Can you increase or decrease the distance of the ethylene bridge in acetylcholine?

Can you increase or decrease the distance of the ethylene bridge in acetylcholine?

• A. They have bad cholinergic activity; ethylene bridge should be retained
• B. Distance changes have no effect on activity
• C. Shortening improves activity
• D. Extending distance increases nicotinic selectivity

Answer: (A)

The distance of the ethylene bridge sets the geometry for how acetylcholine fits its receptor. That two-carbon linker positions the quaternary ammonium correctly relative to the acetyl ester so the molecule can engage the binding pockets of both nicotinic and muscarinic sites. If you shorten or extend this linker, the spatial arrangement becomes misaligned, reducing binding affinity and overall cholinergic activity. So the ethylene bridge should be retained to preserve activity. In practice, changing the linker length generally lowers potency, rather than improving it or creating reliable selectivity differences.

The distance of the ethylene bridge sets the geometry for how acetylcholine fits its receptor. That two-carbon linker positions the quaternary ammonium correctly relative to the acetyl ester so the molecule can engage the binding pockets of both nicotinic and muscarinic sites. If you shorten or extend this linker, the spatial arrangement becomes misaligned, reducing binding affinity and overall cholinergic activity. So the ethylene bridge should be retained to preserve activity. In practice, changing the linker length generally lowers potency, rather than improving it or creating reliable selectivity differences.