Which substitution would still be an active agonist albeit weaker when nitrogen is replaced with a heteroatom (as described above)?

• A. Replacement with arsenic, phosphorus, or sulfur
• B. Replacement with neon
• C. No activity
• D. More potent than acetylcholine

Answer: (A)

A key idea here is that the positively charged center that acetylcholine’s nitrogen provides can be preserved by substituting nitrogen with certain other heteroatoms, yielding analogous cationic species that the receptor can still recognize and activate, though less effectively. Replacing nitrogen with phosphorus, sulfur, or arsenic can create phosphonium, sulfonium, or arsonium-like centers that maintain enough structural and electrostatic similarity to act as agonists, but their different sizes and charge distribution typically reduce potency. Replacing with neon would not give a comparable charged center, so activity would be unlikely. Saying it would be more potent than acetylcholine isn’t supported because these substitutions usually weaken binding and efficacy rather than improve them.