Download Ultrafast Excited-State Dynamics in Nucleic Acids

DNA Photophysics: Ultrafast Excited-State Dynamics in Base Monomers and
Multimers in the Condensed Phase
Bern Kohler, Department of Chemistry, The Ohio State University, 100 W. 18th Avenue,
Columbus, OH 43210 USA
On account of their significant chemical energies, UV photons from the sun pose a
considerable and longstanding threat to the organic molecules vital to life. For example, the
excited electronic states produced in DNA by UV light are at the beginning of a complex
chain of events that can lead to photocarcinogenesis in humans. Femtosecond pump-probe
spectroscopy is providing dramatic new insights into the dynamics of these excitations [1].
Femtosecond transient absorption experiments have shown that the S1 lifetimes of the natural
DNA and RNA bases are subpicosecond due to ultrafast internal conversion [2]. Rapid
electronic energy relaxation greatly reduces the likelihood of photochemical damage. We
have recently shown that covalently modified nucleobases [3] and even individual tautomers
[4] differ dramatically in their excited-state dynamics. The S1 lifetime of 9-methyladenine is
≈ 200 fs, while the isomeric compound, 7-methyladenine has a lifetime of 4.2 ps [4]. These
experimental results are providing rigorous tests of emerging theoretical models for
nonradiative decay by single nucleobases.
Recent femtosecond pump-probe experiments have shown that electronic energy
relaxation in assemblies of two or more bases (“base multimers”) occurs much more slowly
than in monomeric bases. In adenine-containing oligo- and polynucleotides, decay times are
observed that range from hundreds of femtoseconds to several nanoseconds [5]. The
influence of base stacking and base pairing on excited-state depopulation has been
investigated in recent experiments on custom-sequence oligonucleotides. Our current
understanding of the long-lived states seen in base multimers and their implications for DNA
photodamage will be presented.
[1] C. E. Crespo-Hernández, B. Cohen, P. M. Hare, and B. Kohler, Chem. Rev. (2004), 104,
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[2] J.-M. L. Pecourt, J. Peon and B. Kohler, J. Am. Chem. Soc., (2000), 122, 9348;
J.-M. L. Pecourt, J. Peon and B. Kohler, J. Am. Chem. Soc., (2001), 123, 10370.
[3] R. J. Malone, A. M. Miller and B. Kohler, Photochem. Photobiol., (2003), 77, 158.
[4] B. Cohen, P. M. Hare, and B. Kohler, J. Am. Chem. Soc. (2003), 125, 13594.
[5] C. E. Crespo-Hernández, and B. Kohler, J. Phys. Chem. B (2004), 108, 11182.