School of Chemistry - Theses
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ItemThe oxidative damage of biological molecules by air pollutants NO2● and NO3●( 2019)Air pollution is perceived as the world’s greatest environmental risk to human health. According to the World Health Organization (WHO), air pollution is responsible for the deaths of about 7 million people each year. In the industrialised urban environment, nitrogen dioxide (NO2•) and ground-level ozone (O3) are the most oxidising air pollutants. Exposure to these gases has been associated with increased respiratory health problems, such as exacerbation of existing asthma and allergies. While the adverse health effects of air pollution are clear, the precise underlying mechanism through which the pollutants affect biological systems is not well understood. It has been speculated that nitrate radicals (NO3•), which are formed from the reaction of NO2• and O3, play an important role in the oxidative damage of biological systems. Therefore, this thesis explores reactions involving NO3• and biomolecules such as proteins through a combination of kinetic, computational and product studies, in order to gain a better understanding of the fundamental chemical pathways that lead to oxidative damage in biological systems upon exposure to air pollution. The first section of this thesis investigates the reaction of NO3• with aliphatic amino acids and peptides. From laser flash photolysis experiments, it was found that NO3• reacts with aliphatic amino acids and peptides at multiple sites through proton-coupled electron transfer (PCET) at the amide nitrogen, and hydrogen atom transfer (HAT) at the α-carbon or the activated C–H moiety (e.g., tertiary carbons) with the rate of about 1 x 10(6) M−1 s−1. Following the above finding, this thesis proceeds to examine the reaction of NO3• with aromatic amino acids and peptides. A faster rate by a factor of 5–6 suggests that the reaction occurs at the aromatic ring through electron transfer (ET). An unprecedented amide neighbouring group effect was discovered, by which the rate of aromatic ring oxidation is increased considerably when the ring is flanked by two amide groups, instead of one amide and one ester group. Due to this effect, phenylalanine can potentially act as a relay amino acid in a long-distance ET even though the aromatic ring in phenylalanine is not readily oxidisable under biochemical conditions. The third section of this thesis explores NO3• reactions involving proline, where its side chain is covalently bound to the α-amino group. This unique structure increases electron density at the nitrogen and significantly accelerates the rate of ET at this nitrogen by a factor of about 600 compared to the other aliphatic substrates. However, when the amide moiety in proline residue is involved in the amide neighbouring group effect, accelerating the rate of aromatic ring oxidation, the rate of ET at this nitrogen was found to decrease significantly. The final part of this thesis studies the reaction of NO2• with various biological molecules, including short peptide and cholesterol derivatives. It was found that contrary to the widely accepted radical pathway, the reaction of NO2• with these molecules involves an ionic pathway through the dissociation of N2O4 into NO+ and NO3−.
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ItemOxidative and chemical modification of amino acids by nitrogen dioxide, ozone and the reactive paracetamol metabolite NAPQI( 2016)Oxidative damage has been implicated in a wide range of diseases including cardiovascular disease, diabetes, asthma, cancer and atherosclerosis. While this damage is typically caused by ROS or RNS generated in vivo, oxidative damage is also mediated by environmental and secondary oxidants such as NO2•, O3 and reactive drug metabolites. This thesis explores the fundamental reactivity of environmental and secondary oxidants towards model amino acids and peptides. According to the WHO, environmental pollution represents the single greatest environmental risk to human health. Exposure of NO2• and O3, common polluting gases, has been linked to the development of asthma, bronchitis, heart disease, stroke, cancer and COPD. While this link is clear, the precise molecular mechanism through which NO2• and O3 cause these adverse health effects is largely unknown. The first section of this thesis explores the reactivity of NO2•, O3 and NO3• towards model biomolecules. N-Acetyl and C-terminal methyl ester protected amino acids and peptides were treated with NO2•, O3 and NO3•. NO3• was generated in situ from the reaction of NO2• and O3 or from the UV irradiation of CAN. These model biomolecules are irreversibly damaged as a result of exposure. The reaction of NO3• generated from the UV irradiation of CAN yields β-nitrate esters from aromatic amino acids, while NO3• generated in situ generally yields nitration products. In the case of tryptophan, pyrroloindoline and nitrosopyrroloindoline products were obtained. Exposure of NO2• to phenylalanine, glycine, alanine and valine containing peptides was found to lead to an unprecedented modification, rearrangement and ultimate excision of amino acids in a peptide chain. The thesis proceeds to examine the fundamental reactivity of a secondary oxidant, the reactive paracetamol metabolite NAPQI. Paracetamol is one of the most widely used analgesic drugs in the world and overdose represents a significant burden on the health system. NAPQI, generated in high concentrations in the liver by CYP450 enzymes, is known to form protein adducts, which have been linked to the development of liver toxicity. The focus of many studies has been on the role of cysteine – paracetamol adducts, formed from the reaction of NAPQI with cysteine residues. In this work, the reaction of NAPQI with a range of amino acids (Cys, Tyr, Trp, His, Lys, Arg, Met, Gln, Glu, Ser and Val) was examined and it was found that NAPQI forms adducts with Cys, Tyr, Trp and Met. Novel paracetamol – amino acid adducts were isolated and characterised by spectroscopic methods. The final part of this thesis explores the reaction of aromatic amino acids and peptides with CAN under UV irradiation. This method was utilised to form β- substituted amino acids with high diastereoselectivity in a single reaction step. Method development was performed in an effort to improve the yield of the β- nitrate ester products. This included 1H NMR based reaction screening of N- terminal protecting groups, work-up procedure and reaction conditions. From these experiments, it was found that ideal reaction conditions included N-acetyl protection, evaporation in vacuo, an excess of CAN and dilute solution concentrations.