Fig. 4. Activation barrier, chemical pathway and dcol (d4 + d5 + d6 – d3). A very clear correlation between the value of dcol and the reaction pathway is visible. The One Asp mechanism is correlated with small values of d5 + d6 (stronger hydrogen bonding to Asp25A and, consequently, lower Asp25A pKa) as well as small values of d4 and large values of d3, promoting water deprotonation by Asp25B over competing Asp25A. The opposite tendency is observed for the Two Asp mechanism. In the three exceptional cases where these conditions were not fulfilled, the barriers were far too high for any of the two mechanisms to take place.
