To design selective covalent carbamate hBChE inhibitors, we also prepared a series of 6substituted indolyl carbamates, as this approach has been successful before. The dimethyl carbamate 23 inhibited both ChEs and had decreased potency compared to the 7-substituted analogue 6. Increasing the steric bulk led to compounds that lost the ability to covalently modify the enzyme (Table ). To support the covalent mechanism of binding with kinetic evidence, the carbamoylation rate constant (k carb ) and inhibitory constant for the reversible binding K i were determined for 6 from pseudo-first order inhibition rates (k obs ) (Figure , Table ). The carbamate reversibly binds with low nanomolar affinity to the enzyme before the carbamoylation takes place.