title: Ultrafast deactivation of an excited cytosine-guanine base pair in DNA.
abstract: Multiconfigurational ab initio calculations and QM/MM molecular dynamics simulations of a photoexcited cytosine-guanine base pair in both gas phase and embedded in the DNA provide detailed structural and dynamical insights into the ultrafast radiationless deactivation mechanism. Photon absorption promotes transfer of a proton from the guanine to the cytosine. This proton transfer is followed by an efficient radiationless decay of the excited state via an extended conical intersection seam. The optimization of the conical intersection revealed that it has an unusual topology, in that there is only one degeneracy-lifting coordinate. This is the central mechanistic feature for the decay both in vacuo and in the DNA. Radiationless decay occurs along an extended hyperline nearly parallel to the proton-transfer coordinate, indicating the proton transfer itself is not directly responsible for the deactivation. The seam is displaced from the minimum energy proton-transfer path along a skeletal deformation of the bases. Decay can thus occur anywhere along the single proton-transfer coordinate, accounting for the remarkably short excited-state lifetime of the Watson-Crick base pair. In vacuo, decay occurs after a complete proton transfer, whereas in DNA, decay can also occur much earlier. The origin of this effect lies in the temporal electrostatic stabilization of dipole in the charge-transfer state in DNA.
authors.affiliation: Contribution from the Department of Theoretical and Computational Biophysics, Max-Planck-Institute for Biophysical Chemistry, Am Fassberg 11, D-37077 Göttingen, Germany.
authors.forename: Gerrit
authors.initials: G
authors.lastname: Groenhof
authors.forename: Lars V
authors.initials: LV
authors.lastname: Schäfer
authors.forename: Martial
authors.initials: M
authors.lastname: Boggio-Pasqua
authors.forename: Maik
authors.initials: M
authors.lastname: Goette
authors.forename: Helmut
authors.initials: H
authors.lastname: Grubmüller
authors.forename: Michael A
authors.initials: MA
authors.lastname: Robb
mesh_terms: D020029:Base Pairing / Q000187:drug effects* / Q000528:radiation effects; D003198:Computer Simulation; D003596:Cytosine / Q000737:chemistry* / Q000528:radiation effects; D006147:Guanine / Q000737:chemistry* / Q000528:radiation effects; D010777:Photochemistry; D011789:Quantum Theory
chemical_list: D006147:Guanine; D003596:Cytosine