Direct Observation of Radical Intermediates in Protein-Dependent DNA Charge Transport
Charge migration through the DNA base stack has been probed both spectroscopically, to observe the formation of radical intermediates, and biochemically, to assess irreversible oxidative DNA damage. Charge transport and radical trapping were examined in DNA assemblies in the presence of a site-specifically bound methyltransferase HhaI mutant and an intercalating ruthenium photooxidant using the flash-quench technique. The methyltransferase mutant, which can flip out a base and insert a tryptophan side chain within the DNA cavity, is found to activate long-range hole transfer through the base pair stack. Protein-dependent DNA charge transport is observed over 50 Å with guanine radicals formed >10^6 s^(-1); hole transport through DNA over this distance is not rate-limiting. Given the time scale and distance regime, such protein-dependent DNA charge transport chemistry requires consideration physiologically.
© 2001 American Chemical Society. Received November 16, 2000. Publication Date (Web): April 20, 2001. We are grateful to the NIH (GM49216 to J.K.B.) for financial support. We also thank the Deutsche Forschungsgemeinschaft (H.A.W.) the Deutscher Akademischer Austauschdienst (M.P.), and the American Cancer Society (S.R.R.) for postdoctoral fellowships and Mount St. Mary's College Professional Development Fund and NSF (MCB981-7338) for funding of E.D.A.S. We are also very grateful to Drs.S. Kumar and R. J. Roberts of New England Biolabs for wild-type and Q237W mutant M·HhaI.