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Efficient Deactivation of a Model Base Pair via Excited-State Hydrogen Transfer

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Science  03 Dec 2004:
Vol. 306, Issue 5702, pp. 1765-1768
DOI: 10.1126/science.1104038

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Abstract

We present experimental and theoretical evidence for an excited-state deactivation mechanism specific to hydrogen-bonded aromatic dimers, which may account, in part, for the photostability of the Watson-Crick base pairs in DNA. Femtosecond time-resolved mass spectroscopy of 2-aminopyridine clusters reveals an excited-state lifetime of 65 ± 10 picoseconds for the near-planar hydrogen-bonded dimer, which is significantly shorter than the lifetime of either the monomer or the 3- and 4-membered nonplanar clusters. Ab initio calculations of reaction pathways and potential-energy profiles identify the mechanism of the enhanced excited-state decay of the dimer: Conical intersections connect the locally excited 1ππ* state and the electronic ground state with a 1ππ* charge-transfer state that is strongly stabilized by the transfer of a proton.

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