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Which of the following correctly describes the key structural feature of penicillin that is responsible for its antibiotic activity?

1. (A) It contains a beta-lactam (4-membered cyclic amide) ring fused to a thiazolidine ring
2. (B) It contains a gamma-lactam (5-membered cyclic amide) ring fused to a pyrimidine ring
3. (C) It contains a beta-lactam ring fused to a piperidine ring with a sulphur atom in the side chain
4. (D) It contains an alpha-lactam (3-membered cyclic amide) ring fused to an oxazoline ring

Correct answer: (A) It contains a beta-lactam (4-membered cyclic amide) ring fused to a thiazolidine ring

Solution

Penicillin is a bicyclic molecule consisting of a beta-lactam ring (4-membered cyclic amide, highly strained) fused to a thiazolidine ring (5-membered ring containing sulphur and nitrogen). The beta-lactam ring is essential for antibiotic activity; it irreversibly inhibits the transpeptidase enzyme involved in bacterial cell wall synthesis.

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