric) measurements using dilute RhIII solutions at relatively high HCl concentrations, which implies that the RhIII:Clmole ratio is higher than what may be expected in authentic process solutions – an important aspect to consider when optimizing RhIII recovery methods. In addition, RhIII kinetic investigations reported in this study shows that ionic strength and temperature effects are important factors that dramatically influences the rate of RhIII ligand exchange (i.e. RhIII aquation reactions) which, in turn, have contributing effects on the species distribution of [RhCln(H2O)6-n]3-n complexes. Notable differences exist between the speciation diagram reported in this study and those documented in literature, especially at a “free” chloride concentration of 1.0 M. At this “free” chloride concentration, the [RhCl5(H2O)]2- complex anion was found to have an abundance of 34%, while literature reports an abundance of 80%. In order to ascertain its practical relevance, the proposed 103Rh NMR speciation method was extended, for the first time, to authentic industrial Rh feed solutions (Anglo Platinum PLC). Each of the 103Rh resonances was unambiguously assigned, and each species quantified. Moreover, the RhIII species distribution of the industrial Rh feed solution was accurately predicted by the “direct” speciation diagram constructed form 103Rh NMR measurements. After careful optimization of the Heraeus industrial feed solutions (optimal chloride concentration followed by thermal treatment for enhancing RhIII chloride anation reactions), the recovery of Rh via precipitation was repeated. In this instance, Rh recovery improved dramatically, with up to 95% of Rh removed from solution. This improvement is ascribed primarily to the increased “free” (unbound) chloride concentration. The presence of associated PGMs as well as other transition metals would lower the effective “free” chloride concentration, since these metals would act as “chloride binders”. By adjusting the total chloride concentration, RhIII chloride anation reactions is enhanced which leads to the [RhCln(H2O)6-n]3-n (n=5,6) complex anions being the dominant species in solution, therefore leading to improved Rh recovery. Moreover, it was shown that, under carefully controlled conditions, “selective” recovery of Rh is achieved using tris(2-aminoethyl)amine (Tren). Considering the fact that Rh is the last precious metal recovered in all PGM refineries, this can possibly provide a cost-effective route for the “upfront” (early stage) recovery of Rh from industrial PGM feed solutions.