The dissociative electron transfer reactions of a series of α-epoxyketones and tetra-n-butylammonium acetate have been examined by electrochemical and computational techniques. Results for both the direct electrochemical (linear sweep voltammetry and convolution voltammetry) and indirect electrochemical (homogeneous redox catalysis) reductions of the epoxyketones are presented. In cases where the ring-closed radical anion generated by reduction of the epoxyketones is resonance stabilized (aromatic epoxyketones) the mechanism proceeds in a stepwise fashion, where the electron transfer and bond breaking reactions occur in sequential, discrete steps. On the other hand, where there is no additional resonance stabilization afforded to the ring-closed epoxide radical anion (aliphatic epoxyketones) the reaction proceeds in a concerted fashion, where electron transfer and ring cleavage occur simultaneously. The presence (or absence) of resonance stabilization in the ring-opened distonic radical anion plays little role in the kinetics of these dissociative electron transfers. Computations with the Density Functional Theory (B3-LYP and BHandH-LYP) on α-epoxyketones are also presented, and are in good agreement with the electrochemical results. The oxidative dissociative electron transfers of the acetate anion in "dry" and "wet" (0.5 M H2O) acetonitrile were also characterized with direct and indirect electrochemical experiments, again utilizing linear sweep voltammetry, convolution voltammetry, and homogeneous redox catalysis. There is a significant change in the observed oxidation potential of the anion upon addition of water, as well as an apparent decrease in the intrinsic barrier to the electron transfer. The possible transition from a concerted to stepwise mechanism for the dissociative electron transfer of acetate upon addition of water is examined - the electrochemical data is compared to theoretical models for both the concerted and stepwise processes. It is determined that the indirect electrochemical experiments do not proceed through an outer sphere electron transfer. Additionally, it is shown that the difference between the direct oxidation of acetate in anhydrous and wet acetonitrile is unlikely to be the result of transition from a purely concerted mechanism to a purely stepwise mechanism based on thermodynamic considerations. / Ph. D.

Despite advances in technology and understanding of biological systems in the past two decades, modern drug discovery is still a lengthy, expensive, difficult and inefficient process with low rate of new therapeutic discovery. The search for new effective drugs remains a somewhat empirical process. There is compelling need for a more fundamental, mechanistic understanding of human cancers and anticancer drugs to design more appropriate drugs. Radiotherapy is still the major therapy of cancer. It uses high-energy ionizing radiation such as x-rays and charged particle beams to destroy cancer cells. DNA is well known to be the principal biological target of radiotherapy, but the molecular mechanism of ionizing radiation induced DNA damage was elusive. The conventional thought of the ???OH radical as the major origin for ionizing radiation induced DNA damage is questionable. Although various strategies and types of compounds have been designed and developed as potential radiosensitizers to enhance the radiosensitizing efficiency of radiotherapy, none of them have been approved for clinical use. The general outcomes of clinical trials have been disappointing. This thesis presents an innovative molecular-mechanism-based drug discovery project to develop novel drugs for effective radiotherapy of cancer through the emerging femtomedicine approach. Its ultimate goal is to develop more effective radiosensitizers, based on our unique molecular understandings of ionizing radiation induced DNA damage and halopyrimidines as a family of potential radiosensitizers. Direct, real-time observation of molecular reactions is of significant importance in diverse fields from chemistry and biology, environmental sciences to medicine. Femtosecond time-resolved laser spectroscopy (fs-TRLS) is a very powerful, direct technique for real-time observation of molecular reactions. Its key strength lies in short duration laser flashes of a time scale at which reactions actually happen - femtoseconds (fs) (1fs = 10???15 second). Since the late 1980s, its application to study chemical and biological systems led to the births of new subfields of science, called femtochemistry and femtobiology. Recently, femtomedicine has been proposed as a new transdisciplinary frontier to integrate ultrafast laser techniques with biomedical methods