Hydration of the Lowest Triplet States of the DNA/RNA Pyrimidines

The effects of hydration on the lowest triplet states of the DNA/RNA pyrimidines have been studied by including one and two water molecules explicitly. Three configurations for the singly hydrated cytosine moiety were located, and six for the doubly hydrated system. For thymine and uracil, four singly and eight doubly hydrated structures were found. The singlet-triplet energy gaps of all three pyrimidines (cytosine, thymine, and uracil) fall in the low-energy range of ultraviolet radiation (UVA). Energetic excited states can be a step leading to lesions in DNA, such as a mismatched base pairs. Although the adiabatic and vertical electronic excitation energies for all three pyrimidines slightly increase upon inclusion of additional water molecules, this effect upon the excitation energies is much smaller than hydration effects upon the electron affinities and ionization energies of the three nucleobases. Because both the ground state and the triplet state are neutral, the hydration energy difference between the two states is not significant (compared to those between the neutral and charged species), making the excitation energy less sensitive to hydration.