Glucosyl oxocarbenium ion

A case similar to the slow, practically irreversible inhibition of jack bean a-D-mannosidase by swainsonine is represented by the interaction of castanospermine with isomaltase and rat-intestinal sucrase. Whereas the association constants for the formation of the enzyme-inhibitor complex were similar to those of other slow-binding glycosidase inhibitors (6.5 10 and 0.3 10 M s for sucrase and isomaltase, respectively), the dissociation constant of the enzyme-inhibitor complex was extremely low (3.6 10 s for sucrase) or could not be measured at all (isomaltase), resulting in a virtually irreversible inhibition. Danzin and Ehrhard discussed the strong binding of castanospermine in terms of the similarity of the protonated inhibitor to a D-glucosyl oxocarbenium ion transition-state, but were unable to give an explanation for the extremely slow dissociation of the enzyme-inhibitor complex. 

C. Chiappe, G. L. Moro, and P. Munforte, Lifetime of the glucosyl oxocarbenium ion and stereoselectivity in the glycosidation of phenols with 1,2-anhydro-3,4,6-tri-O-mcthyl-u-D-glucopyranose, Tetrahedron, 53 (1997) 10471-10478. 

As a result of these studies, and the approximately 4-fold greater stability attributed to 2-deoxyglycosyl oxocarbenium ions over their 2-hydroxy counterparts, Zhu and Bennet estimated that the lifetime for the 2-deoxyglucosyl oxocarbenium ion is on the order of 2 x 10-11 s, and 10—12 s for the glucosyl oxocarbenium ion in water, consistent with prior results.29... 

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