No description available.

Nitrate esters of aromatic alcohols were synthesized by esterification which involved competition between 0-nitration and aromatic C-nitration. TLC analysis gave a pattern of adsorption affinities for the nitroxy group and other substituents consistent with the molecular conformations. The NMR frequency of the α-protons showed a linear correlation with the accepted group electronegativities of the substituents in molecules with rigid carbon skeletons and gave a value of 4.18 kcal/mole for the nitroxy group. The symmetric and asymmetric IR stretching frequencies of nitroxy groups in dilute cyclohexane solution were shifted to higher values by steric interaction between contiguous groups when the C-ONO₂ bonds were constrained to coplanarity. The UV spectra showed benzenoid, π→π*, and n→π* bands and a solvent perturbation effect assigned to a solvent → solute charge-transfer interaction. The nitrate esters reacted with the solvent when irradiated in solution in the wavelength range of the n→π* excitation. Product analysis indicated that C-C bond cleavage occurred via intermediate alkoxyl radicals. Rate studies showed the following order of reactivity: benzyl nitrate < dl-hydrobenzoin dinitrate < meso-hydrobenzoin dinitrate, < trans-1, 2-acenaphthenediol nitrate < cis-1, 2-acenaphthenediol dinitrate. The rate measurements and ESR spectra gave evidence of intramolecular energy transfer from the naphthalene moiety to the nitroxy groups in the 1, 2-acenaphthenediol dinitrates assigned as a singlet→singlet transfer. Calculations from the apparent first-order rate constants and spectra showed that benzyl nitrate, and meso- and dl-hydrobenzoin dinitrates photolysed with a quantum yield of about 2 in benzene solution. A solvent effect caused k[subscript Et₂O] > k[subscript EtOH] >k[subscript PhH]. On the basis of product analysis, rate measurements, estimated quantum yields and ESR spectra a mechanism for the nitrate ester photolysis was proposed. / Science, Faculty of / Chemistry, Department of / Graduate

Tetra-p-nitrophenyl pyrophosphate, generated in situ by the reaction of di-p-nitrophenyl phosphate with di-p-tolyl carbodiimide in anhydrous dioxane, has been shown to be a new and powerful phosphorylating agent, it has been used for the phosphorylation of various aliphatic alcohols and carbohydrate derivatives, particularly nucleosides, as well as of aliphatic amines and mercaptans. The removal of the protecting groups from the initially formed di-p-nitrophenyl alkyl phosphates has been accomplished by both alkaline hydrolysis and catalytic hydrogenolysis. Various other chemical reactions of the intermediate neutral esters, as typified by di-p-nitrophenyl methyl phosphate, have been studied in considerable detail. Included in these are: mild alkaline hydrolysis, partial hydrogenolysis, the action of anhydrous amines, acidic hydrolysis, base catalysed transesterification, and reductive cleavage with sodium in liquid ammonia. The method has been specifically applied, to the phosphorylation of 2',3'-0-isopropylidine guanosine and has led to the first efficient synthesis of the biologically interesting guanosine-5'-phosphate. Also the enzymatic-ally useful guanosine-5’-mono-p-nitrophenyl phosphate and uridine-5’-mono-p-nitrophenyl phosphate have been prepared and the formation of 3,5’-cyclo-guanosine quaternary salts observed. Several synthetic routes have been developed for the synthesis of 1,2-0-isopropylidine-D-xylofuranose-3,5-cyclic phosphate and of D-xylofuranose-3,5-cyclic phosphate. Hot alkaline hydrolysis of the former followed by acidic removal of the isopropylidine group has led to a mixture of D-xylofuranose-5-phosphate and D-xylopyranose-3-phosphate which were separated by ion exchange chromatography. Both products have been fully characterized and the method constitutes the first successful synthesis of D-xylose-3-phosphate. A previously reported isolation of the latter compound has been reexamined and shown to be in error. Reinterpretation of the data has shown the reported compound to be in fact D-xylulose-5-phosphate. / Science, Faculty of / Chemistry, Department of / Graduate

