1.A A and C are both carbapenems. Syllabus p24 2.D D and E are both cephems. D is resistant as the oxime substituent at C-7 reduces binding to cephalosporins. The unhindered amine can attack the b -lactam ring on another molecule, leading to intermolecular polymerization and inactivation.
Steric constraints prevent intramolecular attack in dilute solutions.
The carboxylate catalyzes intramolecular hydrolysis of the b -lactam. 8.B Vancomycin is too bulky to be transported through porins. Therefore, below pH 4 it is protonated, which protects the b -lactam ring from intramolecular acid hydrolysis, so it is more stable. The latest penams and cephems are largely less effective against wild type bacteria as they are a compromise between potency and resistance to b -lactamases. In other words, cilistatin inhibits DHP-1.