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(a) Natural substrate for a DD-peptidase. (b) Benzylpenicillin (PENG), also known as penicillin G. (c) Perfect penicillin (PPEN), a substrate that combines a β-lactam/penam framework and the natural substrate. (d) Perfect cephalosporin (PCEPH), a substrate that combines a β-lactam/cepham framework and the natural substrate.
