Assuming that the system-to-environment coupling constant is dominated by the thermal fluctuations of the surrounding water molecules, we can estimate the value of γ. Water has a vibrational spectrum in the range 3300-3900 cm -1 . So, assuming that the fastest oscillators in this range dominate, we use an Ohmic spectral density for our environment oscillators having a coupling parameter γ = 3900 cm -1 . We determine the quantum contribution to the reaction rate by monitoring the flux of the density passing through the transition state, see supplementary note 2 for further details. The forward and reverse reaction rate constants, k f and k r , are obtained from,