Reports: GB4 48425-GB4: Atomic Oxygen O(3P) : Photogeneration and Reactions with Biomolecules in Aqueous Media

Ryan D. McCulla, Saint Louis University

         Over the past two years, 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.  In the first year of support, the development of aqueous-soluble photoactivatable precursors of O(3P) was the major focus.  Early in the second year of support, our goal of developing an aqueous-soluble photoactivatable precursors of O(3P) was completed.  With these precursors at hand, we were able to begin to investigate how O(3P) reacts with a number of biomolecules, which continues to be our focus.  This work has been presented in three invited lectures, and the work related to the development of O(3P)-precursors was published in the Journal of the American Chemical Society.  Additionally, this work was 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.

            During the period of support we prepared 2,8-dihydroxymethyldibenzothiophene-S-oxide (1), 4,6-dihydroxymethyldibenzothiophene-S-oxide (2), 4,6‑dicarboxy-dibenzothiophene S-oxide (3), 2,8-sulfonic acid dibenzothiophene S-oxide (4). All of these compounds are soluble in water at mM concentrations, and 4 is essentially completely water soluble.  The photolysis of 1-4 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 generation of O(3P) oxygen in water was confirmed by trapping experiments using molecular oxygen or thiols.  The reaction of O(3P) with molecular oxygen is known to generate ozone, which was observed upon photolysis.  Another interesting finding was that O(3P) in water reacted more efficiently with thiols than sulfides, which is the opposite to the trend observed in the gas phase. 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).  This led us to investigate the cleavage of DNA induced by O(3P) generation.  We observed that binding between the O(3P)-precursor and DNA had a direct correlation with the degree of photoinduced cleavage.  We also have begun to investigate the reactivity of O(3P) with biological thiols. 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 to look for collaborations to apply O(3P) in biological environments in the coming year.

            In June of 2009, 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.  The work has also been published in the Journal of the American Chemical Society, and presented at five American Chemical Society national and regional meetings. Additional publications, which report findings on the reaction of O(3P) and biological molecules, will be submitted in the near future. Presentations at the Gordon Research Conference and American Chemical Society meetings have also led to invitations to review articles 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. The undergraduate research of Medina Krantic, Whitney Grither and Katherine Gray was supported by this grant.

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