A series of five, tetradentate Schiff-base ligands were synthesised and chelated to vanadyl to form oxovanadium(IV) complexes. The ligands, 4,4’-{benzene-1,2-diylbis[nitrilo(1E)phen-1-yl-1ylidene]}- dibenzene-1,3-diol (H2L1), 4,4’-{ethane-1,2-diylbis[nitrilo(1E)phenyl-1-yl-1-ylidene]}dibenzene-1,3-diol (H2L2), 4,4’-{propane-1,2-diylbis[nitrilo(1E)phen-1-yl-1-ylidene]}dibenzene-1,3-diol (H2L3), 4,4’-{(2- hydroxypropane-1,3-diyl)bis[nitrilo(1E)phen-1-yl-1-ylidene]}dibenzene-1,3-diol (H2L4) and 4,4’-{2,2- dimethylpropane-1,3-diyl)bis-[nitrilo(1E)phen-1-yl-1-ylidene]}-dibenzene-1,3-diol (H2L5), characterised by TOF-MS, IR, electronic absorption, 1H and 13C NMR spectroscopy. The ligand H2L5 was also characterised by XRD. The ligands were shown to have a bis-zwitterionic structure in the solid state, and possibly also in solution. Complexes were characterised by Elemental Analysis, TOF-MS, IR, electronic absorption spectra, EPR and 51V NMR spectroscopy. They form mononuclear complexes, with one ligand binding a single vanadyl ion. EPR spectroscopy was performed on both the powdered form and solutions of the complexes. All the complexes displayed axial symmetry, with increasing distortion from an ideal square pyramidal geometry as the size and bulk of the central chelate ring was increased. Isotropic g0 values suggest solvent interaction with the vanadium ion for the coordinating solvent DMSO. Additional distortion on the coordination geometry, presumably from the benzyl groups of the compounds, causes the isotropic hyperfine coupling constants to be greater than expected. Furthermore, the ability of the complexes to bind peroxide species was investigated by following the addition of H2O2 to the complexes using 51V NMR spectroscopy to observe shielding changes at the vanadium nucleus, and 1H NMR spectroscopy to monitor the bulk magnetic susceptibility, via a modified Evan’s NMR method. Similar experiments were done with sodium hydroxide for comparison. As expected, the oxoperoxovanadium(V) complexes were more stable than their progenitor oxovanadium(IV) complexes. Additionally, increasing the distortion from the ideal pseudo square-pyramidal coordination geometry for the vanadyl ion resulted in a greater increase in the apparent stability of the peroxocomplexes. This latter effect is further enhanced by the addition of a hydrogen-bonding group in close proximity to the vanadium nucleus. DFT calculations of the optimized geometries, natural bond orbitals, electronic absorption and infra-red frequencies were performed for both the ligands and the complexes; nuclear magnetic resonance calculations were performed for the ligands as well. The B3LYP/6-311G (d,p) and B3LYP/LANL2DZ level of theories were used for the ligands and complexes respectively, except for electronic transitions, which were calculated using TD-SCF methods for both ligands and complexes. Calculated and experimental results were compared where possible, and showed reasonable agreement for all calculations performed. The exception to this was for the NMR calculations for the ligands, which were poorly simulated. Finally, the in vitro biological activity of the complexes was evaluated for cytotoxicity against the human tumour cell lines: A549, U251, TK-10 and HT29, via an MTT assay. All complexes showed promising anticancer activity, as evidenced by their low IC50 values for the cell lines A549, U251 and TK-10, which are in general, lower than that observed for cisplatin. They did, however, express negligible activity against the HT29 colon adenocarcinoma cell line; showing an apparent selectivity for certain cell lines. These oxovanadium(IV) complexes, thus warrant further evaluation as chemotherapeutic agents. / Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2012.

No description available.

The Cdc25 phosphatases are highly conserved from yeast through humans and play pivotal roles in regulating the activities of cyclin-dependent kinases (Cdks). Cdc25A is one of three human Cdc25 family members, and has previously been shown to be overexpressed in numerous cancers and to transform rodent fibroblasts. Cdc25A therefore represents a rational target for chemotherapeutic development. Further, a thorough understanding of its biology and regulation in normal and transformed cells may facilitate the development of strategies to specifically interfere with the proliferation of cancerous cells. In this work I describe experiments which demonstrate that bisperoxoVanadium compounds, and specifically bpV(Me2Phen), inhibit Cdc25A phosphatase in vitro and in vivo. Further, these compounds cause cell-cycle arrest, are cytotoxic to cancer cells, and slow the growth of tumours in mouse models. With respect to the fundamental biology of Cdc25A, I have identified a sequence element (NLS) responsible for nuclear localization of Cdc25A phosphatase. An analysis of this sequence demonstrated high conservation of flanking phosphoacceptor sites, notably Serine 292. S292 was predicted to be a consensus PKA or CamKII substrate. Using site-directed mutagenesis I have shown that S292 is the sole site of PKA phosphorylation in vitro. The functional importance of S292 phosphorylation was investigated via transfections of phospho-mimetic mutants of S292 (S292E) expressed as GFP-fusion proteins; these studies indicate that S292 phosphorylation may promote nuclear localization. Studies by other groups have indicated that S292 is a phosphorylation site for inhibitory kinases, namely Chk1 and Chk2 (4). I generated a phospho-specific antibody to this site and demonstrate by immunofluorescence and western blotting an unexpected pattern of S292 phosphorylation associated with nuclear bodies and the mitotic apparatus. I provide evidence to suggest that these sites represent local fine-tuning of Cdc25A, allowing Cdk activity to be controlled at the level of specific subcellular structures. These studies highlight the complexity of Cdc25 regulation and indicate a previously unappreciated degree of control of their activity such that these enzymes exist in multiple discrete pools within a given cell.

