Flavins and Their Analogues as Natural and Artificial Catalysts

There has been a tremendous interest in the study of flavins and their derivatives in order to gain information valuable for designing model compounds that can mimic the functions of flavoenzymes. Therefore, this dissertation seeks a) to gain further knowledge about the excited-state behavior of the reduced flavin cofactors and study the repair of cyclobutane thymidine dimer lesions in DNA; b) synthesize and study a fully organic flavin derivative ethyl-flavinium perchlorate (Et-Fl⁺) as catalyst for water oxidation.

In order to better understand the mechanism of light driven catalysis by flavoproteins such as DNA photolyase, we did a comparative study of excited state dynamics of aqueous solutions of two reduced flavin cofactors: flavin-mononucleotide (FMNH₂) and flavin-adenine dinucleotide (FADH₂) using femtosecond and mid —IR transient absorption spectroscopy. FMNH₂ was used as a model system that lacks adenine moiety. We found that FMNH^- exhibits multiexponential decay dynamics due to the presence of two bent conformers of the isoalloxazine ring. However, adenine containing-cofactor FADH^- exhibits an additional complexity in their excited-state dynamics. This is because of an intramolecular electron transfer from the isoalloxazine to the adenine moiety of the molecule due to the close proximity of the two moieties. To investigate the repair dynamics of thymidine dimer (TT-dimer) lesions in DNA by flavins and the role of adenine in the repair process by FADH^-, we conducted experiments in a H₂O/DMSO solvent mixture at pH=8 where reduced flavins exist in the deprotonated form. The flavin excited state decays faster with increasing TT-dimer concentration due to the repair process. However, the decrease in the lifetime was more pronounced in the case of FADH^- than FMNH^-. The faster repair observed in FADH^- could suggest that the adenine moiety of FADH^- contributes to the TT-dimer repair efficiency by facilitating a two-step electron transfer mechanism. In FMNH^-, the flavin excited-state decays because of direct electron transfer from flavin excited-state to TT-dimer, while in FADH^- the decay is a competition between electron transfer to adenine and electron transfer to TT-dimer.

In order to study a fully organic flavin derivative ethyl-flavinium perchlorate (Et-Fl⁺) as a catalyst for water oxidation, we synthesized Et-Fl⁺ and its pseudobase Et-FlOH, according to the literature procedures. We investigated the electrochemical oxidation of Et-FlOH in acetonitrile with the aim of determining whether the two electrochemical oxidation of Et-FlOH releases the OH⁺ cation with the production of Et-Fl⁺ as previously reported. The all purpose was to study Et-Fl⁺ as a potential water oxidation catalyst. To our surprise we found that the two electron oxidation does not release the OH⁺ cation. Instead the production of Et-Fl⁺ is associated with changes in the acid-base equilibrium. Even though we did not observe the release of cation OH⁺, we observed an interesting effect during electrochemical oxidation of Et-Fl⁺ in acetonitrile/water mixture. It exhibits a catalytic peak at 2 V and retains its reduction peaks. We have been investigating this process in details in order to understand the origin of this oxidation catalytic peak.