for advances in fundamental understandings and treatments of major human diseases. This the remarkable opportunity afforded through real-time observation of biochemical reactions at the molecular level. Femtomedicine holds the promise of advances in the radiotherapy of cancer. Several important findings were made in this thesis. First, our results of careful and high-quality fs-TRLS measurements have resolved the long existing controversies about the physical nature and lifetimes of a novel ultrashort-lived electron species (epre???) generated in radiolysis of water. These results have not only resolved the large discrepancies existing in the literature but provided new insights into electron hydration dynamics in bulk water. Such information is important for quantitative understanding and modeling of the role of non-equilibrium epre??? in electron-driven reactions in diverse environmental and biological systems, from radiation chemistry and radiation biology to atmospheric ozone depletion. Second, our fs-TRLS results have unraveled how epre??? plays a crucial role in ionizing radiation induced DNA damage. We found that among DNA bases, only T and especially G are vulnerable to a dissociative electron transfer (DET) reaction with epre??? leading to bond breaks, while the electron can be stably trapped at C and especially A to form stable anions. The results not only challenge the conventional notion that damage to the genome by ionizing radiation is mainly induced by the oxidizing ???OH radical, but provide a deeper fundamental understanding of the molecular mechanism of the DNA damage caused by a reductive agent (epre???). Our findings have led to a new molecular mechanism of reductive DNA damage. Third, halopyrimidines, especially BrdU and IdU, have passed Phase I to II clinical trials as potential hypoxic radiosensitizers, but the outcome of Phase III clinical trials was disappointing. Our results of fs-TRLS studies have provided a new molecular mechanism of action of halopyrimidines (XdUs, X=F, Cl, Br and I) in liquid water under ionizing radiation. We found that it is the ultrashort-lived epre???, rather than the long-lived ehyd???, that is responsible for DET reactions of XdUs. This reaction leads to the formation of the reactive dU??? radical, which then causes DNA strand breaks and cancer cell death. Our results have challenged a long accepted mechanism that long-lived ehyd??? would be responsible for the radical formation from halogenated molecules. Furthermore, we found that the DET reaction efficacy leading to the formation of the reactive dU??? radical is in the order of FdU << CldU < BrdU < IdU. Thus, only BrdU and IdU could be explored as potential radiosensitizers, in agreement with the results of bioactivity tests and clinical trials. Fourth, our fs-TRLS studies have provided a molecular mechanism for the DNA sequence selectivity of BrdU and IdU in radiosensitization. We found the DET reactions of BrdU/ IdU with dAMP*??? and dGMP*??? formed by attachment of epre??? generated by radiolysis of water in aqueous BrdU-dAMP/dGMP and IdU-dAMP/dGMP complexes under ionizing radiation. This new mechanistic insight into the interaction of BrdU and IdU with DNA provides clues to improve the halogen familty as potential radiosensitizers and to develop more effective radiosensitizers for clinical applications. Fifth, based on our molecular mechanistic understandings of DNA damage induced by ionizing radiation and halopyrimidines as potential radiosensitizers, we develop more effective new radisensitizing drug candidates through the femtomedicine approach. We have performed a fs-TRLS study of the DET reaction of a candidate compound (RS-1) with epre???, and found that the DET reaction of epre??? with RS-1 is much stronger than that of IdU (and certainly BrdU and CldU). Moreover, we have tested the radiosensitizing effect of RS-1 against human cervical cancer (HeLa) cells exposed to various doses of x-ray irradiation through DNA damage measurements by gel electrophoresis and cell viability/death assays by MTT. Our results have confirmed that RS-1 can largely enhance the radiosensitivity of treated human cervical cancer (HeLa) cells to x-ray (ionizing) radiation. It is clearly demonstrated that RS-1 has a much better radiosensitizing effect than IdU. Although these are just preliminary results, our results have shown promise of developing more effective radiosensitizers. In summary, our studies have demonstrated the potential of femtomedicine as an exciting new frontier to bring breakthroughs in understanding fundamental biological processes and to provide an efficient and economical strategy for development of new anticancer drugs.

