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Published May 22, 2007 | Published
Journal Article Open

Double proton transfer dynamics of model DNA base pairs in the condensed phase


The dynamics of excited-state double proton transfer of model DNA base pairs, 7-azaindole dimers, is reported using femtosecond fluorescence spectroscopy. To elucidate the nature of the transfer in the condensed phase, here we examine variation of solvent polarity and viscosity, solute concentration, and isotopic fractionation. The rate of proton transfer is found to be significantly dependent on polarity and on the isotopic composition in the pair. Consistent with a stepwise mechanism, the results support the presence of an ionic intermediate species which forms on the femtosecond time scale and decays to the final tautomeric form on the picosecond time scale. We discuss the results in relation to the molecular motions involved and comment on recent claims of concerted transfer in the condensed phase. The nonconcerted mechanism is in agreement with previous isolated-molecule femtosecond dynamics and is also consistent with the most-recent high-level theoretical study on the same pair.

Additional Information

© 2007 by the National Academy of Sciences. Contributed by Ahmed H. Zewail, April 3, 2007 (received for review March 15, 2007). Published online before print May 14, 2007, 10.1073/pnas.0702944104. We thank Dr. H. Paik for his help in the initial setup of the apparatus, and the four reviewers, Profs. Hans-Heinrich Limbach, Richard L. Schowen, Abderrazzak Douhal, and A. Welford Castleman, Jr., for the helpful comments and careful reading of the manuscript. This work was supported by the National Science Foundation. Author contributions: O.-H.K. and A.H.Z. performed research and wrote the paper. The authors declare no conflict of interest. This article contains supporting information online at www.pnas.org/cgi/content/full/0702944104/DC1.

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