Reports: GB4
48425-GB4 Atomic Oxygen O(3P) : Photogeneration and Reactions with Biomolecules in Aqueous Media
Over the past year, our group made significant progress toward our long-term goal of developing atomic oxygen (O(3P)) as a unique oxidant for applications in the life sciences. Atomic oxygen is the putative oxidant formed during photodeoxygenation of a variety of aromatic heterocyclic oxides. However, all of the known heterocyclic oxides that generate O(3P) are insoluble in water. Thus, the successful completion of our long-term objective will require the development of O(3P) precursors with significant solubility in water. The development of aqueous-soluble photoactivatable precursors of O(3P) was the major focus our research this past year. As will be described in more detail later, we have successfully prepared dibenzothiophene-S-oxides with octanol-water partition coefficients that approach one and upon photolysis undergo photodeoxygenation. This work has been presented in three invited lectures and selected for an honor at the Gordon Research Conference for Physical Organic Chemistry, which highlights the impact of this research.
Dibenzothiophene derivatives are one of the major sulfur contaminants in petroleum. Upon photolysis, dibenzothiophene-S-oxide generates atomic oxygen O(3P) and dibenzothiophene. Atomic oxygen (O(3P)) has a distinct reactivity profile that has been thoroughly investigated in the gas-phase; however, reactions of O(3P) in aqueous media are virtually unexplored. The application of O(3P) as a new reactive oxygen species is expected to have numerous applications. For example, the judicious exploitation of reactive oxygen species has yielded invaluable bioanalytical techniques for the investigation of critical problems in molecular biology. Hydroxyl radical (HO•) footprinting techniques reveal the solvent accessibility of DNA, RNA, and proteins to explore the structure, folding, and interactions of biopolymers. Since the chromophore of dibenzothiophene-S-oxide absorbs in a region where biological media are largely transparent, O(3P) can be used for time-resolved footprinting and other applications. Additionally, O(3P) relevance to enzymatic processes as many metal ion stabilized oxygen atoms are responsible for many rapid enzymatic oxidations.
The past year we prepared 2,8-dihydroxymethyldibenzothiophene-S-oxide (1) and 4,6-dihydroxymethyldibenzothiophene-S-oxide (2). The octanol-water partition coefficient (Kow) of 1 and 2 was determined to measure the increase in hydrophillicity compared to the parent molecule (dibenzothiophene-S-oxide The (Kow), of 1, 2 and dibenzothiophene-S-oxide are 1.88, 2.55, and 78, respectively. Thus, the addition of two alcohol functional groups increases the hydrophillicity of 1 and 2 by a factor of 40. Additionally, we have been able to prepare aqueous solutions of 1 and 2 with concentrations greater than 3 mM. We are currently preparing other dibenzothiophene-S-oxide derivatives that are expected to have Kow that are less than one provide solutions approaching 1 M in concentration. The photolysis of 1 and 2 at 254, 294, and 330 nm all result deoxygenation. Surprisingly, the quantum yield of deoxygenation of 1 and 2 in water is 10 times greater than dibenzothiophene-S-oxide in organic solvents. The products of the reactions are the expected sulfides and a mixture of oxidation products. We have begun to investigate the reaction of O(3P) with all of the naturally occurring nucleobases. Under our photolysis conditions with 1 and 2, guanine is oxidized to 8-oxoguaine. We observed no oxidation or degradation of adenine or thymine, and cytosine and uracil form yet unknown oxidation products when exposed to O(3P). All of the materials and supplies used in these experiments were purchased with funds provided by this grant. The impact of this research is expected to amplify as we begin “proof-of-concept” experiments to demonstrate the possible applications of O(3P) in the coming year.
In June, the principal investigator attended the Gordon Research Conference on Physical Organic Chemistry with the support of funds provided by this grant. At the conference, the work described above was presented during the poster session. The poster was one of 7 posters (out of a total of 64 posters) selected for an additional oral presentation during the last session of the conference. At the end of the oral presentation, the session chair had to stop questions from the audience due to time restraints, which indicates there was significant interest in this work from the community. This was a significant opportunity to increase the communities’ exposure to our work; which would not have been possible without the support of this grant. Presentations at the Gordon Research Conference and American Chemical Society meetings have also led to invitations to review articles for the Journal of Organic Chemistry and to give talks at other universities.
This grant has also support the thesis research one graduate student and two undergraduate students. The stipend for the graduate student, James Korang, provided by a one-year research fellowship through the graduate school. In addition to providing the supplies for his thesis research, James was able to travel to travel to an American Chemical Society meeting to present his work with the support of funds from this grant. In the next year, funds from this grant will support travel for James to two ACS meetings. The undergraduate research of Medina Krantic, Whitney Grither and Katherine Gray was supported by this grant.
