In order to calculate the contribution of quantum tunnelling in this activated tunnellingready state, we re-evaluate the PES considering only where the proton is transferring. We then calculate the rate of proton tunnelling from the tunnelling ready state through region 2. We calculate the inner barrier section (between 4-7.5 Å) by taking the image of the start of the barrier from reaction 1 and assuming that the local polymerase environment has thermally induced this conformation change. Using this approach, we calculate the quantum contribution to the reaction rate and find that the overall rate is much larger. The rate is now 1.279 × 10 -1 s -1 , with a κ = 99.0 indicating a large contribution from tunnelling. By substituting hydrogen with deuterium, we find that the tunnelling-corrected reaction rate exhibits a KIE of 10.15, which is compatible with experimental results 2 that predict a threefold reduction in rate. Consequently, by adopting the tunnelling-ready state we are able to bridge the timescale gap between experimental observation (0.6-68 s -1 2 ) and the rate predicted by theory (1.279 × 10 -1 s -1 ). The tunnelling-ready mechanism for proton transfer could be facilitated via a concerted mechanism between the transition of the polymerase domain from the ajar to the closed conformation, which is consistent with Li et al. The conformation change could sample the short-lived tunnelling-ready state where the reactant and products become similar in energy, increasing the quantum contribution to the rate.