What would replacing one of the hydrogens on the urethane nitrogen with a methyl group result in?

• A. An inactive cholinergic agonist; too large for the small hydrophobic pocket available
• B. A potent muscarinic agonist
• C. A selective nicotinic antagonist
• D. An inactive allosteric modulator

Answer: (A)

Small changes to a ligand’s nitrogen can drastically alter receptor binding and activity. Replacing a hydrogen on the urethane nitrogen with a methyl adds steric bulk right at the binding interface. The cholinergic binding pocket that accommodates carbamate-type agonists is small and can tolerate only limited substitution. The extra methyl disrupts the precise orientation and any hydrogen-bonding interactions the urethane N–H would normally provide, so the molecule can’t activate the receptor effectively. The result is loss of agonist activity, making the compound an inactive cholinergic ligand. It wouldn’t become a more potent muscarinic agonist or a selective nicotinic antagonist, and it’s unlikely to act as an allosteric modulator, since that would require fitting a different site with compatible sterics.