We have explored the potential energy profiles of TpT dinucleotides toward formation of a DNA photolesion product, spore photoproduct (SP), along the S-0, S-1, and T-1 states, by means of density functional theory and time-dependent density functional theory. Together with the spin density analysis, the consecutive mechanism for the SP formation can be established. The detailed reaction pathways have been revealed. All the adiabatic reaction pathways proceeding though S-1, T-1, or S-0 alone are shown to be energetically infeasible, while the nonadiabatic pathway involving both the T-1 and S-0 states corresponds to the lowest-energy path and is the most favorable in energy. The nonadiabatic pathway is rate-limited by the step of the hydrogen atom transfer proceeding in the T-1 state with a barrier of 14.2 kcal mol(-1) (11.9 kcal mol(-1) in bulk solution), whereas the subsequent C5-CH2 bond formation toward the final SP formation occurs readily in S-0 after intersystem crossing from T-1 to S-0 via the singlet-triplet interaction. The results provide a rationale for the experimentally observed kinetic isotope effect after deuterium substitution at the 3'-T methyl group of TpT.