No description available.

published_or_final_version / Chemistry / Master / Master of Philosophy

An empirical approach to the study of phosphine compounds was completed using Molecular Mechanics 2 (MM2), and several computer programs written to descibe and analyze the final geometric orientations of the molecules. The calculations were performed on 64 conformers of 16 different phosphines. Results from these calculations were compared to those previously obtained for MNDO and MINDO/3 calculations, and to experimental data. Cone angles calculated from the MM2 optimized geometries, were compared to Tolman's original work, and to values obtained from semiempirical calculations, and to experimental results. In general, it was found that weighted average cone angles best represent the size of phosphine ligands. / Department of Chemistry

The decapeptide, gonadotropin releasing hormone (GnRH), is the central regulator of reproductive function. It binds to receptors on the gonadotrope cells of the pituitary and stimulates release of luteinizing hormone (LH) and follicle stimulating hormone (FSH). Eleven different structural forms of GnRH have now been identified in various animal species. Chimaeric analogues of some of the variant forms of GnRH were synthesized in order to study the functional significance of the most common amino acid substitutions, which occur in positions 5, 7 and 8. Peptide binding affinities for sheep and rat GnRH receptors and potencies in stimulating LH and FSH release from cultured sheep pituitary cells and LH release from cultured chicken pituitary cells were measured. Histidine in position 5 decreased LH releasing potency in chicken cells, but slightly increased receptor binding affinity in rat and sheep membranes. Tryptophan in position 7 had minimal effect on GnRH activity in mammals, but increased LH release in chicken cells. Although differences in the structural requirements of mammalian and chicken GnRH receptors were anticipated, it was also found that rat GnRH receptors exhibited higher affinity for analogues with Tryptophan in position 7, than did sheep GnRH receptors. Substitutions in position 8 revealed the most marked differences in the structural requirements of mammalian and chicken GnRH receptors. Arginine was required for high GnRH activity in mammalian systems, but analogues with neutral substitutions in position 8 were more potent in chicken pituitary cells. The tolerance of position 8 substitutions, combined with the relatively small effects, in chicken cells, of incorporating a D-amino acid in position 6, indicate that the chicken GnRH receptor is less stringent than mammalian receptors in its recognition of peptide conformation. To examine how changes in ligand structure cause changes in receptor binding affinity and receptor activation, it was necessary to know the structures of the GnRH receptors. A protocol was developed for the purification of GnRH binding proteins from detergent-solubilized pituitary membranes, by affinity chromatography. This procedure yielded a protein which migrated as a single band on sodium dodecyl sulfate polyacrylamide gel electrophoresis, but was different from the recently cloned GnRH receptor. To test the proposal that the arginine residue in mammalian GnRH interacts with an acidic receptor residue, eight conserved acidic residues of the cloned mouse GnRH receptor were mutated to asparagine or glutamine. Mutant receptors were transiently expressed in COS-1 cells and tested for decreased preference for Arg⁸-containing ligands by ligand binding and inositol phosphate production. One mutant receptor, in which the glutamate residue in position 301 was mutated, exhibited decreased affinity for mammalian GnRH. The mutant receptor also exhibited decreased affinity for [Lys⁸]-GnRH, but unchanged affinity for [Gln⁸]-GnRH compared with the wildtype receptor, and increased affinity for the acidic analogue, [Glu⁸]-GnRH. This loss of affinity was specific for the residue in position 8, because the mutant receptor retained hiszh affinity for analogues with favourable substitutions in positions 5, 6 and 7. Thus, the Glu³⁰¹ residue of the GnRH receptor plays a role in receptor recognition of Arg⁸ in the ligand, consistent with an electrostatic interaction between these two residues. The Glu³⁰¹ and Arg⁸ residues were not required for the high affinity interactions of conformationally constrained peptides. This indicates that an interaction which involves these two residues may induce changes in the conformation of GnRH after it has bound to the receptor.

In this study, attention has been given to the design and synthesis of novel multidentate ligands for use in the construction of ruthenium-based metathesis catalysts, and their chelating potential has been explored by computer modelling at the Density Functional Theory (DFT) level. Both Kemp’s triacid (1,3,5-trimethyl-1,3,5-cyclohexanetricarboxylic acid) and D-(+)-camphor have been investigated as molecular scaffolds for the development of such ligands. However selective elaboration of the functional groups in Kemp’s triacid proved difficult to achieve, and the research has focused on the development of camphor derivatives. The synthesis of the camphor-based ligands has involved C-8 functionalisation and ring-opening of the bicyclic system to afford tridentate products. The formation of 9-iodocamphorquinone bis(ethylene ketal) together with the desired product, the 8-iodo isomer, has been confirmed by single crystal X-ray analysis of both compounds. Formation of the 9-iodo analogue has provided new insights into the intramolecular rearrangement of camphor skeleton, and the mechanistic implications have been assessed by coset analysis. Attempts to effect nucleophilic displacement of the 8-halogeno groups by nucleophilic donor moieties proved unexpectedly difficult and, coupled with the susceptibility of the carbonyl groups to nucleophilic attack, has led to the formation of novel tricyclic products, viz., 1,6-dimethyl-3-(2-pyridylamino)-4-oxatricyclo[4.3.0.0[superscript 3,7]]-2-nonanone and 6,7-dimethyl-3-(2-pyridylamino)-4-oxatricyclo -[4.3.0.0[superscript 3,7]]-2-nonanone. However the diphenylphosphine group was successfully introduced at C-8 and oxidative ring-opening of the camphor skeleton has afforded the tridentate ligands, 2-(diphenylphosphinoylmethyl)-1,2-dimethyl-1,3-cyclopentanedicarboxylic acid and 2-(diphenylphosphinoylmethyl)-1,3-bis(hydroxymethyl)1,2-dimethylcyclopentane. One- and two-dimensional NMR and, where appropriate, high-resolution MS methods have been used to characterise the products. Three [superscript 13]C NMR chemical shift prediction programmes, viz., ChemWindow and the MODGRAPH neural network and HOSE (Hierachially Ordered Spherical description of Environment), have been applied to representative compounds to assess their efficacy. While the predicted shifts correlated reasonably well with the experimental data, they proved to be insufficiently accurate to differentiate the isomeric systems examined.

