The difference in lifetime with respect to hydrolysis of two covalent syalosyl-enzyme intermediates of two difluorinated sialic acid analogues (1 and 2) bound to Trypanosoma rangeli sialidase is rationalized based on quantum mechanical calculations. The two intermediates differ only in a single functional group, acetamide in the sialidase-1 complex and hydroxyl in the sialidase-2 complex. It is shown that the acetamide group, which is also present in the natural substrate, increases the pK(a) of a catalytic base (Asp60) through electrostatic repulsion with the carbonyl oxygen on the ligand. This oxygen is absent in 2, resulting in a less basic Asp60 residue and, hence, a longer lifetime of the silaidase-2 complex. Presumably, the lifetime of a sialidase inhibitor complex could be increased further by substituents that stabilize the negative charge on (and lowers the pK(a) value of) Asp60 in T. rangeli sialidase.