Cyclic Guanidino-Sugars with Low pKa as Transition-State Analog Inhibitors of Glycosidases: Neutral Instead of Charged Species Are the Active Forms

Cyclic guanidino-sugars with different p K a values are designed and synthesized as transition-state analog inhibitors of galactosidases. Characterization of these structures ( 7, 10, 12 ) reveals that 7 and 10 are in a pH-dependent equilibrium between a furanose form and a mixture of neutral and protonated tetrahydropyrimidine forms. In contrast, the O -linked guanidino-sugar 12 exists as the tetrahydropyrimidine forms above pH 5. The furanose−tetrahydropyrimidine equilibrium can thus be modulated with the appropriate N -substituent which affects the guanidino-sugar p K a value. Enzymatic inhibition by 7, 10, and 12 is also pH-dependent, indicating that the enzymes recognize the tetrahydropyrimidine form. Evidence is presented to support a dominant role for the uncharged form of the six-membered cyclic guanidino-sugar in the inhibition of galactosidases. Though the inhibition potency is moderate ( K i range 4−50 μM), the use of cyclic guanidino-sugars in the study provides new insights into the mechanism of inhibition of glycosidases.