Potential energy surfaces for the hydrolysis of a model for adenosine monophosphate

Glycoside hydrolyses are important bioorganic processes whose mechanisms are of considerable interest to physical organic chemists. A simple glycoside may undergo nucleophilic substitution in three ways.(i) The stepwise SNI mechanism involves ratedetermining heterolysis of the bond to the aglycone leaving group, forming a glycosyl cation which is then subject to nucleophilic attack.(ii) The concerted SN2 mechanism involves a bond to the nucleophile being formed as the bond to the leaving group is broken.(iii) The stepwise pre-association mechanism is similar to SNI except that the nucleophile is present in the rate determining step as a" spectator".
The approximate lifetime of typical glycosyl oxocarbenium ions is 10-12s, and experiment suggests that glycoside hydrolysis occurs s 1, 2 at the borderline of the SNI and SN2 mechanism. Characterization of these processes requires knowledge of the transition …