Nucleobases, conical intersections

Transient absorption experiments have shown that all of the major DNA and RNA nucleosides have fluorescence lifetimes of less than one picosecond [2—4], and that covalently modified bases [5], and even individual tautomers [6], differ dramatically in their excited-state dynamics. Femtosecond fluorescence up-conversion studies have also shown that the lowest singlet excited states of monomeric bases, nucleosides, and nucleotides decay by ultrafast internal conversion [7-9]. As discussed elsewhere [2], solvent effects on the fluorescence lifetimes are quite modest, and no evidence has been found to date to support excited-state proton transfer as a decay mechanism. These observations have focused attention on the possibility of internal conversion via one or more conical intersections. Recently, computational studies have succeeded in locating conical intersections on the excited state potential energy surfaces of several isolated nucleobases [10-12]. 

Surface Hopping, Excited States, Density Functional Theory, Ultrafast Internal Conversion, Conical Intersections, Nucleobases, Base Pairs, Photostability, UV Genetic Damage... 

Along this OH dissociation coordinate, we also find a conical intersection between the ttct state,. S , and the ground state, S0, which could act as an efficient route for internal conversion. Such a scenario has been advocated by Domcke and Sobolewski [23, 84, 86] to be responsible for the photostability of nucleobases. However, in the present case, the free energy activation barrier for OH dissociation was computed to be 52 kJ/mol [47], Hence this de-excitation pathway is unlikely to explain the ultrafast nonradiative decay observed experimentally [5, 11, 37], Shukla and Leszczynski [80] find an activation barrier of 154 kJ/mol for the keto-enol tautomerisation of 7H G. However, this result is for tautomerisation in the tttt state, whereas the ROKS study involves two different excited states [47],... 

It is interesting that the biradical geometries of the purine bases, resulting from the twisting of the N3-C2 bond, are very similar to the geometries of the nucleobases at the S1(tttt )/S0 conical intersections located by higher levels of theory [17-20], There is also a close geometrical similarity between the biradical state of pyrimidine... 

The above results on the covalently modified nucleobases support the theoretical conclusion that ultrafast internal conversion occurs via the conical intersection of the... 

For cytosine, in common with other pyrimidine nucleobases, photostability is the result of an ethylenic-type conical intersection associated with torsion around a C = C double bond. This interpretation remains - but the barrier is reduced - when the solvent is included approximately. 

The five nucleobases are non-fiuorescent species (Longworth et al. 1966), which implies that the photo-energy is dissipated by means of ultrafast internal conversion. Time-resolved spectroscopy experiments have shown that either in gas phase or in solution the time constant for deactivation of the nucleobases excited at the 260 nm 7Ttt <-So band is in the range of 0.5-6ps (Canuel et al. 2005 Crespo-Hernandez et al. 2004 Kang et al. 2002 Ullrich et al. 2004). This means that pathways to conical intersections where radiationless decay occurs are easily available from the Franck-Condon region. 

A great deal of theoretical work has been dedicated to nucleobases and their derivatives to identify conical intersections and reaction paths leading to them. In summary it has been found that these conical intersections can be formed by four different mechanisms ... 

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