Several series of ligands, designed to chelate silver(I) specifically in the presence of base metals, have been synthesised. The ligands include: - dithiodiamide compounds, prepared by the condensation of acetanilide derivatives with 1,2-dibromoethane; propanenitrile and propanoic ester derivatives prepared from pyridine-2-carbaldehyde via the Morita-Baylis-Hillman reaction; and novel malonamide ligands from the reaction of diethyl malonate with a range of primary amines. The malonamide derivatives were prepared under both conventional thermal and microwave-assisted conditions, the latter proving to be highly efficient. The ligands were all characterised using a combination of spectroscopic and, where appropriate, elemental analysis; in one case, the structural assignment was confirmed by single-crystal X-ray analysis. The fragmentation patterns in the electron-impact mass spectra of the malonamide derivatives have been explored using high-resolution and meta-stable peak scanning techniques. Complexes of the malonamide ligands with copper(II) and silver(I) have been synthesised, and examination of these complexes has revealed distinct differences in their co-ordination preferences towards silver(I) and copper(II). Tentative, computer-modelled structures for the complexes have been proposed using the available spectroscopic and elemental analysis data. Computer modelling, at the Molecular Mechanics level, has also been used to assess the capacity of the ligand systems to adopt conformations suitable for the chelation of tetrahedral silver(I). Solvent extraction studies have been undertaken using aqueous metal ion solutions and various organic solvents. The dithiodiamide derivatives typically presented solubility problems, but one of the ligands, N,N´-bis(3-chlorophenyl)-3,6-dithiaoctanediamide, exhibited significant but slow extraction of silver(I) into toluene. The malonamide derivatives, however, proved to be readily soluble in ethyl acetate and, in some cases, exhibited good to excellent selectivity for silver(I) in the presence of the base metals copper and lead. Atomic absorption analysis revealed rapid equilibration times (<15 min) and high extraction efficiencies over a wide pH range (2.78 - 9.0). Metal selectivity has been determined by ICP-MS analysis of the residual silver, copper and lead present in the aqueous phase after 15 min, and one of the ligands, N,N´-bis(2-benzylsulfanylethyl)malonamide, exhibits excellent (≥ 96 %) silver(I) specificity.

Syntheses of Schiff bases were carried out by reacting salicylaldyhde, ortho-vanillin, para-vanillin or vanillin with aniline, 1-aminonaphthalene, 4- and 3-aminopyridine, and also with 2- and 3-aminomethylpyridine. The various Schiff bases obtained from the condensation reaction were reacted with CoCl₂.6H₂0, triethylamine stripped CoCl₂.6H₂0 or Co(CH₃COO)₂ to form cobalt(Il) complexes of ratio 2:1. The complexes obtained from cobalt chloride designated as the "A series" are of the general formulae ML₂X₂.nH₂0 , (L = Schiff base, X = chlorine) while those obtained from cobalt acetate or triethylamine stripped cobalt chloride denoted as "B" and C" are of the general formulae ML₂. nH₂0. The few complexes that do not follow the general formulae highlighted above are: IA [M(HL)₃.Cl₂], (L = N-phenylsalicylaldimine), 4A = (MLCl₂), (L = N-phenylvanaldiminato), 7 A and 21 A (ML₂), (L = N-naphthyl-o-vanaldiminato, and N-methy-2-pyridylsalicylaldiminato respectively), 8A = MLCI, (L = N-naphthylvanaldiminato), 12A = M₂L₃Cl₂, (L = N-4-pyridylvanaldiminato), 15A (MLCI), (L = N-3-pyridyl-o-vanaldiminato). The ligands and their complexes were characterized using elemental analyses and cobalt analysis using ICP, FT-IR spectroscopy (mid and far-IR), NIR-UV/vis (diffuse reflectance), UV/vis in an aprotic and a protic solvents, while mass spectrometry, ¹HNMR and ¹³CNMR, was used to further characterized the ligands. The tautomeric nature of the Schiff bases were determined by examining the behaviour of Schiff bases and their complexes in a protic (e.g. MeOH) and non-protic (e.g. DMF) polar solvents. The effects of solvents on the electronic behaviour of the compounds were also examined. Using CDCl₃, the NMR technique was further used to confirm the structures of the Schiff bases. The tentative geometry of the complexes was determined using the spectra information obtained from the far infrared and the diffuse reflectance spectroscopy. With few exceptions, most of the "A" series are tetrahedral or distorted tetrahedral, while the "B + C" are octahedral or pseudooctahedral. A small number of complexes are assigned square-planar geometry owing to the characteristic spectral behaviour shown. In order to determine their biological activity, two biological assay methods (antimicrobial testing and brine shrimp lethality assay) were used. Using disc method, the bacteriostatic and fungicidal activities of the various Schiff bases and their respective complexes to Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa as well as Aspergillus niger, were measured and the average inhibition zones are tabulated and analysed. Both the Schiff bases and their complexes showed varying bacteriostatic and fungicidal activity against the bacteria and fungus tested. The inhibition activity is concentration dependent and potential antibiotic and fungicides are identified. To determine the toxicity of the ligands and their corresponding cobalt(II) complexes, brine shrimp lethality assay was used. The LD₅₀ of the tested compounds were calculated and the results obtained were tabulated for comparison.