The mechanism and the oxidation potential of the dissociative single electron transfer for tetra-n-butylammonium acetate has been investigated via conventional (cyclic voltammetry) and convolution voltammetry. The oxidation potential for tetra-n-butylammonium acetate was determined to be 0.60 ± 0.10 (vs. Ag/ (0.1 M) AgNO₃) in anhydrous acetonitrile. The results also indicated the mechanism of oxidation was concerted dissociative electron transfer (cDET), rather than stepwise as was previously reported. To further investigate the mechanism, a series of aliphatic and aromatic tetra-n butylammonium carboxylates were synthesized and investigated via convolution and conventional methods under anhydrous conditions (propionate, pivalate, phenyl acetate, and benzoate). The reported results showed high reproducibility and consistency with a concerted dissociative electron transfer for aliphatic carboxylates with a systematic shift in the oxidation potentials (0.60 ± 0.09 V for acetate, 0.47 ± 0.05 V for propionate, and 0.40 ± 0.05 V for pivalate) within the series which is expected trend based on radical stabilization energies of the alkyl groups on the aliphatic carboxylates. Hydrogen bonding was investigated as a possible source for the discrepancy between our results and the reported mechanism of the dissociative electron transfer. Because of the extreme hygroscopic nature of carboxylate salts, it was hypothesized that the presence of small amounts of water might alter the reaction mechanism. Deionized water and deuterium oxide additions to anhydrous acetonitrile were performed to test this hypothesis. The mechanism was noted to shift towards a stepwise mechanism as water was added. In addition, the derived oxidation potentials became more positive with increasing concentrations of water. Several explanations are presented with regards to water effects on the shift in the electron transfer mechanism. Indirect electrolysis (homogeneous redox catalysis) was also employed as an alternative and independent approach to quantify the oxidation potentials of carboxylates. A series of substituted ferrocenes were investigated as mediators for the oxidation of tetra-n-butylammonium acetate. Preliminary data showed redox catalysis was feasible for these systems. Further analyses of the electrochemical results suggested a follow-up chemical step (addition to mediator) that competes with the redox catalysis mechanism. As predicted from theoretical working curves, a plateau region in the i_p/i_pd plots (where no meaningful kinetic information could be obtained) was observed. Products mixture analyses verified the consumption of the mediator upon electrolysis, but no further information with regards to the nature of the mechanism was deduced. In a related study the effects of hydrogen bonding and ions on the reactivity of neutral free radicals were examined by laser flash photolysis. The rate of the β-scission of the cumyloxyl radical is influenced by cations (Li⁺ > Mg²⁺ ≈ Na⁺ > ⁿBu₄N⁺) due to stabilizing ion-dipole interactions in the transition state of the developing carbonyl group. Experimental findings are in a good agreement with theoretical work suggesting metal ion complexation can cause radical clocks to run fast with a more significant effect if there is an increase in dipole moment going from the reactant to the transition state. / Ph. D. / Our work focuses on employing electrochemical techniques to investigate single electron transfer processes, which lead to unstable organic species that contain an odd number of electrons called radicals and radical ions. Many essential biological and environmental pathways are found to occur via radicals, i.e. photosynthesis, atmospheric degradation, enzyme catalyzed reactions in biology, autooxidation, DNA mutations, and more. Electrochemical techniques permit us to investigate the scientific fundamentals of radical processes by generating radicals and radical ions in a controlled manner with a higher efficiency. We have combined electrochemical techniques with established physical organic theories of electron transfer to allow us to determine of the rate and mechanism of electron transfer for a selective group of chemical compounds, specifically anions derived from carboxylic acids (carboxylates). A fundamental understanding of single electron transfer processes for carboxylates allows for a prediction of chemical behavior and the future design of novel chemical compounds for alternative chemical functionality. Our findings are the first to report experimental evidence for a so-called concerted dissociative electron transfer mechanism for carboxylates, where the transfer of an electron is accompanied by the simultaneous breakage of a carbon-carbon bond yielding a radical and carbon dioxide. The mechanism has been shown to proceed in a stepwise fashion only in the presence of water. Our work highlights the environmental effects on radical stability such as water and metals ions.