Elongating the methyl group to an ethyl group on the ester of acetylcholine would result in what?

• A. Worse activity as a cholinergic agonist
• B. No change in activity
• C. Improved activity as a cholinergic agonist
• D. Acts as a cholinergic antagonist

Answer: (A)

The key idea here is how small changes in the ester portion of a cholinergic agonist affect how well it fits and activates the receptor. Acetylcholine relies on a precise shape and size: a compact acetyl ester that positions the ester linkage and the quaternary ammonium group just right to engage the receptor binding site and be recognized by acetylcholinesterase. Extending the alkyl group from methyl to ethyl adds steric bulk near the ester bond, making it harder for the molecule to tuck into the receptor pocket in a way that triggers activation. This typically decreases receptor affinity and efficacy, so the compound acts as a weaker cholinergic agonist. While a bulkier ester can slow hydrolysis by acetylcholinesterase, the initial agonist activity is reduced because the fit is poorer. Therefore, increasing the ester group from methyl to ethyl leads to worse activity as a cholinergic agonist.