The efficiency of proton transfer in Kirby's enzyme model, a computational approach

DFT and ab initio calculation results for proton transfer reactions in Kirby's acetals reveal that the mechanism proceeds via efficient intramolecular general acid catalysis (IGAC) and not through a 'classical' general acid catalysis mechanism (GAC). Further, they show that the driving force for the proton transfer efficiency is the proximity of the two reactive centers (r) and the attack angle (α), and the rate of the reaction is linearly correlated with r2 and sin (180° − α). Acetals with short r values and with α values close to 180° (forming a linear H-bond) are more reactive due to the development of strong hydrogen bonds in their global minimum, transition state, and product structures.

Proximity orientation due to hydrogen bonding in the ground state and transition state is the sole driving force for the remarkable acceleration of proton transfer in Kirby's system.