3. Electronic Theses and Dissertations (ETDs) - All submissions
Permanent URI for this communityhttps://wiredspace.wits.ac.za/handle/10539/45
Browse
3 results
Search Results
Item Synthesis and characterisation of novel [PtII(phen)(Ln-κS)2] complexes: exploring rare monodentate coordination of disubstituted acylthioureato ligands(2019) Kangara, Edmore F.This thesis describes a study directed towards the synthesis and full characterization of novel [Pt(phen)(Ln-κS)2] complexes where disubstituted acylthioureato ligands (Ln), (also referred herein as N,N-di(alkyl/aryl)-N’-acylthioureato ligands), coordinate to the platinum(II) metal centre in a monodentate fashion through the sulfur donor atom and establishing the driving factors behind this rare coordination mode. The study included a systematically chosen ligand library where the ligands varied mainly on the electronic influence of acyl substituents balanced with minor steric variations on the amino end of the ligand. [Pt(phen)(Ln-κS)2] complexes were synthesized in excellent yields via a stepwise method where the ligands were first deprotonated using sodium hydride in anhydrous THF and subsequently reacting the resultant solvated salts with the [PtCl2(phen)] precursor under reflux for one hour in a 1:2.2 (precursor/ligand) stoichiometric ratio. The complexes were characterised using FT-IR (ATR) spectroscopy, 1H- and 13C NMR spectroscopy, high-resolution mass spectrometry, UV-vis spectrophotometry, elemental analysis, and single-crystal X-ray diffraction. The complexes were also probed for their solution conformations in correlation with the solid-state structures obtained through single-crystal X-ray diffraction using variable temperature 1H- and 195Pt NMR spectroscopy in order to gain insight into the persistence of intramolecular π-π stacking interactions in solution as a stabilising factor directing the monodentate coordination. This study showed that [Pt(phen)(Ln-κS)2] complexes with acylthioureato ligands bearing aromatic acyl substituents could at least exist in three conformations while complexes with acylthioureato ligands that have non-aromatic acyl substituents could only have one structural conformation at -50 ° C. However, there is no conclusive evidence of the intramolecular π-π stacking interactions in solution at ambient and higher temperatures. Investigations into whether the electronic effects of acyl substituents could influence the nucleophilicity of the other donor sites in the ligands, particularly oxygen, enough to render their reactivity towards platinum(II) ions more preferable was done using conceptual DFT. The study showed that the sulfur donor atom was tenfold more nucleophilic than any other donor atom in the acylthioureato ligands regardless of acyl substituents making it the most probable site to coordinate with platinum(II) ions. Mechanistic insights into how the reaction proceeded were probed through a series of individual experiments that involved the metal precursor, the ligands, the [Pt(phen)(Ln-κS)2] complexes, the byproduct [Pt(phen)(Ln-κO,S)]+ complexes and acids of different degrees of acidity. These experiments showed that the synthetic reaction proceeded most likely via solvolysis of the precursor resulting in a solvento complex and subsequent anation of the solvento complex with disubstituted acylthioureato anions to get [Pt(phen)(Ln-κS)2]. [Pt(phen)(Ln-κS)2] could also be obtained by reacting the cationic byproduct [Pt(phen)(Ln-κO,S)]+ with the ligand after deprotonation with base. It was also shown that the [Pt(phen)(Ln-κS)2] complexes dissociated under different acids resulting in different dissociation products via protonation-anation reactions. The nature of the dissociation products depended on the strength of the acid and the coordinating properties of the acids’ conjugate bases. This coordination mode could be extrapolated to include platinum(II) complexes bearing other co-ligands like bipyridine and triphenylphosphine, however in medium to low yields. A pilot study into the potential antiproliferative activity of [Pt(phen)(L1-κS)2] and [Pt(phen)(L2-κS)2] (where L1 = N,N-di(ethyl)-N’-(1-naphthoylthioureato) and L2 = N,N-di(butyl)-N’-(1naphthoylthioureato) anions) against an A549 lung cancer line showed that these complexes are very active with IC50 values of 6.4 ± 0.9 and 2.4 ± 0.3 μM respectively, killing cancer cells via apoptosis. This study also revealed that these complexes could potentially interact with DNA as a major groove binder. The knowledge