For the G*-T ⇀ ↽ G-T* (reaction 2 in Fig. ), the barrier is considerably smaller than for the wobble transfer reaction (reaction 1 in Fig. ), 0.356 eV vs 0.926 eV. As shown in Fig. , the region of the PES comprised between 0.0 and 0.5 Å corresponds to the small translation of the two bases towards each other to facilitate the transfer. First, the middle proton in the N-H-N bond transfers (denoted by the arrow in Fig. ). Then, the O-H-O proton transfers, observed in the secondary hump after the transition state. We determine that the reduced mass of the double proton transfer is 1.49 m p at the transition state (see supplementary note Here, the forward reaction barrier, E f = 0.356 eV, and asymmetry ∆E = 0.017 eV are small compared to the wobble-to-enol transfer and compares well with previous calculations (E f = 0.34 eV 3 ). The low reaction barrier leads to a fast proton transfer with a large forward and reverse reaction rate on the order of 10 8 s -1 . We use the OQS quantum dynamics model to determine a quantum-to-classical rate ratio of κ = 18.1 and a KIE of 4.25. The high κ and KIE for this reaction suggest that quantum effects play a significant role in reaction 2 and that, due to the fast proton transfer timescale, the system can quickly reach equilibrium.