Photoinduced electron detachment and proton transfer: the proposal for alternative path of formation of triplet states of guanine (G) and cytosine (C) pair.

A viable pathway is proposed for the formation of the triplet state of the GC Watson–Crick base pair. It includes the following steps: (a) a low-energy electron is captured by cytosine in the GC pair, forming the cytosine base-centered radical anion GC–•; and (b) photoradiation with energy around 5 eV initiates the electron detachment from either cytosine (in the gas phase) or guanine (in aqueous solutions). This triggers interbase proton transfer from G to C, creating the triplet state of the GC pair. Double proton transfer involving the triplet state of GC pair leads to the formation of less stable tautomer G(N2–H)•C(O2H)•. Tautomerization is accomplished through a double proton transfer process in which one proton at the N3 of C(H)• migrates to the N1 of G(−H)•; meanwhile, the proton at the N2 of G transfers to the O2 of C. This process is energetically viable; the corresponding activation energy is around 12–13 kcal/mol. The base-pairing energy of the triplet is found to be ∼3–5 kcal/mol smaller than ...