>Magister Scientiae - MSc / There has been a decline in the global fossil fuel reserves, due to an increasing demand for petroleum. Biofuels can be used as an alternative source of energy whereby biomass is converted to liquid fuels such as bioethanol. There is considerable interest in lipolytic enzymes because of their broad substrate range and for this purpose these enzymes have potential in a variety of biotechnological application. Lipolytic enzymes include esterases (E.C 3.1.1.1) and lipases (E.C3.1.1.3). Esterases preferentially hydrolyse short chain (<C10) ester-containing molecules that are partly soluble in water, while lipases hydrolyse a broad range of substrates preferably water-insoluble fatty acyl molecules (>C10). The aim of this study was to express, purify and characterise the lipolytic enzyme present on afosmid, Try 11, previously isolated from a metagenomic library of a Malawian hotspring, which conferred activity on tributyrin and ethyl ferulate. Bioinformatic analysisof the fosmid insert sequence predicted an open reading frame consisting of 951 bp,designated RHgene34, encoding a 317 amino acid protein with 41 % similarity to theα/β hydrolase fold-3 domain protein of Burkholderia sp. The RHgene34 protein contains conserved motifs of esterases/lipases, such as HGGG (residues 95-98),GxSxG (residues 167 - 171) and the putative catalytic triad composed of Ser157,Asp255 and His285. The gene was cloned and expressed in pET21a(+), and transformed into Escherichia coli Rosetta. p-Nitrophenol (p-Np) fatty acyl esters of different carbon chain lengths were used for kinetic characterisation of RHgene34. Kinetic analysis revealed that RHgene34 had a broad range activity on the p-Npesters, from C2 - C14. RHgene34 operates optimally at 45 ºC, pH 9.0 and has a half life of 30 mins at 45 ºC. This study demonstrates that functional screening combined with the sequence analysis is a useful approach for isolating novel enzymes from ametagenome.

No description available.

An unexpected discovery in 1940 revealed that urea has the peculiar property to form crystalline adducts with many straight chain organic compounds. Since the discovery, considerable time and money have been spent on the elucidation of this phenomenon, and the field of urea adducts has now advanced to the stage at which many empirical rules have been observed but, unfortunately, no sound theory has been formulated. The purpose of this research was to determine the effect, if any, of changes in structure upon the amount of urea required to adduct with esters. The change in structure consisted of changing the position of the carbonyl group in a series of straight chain twenty-carbon esters.

Urea has the interesting and unexpected property of forming solid complexes with many straight-chained organic compounds. So selective is the formation of these complexes with linear aliphatic compounds that it provides a means of separation form the analogous branched and cyclic compounds. Linear esters derived from normal saturated and unsaturated esters give adducts also but not to the same extent. Although many linear aliphatic compounds form adducts, the compounds so formed have varying degrees of stability depending on molecular weight, saturation and the shape of the molecule. The purpose of this investigation is to determine the effect of the position in the molecule of the carbonyl group in the formation of urea adducts with a series of twenty carbon esters.

No description available.

<p> This work was initiated with a view to elucidating the hitherto little-studied thermolysis of esters of oxalic acid. A number of symmetrical diesters were synthesized and several were investigated thoroughly by use of product studies, kinetic methods and kinetic isotope-effect measurements. Both preparative and pyrolytic procedures and results are recorded. The literature regarding previous studies of the effect of heat on oxalate esters is surveyed, and a review of the principles and uses of the investigative methods employed is presented. The results are discussed in the light of more recent work and both general and specific mechanisms for oxalate thermolysis are suggested.</p> / Thesis / Doctor of Philosophy (PhD)

Oxidative storage stability was conducted to determine the optimum antioxidant for the production of catfish methyl esters (CFME) for use as biodiesel. Peroxide value, anisidine value, 2-thiobarbituric reaction substances, acid value, iodine value, UV absorbance, and induction period were measured. Antioxidant, stability parameters changed over a storage time of one year when stored at 25°C. The CFME were not stable to oxidation without the addition of TBHQ. When stored at 25°C, oxidative parameters indicated that CFME was a stable product that met oxidative stability standards when TBHQ was added. When held at 80°C for 28 days, the samples with TBHQ were stable to oxidation. However, the acid value rose above the maximum at 28 days. Thus, CFME can be stable to oxidative breakdown when THBQ was added at 800 ppm, but when exposed to high temperatures, one will have to limit water or protect CFME against hydrolytic rancidity.