Nucleobases, mean excitation energies

Table 5.5 The number of electrons, the Thomas-Reiche-Kuhn sum rule in the length representation So, and the mean excitation energy, /q, for the nucleobases...

Table 5.6 Calculated components and anisotropy of the mean excitation energies of the nucleobases...

The anisotropy of fhe purine nucleobases is larger than that of fhe pyrimidines, which is attributable to the larger physical extension in the direction of the molecular plane. The mean excitation energy of fhe nucleobases is also more anisotropic than in the case of fhe amino acids, which is caused by the fact that the nucleobases include planar heterocycles with conjugated 7T-bonds. 

Comparison with Table 5.5 shows that increasing the complexity of the system, in this case, by forming fhe nucleoside from fhe nucleobase has little effect on the mean excitation energy of fhe sysfem. 

Consider a nucleotide made up of a nucleoside and one fo fhree phosphafe groups. If the nucleoside is composed of a nucleobase confaining N elecfrons, and there are M phosphate groups in the nucleotide, then the mean excitation energy of the nucleotide can be written as follows ... 

The details of fhe cellular environment are also apparently unimportant. Although the directional dependence of the mean excitation energy with regard to the direction of fhe beam wifh respect to the nucleobase plane is significant, in vivo the biomolecules would not be expected to maintain any particular orientation. 

S.P.A. Sauer, J. Oddershede, J.R. Sabin, The Mean Excitation Energies and Their Directional Characteristics for Energy Deposition by Swift Ions on the DNA and RNA Nucleobases, J. Phys. Chem. C 114 (2010) 20335-20341. 

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. 

---