obtained through this study should contribute to the fundamental understanding of the coordination chemistry of disubstituted acylthiourea ligands, chemical and physical properties of various platinum(II) complexes bearing disubstituted acylthiourea ligands coordinated in a monodentate fashion and explore possible applications in the fight against cancer.Item Inhibition of Schistosoma japonicum glutathione transferase by Cibacron Blue: insights from structural, functional and molecular modelling studies(2018) Hlabano, BabongiweSchistosomiasis is a leading neglected tropical disease, caused by blood flukes of the genus Schistosoma. Around 200 million people worldwide are affected, with the majority in Sub-Sahara Africa. Currently, only praziquantel is used for the treatment of schistosomiasis and its exclusive use has led to concerns of rise of praziquantel resistant Schistosomes. There is therefore a need for the development of new anti-schistosomal drugs. Schistosoma species lack the cytochrome P-450 detoxification mechanism, an important mechanism in human detoxification cycle, thus making Schistosoma glutathione S-transferase (GST) one of the main enzyme for detoxification of electrophilic and hydrophobic compounds. Schistosoma japonicum GST (SjGST) is an attractive drug/vaccine target against schistosomiasis. In this study, the mechanism of inhibition of SjGST by Cibacron Blue 3G-A (CB3GA) was investigated. Soluble SjGST was recombinantly expressed and purified successfully to homogeneity. SjGST maintained dimeric structure in the presence of CB3GA. IC50 value of CB3GA was determined to be 100 nM. Michaelis-Manten kinetic studies where performed in the presence and absence of CB3GA and showed that SjGST has high affinity for glutathione compared with CDNB. Lineweaver–Burk plots indicated that CB3GA is an uncompetitive and mixed inhibitor to the G-site and H-site respectively. Induced fit docking predicted that CB3GA binds to the L-site consistent with kinetic inhibition studies. MM-GBSA predicted free binding energy of SjGST and CB3GA was ΔGPred = -310 kJ/mol compared with experimental free energy of binding of ΔGExp = -49 kJ/mol. CB3GA is an efficient inhibitor of SjGST that binds to the dimer interface of SjGST altering catalytic activity of both the G-site and H-site. The unique characteristic of the L-site provides an opportunity for highly specific rational drug design.Item Further elucidating the steroid isomerisation reaction mechanism of GSTA3-3(2017) Robertson, Gary JayGlutathione S-transferase A3-3 is the most catalytically efficient steroid isomerase enzyme known in humans, transforming Δ5-androstene-3-17-dione into Δ4-androstene-3-17-dione. Though its mechanism of action remains unsolved. GSTA3-3 catalyses this reaction with at least ten-fold greater efficiency than GSTA1-1, its closest competitor in the Alpha class of GSTs. In order to examine the differences between Alpha class GSTs and to better elucidate the mechanism of GSTA3-3 the roles of Tyr9 and Arg15 were examined. Tyr9 is the major catalytic residue of Alpha class GSTs and Arg15 is proposed to be catalytically important to GSTA3-3 but never before experimentally examined. While the structure and stability of the Alpha class enzymes are highly comparable, subtle differences at the G-site of the enzymes account for GSTA3-3 having a ten-fold greater affinity for the substrate GSH. Y9F and R15L mutations, singly or together, have no effect on the structure and stability of GSTA3-3 (the same effect they have on GSTA1-1) despite the R15L mutation removing an interdomain salt-bridge at the active site. Hydrogen-deuterium exchange mass spectrometry also revealed that neither mutation had a significant effect on the conformational dynamics of GSTA3-3. The R15L and Y9F mutations are equally important to the specific activity of the steroid isomerase reaction; however, Arg15 is more important for lowering the pKa of GSH. Lowering the pKa of GSH being how GSTs catalyse their reactions. This suggests an additional role for Tyr9, with an important mechanistic implication. Factoring in the inability to detect an intermediate during the reaction, all data are in agreement with the mechanism being concerted and that Tyr9 acts as a proton shuttle. Additionally, there is evidence to suggest that Arg15 is integral to allowing GSTA3-3 to differentiate between Δ5-androstene-3-17-dione and Δ4-androstene-3-17-dione, indicating that Arg15 is a more important active-site residue than previously recognized.