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Time-Resolved Spectroscopic Studies of the Photochemistry of riboflavin, aromatic N-Oxides and the absolute reactivity of hydroxyl radical

Shi, Xiaofeng

Abstract Details

2005, Doctor of Philosophy, Ohio State University, Chemistry.
Both nanosecond and ultrafast laser flash photolysis with UV-visible and infrared detection were used to observe the transient species generated photochemically from a number of photosensitizers. The reactions of these transient species were monitored spectroscopically with the aid of theoretical computation.In the study of photochemical reactions of riboflavin and nucleosides, it was found that triplet riboflavin can be quenched by a silylated guanosine derivative. TRIR spectroscopy demonstrated that a hydroflavin radical is formed by an electron transfer-proton transfer mechanism. This sequential electron transfer-proton transfer between triplet riboflavin and guanosine derivative provides the direct observation of the photoinduced oxidative damage of riboflavin to the DNA nucleobase. The triplet states of isoquinoline N-oxide and benzocinnoline N-oxide react sluggishly with electron, proton and hydrogen atom donors. These triplets will react with hydroquinone by hydrogen atom transfer (proton coupled electron transfer). Triplet 4-nitroquinoline N-oxide reacts readily with electron donors to from the radical anions as previously reported. The radical anion is protonated on the oxygen atom of the N-oxide group to from a neutral radical. The three N-oxides of this study are not expected to serve as photochemical sources of hydroxyl radical. Singlet states of tirapazamine and desoxytirapazamine were identified by picosecond time-resolved absorption spectroscopy. The lifetimes of the S1 states and fluorescence quantum yields of aromatic N-oxides were found to be controlled by reversible cyclization to an oxaziridine. The S1 states of TPZ and dTPZ are reduced to radical anions by KSCN, KI and NaN3. Using LFP-based methodology, we have determined the rate coefficients for the reaction of hydroxyl radical with a number of monocyclic and polycyclic aromatic hydrocarbons in acetonitrile. We observed the reactivities of hydroxyl radical in acetonitrile. For simple aromatic hydrocarbons, the predominant reaction pathway in acetonitrile is the addition of the hydroxyl radical to the aromatic ring, rather than hydrogen-atom abstraction from the phenyl or benzylic C-H positions. Structure-reactivity analysis, based upon frontier molecular orbital and state correlation models indicate that charge-transfer interactions between hydroxyl radical and a given arene play an important role in the stabilization of the transition state for the reaction. Alpha-Alkoxy and hydroxy radicals were generated through thermal and photochemical reactions. Both the product analysis of the thermal reaction and the direct observation of the transient species involved suggest that alpha-hydroxy radical can very efficiently react with TPZ to form the TPZ-H radical, probably through a direct hydrogen atom exchange between TPZ and ketyl radicals. Alpha-alkoxy radicals can not proceed through this mechanism.
Matthew Platz (Advisor)
185 p.

Recommended Citations

Citations

  • Shi, X. (2005). Time-Resolved Spectroscopic Studies of the Photochemistry of riboflavin, aromatic N-Oxides and the absolute reactivity of hydroxyl radical [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1126795561

    APA Style (7th edition)

  • Shi, Xiaofeng. Time-Resolved Spectroscopic Studies of the Photochemistry of riboflavin, aromatic N-Oxides and the absolute reactivity of hydroxyl radical. 2005. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1126795561.

    MLA Style (8th edition)

  • Shi, Xiaofeng. "Time-Resolved Spectroscopic Studies of the Photochemistry of riboflavin, aromatic N-Oxides and the absolute reactivity of hydroxyl radical." Doctoral dissertation, Ohio State University, 2005. http://rave.ohiolink.edu/etdc/view?acc_num=osu1126795561

    Chicago Manual of Style (17th edition)