Study on the mechanism of photodegradation of pharmaceutical products and analogues, development of a novel fluorescence technique for DNA-damage detection.

Title: Study on the mechanism of photodegradation of pharmaceutical products and analogues, development of a novel fluorescence technique for DNA-damage detection.
Authors: Cosa, Gonzalo.
Date: 2002
Abstract: This thesis focuses on different aspects of photobiology. The first chapters address the photophysical and photochemical characterization of two pharmaceutical products, fenofibric acid and ketoprofen, both containing the benzophenone chromophore. These studies have been performed with the aim of better understanding the photostability of these drugs. These compounds are shown to undergo efficient photodecarboxylation in basic solutions, with concomitant formation of carbanions. Time-resolved studies lead us to propose a singlet mediated reaction in the case of ketoprofen, whereas the photoreactivity of fenofibric acid results from the triplet state of this compound; further, dramatic solvent dependence is encountered in the photochemical reactions of the latter drug and its photoproducts. In the case of ketoprofen photodegradation, the photogenerated benzylic carbanion can be monitored, and lifetime measurements at different temperatures and in different solvents have been performed to determine its dynamic parameters. The decay of this carbanion, resulting from protonation by water, is found to occur within a few hundred nanoseconds; and the protonation rate is shown to be dependent on the solvent and the degree of substitution of the carbanion center. The case of alpha-diketones as potential photosensitizers is also addressed with the example study of a di-thienyl alpha-diketone; i.e., 2,2 '-thenil. Direct spectroscopic evidence is given for the formation of a molecular oxygen-triplet adduct (Bartlett-Schenck intermediate), a previously described, though never isolated, reactive oxygen species about which little is known. The formation and yield of this intermediate are discussed in terms of the energy of its triplet precursors. The subsequent chapters in this thesis deal with two other biologically relevant problems where photochemistry and photophysics are employed as a tool in order to better understand the systems under study. Thus the properties of the adrenaline derived radical are evaluated, as a case study for the catecholamine group in general. Absolute rate constants for tert-butoxyl radical scavenging and triplet benzophenone quenching are reported, and a reactivity comparison is established with other intermediates involved in the reaction of melanin formation via catecholamines. A second aspect involves the development of a novel technique to assess DNA-damage, based on fluorescence of dye-DNA complexes. The rigidity imposed by the DNA base pairs on intercalating chromophores is exploited. Its retardation effect on the relaxation of a photoexcited DNA-stain probe is employed to determine the amounts of DNA existing as double and single stranded form; which is ultimately an expression of the damage the DNA has suffered.
CollectionTh├Ęses, 1910 - 2010 // Theses, 1910 - 2010
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