Gas-Phase Formation of Environmentally Persistent Free Radicals from Thermal Degradation of Catechol, Hydroquinone, Phenols and Tobacco
Doctor of Philosophy (PhD)
Catechol, hydroquinone and Phenol are major constituents of the mainstream tobacco smoke. The toxicity of tobacco has been attributed to the ability of catechol and hydroquinone to undergo endogenous or exogenous redox cycling to form semiquinone type radicals responsible of Reactive Oxygen Species (ROS) formation. ROS such as hydroxyl radicals can cause severe oxidative stress on biological tissues and can provoke severe signaling pathways leading to cardiovascular and pulmonary dysfunctions and carcinogenesis. Given that semiquinone type radicals are organic radicals, characterized by their high instability and reactivity; it is somewhat surprising that they can live long enough mostly when associated with atmospheric fine particles to induce the biological damages reported in the literature. Thus identification of the exact nature of the free radicals, their origin, the reason for their stability and persistency, and their health impacts appear to be an increasing environmental issue. Consequently, we have performed studies of the thermal degradation of catechol, hydroquinone and phenol and structurally similar derivatives that have been proposed as progenitors of semiquinone type radicals. Tobacco pyrolysis has also been investigated. We have employed in conjunction with the Electron Paramagnetic Resonance (EPR), the technique of Low Temperature Matrix Isolation in which catechol, hydroquinone, phenols and Tobacco were pyrolyzed in both low and atmospheric pressures reactor that was directly connected to a liquid nitrogen-cooled cold finger located in the EPR cavity of a Bruker EPR spectrometer. Comprehensive potentially persistent free radicals identification associating additional experimental and mathematical tools has led to the acquisition of the EPR spectra of p- Semiquinone, o-Semiquinone, cyclopentadienyl and phenoxy radicals. The hydroxycyclohexadienyl radical, one of the unexpected radicals according to the decomposition mechanism developed earlier, was found during the atmospheric pyrolysis of phenol. The supposedly very labile radical identified was the hydroxycyclopentadienyl. The methylperoxide type radicals were found when trace of oxygen was used during the pyrolysis experiments. The precursors pyrolysis product analysis employing GC-MS revealed the formation of naphthalene, indenol, indene, benzofuran-2-methyl, indenone, fluorene, and acenaphthylene, thus giving additional evidence of the formation of both labile and potentially persistent free radicals.
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Adounkpe, Julien Gnonlonfoun, "Gas-Phase Formation of Environmentally Persistent Free Radicals from Thermal Degradation of Catechol, Hydroquinone, Phenols and Tobacco" (2008). LSU Doctoral Dissertations. 885.