The growing significance of vanadium in medicinal inorganic chemistry is due to the diverse biological activities of its metal complexes, as elaborated in Chapter 1. These biological activities stem from the fact that vanadium is an essential trace element as well as its ability to form active pro-drugs under physiological conditions. To improve the bio-availability of these potential metallopharmaceuticals, the use of biologically relevant ligand systems such as heterocyclic ligands were considered. These chelators should provide the stability and the ability to promote absorption through cell-membranes. The techniques as described in Chapter 2 were employed to analyze and characterize the formulated heterocyclic ligands and their metal complexes. The attained research findings are mainly divided into two studies which involve the explorative coordination chemistry of two classes of ligands: 2-pyridylbenzimidazole (see Chapter 3) and 2-phenylsubstituted heterocyclics (see Chapter 4). An additional brief study is described in Chapter 5 which discusses the attempted coordination of a uracil Schiff base ligand. In Chapter 3, the coordination behaviour of Hpybz (2-pyridylbenzimidazole) towards vanadium in various oxidation states (i.e. +III/IV/V) was explored. The six-coordinate complex cis- [VVO2(Hpybz)(pybz)] (1) was isolated as the CH3OH.(H2O)2 hydrate from the reaction of NH4VO3 and Hpybz in aqueous methanol. The crystal structure shows that the vanadium is bonded to two cis-oxido ligands, and to the two bidentate ligands pybz and Hpybz. This combination of ligands confers six-coordination on the metal centre, which is a rare coordination number for a mononuclear dioxido complex of vanadium(V). From the reaction between Hpybz and VCl3 the cationic complex salt cis-[VIII(OH)2(Hpybz)2]Cl (2) was formed. The ligands in cis- [VIII(OH)2(Hpybz)2]Cl exhibits the same coordination behaviour as in 1, but instead of the dioxido moiety present in 1, two hydroxyl co-ligands are coordinated to the metal centre, with both chelator ligands neutral. Conductivity measurements in DMF affirmed that the compound is a 1:1 electrolyte. A novel binuclear mixed-valence oxidovanadium compound, (μ- O)[VVO(pybz)2.VIVO(Hpybz)(acac)] (3), was obtained from the reaction of Hpybz with VO(acac)2. ESR analysis illustrates paramagnetic behaviour typical of a type I dimer. The metal compound, VO(Hpbyz)2SO4 (4).H2O was isolated in a good yield from the reaction of two equivalents of Hpybz with vanadyl sulfate. Chapter 4 reports the isolation of oxidovanadium compounds with 2-phenylsubstituted benz(imidazole/othiazole/oxazole) chelators. The 2:1 molar reaction between NH4VO3 and 2- hydroxyphenylbenzothiazole (Hobs) led to the formation of a polynuclear vanadium(IV) complex, [VO(obs)2]n (1). The atmospheric oxygen-induced oxidation reaction of VCl3 and 2- hydroxyphenyl-1H-benzoxazole (Hobo) afforded a similar oxidovanadium compound, [VO(obo)2]n (2). A characteristic eight-line isotropic signal was observed in the ESR spectrum of 2 in DMF while, due to the poor solubility of 1, a singlet was attained upon analysis of the single crystals. A diamagnetic dioxidovanadium(V) complex, cis-[VO2(obz)py] (3) (Hobz = 2- hydroxyphenyl-1H-benzimidazole) was isolated from the reaction of NH4VO3 and Hobz in a methanolic solution. A broad singlet is found in the 51V NMR spectrum at -520.7 ppm for the d0-vanadium centre. The intra-ligand (π-π*) relaxations [466 nm for 1, 376 nm for 2 and 469 nm for 3] could be observed in the emission spectra which were obtained in anhydrous DMF. In an effort to synthesize a coordination compound of vanadium, the reaction of a heterocyclic ligand, 2-mercaptophenyl-1H-benzimidazole (Hsbz) with vanadyl sulfate resulted in an unexpected reaction product, [C26H20N4S2].[SO4].4H2O (4). In Chapter 5, the metal-induced cyclization of 5-amino-6-[(Z)-(2-hydroxybenzylidene)amino]- 1,3-dimethylpyrimidine-2,4-(1H, 3H)-dione (H3duo) by NH4VO3 resulted in the formation of a cyclized benzimidazole derivative, 8-(2-hydroxyphenyl)-1,3-dimethyl-1H-purine-2,6-(3H, 7H)- dione (1). The IR spectra of H3duo and its cyclized form are nearly identical where only minor shifts in the significant bands are observed. The molecular transformation was more evident when comparing the 1H NMR spectra of H3duo and 1. / Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2013.