Chemical structure of<unk>  G. The sulfur and nitrogen of the five-membered thiazolidine ring are shown in yellow and blue respectively. The image shows that the thiazolidine ring and fused four-membered β-lactam are not in the same plane. The term "penam" is used to describe the common core skeleton of a member of the<unk> s. This core has the molecular formula R-C9H11N2O4S, where R is the variable side chain that differentiates the<unk> s from one another. The<unk> am core has a molar mass of 243 g/mol, with larger<unk> s having molar mass near 450—for example, cloxacillin has a molar mass of 436 g/mol. 6-APA (C8H12N2O3S) forms the basic structure of<unk> s. It is made up of an enclosed dipeptide formed by the condensation of L-cystein and D-valine. This results in the formations of β-lactam and thiazolidinic rings. The key structural feature of the<unk> s is the four-membered β-lactam ring; this structural moiety is essential for<unk> 's antibacterial activity. The β-lactam ring is itself fused to a five-membered thiazolidine ring. The fusion of these two rings causes the β-lactam ring to be more reactive than monocyclic β-lactams because the two fused rings distort the β-lactam amide bond and therefore remove the resonance stabilisation normally found in these chemical bonds. An acyl side side chain attached to the β-lactam ring. A variety of β-lactam antibiotics have been produced following chemical modification from the 6-APA structure during synthesis, specifically by making chemical substitutions in the acyl side chain. For example, the first chemically altered<unk> , methicillin, had substitutions by methoxy groups at positions 2’ and 6’ of the 6-APA benzene ring from<unk>  G. This difference makes methicillin resistant to the activity of β-lactamase, an enzyme by which many bacteria are naturally unsusceptible to<unk> s.
Penicillin
Carbenicillin
Extended-spectrum penicillin
Tetracycline
Phenoxymethylpenicillin
Benzylpenicillin
Piperacillin
Methicillin