How potent is the R enantiomer of beta-methyl substitution of acetylcholine (methacholine) compared to the S enantiomer?

• A. Equally potent
• B. 20-fold less potent
• C. 2-fold more potent
• D. 10-fold less potent

Answer: (B)

Understanding that receptors are chiral and interact with a drug in a highly stereospecific way is key. The beta-methyl substitution in methacholine creates two mirror-image forms that fit the muscarinic receptor differently. The S enantiomer aligns its substituents in a way that maximizes the essential interactions with the receptor’s binding pocket, yielding stronger binding and activation. The R enantiomer, on the other hand, misplaces those groups, causing steric crowding or poorer contacts, so it binds more weakly and produces less effect at the same concentration. That reduced affinity translates to much lower potency—about 20-fold less than the S enantiomer. This kind of large enantioselectivity is common when a receptor site is highly stereospecific.