The complexes potentially formed by 9-alkyl adenine with acetic acid were characterized, and the feasibility of them undergoing a two-proton transfer via the hydrogen bonds that bind the two components was determined, in light of the chemical simultaneity criterion. In this way, the ability of adenine to undergo a two-proton transfer in its first pi,pi* excited state via Watson-Crick pairing was ruled out. On the other hand, the process is feasible when the adenine binds to acetic acid via Hoogsteen pairing.