On the formation of cyclobutane pyrimidine dimers in UV-irradiated DNA: Why are thymines more reactive?

The reaction pathways for thermal and photochemical formation of cyclobutane pyrimidine dimers in DNA are explored using density functional theory techniques. Although it is found that the thermal [2 + 2] cycloadditions of thymine + thymine (T + T --> Tlozenge T), cytosine + cytosine (C + C --> Clozenge C) and cytosine + thymine (C + T --> Clozenge T) all are similarly unfavorable in terms of energy barriers and reaction energies, the excited-state energy curves associated with the corresponding photochemical cycloadditions display differences that-in line with experimental findings-unanimously point to the predominance of Tlozenge T in UV-irradiated DNA. It is shown that the photocycloaddition of thymines is facilitated by the fact that the S, state of the corresponding reactant complex lies comparatively high in energy. Moreover, at a nuclear configuration coinciding with the ground-state transition structure, the excited-state energy curve displays an absolute minimum only for the T + T system. Finally, the T + T system is also associated with the most favorable excited-state energy barriers and has the smallest S-2-S-0 energy gap at the groundstate transition structure.